keefer 2002, landslides caused by earthquakes; a historical review

8/10/2019 Keefer 2002, Landslides Caused by Earthquakes; A Historical Review

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INVESTIGATING LANDSLIDES CAUSED BY EARTHQUAKES AHISTORICAL REVIEW

DAVID K. KEEFERU.S. Geological Survey, 345 Middleeld Road MS 977, Menlo Park, CA 94025, USA

E-mail: [email protected]

(Received 17 January 2002; Accepted 11 February 2002)

Abstract. Post-earthquake eld investigations of landslide occurrence have provided a basis forunderstanding, evaluating, and mapping the hazard and risk associated with earthquake-inducedlandslides. This paper traces the historical development of knowledge derived from these investiga-tions. Before 1783, historical accounts of the occurrence of landslides in earthquakes are typicallyso incomplete and vague that conclusions based on these accounts are of limited usefulness. For

example, the number of landslides triggeredby a given event is almost always greatly underestimated.The rst formal, scientic post-earthquake investigation that included systematic documentation of the landslides was undertaken in the Calabria region of Italy after the 1783 earthquake swarm. Fromthen until the mid-twentieth century, the best information on earthquake-induced landslides camefrom a succession of post-earthquake investigations largely carried out by formal commissions thatundertook extensive ground-based eld studies. Beginning in the mid-twentieth century, when the useof aerial photography became widespread, comprehensive inventories of landslide occurrence havebeen made for several earthquakes in the United States, Peru, Guatemala, Italy, El Salvador, Japan,and Taiwan. Techniques have also been developed for performing retrospective analyses years ordecades after an earthquake that attempt to reconstruct the distribution of landslides triggered by theevent. The additional use of Geographic Information System (GIS) processing and digital mappingsince about 1989 has greatly facilitated the level of analysis that can applied to mapped distributionsof landslides. Beginning in 1984, syntheses of worldwide and national data on earthquake-inducedlandslides have dened their general characteristics and relations between their occurrence andvarious geologic and seismic parameters. However, the number of comprehensive post-earthquakestudies of landslides is still relatively small, and one of the most pressing needs in this areaof researchis for the complete documentation of landslides triggered by many more earthquakes in a widervariety of environments.

Keywords: Debris ows, earthquakes, ground failure, historical landslides, landslides, landslideinventories, lateral spreads, liquefaction, review, rock falls, seismic slope stability, slope failure

1. Introduction

Much has been learned about earthquake-induced landslides since the rst formal,scientic post-earthquake investigation was undertaken following the earthquakeswarm in Calabria, Italy in 1783. Knowledge has come from eld studies followingmany earthquakes throughout the world, and these studies have generally becomemore detailed and comprehensive with time as increased resources and new tools,

Surveys in Geophysics 23: 473510, 2002. 2002 Kluwer Academic Publishers. Printed in the Netherlands.


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474 DAVID K. KEEFER

such as aerial photography and Geographic Information Systems (GIS) processing,have become available.

The primary purpose of this paper is to review the history of the most compre-hensive investigations of earthquake-induced landslides. The review concentrateson (1) those earthquakes for which the landslides have been documented mostcomprehensively, and (2) those studies that have synthesized the primary datafrom post-earthquake investigations to determine general characteristics of land-slide occurrence. Studies of both these types have provided the basis for increasingnumbers of seismic landslide-hazard and slope-stability analyses on a variety of scales, but no attempt is made to review those analyses here. This paper, however,does additionally review less extensive but signicant recent ndings on threeselected topics: the occurrence of landslides at great distances from earthquakesources, eld verication of Arias intensity thresholds, and delayed activation of landslide movement by earthquake shaking. This selection of topics and studies issubjective and not exhaustive; much has also been learned from other studies that

are omitted from this review because of limitations of time and space.The rst extensive synthesis of data on the occurrence of landslides in earth-

quakes was completed in 1984, for earthquakes that had occurred through May of 1980 (Keefer, 1984). The present paper thus initially discusses investigations of landslides caused by earthquakes through May of 1980. Results from that initialsynthesis are then summarized. Following that, comprehensive landslide inventor-ies from events that occurred after 1980 are described, and then more recent studiessynthesizing worldwide and national data are discussed. The nal main sectionreviews the ndings on landslides at great distances, Arias intensity thresholds,and delayed landslide activation.

2. From ancient times through May 1980

Landslides caused by earthquakes have been documented from at least as early as1789 BCE in China (Hansen and Franks, 1991) and 373 or 372 BCE in Greece(Seed, 1968). However, before about 200 years ago, accounts of earthquake-induced landslides in historical documents are generally incomplete for any givenevent, imprecise concerning locations of landslides, and vague about the charac-teristics of material and movement. While sometimes valuable from a historicalperspective or for local hazard analysis, such accounts give at best an incompleteand at worst a misleading picture of the extent, nature, and abundance of landslidescaused by any given seismic event.


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INVESTIGATING LANDSLIDES CAUSED BY EARTHQUAKES 475

2.1. E ARLY FORMAL INVESTIGATIONS

The earthquake swarm in Calabria, Italy in 1783 (estimated maximum magnitude,M I 1 = 7) led to the rst formal scientic study of a large earthquake when theNeapolitan Academy of Sciences and Fine Letters appointed a commission to con-duct eld studies in the epicentral area (Sarconi, 1784, quoted by Cotecchia et al.,1986; Davison, 1936; Cotecchia et al., 1986). These reports, a subsequent synthesisby Sir Charles Lyell, in his Principles of Geology (1874), and retrospective studiesby Cotecchia and Melidoro (1974) and Cotecchia et al. (1986) indicate that land-slides, including several that were exceptionally large, were widespread. Amongother effects, at least 215 landslide-dammed lakes were documented (Vivenzio,1788, quoted by Cotecchia et al., 1986).

Throughout the nineteenth and early twentieth centuries, various other scienticcommissions carried out eld investigations after several other major earthquakes,and landslides were commonly documented along with other earthquake effects.

Although undertaken without the benet of aerial observation or aerial photo-graphy, several of these investigations provided extensive information on thelocations, though not always on the characteristics, of landslides caused by theearthquakes. Examples of these early reports include those on the M I 7.3 Charle-ston, South Carolina, earthquake of 1886 (Dutton, 1889), the M 8.3 Assam, India,earthquake of 1897 (Oldham, 1899), the M 7.8 San Francisco, California, earth-quake of 1906 (Lawson, 1908), and the M 8.1 Bihar, India-Nepal, earthquake of 1934 (Geological Survey of India, 1939). All of these reports contain descriptionsof landslides occurring throughout the relatively large areas that were surveyedon the ground, although not all localities that probably produced landslides weredescribed. Notable for its detailed descriptions of landslides is the report of Lawson(1908) on the San Francisco earthquake. From this and other contemporary reportson the earthquake (Youd and Hoose, 1978), it is possible to infer that thousandsof landslides occurred throughout an area of 32,000 km 2. Many localities are alsodescribed in enough detail to determine the types and source characteristics of thelandslides.

2.2. R ETROSPECTIVE STUDIES

Some other early earthquakes have been the subjects of retrospective studiesconducted decades after the events that have sought to reconstruct the landslideoccurrence. Even eld investigations conducted soon after an earthquake mayencounter problems separating out landslides triggered by the earthquake from,for example, landslides triggered by recent rainstorms. Retrospective studies face

1 Earthquake magnitude designations are as follows: M s signies a Richter surface-wave mag-nitude, M L a Richter local magnitude, M a moment magnitude, M I a magnitude estimated fromintensity information, and M a general magnitude determination of type not specied in the originalsource. Moment magnitude, M , is used preferentially throughout this paper.


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476 DAVID K. KEEFER

the additional challenges of separating earthquake-induced landslides from land-slides possibly triggered by other, unrelated events. The condence in identifyinglandslides caused by an earthquake several decades in the past can be increasedif even incomplete reports or eyewitness accounts recorded soon after the eventare available; these can be used to calibrate landslide identication by establishingapparent geomorphic ages and degrees of surface alteration for landslides triggeredby that earthquake.

Early retrospective studies include the investigation of a 1935 earthquake inNew Guinea ( M = 7.9), for which Simonett (1967) pioneered a statistical method todifferentiate presumed earthquake-induced landslides from landslides presumablytriggered by other events, such as rainstorms. He concluded that a distribution inwhich the concentration of landslides decreases away from a central point or zoneindicates earthquake triggering, and this conclusion is conrmed by typical land-slide distributions associated with recent earthquakes. Other retrospective studieswere used to identify landslides caused by several earthquakes in northern Cali-

fornia (Youd and Hoose, 1978), landslides in British Columbia caused by an M s7.2 earthquake in 1946 (Matthews, 1979; Rogers, 1980), and landslides causedby earthquakes in 1929 and 1968 in New Zealand (Adams, 1980; Pearce andOLoughlin, 1985; Hancox et al., 2002).

The retrospective approach has been applied most extensively to the 18111812New Madrid, Missouri, earthquake sequence ( M I 8.1, 7.8, and 8 for the threemain shocks; Johnston, 1996). Contemporary accounts, such as those in Mitchill(1815) and Penick (1981), indicate that many landslides occurred on riverbanksand bluffs of the Mississippi River during the earthquake. Fuller (1912) synthesizedinformation from the available contemporary accounts and carried out eld studiesin the affected region in 1904 and 1905. He found abundant evidence of relatively

recent landslides along a 56-km-long stretch of the bluffs. Noting that uprighttrees on landslide surfaces were also of a fairly uniform age of a little less than100 years, he concluded that the landslides had occurred during the 18111812earthquake sequence.

To test the hypothesis that these landslides were a result of the earthquakesequence, Jibson (1985) and Jibson and Keefer (1988, 1989, 1993) prepared alandslide inventory and performed statistical and geotechnical analyses. The in-ventory identied about 200 large, complex landslides that appeared to be aboutthe same age, and statistical analysis showed signicant correlation between theconcentrations of these landslides and distances from the inferred hypocenters of the largest three 18111812 earthquakes (Jibson, 1985; Jibson and Keefer, 1988,1989). Additionally, detailed slope-stability analyses of two representative land-slides showed that neither would have moved under any reasonable combinationof non-seismic conditions whereas inferred ground shaking from the 18111812events would have induced large displacements in both (Jibson, 1985; Jibson andKeefer, 1993). The convergence of evidence from inventory mapping, statistical


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analysis, and geotechnical investigations led to the conclusion that these landslidesalmost certainly moved during the 18111812 earthquakes.

2.3. C OMPREHENSIVE LANDSLIDE INVENTORIES THROUGH 1980

Aerial photographs, which started becoming generally available in the 1930s and1940s, have become the single most important tool for documenting the occur-rence of landslides in earthquakes. They were probably used systematically for therst time to document earthquake effects immediately following the M 7.3 Fukui,Japan, earthquake in 1948 (Collins and Foster, 1949). Where landslides are visiblefrom the air, systematic and complete post-earthquake aerial photography poten-tially allows landslides from a seismic event to be completely documented andprecisely located. This in turn permits landslide occurrence to be correlated withtopographic, geologic, and seismic parameters, thus providing the basic data forgeneral assessments of landslide susceptibility and hazard. Aerial photographs do,of course, have limitations for documenting landslide occurrence. These include alower limit on the size of landslides that can be detected, lack of visibility in heavilyvegetated areas, difculty in differentiating landslides triggered by an earthquakefrom landslides triggered by other events, difculty in discerning landslide types,and lack of complete airphoto coverage of affected areas after most earthquakes.For these reasons, ground-based eld studies have also been crucial in preparingmost comprehensive landslide inventories.

Probably the rst essentially complete post-earthquake inventory of landslideswas produced for the relatively small 1957 Daly City, California, earthquake ( M L =5.3). For this event, Bonilla (1960) plotted the locations of all 23 known coseismiclandslides, which occurred in an area of about 10 km 2. He reported the dimen-

sions, type of material, and type of movement for each landslide, thus providing anexcellent model for later post-earthquake investigations.The much larger 1964 Alaska earthquake ( M 9.2) provided much data on land-

slides even though the area affected was so large (269,000 km 2; Plafker et al.,1969) and much of it so remote that a complete inventory of the landslides was notattempted. Nevertheless, thousands of landslides were documented by many invest-igators, and results of the investigations of landslides along with other earthquakeeffects were published in an eight-volume report by the National Academy of Sciences (National Research Council Committee on the Alaska Earthquake, 19681973) and many other reports and articles (for a list, see Keefer and Tannaci, 1981).Detailed descriptions and analyses were carried out on a wide variety of landslides,including large rock avalanches, subaqueous landslides, and several landslides thatcaused considerable damage in Anchorage, the largest city in Alaska.

The M 7.9 earthquake in the Rio Santa region of Peru on May 31, 1970, pro-duced the most destructive single earthquake-induced landslide in historic times the rock avalanche from Nevados Huascarn in the Andean Cordillera Blanca,which killed more than 18,000 people (Plafker et al., 1971). The earthquake also


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478 DAVID K. KEEFER

generated thousands, or possibly tens of thousands of other landslides throughoutan onshore area of about 30,000 km 2 (Plafker et al., 1971). An extensive airphotoinventory, the rst of its kind produced for a large earthquake, was prepared for anarea of 8,300 km 2 in the most heavily affected region; it showed the locations of about a thousand of the sources of the largest landslides as well as one extensivearea of numerous rock falls (Plafker et al., 1971). Landslides of different types weredifferentiated by use of different map symbols, and the overwhelming majority of the landslides were identied as rock falls, rock slides, or disrupted soil slides(Plafker et al., 1971). The report accompanying the map (Plafker et al., 1971) con-tained much additional data on the types of landslides, the distribution of landslideseither outside the limits of airphoto coverage or too small to be discerned, and thegeologic environments of the landslides sources.

Several more post-earthquake inventories prepared between 1970 and 1980,inclusive, substantially expanded knowledge of the occurrence of landslides inearthquakes. The rst of these was prepared after the M 6.7 San Fernando, Cali-

fornia, earthquake of February 9, 1971 (Morton, 1971, 1975) and showed thelandslides that occurred throughout about 250 km 2 of the most heavily affectedregion. Morton (1975) noted that several thousand landslides occurred in this area,but only the larger ones and those whose displacement was sufcient to producelandslide morphology or to give rise to bare areas stripped of vegetation could bemapped from the aerial photographs. Additionally, Morton (1975) indicated thatinnumerable minor rockfalls in roadcuts occurred outside the mapped area, andother sources indicated that landslides from this earthquake occurred throughoutan area of 3400 km 2 (Keefer, 1984). Nevertheless, this study provided detaileddata on earthquake-induced landslides, including types of movement and mater-ial involved. The study also concluded that the types of landslides produced by

the earthquake were different from the large rotational or complex landslidespreviously found to be characteristic of this area (Morton, 1971, 1975).Probably the rst attempt to map essentially all of the landslides produced by

a large earthquake was the inventory prepared by Harp et al. (1981) after the Feb-ruary 4, 1976, Guatemala earthquake ( M 7.5). Mapping from aerial photographscovering an area of 16,000 km 2 and ground-based eld studies revealed more than10,000 landslides (Harp et al., 1981), a number later estimated by E.L. Harp (un-published data) to be about 50,000. Nearly all of these landslides were rock falls ordisrupted soil slides, and 90 percent of these occurred in weakly-cemented Pleisto-cene pumice deposits or the residual soils developed on them. Analyses by Harp etal. (1981) related the landslide occurrence to bedrock lithology, slope inclination,topographic amplication of ground motion, seismic intensity, and regional-scalefracture systems in the bedrock. Harp et al. (1981) also found that only one of the many pre-existing, large and deep-seated landslides mapped in the region wasreactivated by the earthquake shaking.

Landslides from the 1976 Friuli, Italy, earthquake sequence ( M S of the largestshock = 6.36.5) were also documented in detail (Ambraseys, 1976; Govi, 1977a,


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1977b; Govi and Sorzana, 1977) from airphoto interpretation and ground-basedeld work. Landslides, which were almost entirely rock falls, were mappedthroughout an area of about 500 km 2 in the epicentral region, with sources, travelpaths, dimensions, and locations of deposits shown for masses as small as singleboulders. Such detailed and complete mapping permitted analyses relating land-slide occurrence and hazard to topographic and geologic parameters with a highdegree of condence (Govi, 1977b). In all, landslides were produced by the earth-quake sequence throughout an area of 2100 km 2 (Keefer, 1984), and landslidesthroughout this larger area were documented in a detailed ground-based eld studyby Ambraseys (1976).

A complete inventory of landslides (Wilson et al., 1985) was produced afterthe January 1980 Mount Diablo earthquake sequence in the San Francisco Bayregion of California (maximum M = 5.8). This inventory, produced from extensiveground-based eld work and limited observation from light aircraft, showed thelocations and types of 103 mostly small landslides that occurred throughout an

area of 500 km 2.Another virtually complete inventory of landslides was produced after the May

1980 earthquake sequence at Mammoth Lakes (maximum M = 6.2) in the easternSierra Nevada of California. The steep and high, glaciated slopes in this regionproduced abundant rock falls and rock slides and a few landslides of other typesthroughout an area of 1220 km 2, virtually all of which were documented from acombination of airphoto interpretation, aerial observation, and ground-based eldwork. A map produced by Harp et al. (1984) shows 5244 landslides in the epicent-ral area, and an additional 9 rock falls and rock slides were reported from YosemiteValley, immediately to the West (Wieczorek and Jger, 1996).

3. 1984 synthesis of data from earthquakes through 1980

In an attempt to provide a general framework for understanding the occurrence of landslides in earthquakes, Keefer (1984) analyzed data from a sample of historicalearthquakes ranging in time from the 18111812 New Madrid earthquake sequencethrough the May 1980 Mammoth Lakes earthquake sequence. The primary datawere taken from a sample of 40 worldwide earthquakes having magnitudes inthe range between 5.2 and 9.5, chosen to represent a wide variety of geographic,geologic, and seismic environments. The earthquakes chosen included all thosefor which comprehensive landslide inventories were available as well as others forwhich data on landslides were relatively extensive or for which some aspect of the landslide occurrence (such as landslide damage or landslide mechanisms) wasespecially signicant. To determine the smallest earthquakes and lowest shakingintensity levels associated with landslides, intensity reports from several hundredmostly small earthquakes in the United States were also studied.


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480 DAVID K. KEEFER

Earthquake-induced landslides were classied into fourteen individual typesand three main categories on the basis of type of material, type of landslide move-ment, degree of internal disruption of the landslide mass, and geologic environment(Table I). The rst major category Disrupted Slides and Falls, or DisruptedLandslides includes highly to very highly disrupted landslides, consisting of boulders and masses of rock fragments, small blocks of soil, and (or) individual soilgrains that move downslope by falling, bouncing, and (or) rolling (rock falls andsoil falls), translational sliding (rock slides and disrupted soil slides), or complexmechanisms involving both sliding and uid-like ow (rock avalanches and soilavalanches). Landslides in this category typically originate on steep slopes, travelrelatively fast, and are capable of transporting material far beyond the bases of thesteep slopes on which they originate. Except for rock avalanches, all of which havevolumes greater than 0.5 106 m3, landslides in this category are also typicallythin, with initial failure depths of less than 3 m (Table I).

The second major category Coherent Slides, or Coherent Landslides in-

cludes translational slides (rock block slides and soil block slides), rotational slides(rock slumps and soil slumps), and slow earth ows, which move by a combinationof translational sliding and ow. These landslides exhibit a slight to moderateamount of internal disruption, typically consisting of a few moving blocks, eachof which may be little deformed except for localized internal ssuring. These typesof landslides occur most commonly on moderately steep slopes, typically moverelatively slowly, and displace material less than 100 m. These landslides are alsorelatively thick, with typical initial failure depths greater than 3 m (Table I).

The third major category Lateral Spreads and Flows includes thoselandslides for which uid-like ow is the predominant movement mechanism.Landslides in this category initiate only in soil materials and involve either blocks

of relatively intact material moving on a subsurface liqueed zone (soil lateralspreads) or more completely liqueed masses that move by uid-like ow through-out (rapid soil ows). In many cases, these landslides are the results of soilliquefaction in saturated sands, gravels, or silts; occasionally they result fromseismically-induced disturbance in sensitive (i.e., thixotropic) clays. Also includedin this category are all underwater (subaqueous) landslides, most of which arecomplex and can involve elements of rotational or translational sliding as well aslateral spreading or ow. Landslides in this category commonly initiate and moveon gentle to nearly level slopes, move rapidly, and can transport material largedistances (Table I).

Keefer (1984) gave order-of-magnitude estimates for the numbers of landslidesof each type triggered by each earthquake studied. Total numbers of landslides gen-erally increased with earthquake magnitude, ranging from a few tens of landslidesat most for earthquakes with M < 5.5 to several thousands at least for earthquakeswith M > 8.0, although numbers were highly variable among individual eventswithin each magnitude range. This variability almost certainly resulted from realvariations in the numbers of landslides triggered as well as from the incomplete


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T A B L E I

C h a r a c t e r i s t i c s o f e a r t h q u a k e - i n d u c e d l a n d s l i d e s ( M o d i e d f r o m K e e f e r 1 9 8 4 , 1 9 9 9 )

N a m e

T y p e o f

I n t e r n a l

W a t e r c o n t e n t

T y p i c a l

M i n i m u m T y p i c a l

T y p i c a l

T y p i c a l

m o v e m e n t

d i s r u p t i o n

D

U P S S

d e p t h s

s l o p e ( ) v e l o c i t i e s

v o l u m e s

d i s p l a c e m

e n t s

D i s r u p t e d l a n d s l i d e s

R o c k f a l l s

B o u n c i n g , r

o l l i n g , f r e e f a l l

H i g h o r v e r y h i g h

S h a l l o w

4 0

E x t r e m e l y r a p i d

M o s t l e s s t h a n 1

1 0 4

m 3 ;

m a x i m u m r e p o r t e d 2

1 0 7

m 3

M a y f a l l t o b a s e o f s t e e p

s o u r c e s l o p e a n d m o v e a s f a r

a s s e v e r a l t e n s o r h u n d r e d s o f

m e t e r s f a r t h e r , o n r e l a t i v e l y

g e n t l e s l o p e s

D i s r u p t e d r o c k s l i d e s T r a n s l a t i o n a l s l i d i n g

H i g h

S h a l l o w

3 5

R a p i d t o v e r y r a p i d


1 0 4

m 3 ;

m a x i m u m r e p o r t e d 2

1 0 9

m 3

M a y s l i d e t o b a s e o f s t e e p

s o u r c e s l o p e a n d s e v e r a l t e n s

o r h u n d r e d s o f m e t e r s f a r t h e r ,

o n r e l a t i v e l y g e n t l e s l o p e s

R o c k a v a l a n c h e s

C o m p l e x , i

n v o l v i n g s l i d i n g ,

o w , a

n d o c c a s i o n a l l y f r e e

f a l l

V e r y h i g h

D e e p

2 5

V e r y r a p i d

t o e x t r e m e l y r a p i d 5

1 0 5 2

1 0 8

m 3

o r

m o r e

S e v e r a l k i l o m e t e r s

S o i l f a l l s

B o u n c i n g , r

o l l i n g , f r e e f a l l

H i g h o r v e r y h i g h

S h a l l o w

4 0

E x t r e m e l y r a p i d

M o s t l e s s t h a n 1 , 0 0 0 m

3 ;

m a x i m u m v o l u m e s n o t w e l l

d o c u m e n t e d

M o s t c o m

e t o r e s t a t o r n e a r

b a s e s o f s t e e p s o u r c e s l o p e s

D i s r u p t e d s o i l s l i d e s T r a n s l a t i o n a l s l i d i n g

H i g h

S h a l l o w

1 5

M o d e r a t e t o r a p i d


1 0 4

m 3 ;

m a x i m u m r e p o r t e d 4 . 8

1 0 7

m 3

M a y s l i d e t o b a s e o f s t e e p

s o u r c e s l o p e a n d s e v e r a l t e n s

o r h u n d r e d s o f m e t e r s f a r t h e r ,

o n r e l a t i v e l y g e n t l e s l o p e s

S o i l a v a l a n c h e s

C o m p l e x , i

n v o l v i n g s l i d i n g ,

o w , a

n d o c c a s i o n a l l y f r e e

f a l l

V e r y h i g h

S h a l l o w

2 5

V e r y r a p i d

t o e x t r e m e l y r a p i d V o l u m e s n o t w e l l

d o c u m e n t e d ; m a x i m u m

r e p o r t e d 1 . 5

1 0 8

m 3

S e v e r a l t e n s o f m e t e r s t o

s e v e r a l k i l o m e t e r s b e y o n d

s t e e p s o u r c e s l o p e s

C o h e r e n t l a n d s l i d e s

R o c k s l u m p s

R o t a t i o n a l s l i d i n g

S l i g h t o r m o d e r a t e

?

D e e p

1 5

S l o w t o r a p i d

M o s t b e t w e e n 1 0 0 a n d a f e w

m i l l i o n m

3 ; m a x i m u m a t

l e a s t t e n s o f m i l l i o n s o f m

3

T y p i c a l l y

l e s s t h a n 1 0 m ;

o c c a s i o n a l l y 1 0 0 m o r m o r e

R o c k b l o c k s l i d e s

T r a n s l a t i o n a l s l i d i n g


?

D e e p

1 5


M o s t b e t w e e n 1 0 0 a n d a f e w

m i l l i o n m

3 ; m a x i m u m a t


3

T y p i c a l l y

l e s s t h a n 1 0 0 m ;

m a x i m u m

d i s p l a c e m e n t s n o t

w e l l d o c u m e n t e d

S o i l s l u m p s

R o t a t i o n a l s l i d i n g


?

D e e p

7


M o s t b e t w e e n 1 0 0 a n d 1

1 0 5

m 3 ;

o c c a s i o n a l l y 1

1 0 5 t o s e v e r a l m i l l i o n m

3

T y p i c a l l y

l e s s t h a n 1 0 m ;

o c c a s i o n a l l y 1 0 0 m o r m o r e

S o i l b l o c k s l i d e s

T r a n s l a t i o n a l s l i d i n g


?

?

D e e p

5

S l o w t o v e r y r a p i d


1 0 5

m 3 ;

m a x i m u m r e p o r t e d

1 . 1 2

1 0 8

m 3

T y p i c a l l y

l e s s t h a n 1 0 0 m ;

m a x i m u m



S l o w e a r t h o w s

T r a n s l a t i o n a l s l i d i n g a n d

i n t e r n a l o w

S l i g h t

G e n e r a l l y s h a l l o w ;

o c c a s i o n a l l y d e e p

1 0

V e r y s l o w t o m o d e r a t e ;

o c c a s i o n a l l y

, w i t h v e r y r a p i d

s u r g e s


1 0 6

m 3 ;

m a x i m u m r e p o r t e d

b e t w e e n 3

1 0 7

a n d 6

1 0 7

m 3

T y p i c a l l y

l e s s t h a n 1 0 0 m ;

m a x i m u m




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482 DAVID K. KEEFER

T A B L E I

C o n t i n u e d

N a m e

T y p e o f

I n t e r n a l

W a t e r c o n t e n t

T y p i c a l

M i n i m u m T y p i c a l

T y p i c a l

T y p i c a l

m o v e m e n t

d i s r u p t i o n

D U P S S

d e p t h s

s l o p e ( ) v e l o c i t i e s

v o l u m e s

d i s p l a c e m

e n t s

L a t e r a l s p r e a d s a n d o w s

S o i l l a t e r a l s p r e a d s

T r a n s l a t i o n o n u i d b a s a l

z o n e

G e n e r a l l y m o d e r a t e ;

o c c a s i o n a l l y s l i g h t o r h i g h

V a r i a b l e

0 . 3

V e r y r a p i d


1 0 5

m 3 ; l a r g e s t r e p o r t e d 9 . 6

1 0

6 m

3 .

T y p i c a l l y

l e s s t h a n 1 0 m ;

m a x i m u m

r e p o r t e d 6 0 0 m

R a p i d s o i l o w s

F l o w

V e r y h i g h

? ? ?

S h a l l o w

2 . 3

V e r y r a p i d t o e x t r e m e l y r a p i d V o l u m e s n o t w e l l

d o c u m e n t e d ; l a r g e s t a r e a t

l e a s t s e v e r a l m i l l i o n m

3 .

A f e w m t o s e v e r a l k m

S u b a q u e o u s l a n d s l i d e s

G e n e r a l l y l a t e r a l s p r e a d i n g o r

o w ; o c c a s i o n a l l y s l i d i n g

G e n e r a l l y h i g h o r v e r y h i g h ;

o c c a s i o n a l l y m o d e r a t e o r

s l i g h t

V a r i a b l e

0 . 5

G e n e r a l l y

r a p i d t o e x t r e m e l y

r a p i d ; o c c a s i o n a l l y s l o w t o

m o d e r a t e

V o l u m e s n o t w e l l

d o c u m e n t e d ; l a r g e s t a r e a t


3 . N o t w e l l d o c u m e n t e d , b u t

s o m e m o v e m o r e t h a n 1 k m

N o t e s : N a m e s : r o c k s i g n i e s b e d r o c k t h a t i s r e l a t i v e l y

r m a n d i n t a c t p r i o r t o l a n d s l i d e i n i t i a t i o n , a

n d s o i l s i g n i e s l o o s e , u n c o n s o l i d a t e d o r p o o r l y

c e m e n t e d a g g r e g a t e s o f p a r t i c l e s t h a t m a y o r m a y n o t c o n t a i n o r g a n i c m a t e r i a l s

. I n t e r n a l d i s r u p t i o n : s l i g h t s i g n i e s l a n d s l i d e c o n s i s t s o f

o n e o r a f e w

c o h e r e n t b l o c k s ; m o d e r a t e s i g n i e s s e v e r a l c o h e r e n t b l o c k s ; h i g h s i g n i e s n u m e r o u s s m a l l b l o c k s a n d i n d i v i d u a l s o i l g r a i n s a n d r o c k f r a g m e n t s ; v e r y

h i g h s i g n i e s n e a r l y c o m p l e t e d i s a g g r e g a t i o n i n t o i n d i v i d u a l s o i l g r a i n s o r s m a l l r o c k f r a g m e n t s . D

e p t h : s h a l l o w s i g n i e s g e n e r a l l y < 3 m d e e p ; d e e p

s i g n i e s g e n e r a l l y > 3 m d e e p . W

a t e r c o n t e n t : D = d r y ; U = m o i s t b u t u n s a t u r a t e d ; P S = p a r t l y s a t u r a t e d ; S = s a t u r a t e d . V e l o c i t y : v e r y s l o w =

1

1 0

6 3

1 0

6 m / m i n ; s l o w = 3

1 0

6 3

1 0

5 m / m i n ; m o d e r a t e = 3

1 0

5 0 . 0

0 1 m / m i n ; r a p i d = 0 . 0 0 1 0 . 3

m / m i n ; v e r y r a p i d = 0 . 3 1 8 0 m / m i n ; e x t r e m e l y

r a p i d = > 1 8 0 m / m

i n . (

T e r m i n o l o g y a f t e r V a r n e s , 1 9 7 8 ) .


11/38


TABLE II

Relative abundance of earthquake-induced landslides (from Keefer, 1984)

Landslide type, listed in order of decreasing total numbers

Very abundant: > 100,000 in the 40 historical earthquakesRock fallsDisrupted soil slidesRock slides

Abundant: 10,000 to 100,000 in the 40 historical earthquakesSoil lateral spreadsSoil slumpsSoil block slidesSoil avalanches

Moderately common: 1,000 to 10,000 in the 40 historical earthquakesSoil fallsRapid soil owsRock slumps

Uncommon: 100 to 1,000 in the 40 historical earthquakesSubaqueous landslidesSlow earth owsRock block slidesRock avalanches

reporting from some of the earthquakes in the sample. Three types of disruptedlandslides rock falls, disrupted soil slides, and rock slides were the most abund-ant types of earthquake-induced landslides (Table II). These landslides comprisedabout 80 percent of all the reported landslides (35, 26, and 20 percent, respectively),and the six types of disrupted landslides together comprised about 86 percent of all the landslides reported. Coherent slides comprised about 8 percent, and lateralspreads and ows comprised about 6 percent of the total number of landslidesreported from the 40 earthquakes.

Types of slopes and geologic environments that produced earthquake-inducedlandslides varied widely, ranging from overhanging slopes in well-indurated bed-rock to unconsolidated sediments with nearly level surfaces. Minimum slopeinclinations for the various types of landslides ranged from 0.3 to 40 (Table I).In general, materials most susceptible to earthquake-induced landslides were (1)weakly cemented, weathered, sheared, intensely fractured, or closely jointed rocks,(2) better-indurated rocks having prominent discontinuities, (3) sandy residual or


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484 DAVID K. KEEFER

colluvial soils, (4) saturated volcanic soils containing sensitive clay, (5) loess, (6)cemented soils, (7) granular deltaic sediments, (8) granular ood-plain alluvium,and (9) uncompacted, or poorly compacted, granular articial ll.

Approximate magnitudes of the smallest earthquakes causing landslides of vari-ous types were 4.0 for rock falls, rock slides, soil falls, and disrupted soil slides;4.5 for soil slumps and soil block slides; 5.0 for soil lateral spreads, rapidsoil ows, subaqueous landslides, rock slumps, rock block slides, and slow earthows; 6.0 for rock avalanches; and 6.5 for soil avalanches. Because all types of earthquake-induced landslides can also be triggered by nonseismic agents, Keefer(1984) noted the possibility that landslides of all types could occasionally occur inearthquakes smaller than those indicated.

The areas affected by landslides in the earthquakes (Figure 1a) correlated withearthquake magnitude, although such other factors as focal depth, specic ground-motion characteristics of individual earthquakes, and geologic conditions werealso important (Keefer, 1984). However, as far as could be determined from the

available data, regional variation in seismic attenuation was not a signicant factor.A non-linear upper bound enclosing the data showed the maximum size of thearea likely to be affected by landslides increased from nil at M 4 to 500,000km2 at M = 9.2 (Figure 1a). A subsequent study (Keefer and Wilson, 1989) usingdata from this sample and seven additional earthquakes, related area affected bylandslides, A, to magnitude, M , with the least-squares linear regression mean,

log10 A = M 3.46( 0.47)

for 5.5 < M 9.2, where A is in square kilometers. Maximum distances fromepicenters and fault ruptures were also determined for landslides in each majorcategory for each earthquake. These distances also correlated with magnitude, and

non-linear upper bounds enclosing all the data were determined for each landslidecategory (Figures 2 and 3).Minimum shaking intensities on the Modied Mercalli (MMI) scale for land-

slides in each category were determined for each earthquake by comparing mapsshowing landslide distribution with isoseismal maps (Figure 4). Substantial scat-ter in these data presumably reected differing application of the intensity scaleby various investigators, among other factors. Nevertheless, disrupted landslidesoccurred on average at one intensity grade lower than landslides in the other cat-egories; for them, the predominant minimum intensity was MMI VI, and the lowestreported intensity in any earthquake was MMI IV; whereas, for landslides in theother two categories the predominant minimum intensity was MMI VII, and thelowest reported intensity was MMI V (Keefer, 1984; Figure 4).

Landslides were responsible for highly variable, but in many cases signic-ant, numbers and proportions of casualties and levels of economic damage in thevarious earthquakes. In at least eight of the earthquakes, landslides caused half or more of the deaths (Keefer, 1984). The greatest number of deaths caused bylandslides occurred during the M 7.8 Kansu, China, earthquake in 1920, in which


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Figure 1a. Relations between area affected by landslides and earthquake magnitude. Circles aredata from earthquakes discussed by Keefer (1984) and Keefer and Wilson (1989). Dashed line isapproximate upper bound from Keefer (1984), curved to approach A = 0 at M = 4. Solid line isleast-squares linear regression mean from Keefer and Wilson (1989). Magnitude determinations forindividual earthquakes are given in Keefer (1984) and Keefer and Wilson (1989); most magnitudessmaller than 7.5 are Richter surface-wave magnitudes ( M S ), and most magnitudes of 7.5 or largerare moment magnitudes ( M ).

rapid ows in loess, some of which traveled several kilometers, killed most of the240,000 people who died in that seismic event (Wang and Xu, 1984). The 1970rock avalanche from Nevados Huascarn, in the Andes of Peru, which transportedat least 50 106 m3 of material 16 km at an average velocity of 280 km/hr, killedabout 18,000 people (Plafker et al., 1971; Plafker and Ericksen, 1978). Rapid soilows and a rock avalanche also caused substantial loss of life in the M S 7.6 Khaitearthquake in Tajikistan in 1949. There, ows in loess probably triggered by acombination of the main shock, aftershocks, and heavy rainfall killed an estimated15,000 people, and a rock avalanche triggered by the main shock buried the town of Khait, killing at least 3,000 people (Seed, 1968; A.M. Sarna-Wojcicki, unpublisheddata).

Signicant numbers and proportions of deaths were also attributable to land-slides in many other historical earthquakes (Keefer, 1984). Although all types of landslides posed some degree of hazard to human life, at least 90 percent of thedeaths in the 40 earthquakes studied were caused by rock falls, rock avalanches,and rapid soil ows (Keefer, 1984), even though the latter two types of landslidesoccur in relatively small numbers (Table II). Rock avalanches and rapid soil ows


14/38

486 DAVID K. KEEFER

Figure 1b. Relations between area affected by landslides and earthquake moment magnitude. Circlesare data from earthquakes discussed by Rodr guez et al. (1999), plotted using moment magnitude(M ); open circle is 1988 Saguenay, Quebec earthquake. Solid line is upper bound of Keefer (1984;see Figure 1a). Dashed line is upper bound of Rodr guez et al. (1999). Triangle is datum from 1963Peria, New Zealand, earthquake, for which area exceeds upper bounds, plotted using Richter localmagnitude ( M L ), from Hancox et al. (1997, 2002).

are particularly dangerous because they can transport material several kilometersat high velocities over gently sloping ground (Table I); also, rock avalanches areall relatively large. The high risk from rock falls derives both from their transportof rock materials at high velocities (Table I) and their abundance (Table II).

Landslides also caused signicant economic losses in many of the earthquakes.Examples include the 1964 earthquake in Niigata, Japan ( M 7.5), where landslidesand related soil-liquefaction phenomena caused an estimated $US 800 million indamage 2 (Lee et al., 1977), the 1964 Alaskan earthquake ( M 9.2), in which land-slides caused an estimated $US 279 million in damage (W.R. Hansen, unpublisheddata), and the 1970 Peru earthquake ( M 7.9) in which landslides buried thousands

of residences and other buildings and destroyed substantial parts of the regionaltransportation and utility systems (Plafker et al., 1971).

2 Damage estimates are in $US of the time of the earthquakes.


15/38


Figure 2. Approximate upper bounds for maximum epicentral distances of landslides in three majorcategories related to earthquake magnitude, from Keefer (1984), and data from earthquakes for whichmaximum epicentral distances of landslides exceed the relevant upper bound. Dashed line is upperbound for disrupted landslides, dot-dash line is upper bound for coherent landslides, and dotted lineis upper bound for lateral spreads and ows. Magnitudes used in determining upper bounds weretypically moment magnitude ( M ) for M 7.5 and Richter surface-wave magnitude ( M S ) for M 10 8 m3 occurred only in earthquakes having M > 7.5and intensities of MMI IX or higher (Hancox et al., 2002).

Papadopoulos and Plessa (2000) compiled data on maximum epicentral dis-tances of landslides in 47 earthquakes in Greece that occurred between 1650 and1995. The authors noted that, for earthquakes before 1911, errors in epicentrallocation were as great as about 30 km, and errors in magnitude were as great as 0.5units. Thus, for many of the earthquakes, errors in epicentral location were of thesame order as the measured epicentral distances of landslides, and the error rangein magnitude determination was also signicant. Nevertheless, with the exception


27/38


of an M S 3.8 aftershock (which could be considered as part of an earthquake se-quence with a main shock M S of 5.5), Papadopoulos and Plessa (2000) showedthat the maximum epicentral distances for landslides are in the same range as theworldwide data. They proposed a straight-line upper bound for the Greek data,

log(R e) = 2.98 + 0.75M S

for a range of magnitudes between approximately 4.5 and 7.5, where Re is themaximum epicentral distance to a landslide, in kilometers.

Prestininzi and Romeo (2000) analyzed data on landslides in earthquakes thathave occurred in Italy since 461 BCE. Most of these earthquakes were from pre-instrumental times and thus were associated with three sources of uncertainty.First, pre-instrumental magnitudes were determined by converting from epicentralintensities. Second, uncertainties in epicentral location were as large as 50 km forthe older events. Third, the reliability of reporting of landslides was questionable

for most events, and so the reported numbers of landslides were probably only asmall fraction of those that actually occurred. These effects caused large uncertain-ties in the determination of such measures as the maximum epicentral distances tolandslides. Nevertheless, most of these data were also in the same range as the otherworldwide data. A straight-line upper bound was developed relating epicentral in-tensity to distance to landslides, and minimum shaking intensities for landslideswere determined. These shaking intensities had a range of IV to XI and a modalvalue of IX on the Mercalli-Cancani-Sieberg (MCS) scale.

6. Selected other recent ndings

6.1. L ANDSLIDES AT GREAT DISTANCES

Five earthquakes in intraplate regions of North America have produced landslidesat distances signicantly greater than indicated by the upper bounds shown inFigures 2 and 3. One of these earthquakes was in the Saguenay, Quebec, regionof eastern North America, as was discussed above. Four of the earthquakes were inthe Colorado Plateau region of western North America. On the Colorado Plateau,the M L 5.3 San Rafael Swell, Utah, earthquake of August 14, 1988, producedpossibly hundreds of rock falls (Case, 1988). Most of these were within about40 km of the epicenter, but isolated rock falls were reported from at least as faras 113 km and possibly as far as 129 km from the epicenter (Case, 1988; Figure2). The M 5.7 St. George, Utah, earthquake of September 2, 1992 generated about10 m of displacement on a 14 106 m3 block slide 44 km from the epicenter(Jibson and Harp, 1996; Figure 2). An M L 4.6 earthquake in western Coloradoon September 13, 1994 triggered rock falls 87 km from the epicenter (Jibson andHarp, 1996; Figure 2). The Western Arizona earthquake of April 29, 1993 ( M L= 5.3) generated one rock fall 169 km from the epicenter (Harp et al., 1993; D.


28/38

500 DAVID K. KEEFER

K Keefer and E. L. Harp, unpublished data; Figure 2). This earthquake producedfew other landslides and none in the area immediately around the epicenter (D.K. Keefer and E. L. Harp, unpublished data.). The most distant rock fall detachedfrom a vertical cliff face along a prominent vertical joint that had been open beforethe earthquake as indicated by the presence of live tree roots in place along thepreserved joint surface (D.K. Keefer, unpublished data).

The reasons for the triggering of these landslide at great distances are not wellknown, but may involve anomalous seismic attenuation in the intraplate regionof North America, extraordinary susceptibility of some sites in these regions of low seismicity, or a combination of both. For example, the most distant landslidestriggered by the Saguenay, Quebec, earthquake involved very sensitive clays andll embankments founded on loose, saturated materials and inclined at near theirangles of repose (Rodrguez et al., 1999). Additionally, shaking in the failure local-ities may have been amplied by the arrival of shear waves reected from interfacesin the lower crust simultaneously with direct radiation from the source (Rodrguez

et al., 1999). The block slide triggered by the St. George, Utah, earthquake wasone of only two coherent slides triggered by the earthquake and involved a slopein clay, saturated by above-normal pre-earthquake precipitation, that was beingundercut by active uvial erosion (Jibson and Harp, 1996). The condition of theslope producing the rock fall in the Western Arizona earthquake indicated that therock mass that failed was probably partly detached prior to the earthquake and thushighly susceptible to falling during even relatively weak shaking.

6.2. A RIAS INTENSITY THRESHOLDS

An instrumentally-based measure of seismic intensity developed by Arias (1970)

was rst used for analyzing the occurrence of landslides by Wilson and Keefer(1985), and its use has become relatively widespread for that purpose since thattime. This Arias intensity for any given strong-motion recording is expressed as:

I a = / 2g T d

0[a(t) 2]dt

where I a is the Arias intensity, t is the time, a(t) is the ground acceleration asa function of time, T d is the total duration of the strong-motion record, and g isthe acceleration of gravity. Arias intensity is thus measured in units of velocity.Based on theoretical considerations, statistical analysis of strong-motion attenu-ation, and empirical data on landslide limits in historical earthquakes, Wilson andKeefer (1985) suggested threshold Arias intensity values of 0.15 m/s for disrup-ted landslides and 0.5 m/s for coherent slides, lateral spreads, and ows (TableIV). Additional analysis by Keefer and Wilson (1989) suggested threshold Ariasintensity values of 0.11 m/s for disrupted landslides, 0.32 m/s for coherent slides,and 0.54 m/s for lateral spreads and ows, whereas Wilson (1993) found the best-


29/38


t threshold Arias intensity value for disrupted landslides to be 0.10 m/s (TableIV).

Harp and Wilson (1995) used data from two seismic events in southern Cali-fornia the October 1, 1987 Whittier Narrows earthquake ( M 6.1) and thedouble-shock October 24, 1987 Superstition Hills event ( M 6.2 and 6.6) to correl-ate the mapped limits of landslide occurrence with Arias intensity values calculatedfrom strong-motion records. In the Superstition Hills event, most of the landslideswere soil falls and disrupted soil slides, and most were generated by the M 6.6shock. Interpolation of data from 19 strong-motion records showed that the meanthreshold Arias intensity for the occurrence of these disrupted landslides was 0.3m/s, with a range in thresholds of 0.1 to 0.5 m/s (Harp and Wilson, 1995; Table IV).All of the landslides generated by the Whittier Narrows earthquake were rock falls,rock slides, soil falls, or disrupted soil slides (Harp and Wilson, 1995). Interpola-tion of data from 83 strong-motion records showed a wider range of threshold Ariasintensities for these disrupted landslides, which varied with rock type (Harp and

Wilson, 1995). For weakly cemented sandstones and conglomerates of Mioceneand Pliocene age, threshold Arias intensity values were in the range between 0.06and 0.7 m/s (Table IV). In an apparent paradox, threshold Arias intensities forlandslides in well cemented Mesozoic and Precambrian rocks were substantiallylower, with a range of 0.01 to 0.07 m/s and a mean of 0.04 m/s (Harp and Wilson,1995; Table IV). The discrepancy in threshold values between the two classes of rocks was explained by the pervasive occurrence of prominent, open ssures andthe abundance of loose rocks perched on steep slopes in areas underlain by theolder, better cemented rocks (Harp and Wilson, 1995).

6.3. D ELAYED INITIATION OR REACTIVATION OF LANDSLIDE MOVEMENT

Although many earthquake-induced landslides initiate in dry materials (Keefer,1984; Keefer and Wilson 1989), seismic shaking also commonly combines withexisting or transiently elevated pore-water pressures to destabilize slopes. Thiscombination of agents can lead to the initiation or reactivation of landslidemovement, especially in coherent slides, several hours or days after strong ground-shaking has stopped. However, only a few such cases have been documented.Probably the earliest well-documented example was that of a large earth ow, 120to 240 m wide and 800 m long, that was reactivated 5 days after the main shock of the M 7.3 Hebgen Lake, Montana, earthquake in 1959 (Hadley, 1964). This earthow moved about 30 m during the month after the earthquake, and Hadley (1964)inferred that the reactivated movement was due to an increase in ground-waterow and a coincident increase in the local slope of the ground surface caused bycoseismic tectonic deformation.

The most numerous reports of delayed initiation or reactivation of landslideswere associated with the November 23, 1980 Irpinia, Italy, earthquake ( M = 6.9). Inthe region affected by that earthquake, several large earth ows and other coherent


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502 DAVID K. KEEFER

T A B L E I V

A

r i a s i n t e n s i t y t h r e s h o l d s f o r l a n d s l i d e s

L a n d s l i d e c a t e g o r y

A r i a s i n t e n s i t y t h r e s h o l d ( m / s )

R e f e r e n c e

M o d e l i n g s t u d i e s

D i s r u p t e d

0 . 1 5

W i l s o n a n d K e e f e r , 1 9 8 5

D i s r u p t e d

0 . 1 1

K e e f e r a n d W i l s o n , 1

9 8 9

D i s r u p t e d

0 . 1 0

W i l s o n , 1

9 9 3

C o h e r e n t

0 . 5


C o h e r e n t

0 . 3 2


9 8 9


0 . 5



0 . 5 4


9 8 9

2 4 O c t . 1

9 8 7 S u p e r s t i t i o n H i l l s , C a l i f o r n i a e a r t h q u a k e

D i s r u p t e d

0 . 3 ( m e a n )

H a r p a n d W i l s o n , 1

9 9 5

D i s r u p t e d

0 . 1 0 . 5 ( r a n g e )


9 9 5

1 O c t . 1

9 8 7 W h i t t i e r N a r r o w s , C a l i f o r n i a e a r t h q u a k e

D i s r u p t e d

0 . 0 6 0 . 7 (

w e a k l y c e m e n t e d s a n d s t o n e s a n d c o n g l o m e r a t e s )


9 9 5

D i s r u p t e d

0 . 0 4 ( m e a n , w e l l - c e m e n t e d r o c k s )


9 9 5

D i s r u p t e d

0 . 0 1 0 . 0 7

( r a n g e , w

e l l - c e m e n t e d r o c k s )


9 9 5


31/38


slides, with volumes as great as 28 106 m3, began moving a few hours to a fewdays after the main shock (Agnesi et al., 1983; Cotecchia and Del Prete, 1984;DElia et al., 1985; Carrera et al.,1986; Cotecchia, 1986; Del Gaudio et al., 2000).The post-earthquake movement on these landslides was inferred to have beencaused by increased spring ow and pore-water pressures associated with tectonicdeformation of the area in which the landslides occurred (Cotecchia and Del Prete,1984; Cotecchia, 1986; Wasowski et al., this volume). These hydrologic changeswere also probably partly responsible for an anomalous distribution of landslidesin part of the affected region, where landslide concentrations were relatively higherat greater distances from the fault rupture (Wasowski et al., this volume).

Similar observations of delayed reactivation owing to changes in ground-waterconditions were reported by Jibson et al. (1994) from the epicentral area of theM 7.1 April 29, 1991 earthquake in the Racha region of the Republic of Georgia.There, two large earth ows showed only small displacements during the mainshock but began moving at velocities as great as several m/day 2 to 3 days after the

earthquake (Jibson et al., 1994).Following the October 28, 1983 Borah Peak, Idaho, earthquake ( M = 7.0),

Keefer et al. (1985) reported that a rapid soil ow with an estimated volume of 0.2 106 m3 had initiated 36 to 48 hours after the main shock. They attributedthis initiation to saturation of the source colluvium owing to increased surface andsubsurface water ow resulting from the earthquake, which Wood et al. (1985),in turn, concluded was probably due to sudden release of elastic strain in aquifersduring the earthquake.

7. Summary

Although earthquake-induced landslides have been described in documents formore than 3700 years, accounts from earthquakes before the late eighteenth cen-tury are incomplete concerning landslide numbers and vague concerning landslidecharacteristics. They are thus typically misleading concerning the true abundanceof landslides and range of characteristics. Beginning with studies of the 1783Calabria, Italy, earthquake sequence, more complete and precise data concerningthe occurrence of landslides in earthquakes have become available. The historicaldevelopment of knowledge concerning landslides triggered by earthquakes can bedivided into several periods. The rst period, from 1783 until the rst applicationof aerial photography, was characterized by ground-based studies of earthquakeeffects, typically carried out by formal scientic commissions. These formal stud-ies typically identied a large, but not necessarily comprehensive, sampling of localities where landslides had occurred. In some, but not all cases, landslide char-acteristics were also described in enough detail that the general range of landslidecharacteristics could begin to be determined. More recently, some nineteenth tomid-twentieth century earthquakes have been studied using retrospective analyses,


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504 DAVID K. KEEFER

in which the landslide occurrences associated with the event are inferred yearsto decades later, using contemporary accounts, mapping from aerial photographs,statistical studies, and (or) geotechnical analyses.

The rst use of aerial photographs to map earthquake effects immediately afterthe event probably occurred in 1948. Since that time, the use of aerial photographyhas greatly facilitated the compilation of post-earthquake landslide inventories, al-though because of the limitations of aerial photography, ground-based eld studiescontinue to be crucial in preparing accurate and comprehensive landslide maps.Beginning with a small California earthquake in 1957, extensive to relatively com-plete inventories of landslides have been prepared for a relatively small number of earthquakes. Through the 1960s and 1970s the best landslide inventories typicallywere complete only for a central affected area, although the rst virtually completeinventory of a large earthquake was prepared for the M 7.5 Guatemala earthquakein 1976.

Beginning in 1980, virtually complete landslide inventories have been prepared

for several additional earthquakes in California, El Salvador, Japan, Italy, andTaiwan. Most of these used aerial photography in combination with ground-basedeld studies, although studies of the most recent of these events, in Taiwan, havealso used satellite imagery, and three of the others (including the two smallest)were compiled largely from ground-based eld studies without aerial photography.Since 1989, digital-mapping and GIS techniques have come into common use formapping earthquake-induced landslides, and the use of these techniques has greatlyenhanced the level of analysis that can be applied to earthquake-induced landslideoccurrence.

The rst general synthesis of data on earthquake-induced landslides (Keefer,1984) dened the general characteristics of these landslides, derived relations

between landslide occurrence on the one hand and geologic and seismic parameterson the other hand, and identied the types of hazards associated with the landslides.Since then, additional synthesis of worldwide data (Rodrguez et al., 1999) andnational data from New Zealand (Hancox et al., 1997, 2002), Greece (Papado-poulos and Plessa, 2000), and Italy (Prestininzi and Romeo, 2000) have providedadditional data on landslide characteristics and hazards and have extended, revised,and rened these relations. Recently completed studies have also identied areaswith anomalous landslide distributions, have provided data for correlating the oc-currence of landslides with a measure of local ground motion, and have veried theoccasional delayed triggering of landslides as a consequence of seismic shaking.

The documentation and synthesis of data on landslide occurrence in earth-quakes has led to greatly increased understanding of the hazards associated withearthquake-induced landslides and to the development of models and methods forhazard mapping and evaluation. However, the number of earthquakes with relat-ively complete data on landslide occurrence is still small, and one of the mostpressing research needs is for complete landslide inventories for many more eventsin a wider variety of environments. Whereas many additional inventories in all


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regions are needed, data from seismically active regions of continental Asia and theMiddle East, the Andean region of South America, the tropical western Pacic, andintraplate regions worldwide would be especially desirable. Such additional data,coupled with the increasing use of GIS and other current analytical tools shouldlead to substantial additional renements in models relating seismic shaking andgeologic conditions to slope failure, and thus to our ability to minimize damageand loss of life from seismically generated landslides.

Acknowledgements

This article was improved by the thoughtful reviews of Vincenzo Del Gaudio, LynnHighland, Randall Jibson, and Janusz Wasowski.

References

Adams, J.: 1980, Contemporary Uplift and Erosion of the Southern Alps, New Zealand, GeologicalSociety of America Bulletin 91(1), 24 and (2), 1114.

Agnesi, V., Carrara, A., Macaluso, T., Monteleone, S., Pipitone, G., and Sorriso-Valvo, M.: 1983,Elementi Tipologici e Morfologici dei Fenomeni di Instabilit dei Versanti Indotti dal Sisma del1980 (Alta Valle del Sele), Geologia Applicata e Idrogeologia 18(1), 309341.

Ambraseys, N.N.: 1976, The Gemona di Friuli Earthquake of 6 May 1976, UNESCO RestrictedTechnical Report RP/1975-76/2.222.3, Part II.

Arias, A.: 1970, A Measure of Earthquake Intensity, in R.J. Hansen (ed.), Seismic Design for Nuclear Power Plants , Massachusetts Institute of Technology Press, Cambridge, Massachusetts, pp. 438483.

Bonilla, M.G.: 1960, Landslides in the San Francisco South Quadrangle, California, U.S. GeologicalSurvey Open-File Report.Bozzano, F., Gambino, P., Prestininzi, A., Scarascia Mugnozza, G., and Valentini, G.: 1998, Ground

Effects Induced by the Umbria-Marche Earthquakes of SeptemberOctober 1997, Central Italy,Proceedings of the 8th International Congress of the International Association for EngineeringGeology and the Environment, Vancouver, Canada, 2125 September, 1998, A.A. Balkema,Rotterdam, pp. 825830.

Carrara, A., Agnesi, V., Macaluso, T., Monteleone, S., and Pipitone, G.: 1986, Slope MovementsInduced by the Southern Italy Earthquake of November 1980, Geologia Applicata e Idrogeologia21(2), 237250.

Case, W.F.: 1988, Geologic Effects of the 14 and 18 August, 1988 Earthquakes in Emery County,Utah, Utah Geological and Mineral Survey, Survey Notes 22, pp. 814.

Cheng, J.D., Chen, B.K., Su, R.R., and Yeh, C.L.: 2000, Landslides Induced by the DisastrousEarthquake on September 21, 1999 in Central Taiwan (Abs.), European Geophysical Society25th General Assembly, Nice, France, 2000, Geophysical Research Abstracts.

Collins, J.L. and Foster, H.L.: 1949, The Fukui Earthquake Hokuriku Region, Japan, 28 June 1948,Volume I, Geology, U.S. Army Ofce of the Engineer, General Headquarters, Far EastCommand.

Cotecchia, V.: 1986, Ground Deformations and Slope Instability Produced by the Earthquake of 23November 1980 in Campania and Basilicata, Proceedings of the International Symposium onEngineering Geology Problems in Seismic Areas, Bari, Italy vol. 5, pp. 31100.


34/38

506 DAVID K. KEEFER

Cotecchia, V. and Del Prete, M.: 1984, The Reactivation of Large Flows in the Parts of Southern ItalyAffected by the Earthquake of November 1980, with Reference to the Evolutive Mechanism,Proceedings of the Fourth International Symposium on Landslides, Toronto, vol. 2, pp. 3337.

Cotecchia, V., Guerricchio, A., and Melidoro, G.: 1986, The Geomorphogenetic Crisis Triggered by

the 1783 Earthquake in Calabria (Southern Italy), Proceedings of the International Symposiumon Engineering Geology Problems in Seismic Areas, Bari, Italy, vol. 6, pp. 245304.

Cotecchia, C. and Melidoro, G.: 1974, Some Principal Aspects of the Landslides of Southern Italy, Bulletin of the International Association of Engineering Geology 9, 2332.

Davison, C.: 1936, Great Earthquakes , Thomas Murby & Co., London.Del Gaudio, V., Trizzino, R., Calcagnile, G., Calvaruso, D., and Pierri, P.: 2000, Landsliding in

Seismic Areas: The Case of the Acquara-Vadoncello Landslide (Southern Italy), Bull. Eng. Geol. Env. 59, 2337.

DElia, B., Esu, F., Pellegrino, A., and Pescatore, T.: 1985, Some Effects on Natural Slope StabilityInduced by the 1980 Italian Earthquake, Proceedings of the Eleventh International Society of Soil Mechanics and Foundation Engineering Conference, San Francisco.

Dutton, C.E.: 1889, The Charleston Earthquake of August 31, 1886, U.S. Geological Survey NinthAnnual Report 18871888, pp. 203528.

Esposito, E., Pordo, S., Simonelli, A.L., Mastrolorenzo, G., and Iaccarino, G.: 2000, Landslidesand Other Surface Effects Induced by the 1997 Umbria-Marche Seismic Sequence, EngineeringGeology 58 , 353376.

Fukuoka, H., Sassa, K., and Scarascia-Mugnozza, G.: 1997, Distribution of Landslides Triggered bythe 1995 Hyogo-ken Nanbu Earthquake and Long Runout Mechanism of the Takarazuka Golf Course Landslide, Journal of Physics of the Earth 45, 8390.

Fuller, M.L.: 1912, The New Madrid Earthquake, U.S. Geological Survey Bulletin 394, reprintedition.

Geological Survey of India: 1939, The Bihar-Nepal Earthquake of 1934, Memoirs of the GeologicalSurvey of India 73.

Govi, M.: 1977a, Carta delle Frane Prodotte dal Terremoto (Map Showing the Landslides Triggeredby the Earthquake), Rivista Italiana di Paleontologia e Stratigraa 83 , Plate 1.

Govi, M.: 1977b, Photo-interpretation and Mapping of the Landslides Triggered by the FriuliEarthquake (1976), International Association of Engineering Geology Bulletin 15, 6772.

Govi, M. and Sorzana, P.F.: 1977, Effetti Geologici del Terremoto, Frane, Rivista Italiana diPaleontologia e Stratigraa 83 , 329368.Hadley, J.B.: 1964, Landslides and Related Phenomena Accompanying the Hebgen Lake Earthquake

of August 17, 1959, U.S. Geological Survey Professional Paper 435, pp. 107138.Hall, J.F. (ed.): 1994, The Northridge Earthquake January 17, 1994 Preliminary Reconnaissance

Report, Earthquake Engineering Research Institute Publication 94-01.Hancox, G.T., Perrin, N.D., and Dellow, G.D.: 1997, Earthquake-induced Landsliding in New Zeal-

and and Implications for MM Intensity and Seismic Hazard Assessment, Institute of Geologicaland Nuclear Sciences, Ltd., GNS Client Report 43601B.

Hancox, G.T., Perrin, N.D., and Dellow, G.D.: 2002, Recent Studies of Historical Earthquake-Induced Landsliding, Ground Damage, and MM Intensity in New Zealand, Bulletin of the New Zealand Society for Earthquake Engineering , vol. 35, no. 2, pp. 5994.

Hansen, A. and Franks, C.A.M.: 1991, Characterisation and Mapping of Earthquake TriggeredLandslides for Seismic Zonation, Proceedings of the Fourth International Conference on SeismicZonation, Stanford, California, vol. 1, pp. 149195.

Harp, E.L. and Jibson, R.W.: 1995, Inventory of Landslides Triggered by the 1994 Northridge,California Earthquake, U.S. Geological Survey Open-File Report 95-213.

Harp, E.L. and Jibson, R.W.: 1996, Landslides Triggered by the 1994 Northridge, California,Earthquake, Bulletin of the Seismological Society of America 86(1B), S319S332.


35/38


Harp, E.L., Jibson, R.W., and Keefer, D.K.: 1993, Seismically-induced Landslides Triggered at Ex-traordinary Distances: Evidence from the Springdale, Utah, Landslide, Geological Society of America Abstracts with Programs, vol. 25, no. 6, p. A32.

Harp. E.L. and Keefer, D.K.: 1990, Landslides Triggered by the Earthquake, in M.J. Rymer and W. L.

Ellsworth (eds.), The Coalinga, California, Earthquake of May 2, 1983 , U.S. Geological SurveyProfessional Paper 1487, pp. 335347.

Harp, E. L., Tanaka, K., Sarmiento, J., and Keefer, D. K.: 1984, Landslides from the May 2527,1980, Mammoth Lakes, California, Earthquake Sequence, U.S. Geological Survey MiscellaneousInvestigations Series Map I-1612.

Harp, E.L. and Wilson, R.C.: 1995, Shaking Intensity Thresholds for Rock Falls and Slides: Evidencefrom 1987 Whittier Narrows and Superstition Hills Earthquake Strong-motion Records, Bulletinof the Seismological Society of America 85, 17391757.

Harp, E.L., Wilson, R.C., and Wieczorek, G.F., 1981, Landslides from the February 4, 1976,Guatemala Earthquake, U.S. Geological Survey Professional Paper 1204-A.

Hecker, S., Ponti, D.J., Garvin, C.D., and Hamilton, J.C.: 1995, Characteristics and Origin of GroundDeformation Produced in Granada Hills and Mission Hills During the January 17, 1994 North-ridge, California, Earthquake, California Department of Conservation, Division of Mines andGeology Special Publication 116, pp. 111131.

Holzer, T.L. (ed.): 1998, The Loma Prieta, California, Earthquake of October 17, 1989 Liquefac-tion, U.S. Geological Survey Professional Paper 1551-B.

Japan Geotechnical Society, 1996, Special Issue on Geotechnical Aspects of the January 17, 1995Hyogoken-Nambu Earthquake, Soils and Foundations, The Japanese Geotechnical Society,Tokyo.

Jibson, R.W.: 1985, Landslides Caused by the 1811-12 New Madrid Earthquakes, Ph.D. Dissertation,Stanford University.

Jibson, R.W. and Harp, E.L.: 1996, The Springdale, Utah, Landslide: An Extraordinary Event, Environmental & Engineering Geoscience 2, 137150.

Jibson, R.W., Harp, E.L., and Michael, J.A.: 1998, A Method for Producing Digital ProbabilisticSeismic Landslide Hazard Maps: An Example from the Los Angeles, California, Area, U.S.Geological Survey Open-File Report 98-113.

Jibson, R.W., Harp, E.L., and Michael, J.A.: 2000, A Method for Producing Digital Probabilistic

Seismic Landslide Hazard Maps, Engineering Geology 58 , 271289.Jibson, R.W. and Keefer, D.K.: 1988, Landslides Triggered by Earthquakes in the Central MississippiValley, Tennessee and Kentucky, U.S. Geological Survey Professional Paper 1336-C.

Jibson, R.W. and Keefer, D.K.: 1989, Statistical Analysis of Factors Affecting Landslide Distributionin the New Madrid Seismic Zone, Tennessee and Kentucky, Engineering Geology 27, 509542.

Jibson, R.W. and Keefer, D.K.: 1993, Analysis of the Seismic Origin of Landslides: Examples fromthe New Madid Seismic Zone, Geological Society of America Bulletin 105 , 521536.

Jibson, R.W., Prentice, C.S., Borissoff, B.A., Rogozhin, E.A., and Langer, C.J.: 1994, Some Obser-vations of Landslides Triggered by the 29 April 1991 Racha Earthquake, Republic of Georgia, Bulletin of the Seismological Society of America 84, 963973.

Johnston, A.: 1996, Seismic Moment Assessment of Earthquakes in Stable Contiental Regions III.New Madrid 18111812, Charleston 1886, and Lisbon 1755, Geophysical Journal International126 , 314344.

Kamai, T.: 1995, Landslides in the Hanshin Urban Region Caused by the 1995 Hygoken-NanbuEarthquake, Japan, Landslide News 9, 1213.

Kamai, T., Wang, W.N., and Shuzui, H.: 2000, The Landslide Disaster Induced by the Taiwan Chi-Chi Earthquake of 21 September 1999, Landslide News 13, 812.

Keefer, D.K.: 1984, Landslides Caused by Earthquakes, Geological Society of America Bulletin 95,406421.


36/38

508 DAVID K. KEEFER

Keefer, D. K. (ed.): 1998, The Loma Prieta, California, Earthquake of October 17, 1989 Landslides:U.S. Geological Survey Professional Paper 1551-C.

Keefer, D.K.: 1999, Earthquake-induced Landslides and Their Effects on Alluvial Fans, Journal of Sedimentary Research 69, 84104.

Keefer, D.K.: 2000, Statistical Analysis of an Earthquake-induced Landslide Distribution the 1989Loma Prieta, California Event, Engineering Geology 58, 213249.

Keefer, D.K. and Manson, M.W.: 1998, Regional Distribution and Characteristics of LandslidesGenerated by the Earthquake, in D.K. Keefer (ed.), The Loma Prieta, California, Earthquakeof October 17, 1989 - Landslides , U. S. Geological Survey Professional Paper 1551-C, pp. 732.

Keefer, D.K. and Tannaci, N.E.: 1981, Bibliography on Landslides, Soil Liquefaction, and RelatedGround Failures in Selected Historic Earthquakes, U.S. Geological Survey Open-File Report81-572.

Keefer, D.K. and Wilson, R.C.: 1989, Predicting Earthquake-induced Landslides, with Emphasis onArid and Semi-arid Enviroments, in P.M. Sadler, and D.M. Morton (eds.), Landslides in a Semi-arid Environment , Inland Geological Society of Southern California Publications, Riverside,California, vol. 2, part 1, pp. 118149.

Keefer, D.K., Wilson, R.C., Harp, E.L., and Lips, E.W.: 1985, The Borah Peak, Idaho Earthquake of October 28, 1983 Landslides, Earthquake Spectra 2, 91125.

Lawson, A.C. (ed.): 1908, The California Earthquake of April 18, 1906: Report of the CaliforniaState Earthquake Investigation Commission, Carnegie Institution, Washington, D.C., Publication87, reprint edition.

Lee, K.L., Marcuson, W.F. III, Stokoe, K.H. II, and Yokel, F.Y. (eds.): 1977, Research Needs and Pri-orities for Geotechnical Earthquake Engineering Applications, Report of Workshop at Universityof Texas, Austin, National Science Foundation Grant No. AEN77-09861.

Lee, W.H.K.: 2001, Strong-motion Instrumentation and Data, in J. Uzarski and C. Arnold, (eds.),Chi-Chi, Taiwan, Earthquake of Sep

keefer 2002, landslides caused by earthquakes; a historical review

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