Geol 15 Activity 7Paleoseismology

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The following exercise is taken from:

N. Pinter, Exercises in Active Tectonics: An Introduction to Earthquakes and rectonicGeomorphology, Prenctice Ha1l, Upper Saddle River, NJ, 1996, pp. I I l_121.

PURPOSE

. Paleojeismology- is the study of earthquakes in the recent geologic past. Inpamcuar' me rocus ot research ls on the occurrence, size, and timing 9f orehistoricegthquate-s. The history of past earthquakes is the best tool'for prcdictiis thE tocaii;n.size,. and frequency _of futurg carthquakes. past eartbquakes ari rccordet best wher;rautrng corncrdes with active deposition. Fault trenching is a method for exposins faultedsediments.and deciphering the history of earthquake actiiity. me purpose oi tuis?i;*;f.to 11lll3ro" you with the technique of fault trenching, illusraie stratigraphic evidenceot eartlquafes, and sbow you how to interprct that evidence.

INTRODUCTION

- ,. .fr:.plt""iple challenge in many fi-elds of geology is secing the features to besu(ueo. -t'leld geologlsts scour :rn arca for useful outcrops and intemolate their daabetween these fixed points; petrolcum geologists drill kilometers through s'edimentarv rockto rnler the strahgraphic and structural rclationships beneath the surfacE; and seismolloeistsus€ seNtrllc waves-to lnterpret the composition of the Earth's corc and mantle. Gcoloiistswno study acuve taults arc at the same disadvautage - earthquakes that occur todav-mavru-pture the surface, but erosion quickly erases that &idence in many settinsa. Th;1rt"i,of past eanhquakes is.rhe key to predicting future earthquakit, t"i th"t hiltd;d-li;ouneo Denealn Lne surtace.

. . F"uJt trenching is a technique de-veloped in the last couple of decades to revealevroence ot past eanhquakes in neai-surface sediments. The principle is that if the faultwon't come up lo the geologist, the geologist will go down to tlie fauft. usins a bulldozer.Dackhoe, -or p-rck and sbovel, one or more long ditches are cut across a fault thit is active orsuspected to be active. Ascientific fault trenah is not just a hole in ttre grouno, uut stroutahave the following features:

Approximate Columnarage (A.D.) section

ExerciUnits

l 9 l 0

1357

t650

1480l 4 t 0133512351 1 0 0

r050

995

l li " r ' :

. .;*:*,- ,-

--- Contact flong dash if gradational;short lt approxlmate)

- Peat'..:.::. ' :L massive**--. Clay luninatcd

1lijr:-i:i |I|aSSIVC

*--, Silt lanilarcdorargc

Fine sand

Mcdium to granulc sand

'.li'" Pcbble and cobbles

- micaceous, disturbcd by roots

.r Charcoal fragncnts

.z 4 Wood fragments

_ ,.100

150

Figure 9.4. Stratigraphic column at Pallett Cr€ek tretrch site. (From Sieh et al, 1989).

Fault Trenching

$gurc 9.4 on the previous page is a generalized description of the stratisxaDhv atPalren cjeck, includitrg the rcsults of radiocarbon dates from several of thc-difrerentla-yers. t he ages ot layeni without radiocarbon &tes can be bracketed usins the Drincioleof -superposition. For example, layer 55 must have been deposited aftcr t-050 A.O. 6utbcfore llfi)A.D. .You will use these datcs to dctcrmine tbe agis of some of thc canhsuakeevenB on the san Andreas fault at this site. Tbe principle of-cross-cutting rclationshipslithe most useful tool for determining th9 age of anearthluake. For

"-"dt", ifl;t;3t i;

displacct bla fault ruptue but layJr 6l is not cut, thei tbat rupore mu'st htv; ;cur€dafter ll00 A.D. but befbrc 1335 A.D.

Frgure 9.5. Exposure 7 at Pallett Creek.

, . th" figure,above (Figure 9.5) iltustratcs at least rwo different carthquake cvcnts.Locate the arca labeled F-7- l in Figurc 9.5.

I ) Wbat kind of featue is F-7- I ? CoX{uui& \t^\F

2) Assune that the numbers on the lcft side of Figure 9.4 arc numetical agcs that you canuse to bracket differcnt strata as wcll

-as earthquake events.

- For eiamole.

statigraphic unit 13 formed between 150 aod 400 h.D. What are tle maxtfumqsd minimnm trg6s that bracket the earthquake rher fo,rmed F_Z.l?

7'to - 1s (

3) Ent€r that age informarion into Table 9.1 on the ncxt pagc for Earthquake Event F

4) Examinc Figurc 9.6. Dctcsmirc thc maxinum and miainum agcs for EartlquakcEvcnts N and T. EDtrr this informltioD in Tablc 9. I .

5) Complcir Tablc 9. l. To e,stimafc thc actual datc of cacb csrthquakc evcnt, you willavcrege thc maximutrr and minimum cstimrtcs. For craEple, Eartlquatc Evctrt Xoccurred some timc bctwccr 1753 and l8l7 A.D. Using this mcthod, you'llestimalc thst thc carlhquake occurred around 1785 A.D. In addition, you willcstimatc thc rccuncncc interval botweoo each pairofcarfhquakcs. For cxanplc tbhtcwal betwccn Event X (1785 A.D.) and Event Z (1857 A.D.) is 72 yean.

6) Estimatc the ovcrage rccurrcnce inkrtul for carthqualcs on this scgmcDt of thc SanAndr€as fault by avcraging thc tilrcs bctwccn all of tbc canhquatas recordcd aPaucs Grck- EDt r ods cstirldc at thc bonom ofTbblc 9.1.

Tablc 9-1. Eartbquakc cvcnts at Psllcn Creck. (Dalcs from Sieh ct d, 1989)

E rllqurb Ovailvlnr unilEvcnr Udc.lin8 onit

avlragc agc cstiluta Raotficoca(lmin.+ m|r.F2) Irlcrvd 6d)

mirl !!. aA.D.)

rn r. r8r (A.D.)

t 8

88t l

JtD- 9, I t5?

7l5 8

t1t$tr. 3asl\s(5

1145

T

R.

N

I

F

D

b.t tLs

55+

5\l1L

1S\ix

l 8

rllx1t"5z5l

I1.t6i too

qil1q7

Averlge Recurrence Intervsl =

34

n

+3'l

6715?1

3et

ls' {5

roqB

r${ol\169

1495t465

tTEl6z

l l651035

l o tSi6-s-t

t0l39E l

a75

___JgL

6U658

,m2 t 0

Loob3b1

I7l Plot the ages of all of the eanhquakes in Table 9.1 in the graph below. Because each

age. is merely an estfuiuted range, you will need to draw each earthquake as ahorizontal bar. Earthquake Event B is ploned for you as an example.

\ge (A.D.)H:e=FP:n!E= = = = 5 5 5 5 5 =

8) Canlou connect all or most of the eanhquakes in the graph above with a straight line?Tbe characteristic ealthquakz model iays that the iarie faults are characte-rized byearthquake,s with roughly the same magnitude that occur at roughly equat timeintervals. How well does this model seem to work for this segmeit of the SanAndrcas fault?

N D . No{ er1 ,^f.I9-.

An additional piece of evidence that you have is that the San Andrcas fault systemmoves at 'an average rate of about 3.2 cnlyear. Filue 9.1 on the ncxt page shows theinterrelationship between recurence interval, long-term slip rate, and characteristiceanhquake magnitude. Knowing any two of these parameters for a given fault system,you can estimate the third. For example, a fault that slips at an average rate of I mrr/yr andhas an carthquake_rccurrence interval of 1000 years is characterized by earthquakes withmagnirudes around 7.0.

Fault Trenching

10,000,000

r,000,000

InactiYc faul6 or fauwith extrcmcly lo!

Low actiYity ratc(0.014.1 run/yd

Modcraic activity(0.1-1.0 ru/yr)

High activity tatc(l.Gl0 mri/yr)

Vcry hith activity(10-100 mn/yr)

t10,000s

ca)

!,

{)rt

.)

100,000I

,0[a \

\

to

tcoExucmc activity ratr

S€ldom scen exc.major plaic bouqd

Be'lrl'E C I t - .gE ,1t"."

6-5 649t.0 l,ltsMagdtude

Figure 9.7. General rclationship between recurrcnce intewal, long-rcrm- stp rate, and characrcristifearthquake magnitude. Earthquakemagninrae 6orimntal axis) is fouud by ploning the intersection ofrecufience interval (vertical axis) and slip rate (diagonal lines).(After Slemmons and DePolo, l98O

9) Using the long-term slip rare of the'satr Rndrcas fault, yout' eitimarc of thl averagerecurrence interval at Palleft Creek, and Figure 9.7, what is the characteristicearthquake magnibde for earthquakes along this scgrrcnt of the fault?

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