the karaginskiy earthquake of january 21, 1976 ...

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THE KARAGINSKIY EARTHQUAKE OF JANUARY 21, 1976, BERING SEA V. M. ZOBIN and Yu. D. MATVIENKO lrinf,il,111.e of' Volc1111olog.v, fkiy curLhquukc of Junuary 21, 1976, occurred HL 18 h 02 ínin GT (G h 02 min LT in the morning of .January 22). The instrument.11! cpkcntcr was in Lhc i-;hullow Lit.ke St.ntit. which ::-;epuruleR Karaginskiy Islancl and Kamchatlrn (Figure 1 ). The mngnlt.udc wns M=fi.2, thc mnxinrnm intcnRit.y on KnraginRkiy lRland bcing VTTT. That. was thc largcHL instrumentally recorded earthquake in Lhe Lilke Strait. We discuss dctailed characteristics of thc rupLur¡~ procesH, macroscismic cffects, and the seismotectonic envfronmcnt.

SEISMOTECTONIC ENVIRONMENT OF THE EARTHQUAKE The source lics beneaLh the shallow (less than 50 m) shelf or Karaginskiy island within the seismic province encircling the Kamchalka-Kommandorskie Islands border of the I3ering Sea. Tlie p1·ovince li(!S no1·i.h of the juncLion of the Kuril-Kamchatkn and lhe i\lculian sei::-;mic dipping zones in Lhe margin of Lhe KamchaLka Uasin in Lhc Uc!1·ing Sea. Gcologists belicve [7] that Lhe western coast of Lhe Bering Sea, including Knraginskiy lsland, constitutes an independenl unit., thc Nort.h kmnchat.ka Olyut.orka block. It.s Rl.ructure is very differcnt. from southern and central Kamchatka. The frontal porLion of t.hc block has no trench and undcrgoes slow uplifting (1.1-1.2 mm/yr), thc uplift itsc~Jf bcing apparently dome-like in charactcr. The morphostructures there resemble the forms adjacent on the sout.h bul are different in that the relief forms are senilc, indicating an ab¡¡ence 1112

KARAGINSKIY EAHTHQUAKE Ol•' .TANUAHY 21, l97G

103

of conL1·11sLi11g neol.ecLonic movements. According Lo l 11, Lhc KamdwLka l\118in, for which the east.ern shore of Karaginskiy l. is part of the Knmchatka continental slope, ·evolved as a local spreading center from Pr1rly Creluceous Lo late Mioccne Lime. This process occurrcd simultarll'ously with the subducLion of Lhe oceanic slab under Karnchal.ka.

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Fil{ure 1 Epicenters of the ~ainehock and aftcrshocks for the January 21, 1976 carthquake. l uinshock epicenler¡ 2, 3 -. epiccntcrs of afte~shocke with KF=I0-11 (21 and KF=l2 (JI; 4 - focal 11chan1sB d1ngraB (shadcd IB the aren whcrc· ftral P waves werc rccordcdl; 5 - isobalh, B. The 11rlhquakee are nuBbered in their teBporal order.

Thc scismicily of lhe norlhwestern coast of the Bering Sea reflects thiH slow-going bul. inlensive teclonic process. Figure 2 present.s 1.he 11ci1;micity of the region for 1.he last three decades when detailed nciHmic invcsligutions providcd complete reporting of Kt.~9.5 carlhquakl!S.

Enrl.hquake epiccntcrs encircle bot.h the Kornmandorskie and the Knmchatka coastal zonc of the Kamchatka Basin. There is a nearly ronl.in11011s zonc of cpiccntcrs from ctrnt of Mcdnyi J. to Cape Govcn in J\umchalka. Somc isolaL(~d carlhquakes occur as fur easl. as Lhc Olyutorka uuy f4]. The only aseismic area thus far is the Shirshov l'idgc. No events of encrgy class 11 (M=3) or greater were recorded within the basin itself in the area bounded by the 3500 m isobath on thc south and 2000 rn on the west and north, as well as al great

104

V. M. ZODIN AND YU. D. MATVIENKO

depths, durint( the period 1962-1989. /¡

ll/!!/1969Lo 1/!011916

/

+ 1\

º"K 1 ( ) 11

1

o \\ . 10

I

/

/"-,

\_/

/

1

J

\00 l•

Figure 2 ScisBic activity in the western Bering Sea in 1962-1988. a - 1/1/1962 to 11/21/1111, b - 11/22/1969 to 1/20/1976, e - 1/21/1976 to 12/31/1988. i - energy class; 2 - source w& ,r 1 largc earthquake; 3 - isobath, .1. 7" and 7• are aftershocks of event 7. Nu~bers denote epic1fttrn o[ carLhquakes with cnergy ciaBs 12 or grcatcr in lhcir teMporal order. The observed magnitudes are rather high. The last 30 Y"11r1 witnessed events with magnitude 7.7 (November 22, 1969, in I~ Ozernyi Day), 7.5 (December 28, 1984, off Cape Kamchatskiy), nr1d i.I (February 29, 1988, belween Mednyi and Bering islands). The fon1l deplhs range from O to 40 km near the Kommandorskie Islands nnrl off

KARAGINSKIY EARTHQUAKE OF JANUARY 21, 1976

105

Knmchatka and are somewh11t deeper, reaching 70-80 km, under the strnit betwcen Kamchalk11 and Bering lsland. ,110·

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Yll(urc J Focal mcchanisms oí earthquakes in the Kamchatka basin. 1, 2 - epicenters oí 11rll:11!es ~ilh K~13.5 (!) and K =12.0 lo 13.4 (2); 3 - isobalh, 1; 4 - arca oí source aechanism 1~11J. Nnbers fol)ow the temporal order oí the earthquakes.

Enrthqunketi in Lhe Kmnchatkn basin are lcss írcquenl than furlhl'r tiouth where they occur in the Kuril-Kamchalka Benioff zone. Tht- l.r-\·alue in the relation logN = a + bK for the Kamchatka basin is

106

V. M. ZOBIN AND YU. D. MATVIENKO

0.57 for thc periori 1962-1988 as against 0.48-0.50 far the Benioff zone. Thcre were whole years in which no cvents with K~12 (M~'1.5) occurred along the Bering Sea coast of Kamchatka and thc Kommandorskie Tslands. Seismic activity on the slope of the Kamchatkn basin occurs in bursts, exr:ept for the arca off Cape Kamchatskiy where the seismic province of the Kamchatka basin borriers the Kuril-Kamchat.ka Benioff zonc. The western coast of the Bering Sea was nearly aseismic durinl( the period from 1962 to Novcmber 1969 (Figure 2, a), so that thc occurrence of the November 22, 1969, M=7.7, Ozernoi earthquake WRS a complete rrnrprise. This first pulse of a scismic burst rnpidly died away (during a year), and the western coast was aBeismic again unt.il t.he ,January 21, 1976, M=6.2, Karaginskiy earthquake (Figure 2, e). Thc Jast. burst of seismic act.ivity was recorded on Deccmber 18, 1984 (M=7.5) at t.he intersection of the Kamchatka basin scismlc province and the Kuril-Kamchatka and Aleutian Benioff zones. Figure 3 presents fault. plane solutions in the aren for the period 1962-1988. Nearly all events with K~13.5 (M~6.0) as well as sorne events with K~l2 are shown. The movements are rather diverse, even though they exhibit a ccrtain spatial pattern.

Table l

Data on the January 21, 1976 earthquake and its larger nflcrshocks.

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11 11 11

1·1 •) 111,s 11 ,K

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1

Holc, Figures in parcnlheses correspond to ntmbers in Figure 1.

Earthqualrns along the coast of eastern Kamchatka from Cape Kamchatskiy lo Olyutorka Bay show thrust faulting oricnted bot.h along (8, 20, 7, 21) and across (23, 5) the coastline trend. At the same time it should be noted that events 23, 5 are uit.uated near shnrp bends of the isobnlhs, the fault. planes generally striking nlong Lhe isobaths.

DESCRIPTION OF THE JANUARY 21, 1976 EARTHQUAKE SOURCE MECHANISM

Hypocentor

position.

Table

prcfwnts

dctcrminations of

Lhe

KARAGINSKIY EARTHQUAKE OF JANUARY 21, 1976

107

J11nu111·y 21 hypocenter by different seismological agencies. Whereas

thc cpicenter location is not much in doubt, the estimates of the focal drplh seem to he questionahle. The ncarest seismograph st.ation, Ossora, is 50 km from a possible epicenter position, but it recorded [' phase alone. The next nearest station, Krutoberegovo, is as far as 330 km. So the focal depth could not be determined with certainty. The estimate based on data from the Kamchatka regional network was O km. The USSR teleseismic network did not give any esLimale at all. The WWSSN estimate was 33 km, which usually means that a hypocenter is within the crirnt. The scismograms presenled in Figure 1 show t.hat P arrivals are distinct both at near and distant stations. Thls is indicativo of a deep focus, most likely in the lower crust, at 20-30 km depth. P waveform. Figure 4, e presents a deconvolved .P wave derived by Lhe technique in [2) from an SKD record (SKD - long period Kirnos

iru1lrurnent). The pulse at the source is a well-defined unipolar one. ll can he inferred that the source rupture was smoolh and single. Source development. Figure 1 shows the spatial distribution of 11flcrnhock epicenl.ers recorded during the first day of activity (2 to li) and lhose which occurred later, during a year after lhe main Rhock. The aftershocks have KF~9.5. The aftershocks of the first day dclinc11Le an ellipsoidal region with the axes of 30 km and 15 km, lhe lnrgcr axis lookin~ NNE. This can be assumed to be the source region of lhc January 21 eart.hquakes. Thc source region extends along the western coast qf Karaginskiy IRlnnd and is bounded by Lhe 50 m isobath on the we~t and by the curving shoreline of the island on the north and easl. The vertical extent of the mainE1hock source region has not been ct1lirnaled, because the focal depths of the aftershocks were inaccur11te.

Figure 1 shows the fault plane stereogram for the Focal mechanism. mninshock. The source parameters are as follows: Fault planes Axes of principal sLresses NPI NPII T N P STR DP SLIP STR DP SLIP PL AZM PL AZM PL AZM 248 90 -92 143 2 -15 ~5 3~0 2 248 45 157 Thc nlxcss sysLem has a compression axis strildng SSF. nnd an PXLension axis striking NNW, both dipping at 45 ·, Onc of !.he t.wo possible l'Upl.ure planes is verLical and strikes ncarly east-west and the oLhcr is almosL horizontal. Slip occurred along the horizontal plnne ami normal faulting along !.he vertical. The preferrcd rupturc plane is the vertical plane slriking NEE, clase to thc regional NE ntructural trend and to Lhe larger axis of the source region. A comparison with the mechanisrris of the other earthquakes of the rrgion (Figure 3) shows that the normal (or shear) faulting of the

108

V. M. ZODIN AND YU. D. MATVIENKO

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Figure 4 P waveforns of the January 21, 1976, H=6.2, Karaginskiy earthquake as recordcd b¡ short period (a) and long period (b) instrumenta, and the deconvolved P wave (e). 1 - seimgm; 2 - deconvolved record.

KAHAGINSKIY EARTHQUAKE OF JANUAHY 21, 1976

109

Karai.;inskiy earthquake is different from the surrounding thrust and lhrusL-sLrike-slip sources. It is not unlikely Lhat this circumstance iR relu.ted to the peculiar position of the Karaginskiy source which is ~•·p1u·aLed from Lhe slope of the Kamchatkn bnsin by Karaginskiy Iuland.

Mnl-{nitwle and energy estímntes are given in Table l. The m11gnit11dc based on surface wave data varies in the range 5.6-6.2, 1rnd th11t derived from body waves is 5.4 to 5.8. The energy class is 12.2 from shear waves, 10.l from compressional waves and 12.9 from roda wuves. The coda wavc value seems to be in belt.er agreemenL wilh Lhc high magnitude of the event.

THble 2

Source para~eters of thc January 21, 1976 earthquake. Bascd on P wave spectra

Based on first-day aftershocks (with

Sou rcc lcngth, km

28

30

2 So 1uce are a, ka

615

353

Stress drop, kPa

25

180

S1i p, cm

0.8

4. ~

FHueter

Source pBrBmeters were determined using the Fourier amplitude 11pccLra of compressional and Ruylcigh waves, and the source size inferred from the one-day aftershock zone. The source parameLer v11l11cs are listed in Table 2. We hnve used rccordings of Lhe vertical SKD instrument al Petropavlovsk-Kamchatskiy for P wnve spcctra and ni. thc SLekolnyi station for Hayleigh wave spectra. The estimules of thc scisrnic moment based on the two waves are similar. Like it was rr•pot'Led in the prcvi01rn pnpers, e. g., in (61, the dimensionR of the Rourcc urca and stress drop eHl.imaLed from P wave spectra and from t he onc-day aft.ershock zone are nol.iceably different. AL the samc time lho relalions connecting thf~ i:;ourcc parameLers of the January 21, l'.l7li earLhqunke are not much diffcrcnt from the average regionnl relHLions for thc Kamchatkan carthquakes [5j.

MACROSElSMIC EFFECTS Tite earLhquake was felt on Karaginskiy Island and along the eastern coasL of Kamchatka. Ten locahties where iL was felL Lo varying extents nre indicated in Figure 5.

110

V. M. ZODIN /\ND YU. D. M/\TVIENKO

Below follows a summery intensily VI-VIII zone.

of

microseismic

effects

wiLhin

the

N

Karoginsl iJ J.

2

Figure 5 HacroseisBic map of the January 21, 1976 earthquake, H=6.2. mainshock cpicenter. Numbers refer to the site nuMbering in the maln text.

- intensity; 2 ·

Whcn analyzing the damagc for building 1.1.nd Htrur,turcs in settlemcnt.s within the intensity VI-VII zone of thc Ul69 M=7."7 Ozernoi earthquukc, we examined the buildings that had been erected after 1969. l. Yagodnoc (A='1 km, int.cnsiLy VIII). Thc carthquake was fclt nR a scquence of distinct shocks lasting for ovcr 11 minute. Thc vibrations were accompanied by a sound that at first rcsemblcd 11 gunshot rcport and afterwards thunder rumblings. Heavy furniturr topplcd down. Brick stovcs werc completely destroyed in all log houses (type-C buildings on the MSK-64 scale), logs clase to the foundations wcre displaced in many houses. Stoves were completely destroyed in lhe only one-storey concrete building (type B). The butt ends of thr building collapsed (Figure 6) and the frume was breached in many places. Wood window frames and fixed sashes were destroyed. All brick chimneys in ali buildings tumbled down. Cracks as wide as 5 cm appeared in the ground after the

r KARAGINSKIY EARTHQUAKE OF JANUARY 21, 1976

r11rt.hqu11kc. Thc craclui were

11

11 l

fcw tens of meLers long.

Figure G 2. OsLrovnoi Kombinat·(~=lO km, intensity VII-VIII).. The earthquakc accompanied by a low rumblc, furniturc toppleli down. Through cracks as wide as 2 cm appeared in the frames in all lyp¡~-íl buildings (cast concrete). The roof timbers carne apart in lype-C wood houses. Typical of ali buildings were collapse of brick ntovcH bascd on cement mortar and chimneys falling northwest. 'l'h