geologic setings for earthquakes seminar main work

8/8/2019 Geologic Setings for Earthquakes Seminar Main Work

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ABSTRACT

Earthquakes are sudden movements in the earths crust. They occur along faults when

stresses building up in the crust is suddenly released. Most of the earthquakes occur along

plate boundaries between two moving plates. Earthquakes have devastating effects on the

environments and lives of people when they occur unexpectedly. Therefore various

methods have been developed to accurately predict earthquakes in order to reduce its

hazardous effects. Earthquakes prediction means a future statement regarding the future

seismic activity in a region based on the result of observed geophysical data measured

from the seismic zone. Predictions could be made covering a long term period using

various theories i.e. elastic rebound theory, the seismic gap theory; or short term using

earthquake precursors i.e. foreshocks, strain accumulation, changes in the surface

geometry of the earth, variations in ground water levels etc. all these predictions depends

on the parameters being measured. One of the long term predictions is based on the use of

seismic gaps. Seismic gaps are faults along a tectonically active area where no large

earthquake has occurred over 30 years although it is known that elastic strain is building

up in the zone and this zone has a high earthquake potential. A theory; the seismic gaptheory and various models have been established to govern and test the accuracy of

seismic gaps in its use for the prediction of earthquakes. Although various controversies

trail its use as stated by various researchers, the seismic gap method has been applied in

predicting the occurrence of earthquakes in various earthquake prone zones with little

success e.g. the Guerrero seismic gap in Mxico, the Loma prieta seismic gap in

California, USA. Notwithstanding, various problems hinder the effective earthquake

predictions. They range from political, geological, and many other factors.


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CHAPTER ONE

INTRODUCTION

Over the past few decades, seismologists have made enormous progress in understanding

the physical processes that govern the occurrence of earthquakes. For example, it is

known that most earth tremors are generated by stresses that accumulate along the

boundaries between the giant tectonic plates that constitute the earths outermost rigid

layer. Seismologists have estimated the sense of ground displacement to be expected in

many seismically active regions, and experience has shown that the number of fatalities

and level of damage created by an earthquake are dependent on the size and location of

the event. Nevertheless, one of the major goals of seismology in earthquake prediction

remains as elusive as ever. Although some seismologists claim that some earthquakes are

predictable in the not-too-distant future, others suggest that their occurrence is essentially

random and research into earthquake prediction might be futile.

EARTHQUAKES

Earthquakes are one of the most damaging natural phenomena to affect the earth. In a

narrow sense, an earthquake is a sudden fracture in the earths interior, together with the

resulting ground shaking; in a broad sense, it is a long-term complex stress accumulation

and release process occurring in a highly heterogeneous medium. Advances have been

made in understanding crustal deformation and stress accumulation processes, rupture

dynamics, rupture patterns, friction and constitutive relations, interactions between faults,

fault-zone structures, and nonlinear dynamics. Thus it should be possible to predict to

some extent the seismic behavior of the crust in the future from various measurements


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taken in the past and in the present. However, the incompleteness of our understanding of

the physics of earthquakes in conjunction with the obvious difficulty in making detailed

measurements of various field variables in the earth makes accurate prediction difficult.

Table 1: Table showing some notable earthquakes in the world.

GEOLOGIC SETTING FOR EARTHQUAKES

The overall framework that guides the discussion of earthquake occurrence is the theory

of plate tectonics, a large-scale picture of the earths basic workings originally set forth in

the 1960s and 1970s. In this conceptual framework, the rocks making up the outer layers


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of the earth are broken into a patchwork of ever-shifting tectonic plates. Some of these

plates are enormous-the rocks underlying much of the Pacific Ocean, for example, lie on a

single IO,OOO-km wide Pacific Plate-whereas others may span only a few hundred

kilometers. What distinguishes a plate, however, is that it moves as a cohesive body across

the surface of the earth. As a plate moves, it grinds or knocks against its neighbors; this

plate-to-plate interaction produces the majority of the worlds earthquakes. With a few

significant exceptions, identifying the most likely breeding ground for damaging

earthquakes is thus synonymous with finding the boundaries of tectonic plates. The two

types of plate boundaries associated with damaging earthquakes in the world are

subduction zones and strike-slip faults. In addition, there are intraplate earthquakes, whose

origins are less well understood.


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FIG 1:World wide location of Earthquake epicenters at the boundaries of the Worlds

tectonic plates.

Fig :cross section illustrating the main types of plate boundaries (Cross section by Jos F.

Vigil from This Dynamic Planet -- a wall map produced jointly by the U.S. Geological

Survey, the Smithsonian Institution, and the U.S. Naval Research Laboratory.)


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CHAPTER TWO

EARTHQUAKE PREDICTION

Because earthquakes occur suddenly, often with devastating consequences, earthquake

prediction is a matter of great interest among the public and emergency service officials.

However, the term earthquake prediction is often used to mean two different things. In

the common usage, especially among the public, earthquake prediction means a highly

reliable, publicly announced, short- term (within hours to weeks) prediction that will

prompt emergency actions e.g. alert, evacuation, etc. The issue whether the quality of

prediction is good enough to be used is in question.

In the second usage, earthquake prediction means a statement regarding the future

seismic activity in a region, and the requirement for high reliabilty is somewhat relaxed in

this usage. The reliability of a specific prediction process, and the amount and quality of

data present can nit be disregarded as the prediction should entail prognostic parameters,

that is the location, the time of occurrence and its magnitude, for some time before it

takes place.

EARTHQUAKE PREDICTION METHODS

Earthquake prediction methods are many but they have been generally characterized into

two. They are:

1. Short-term predictions

2. Long-term predictions


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2.2.1 Short term predictions

Short-term predication involves monitoring of processes that occur in the vicinity of

earthquake prone faults for activity that signify a coming earthquake. Anomalous events

or processes that may precede an earthquake are calledprecursorevents and might signal

a coming earthquake. Despite the array of possible precursor events that are possible to

monitor, successful short-term earthquake prediction has so far been difficult to obtain.

This is likelybecause:

a) The processes that cause earthquakes occur deep beneath the surface and are

difficult to monitor.

b) Earthquakes in different regions or along different faults all behave differently,

thus no consistent patterns have so far been recognized.

Among the precursor events that may be important are the following:

1. Ground Uplift and Tilting - Measurements taken in the vicinity of

active faults sometimes show that prior to an earthquake the ground is uplifted or

tilts due to the swelling of rocks caused by strain building on the fault. This may

lead to the formation of numerous small cracks (called microcracks). This

cracking in the rocks may lead to small earthquakes called foreshocks.

2. Foreshocks - Prior to a 1975 earthquake in China, the observation of

numerous foreshocks led to successful prediction of an earthquake and evacuation

of the city of the Haicheng. The magnitude 7.3 earthquake that occurred,

destroyed half of the city of about 100 million inhabitants, but resulted in only a

few hundred deaths because of the successful evacuation.

3. Water Levels in Wells- As rocks become strained in the vicinity of a

fault, changes in pressure of the groundwater (water existing in the pore spaces


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and fractures in rocks) occur. This may force the groundwater to move to higher

or lower elevations, causing changes in the water levels in wells.

4. Emission of Radon Gas - Radon is an inert gas that is produced by the

radioactive decay of uranium and other elements in rocks. Because Radon is inert,

it does not combine with other elements to form compounds, and thus remains in a

crystal structure until some event forces it out. Deformation resulting from strain

may force the Radon out and lead to emissions of Radon that show up in well

water. The newly formed microcracks discussed above could serve as pathways

for the Radon to escape into groundwater. Increases in the amount of radon

emissions have been reported prior to some earthquakes.5. Changes in the Electrical Resistivity of Rocks: Electrical resistivity

is the resistance to the flow of electric current. In general rocks are poor

conductors of electricity, but water is more efficient a conducting electricity. If

microcracks develop and groundwater is forced into the cracks, this may cause the

electrical resistivity to decrease (causing the electrical conductivity to increase). In

some cases a 5-10% drop in electrical resistivity has been observed prior to an

earthquake.

6. Unusual Radio Waves - Just prior to the Loma Prieta Earthquake of

1989, some researchers reported observing unusual radio waves. Where these

were generated and why, is not yet known, but research is continuing.

7. Strange Animal Behavior- Prior to a magnitude 7.4 earthquake in

Tanjin, China, zookeepers reported unusual animal behavior. Snakes refusing to

go into their holes, swans refusing to go near water, pandas screaming, etc. This

was the first systematic study of this phenomenon prior to an earthquake.

Although other attempts have been made to repeat a prediction based on animal

behavior, there have been no other successful predictions.


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2.2.2 Long-Term Predictions

Long-term forecasting is based mainly on the knowledge of when and where earthquakes

have occurred in the past. Thus, knowledge of present tectonic setting, historical records,

and geological records are studied to determine locations and recurrence intervals of

earthquakes. Two aspects of this are important.

1. Paleoseismology: The study of prehistoric earthquakes. Through study of the

offsets in sedimentary layers near fault zones, it is often possible to determine recurrence

intervals of major earthquakes prior to historical records. If it is determined that

earthquakes have recurrence intervals of say 1 every 100 years, and there are no records of

earthquakes in the last 100 years, then a long-term forecast can be made and efforts can be

undertaken to reduce seismic risk.

2. Seismic gaps - A seismic gap is a zone along a tectonically active area where no

earthquakes have occurred recently, but it is known that elastic strain is building in the

rocks. If a seismic gap can be identified, then it might be an area expected to have a large

earthquake in the near future.

Fig 2: diagram showing seismic gaps in the south American plate.


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CHAPTER THREE

DEFINITION OF SEISMIC GAPS

Fig 3: Aerial photograph showing presence of seismic gaps between the North

American plate and the Pacific plate.

The term seismic gap is taken to refer to any region along an active plate boundary that

has not experienced a large thrust or strike-slip earthquake for more than 30 years. A

region of high seismic potential is a seismic gap that, for historic or tectonic reasons, is

considered likely to produce a large shock during the next few decades. The seismic gap

technique provides estimates of the location, size of future events and origin time to

within a few tens of years at best. Referring to the diagram below:


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Fig 4: schematic diagram showing the Loma Prieta, San Francisco and Parkfield seismic

gap before and after earthquake occurrence.

A: Activity from January 1969 to July 1989. Note how there seem to be gaps in the

activity, where few or no earthquakes occurred, around Loma Prieta, the San Francisco

peninsula, and just South of Parkfield.

B: The Loma Prieta earthquake and the first three weeks of aftershocks. The frenetic burst

of activity associated with this major earthquake neatly filled in most of the Loma Prieta

gap.


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DEVELOPMENT OF THE SEISMIC GAP THEORY AND

MODEL

3.2.1 Seismic Gap Theory

The seismic gap theory has enjoyed intuitive appeal since the early work of Reid (1910).

He suggested that a large earthquake releases most of the stress on a given part of a fault

and that further earthquakes could be expected when that stress has reaccumulated

through tectonic motion. The acceptance of plate tectonics in the 1960s as a believable

mechanism for resupplying stress added more intuitive arguments for the seismic gap

theory. Using this, Fedotov (1968) identified several plate boundary regions that had

experienced large historical earthquakes and named several zones as likely to have

earthquakes in the near future.

This crude form of long-term prediction used in the absence of quantitative data (seismic

gap theory) simply states that large events along a specific plate boundary segment will be

widely separated in time. This is not to suggest, however, that the seismic gap theory is

more advanced than a first, primitive step towards earthquake prediction.

Most work on seismic gap theory was published before 1982. Considerable subsequent

work (e.g., Sykes and Nishenko, 1984) has focused on making time-varying probabilistic

predictions for fault segments along the main plate boundary in California, offshore

western Canada, southern Alaska and the Aleutians. Those calculations took into account

the rate of loading for each fault segment, the size and date of its last large event, its

average repeat time and their standard deviation, which simple seismic gap theory doesnot. Many of the newer calculations not only were more quantitative but also they took

into account pronounced gradients in slip in great earthquakes, such as from about 2 to 6

m along the rupture zone of the 1906 California shock, which gap theory could not.


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The application of simple gap theory in about 1981 merely yielded the result that the

entire 1906 zone had not ruptured in many decades. Lindh (1983) and Sykes and

Nishenko (1984) proposed that reloading had brought stresses along the southeastern end

of the 1906 rupture zone close to their pre-1906 levels whereas stresses along segments to

the north of San Francisco where slip was highest in 1906 were still far below their pre-

1906 levels. Similarly, that portion of the rupture zone of the 1968 Tokachi-oki

earthquake where slip was smallest in 1968 broke in the great thrust earthquake of

December 1994 (Tanioka et al., 1996). Thus, Lynn ( Lynn R. Sykes et al) concluded that

time-varying, long-term probabilistic predictions that included information treating

pronounced gradients in slip in great earthquakes represented an advancement with

respect to simple gap theory.

3.2.2 THE SEISMIC GAP MODEL.

Fig 5 : Seismic Gap Model of McCann, Nishenko, Sykes, and Krauss, 1979.

The seismic gap model ( McCann et al. 1979: Nishenko 1989: 1991) assumes that

characteristic earthquakes are quasi periodic with a characteristic recurrence time.


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According to the gap model, plate boundaries, like faults, are divided into various

segments each with its own characteristic earthquake (Fedotov, 1968: McCann et al,.

1979: Nishenko, 1989: 1991).

The characteristic earthquakes are assumed large enough to dominate the seismic

moment release and substantially reduce the average stress. The standard explanation for

quasi periodicity is that the stresses which cause earthquakes are slowly building up by

plate movements after one event (Nishenko and McCann, 1981. P. 21): a new, strong

earthquake is less probable until the stress or deformational energy reaches a critical value

(Shimazaki and Nakata, 1980). A seismic gap according to the model is a fault or plate

segment for which the time since the previous characteristic earthquake is close to or

exceeds the characteristic recurrence time. McCann et al.(1979) adopted the gap model

and produced a colored map of earthquake potential for nearly a hundred circumPacific

zones (as shown above). They assumed that seismic potential increases with the absolute

time since the last large earthquake. Nishenko (1989; 1991) for the first time refined the

seismic gap model into one that could rigorously be tested. He specified the geographical

boundaries, characteristic magnitudes, and recurrence times for each seismic gap segment.He used a quasiperiodic recurrence model to estimate conditional earthquake probabilities

for 125 plate boundary segments around the Pacific Rim. The accuracy of the newer

probabilistic methods, however, does require more information about initial conditions,

such as the distribution of slip in the last large shock produced during earthquake

occurrence, particularly for great events that rupture several fault segments. The seismic

gap model has been applied to make long-term forecasts for many faults and plate

boundaries around the world even though this model has failed to successfully predict an

earthquake occurrence.


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Fig 6: Probabilities of a major quake between 1988 and 2018 along the San Andreas Fault,

California, USA.

CONTROVERSY REGARDING THE SEISMIC GAP MODEL

The seismic gap model have encountered many problems (Kagan and Jackson, 1991)

compared the model of McCann et al(1979) against predicting earthquakes. They found

that large earthquakes were more frequent in those zones where McCann et al. had

estimated low seismic potential. Those zones proved to be ones with consistently high

seismicity. The test required that definitions of the qualifying earthquakes and terms like

earthquake potential be supplied in retrospect. Understandably, this led to strong debate

(Nishenko and Sykes, 1993; Jackson and Kagan, 1993).


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The validity of the seismic gap model was vigorously contested in a Nature debate on

earthquake prediction (see http:// www.nature.com/nature/debates/earthquake/equake_

frameset.html). Stein and Newman ( 2004) and Stein et al (2005 ,p.432) suggested that

evidence for characteristic recurrent earthquakes can result from three possible selection

biases:

Short instrumental earthquake history.

Errors and biases in estimating the size or frequency of the largest earthquakes from

paleoseismic records, and

Selection of the spatial extent of the seismic zone considered.

Kagan and Jackson (1995) found that earthquakes after 1989 did not

support Nishenkos (1989, 1991) gap model. Rong et al. (2003) concurred. They found

that the rate of earthquakes meeting the characteristic threshold was significantly less

than the number predicted by Nishenko (1989; 1991): nineteen were predicted but only 5

occurred for the period 1989-2001. Had the 2004 Parkfield event happened before the end

of 2001, which would have been more favorable to the seismic gap model, there would be

one more success. Even then the hypothesis would have failed at the 95% confidence

level.

PROBLEMS IN THE SEISMIC GAP MODEL

Three main reasons accounts for the overestimation of the seismic gap model:

1. This historic method requires that at least two earthquakes of equal

characteristic magnitudes occur in a zone in the past century, introducing a selection bias

in assigning recurrence times to the earthquakes. If there have been two or more

earthquakes similar in magnitude to the largest in the zone, the mean recurrence rate is

estimated from the time intervals between events, and the probability of future events is


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calculated using a time-dependent model of earthquake recurrence. If there are fewer than

two such earthquakes, no forecast is made, except for a few zones where the direct

method can be used. Thus forecast would be made for zones experiencing more than the

expected number of earthquakes, and zones experiencing fewer numbers would be

preferentially neglected.

2. Open time intervals are neglected in this method, and

3. The direct method assumes that all slip on a fault is accomplished by

characteristic earthquakes, but this slip in fact occurs in various sizes of a few centimeters

per year.

THE ROLE OF SEISMIC GAPS IN EARTHQUAKE

PREDICTION.

Seismic gaps are most frequently used for long term predictions of earthquakes in

the absence of quantitative seismological data and long periods between earthquake

reoccurrences in the fault zones. The slip history and tectonic differences of the seismic

gaps can be used in assessing and predicting the seismic potential of the region where the

gaps are located. By measuring and monitoring the geophysical activities or precursors on

the fault e.g. the rate at which strain builds up in rocks, is another factor used to determine

the earthquake probability along a section of a fault. Seismic gaps can give clues about an

impending earthquake although it lacks preciseness e.g. earthquake recurrence rates in a

region can indicate that the fault involved ruptures repeatedly at regular intervals to

generate similar quakes. It has been used to forecast earthquakes on subduction zones and

some strike-slip plate boundaries e.g. San Andrea's fault. Also Probability forecasts are

also based on the location of seismic gaps. Long term forecast of large earthquakes using


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the seismic gap method is useful in understanding the long term behavior of seismic

zones.

The seismic gap model has been used in many recent estimates of U.S earthquake

probability and seismic hazard. Without doubt it influences earthquake hazard

assessments in other countries as well. Modifications and complexities have been added,

but the validity of the results still depends largely on the validity of the characteristic

model. Long term predictions with this method are generally considered to have been

successful for several large earthquakes with M > 7.5 e.g., the 1972 sitka, Alaska,

earthquake ( Kelleher, 1970), the 1973 Nemuro-Oki, Japan, earthquake, the 1985

Valpariso, Chile, earthquake( Kelleher, 1972; Nishenko, 1985).


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CHAPTER FOUR

APPLICATION OF SEISMIC GAPS IN EARTHQUAKE

PREDICTION: CASE STUDIES.

4.1.1 THE SHUMAGIN SEISMIC GAP, ALASKA PENINSULA

The Shumagin seismic gap, a segment of the plate boundary along the eastern Aleutian

arc, has not ruptured during a great earthquake since at least 18991903. Because at least

77 years have elapsed since the Shumagin Gap last ruptured in a great earthquake and

repeat times for the 1938 rupture zone and part of the Shumagin Gap are estimated to be

50 to 90 years, a high probability exists for a great earthquake to occur within the

Shumagin Gap during the next one to two decades. Reconsideration of the rupture zones

of the Aleutian earthquakes of 1938, 1946, and 1948 suggests that those events did not

break the interplate boundary beneath the Shumagin Islands. Thus, the Shumagin seismic

gap extends from the western end of the 1938 rupture zone to the eastern end of that of

1946. These boundaries also coincide with transverse structural features. At least the

eastern half of the Shumagin Gap broke in great earthquakes in 1788 and 1847 and

possibly in 18981903. The Shumagin Gap is probably not the result of aseismic slip;

rather, plate motion is accommodated there seismically and episodically and can be

expected to produce large earthquakes in the future. Although there is no definitive

evidence of longterm precursors of a possible future earthquake, several observations

suggest that the Shumagin Gap is in an advanced stage of the earthquake cycle. Both

teleseismic and local network data indicate a near absence of seismic activity (M: 2.0)

above a depth of 30 km along the main thrust zone within the gap; this is in strong

contrast to adjacent portions of the arc where seismic activity is scattered across most of

the main thrust zone. Two earthquakes with high stress drops (600900 bars), which


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occurred at the base of the main thrust zone, may indicate the accumulation of a

considerable amount of strain energy within the gap. A possible seismic gap at the eastern

end of the aftershock zone of the Aleutian earthquake of 1957 has been identified near

Unalaska Island. An earthquake that nucleates in the Shumagin Gap could also rupture the

possible Unalaska Gap to the west, the 1938 aftershock zone to the east, or both, with

resultant magnitude up to Mw = 9.0. Alternatively, the Shumagin Gap alone, or in one of

the above combinations could rupture in a series of very large earthquakes instead of a

single great shock. Past AlaskaAleutian earthquakes, including those of 1788, 1938,

1946, 1957, 1964, and 1965, have generated very large tsunamis. Future large earthquakes

in the Shumagin Islands region could generate wave heights of several tens of metersalong shorelines near the rupture areas. The Shumagin Gap is one of two major gaps along

the United States portion of the AlaskaAleutian plate boundary and is one of the few

areas in the United States where processes leading to a great earthquake are likely to be

observed within a reasonable span of time.

4.1.2 TOKAI ANDIBARAKI-OKI SEISMIC GAPS, JAPAN.

In Japan, the sole case in which the administrative authorities have acknowledged the

possibility of prediction is the Tokai earthquake. In actuality, even assuming the

appearance of preslip, the Tokai earthquake is the only earthquake with observation

conditions that make it possible to capture this phenomenon. In March 1998, the

Earthquake Assessment Committee for Tokai earthquakes revised its standard for

convening, which is invoked when anomalous activity is detected. If preslip is detected,

the Assessment Committee will convene, based on an assumption that an earthquake will

occur within 72 hours. It can be said that this is one result of progress in simulation

techniques. Of course, this is ultimately only a result of simulation and is not backed by

fact. Therefore, even assuming actual preslip is detected in the Tokai seismic region, there


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is a remaining element of anxiety, in that the prediction based on this will itself be an

unrehearsed performance, i.e., a prediction of a major disaster based on an unproven

assumption. There are several problems, including this, which stand in the way of

prediction of a Tokai earthquake. The fact that expectations for prediction of a Tokai

earthquake are placed on the appearance of preslip is based on the prior example of the

1944 Tonankai Earthquake. The day before this earthquake, an unexpected change in

inclination was observed in measurements of the water level around Kakegawa.

However, questions were raised as to whether this observation was a true crustal

movement or not. Moreover, even assuming this change in inclination was actual, no

convincing reason was given for the fact that the observation was made at the Kakegawa,

which is located at a considerable distance from the source.

According to the forecast by HERP, the 30 year probability of a Tokai earthquake is 87%

(reference value). This result is based on the fact that the average interval between Tokai

earthquakes, which have occurred four times since the 1498 Meio Earthquake during the

Muromachi period, is 119 years. Actually, however, apart from the previous Ansei Tokai

Earthquake of 1854 and the Hoei Earthquake of 1707 which preceded it, there are

questions about the existence of earlier Tokai earthquakes than this, and the interval

between occurrences may be longer. In other words, there is a possibility that the current

probability is much smaller than that in the HERP forecast. This is also related to the

suggestion that the relative velocity of the plates in the assumed source area of a Tokai

earthquake may be smaller than originally assumed (4cm/yr). For example, there is a

theory that part of the Philippine Sea Plate on which the Izu Peninsula rides is a

microplate that has separated from the main body. According to this theory, the relative

velocity is assumed to be 2cm/yr, and if this is true, the interval between the occurrence of

earthquakes would be two times the issued value.

Furthermore, many researchers think that there is a high possibility of linkage between

Tokai earthquakes and Tonankai and Nankai earthquakes. This is because, historically,


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there are no examples in which a Tokai earthquake occurred independently. In the current

condition, the probability of the next Tonankai earthquake is 60%. From this forecast, the

crisis point should come around 2030. Assuming linkage between these earthquakes, the

next Tokai earthquake will be triggered by a Tonankai earthquake, which means that its

occurrence should be delayed until another Tonankai earthquake occurs. As this

discussion suggests, many difficult questions regarding the feasibility of predicting a

Tokai earthquake remain unanswered. However, this notwithstanding, it can be said that

the Tokai earthquake is the closest to prediction. Beginning around the year 2000, the

largest slow slip in the history of observation occurred directly under Hamana Lake,

which is adjacent to the assumed source, and simultaneously with this, significant changes

were seen in the condition of locking in the source area. Using the dense GPS network,

which is prominent even in Japans high density observation network, it has become

possible to grasp such movements in detail. In other words, changes in the current

condition are being captured on a moment-by-moment basis only in the case of the Tokai

earthquake. Moreover, simulation research on the Tokai earthquake is gradually

approaching a realistic level. Conversely, if prediction of a Tokai earthquake is thought

impossible even in light of these conditions, this is equivalent to denying the possibility of

earthquake prediction as such. In this sense, the Tokai earthquake must be considered a

touchstone for earthquake prediction in general.

On May 8, 2008, an M7.0 earthquake occurred off the coast of Ibaraki city. Here, it is

known that six earthquakes have occurred virtually periodically at intervals of more than

20 years from the first historical earthquake in 1896 up to the present (which signifies the

presence of a gap). Furthermore, from an analysis of the seismic waveform, it is also

known that the same asperity ruptured in the most recent earthquake and the previous

M7.0 earthquake which occurred in 1982. It has been reported that this asperity appears to

have formed with a relationship to a subducting seamount. The understanding of the


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Ibaraki-oki earthquakes has increased dramatically based on these facts and discoveries

and the new recognition of the process by which earthquakes occur developed up to the

present. Changes in seismic activity were detected before the event in this earthquake, and

there is to expect that some type of advance information will be possible when the next

Ibaraki-oki earthquake arrives in about 20 years.

FIG 7: Seismic hazard map of Japan showing the distribution of the probability of seismic

motion of seismic intensity 6-Lower or higher within the next 30 years using seismic gaps.


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4.1.3 THE MICHOACAN AND GUERRERO SEISMIC GAPS,

MEXICO

Fig 8: Map of southern Mxico showing the Michoacan and Guerrero seismic gaps.

The map above shows the southern coast of Mexico. Here the Cocos plate is subducting

beneath the North American Plate along the Acapulco Trench. Prior to September of

1985, it was recognized that within recent time there had been major and minor

earthquakes on the subduction zone in a cluster pattern. For example, there were clusters

of earthquakes around a zone that included a major earthquake on Jan 30, 1973, another

cluster around an earthquake of March 14, 1979, and two more cluster around earthquakes

of July 1957 and January, 1962. Between these clusters were large areas that had

produced no recent earthquake activity. Thezones with low seismically were identified as seismic gaps. Because the faulting had

occurred at other places along the subduction zone it could be assumed that strain was

building in the seismic gaps, and an earthquake would be likely in such a gap within the

near future. Following a magnitude 8.1 earthquake on September 19,1985, a magnitude


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7.5 aftershock on Sept. 21, and a magnitude 7.3 aftershock on Oct. 25, along with

thousands of other smaller aftershocks, the Michoacan Seismic gap was mostly filled in.

Note that there still exists a gap shown as the Guerrero Gap and another farther to the

southeast, the Acapulco seismic gap. Over the next 5 to ten years, earthquakes are

expected to occur in these gaps.

4.1.4 THE SAN FRANCISCO, LOMA PRIETA, AND

PARKFIELD SEISMIC GAPS,UNITED STATES OF AMERICA.

Shown below are two cross-sections along the San Andreas Fault in northern California.

The upper cross section shows earthquakes that occurred along the fault prior to October

17, 1989. Three seismic gaps are seen, where the density of earthquakes appears to be

lower than along sections of the fault outside the gaps. To the southeast of San Francisco

is the San Francisco Gap, followed by the Loma Prieta Gap, and the Parkfield Gap.

Because of the low density of earthquakes in these gaps, the fault is often said to be

locked along these areas, and thus strain must be building as the tectonic plates keep

moving. This has lead researchers to believe that there is a high tendency for a high

magnitude earthquake to occur in these areas in the nearest future.


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Fig 9: cross section of the San Andreas fault showing seismic gaps


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SUMMARY AND CONCLUSION

Because earthquakes occur suddenly, often with devastating consequences, earthquake

prediction is a matter of great interest in order to help understand the mechanism of

earthquake occurrence in the world today. In earthquake prediction, which has been

promoted during the past 10 years based on a new direction, great progress has been made

in elucidating the process by which earthquakes occur based on a physical science

approach, focusing particularly on the areas on a regional fault known as seismic gaps

which are considered as regions of high seismicity. Seismic gaps are considered as a

useful guide to understanding the location, timing and magnitudes of earthquake

occurrence and a lot of research has been done to identify the processes that occur along a

seismic gap which can help in accurately predicting earthquakes.

Seismic gaps occur in major regional faults at the boundaries of the worlds tectonic

plates and are seen as regions were stress accumulation increases with time on these faults

as a result of plate movement. They are areas with great potential occurrence of

earthquakes of large magnitudes and intensity when the accumulated stress is finally

released on the fault. This method of predicting earthquake is under uncertainties in its

effectiveness although that has not deterred some countries from applying it in identifying

earthquake prone regions e.g. Sitka, Alaska; Nemuro-Oki, Japan; Oaxaca, Mexico; Loma

prieta, California etc

In reality, however, the definitive prediction of earthquakes has not been achievable

especially when seismic gaps are used. Major problems are not limited to an inadequate

amount of information alone but also an the application of results from geophysical data

gotten from the analysis of earthquake precursors. There is also an uncertainty

surrounding the use of seismic gaps.

In conclusion, to make prediction efforts more effective, it is important to understand the

physics of earthquakes, and solid basic research should be promoted. However it is

important to know that more knowledge may not necessarily lead to better prediction


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capability. We may only understand better why it is so difficult to accurately predict short

term earthquakes behavior and try to clear the uncertainty surrounding the long term

behavior. Because earthquake prediction is a matter of serious concern among the public

and emergency services officials, it is the important responsibility of researchers to

communicate what is possible and what is not possible at the present as concerning this

natural phenomena.


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REFERENCES

Berberian, M., Opinion: Earthquake management in Iran, Iranian analysis quarterly,

vol 1 No.2, January 2004

Geller, R. S. (1997d). earthquake prediction: A critical review, Geophys. J. Int. pp131,

425-450.

Hiroo Kanamori. Earthquake prediction: An overview.

Jackson, D.D., and Y.Y. Kagan., 2006. The 2001 parkfield earthquake, the 1985

prediction and characteristic earthquakes: Lessons for the future. Department of earth

and space sciences, university of California, los Angeles, California.

Kagan, Y.Y., and D.D. Jackson, 1991. Seismic gap hypothesis: Ten years after, J.

Geophy. Res., 96, 21, 119-21, 131.

Lynn R. Sykes., Bruce E. Shaw and Christopher H. Scholz, 1999. Rethinking earthquake

prediction., Pure and Applied Geophysics. Vol 155 pp 207-232.

McCann, W. R., S.P. Nishenko, L. R. Sykes, and J. Krause, 1979. Seismic gaps and

plate tectonics: seismic potential for major boundaries, Pure and Applied geophysics.,

117, 1082-1147.

Prof. Stephen A. Nelson, 2009. Earthquake prediction and control: lecture note on

Natural disasters., pp 1-5.


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Rodolfo Console., Daniela Pantosi and Giuliana D. Addezio, 2002. Probabilistic

approach to earthquake prediction: Annals of geophysics, Vol 45, No 6 pp723-731.

Sykes, L. R., 1971. Aftershock zones of earthquakes, seismicity gaps, and earthquake

prediction for Alaska and the Aleutians. J. Geophysics. Res., 76, 8021- 8041.

William Lowrie, 2007. Seismology and the internal structure of the earth: fundamentals

of geophysics., (2nd Ed). Cambridge university press, Newyork, USA. Pp 121- 206.

Y.Y. Kagan and D. D. Jackson, 1995. New seismic gap hypothesis: five years after.

Journal of geophysical research, vol. 100, No. B3, pp 3943-3959, 1995.

Wyss, M. (Ed) 1979. Earthquake prediction and seismicity patterns ( reprinted from Pyre

and Applied Geophysics., vol 117, No. 6, 1979). Birkhauser Verlag, Basel, 237pp.

geologic setings for earthquakes seminar main work

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