EERI SPECIAL EARTHQUAKE REPORT -AUGUST 1993 2 EERI SPECIAL EARTHQUAKE REPORT-AUGUST 1993 of Okushiri Island which lies west ~ARTHQUAKE MECHANISM of the Hokkaido mainland and south-southwest of the epicenter ..,-- --,.- , -, ,. -,. ", Shakotan-oki earthquake, which The earthquake began on July 12 (Figures 3a and 3b). The amount at 22: 17 local time at latitude 42 occurred immediately to the north of subsidence initially ranged from 47' North, 139° 12' E at a depth of the 1993 event, had a similar about 20 cm to 100 cm, but about of 27 km. The aftershocks define focal mechanism, but smaller mag- half of that subsidence has been a plane about 150 km long (north nitude and fault dimensions (Sata- recovered by uplift of the island. south) and 50 km wide (east-west) ke 1986). The occurrence of the Extensive marine seismic surveys dipping eastward at a shallow 1983 event led to the suggestion by the Japan Maritime Safety angle (about 10 ° ) , with most of of nascent subduction along the Agency following the earthquake the aftershock foci occurring at Japan Sea coast off northern Hon- have revealed extensive imbricate I shu (Kobayashi, 1983; Nakamura, depths between 10 km and 20 km , faulting of the sea floor extending based on preliminary information 1983) which the 1993 earthquake from east of Okushiri Island west- from Hokkaido University (Figure tends to confirm. The Sea of ward to the inferred surface ex- 1 ). A centroid depth of 1 5 km (a Japan side of this plate boundary pression of the subduction zone. 1"point source representative of the is the Eurasian plate" After the It is inferred that Okushiri Island is overall faulting) was estimated by 1983 earthquake, the northern located on a graben structure with- Harvard. The seismic moment of Japan side of the plate boundary in the accretionary wedge of the 5.6 x 1027 dyne cm corresponds tO was initially interpreted as the overriding plate and that this gra- an Mw of 7.8. Assuming a fault North American plate, but it may ben structure experienced coseis- length of about 150 km and a fault instead be the recently proposed mic subsidence. The sea floor width of 50 km based on after Okhotsk plate (Seno, 1987; De- structures discovered during this shock data, this moment and fault Mets et al., 1992)" survey resemble those that have geometry indicates an average slip been found in the sea floor off the of 2.5 m on the fault plane. The The earthquake caused subsidence Pacific coast of Japan. D ( ~MT focal mechanism estimated / ~ North AmeriCan-* J>y Harvard, representing the fault- / ~ or Okhotsk?) ,cI( I Plate f - ing process as a whole, has one / I nodal plane dipping eastward at 24 ° from the horizontal. This HOKKAIDO ( fault plane orientation is compat- ible with the aftershock distribu- 12 July 1993 M:7.8 tion and consistent with subduc- tion of the floor of the Sea of v.JI Japan beneath northern Japan. However, the source process Eurasian Plate seems to have been complex, with the initial rupture occurring on a Plate Boundaries 78 steeper eastward dipping plane or even possibly on a shallow west- () ward dipping plane (Tanioka et al., Pacific Plate 1993; Kikuchi, 1993). \ 2~ The subduction plate boundary along the Japan Sea coast of SHI~ northern Honshu and Hokkaido has J ~ been recognized only recently KYUSHU (Figure 2). The 1983 Nihon Kai , 48 Chubu earthquake occurred imme- 68/// '--- Philippine Sea Plate diately to the south of the 1993 ,event; the magnitude, fault dimen- 0 SOOkm )ions, and focal mechanism were d very similar to those of the 1993 Figure 2-Rupture zones of offshore earthquakes in Japan larger than earthquake (Satake, 1985; Kana- magnitude 7.4 since 1923. Source: modified from Japan Meteorological mori and Astiz, 1985). The 1940 . Agency. ~ EERI SPECIAL EARTHQU:AKE REPORT -AUGUST' 993 Map of Sea Floor Around Okushiri Island Inferred Cross Section of Okushiri Island and \ i I Coseismic Crustal Deformation Model \ y»" ~~EPicenter of Hokkaido ~~ ~ INanseiOki Okushiri Island Sea level :~~~~~~~~~~~:::---O \;:.:- ,~a~t:~ apan ( Sea \, . I ~ ~ ~~ ~ 0'""'0'0000000'"0000 'Epicenter .--i :; , , -~ " .:Y', ~ -\;~;ijo~*i!io!o.'°'",""1 Okushiri Ridge IStudy AreC" ) \ Eurasian --I ESA h, Plate /6 North American Plate -' I ~~r~~t~~;t-"ke;~~i] Main Fault / / ./ Major Active Fault \(i Figure 3 -Sea floor faults inferred from marine surveys after the ,,:::::;::::::Large Scale Landslide Scar ,,~i;,\~;;': ":::::::::" earthquake. Source: Yomiuri Shlmbur, Japan MaritIme Safety Agency RECONNAISSANCE TEAM A combined EERI-UJNR team sur- to the field led by Yutaka lida the tsunami report, which was co- veyed damage and other effects (PWRI), leader, Kazuhiko authored by E. Bernard, F. Gonza- from the earthquake and tsunami. Kawashima (Head of the Earth- lez, and D. Sigrist of the U.S. side The team arrived in Japan on July quake Engineering Division, PWRI), of the UJNR team, and Hiroichi 18, and conducted field reconnais- Susumu lai (Head, Geotechnical Tsuruya, Port and Harbor Research sance and data gathering between Earthquake Engineering, P&HRI) S. Institute, Ministry of Transport, July 20 and 24. Unjoh (PWRI), T. Nakajima (PWRI), and Kenji Kato, University of and R. Tsunaki (PWRI). Tokyo, of the Japan side of the EERI team members included Les UJNR team, and the landslides Youd, Brigham Young University, The acronym UJNR refers to the report, which was authored by E. EERI team leader (geotechnical U.S.-Japan Panel on Natural Harp, USGS, of the UJNR team. D aspects); Jane Preuss, Urban Resources, a cooperative inter- Regional Research (tsunamis, land national program that celebrated use and response planning); Paul its 25th year in 1993. The UJNR Somerville, Woodward-Clyde Panel on Wind and Seismic Effects Consultants (seismology) ; and is one aspect of the larger UNJR Charles Scawthorn, EOE Inc. program. Members of the Japan- (structural engineering and fire side Wind and Seismic Panel response). Other members of the arranged outstanding logistical The research, publication, and UJNR team included Riley Chung support for the U.S. team, for distrlDution of this report was (NIST), leader; Richard Bukowski which all members were very funded by National Science (NIST); Eddie Bernard (NOAA); grateful. The UJNR cooperation Foundation Grant #BCS-9215158 Frank Gonzalez (NOAA); Dennis also made it possible for the U.S. as part of the EERI Learning from Sigrist (NOAA); Charles Barnes team members to work closely Earthquakes Project. (American Plywood Association); with Japanese investigators. David Tyree (American Forest and Complete copies of reconnaissancefi Paper Association); Edwin Harp The reports in this special edition reports submitted to EERI are V (USGS), and Peter Yanev, EOE Inc of the EERI newsletter were archived at the EERCINISEE lIbrary, (NCEER). Several Japanese authored by the EERI team Call EERC at 510-231-9401 for I engineers accompanied the team 'members, with the exception of further information. 4 ~ EERI SPECIAL EARTHQUAKE REPORT -AUGUST 1993 damage, and few gravestones ( ~TRONG GROUND MOTION were overturned, suggesting that the ground motions in this region Strong ground motions were re- island lies just east of the lower were not unusually large. corded on a variety of instrument edge of the rupture surface, which types by numerous agencies in- had a depth of about 20 km. At Strong ground motion recordings cluding the Hokkaido Development the Aonae cemetery, at the south were made at several localities in Department, the Japan Meteoro- end of the island, 80% of the the eastern part of southwestern logical Agency I Japan Rail, the tombstones were overturned, and Hokkaido and farther to the north Ministry of Construction, and the in houses furniture was over- near Sapporo. The largest peak Port and Harbor Research Institute. turned, suggesting peak accelera- acceleration, slightly less than There were no strong motion re- tions in the range of 0.4g to 0.5g. 0.5g, was recorded at Kuromat- cordings from Okushiri Island or On the mainland of Hokkaido be- sunai which lies midway between the immediately adjacent south- tween Esashi and Motsutanosaki, Suttsu on the Japan Sea coast and west coast of Hokkaido. Okushiri there was no major structural Oshamanbe on Uchiura Bay. That locality is about 80 km from the nearest point on the fault. This 1 recording had an unusually large ratio of peak horizontal to vertical motions (a factor of about 5). 0 -Croule Suttsu, which is at a similar dis- -Youngl tance, had peak horizontal accel- -Cohee erations of about 0.2g and peak 00 displacements of about 8 cm. The ,0Q:DoO duration of strong ground motion 00 '0 was approximately one to two O~ minutes at most locations within ( ~ 0 .1 100 km of the rupture surface, as ~ 0 0 expected from an earthquake of :,j II 0 '\ ~ this size. The duration of high GI 0 - GI frequency motion was markedly CJ ~ longer at stations east of the epicenter than at stations to the ~ 0 § southeast, which may reflect the .~ effect of rupture directivity from 0 00 = 0 north to south. 0 -= o~ GI 0. 01 ~ Very preliminary peak horizontal acceleration values without instru- 00 ment corrections are plotted as a function of closest distance to the 0 fault in Figure 4; these values will change after the data have been corrected and checked. Theyare compared with attenuation rela- tions for subduction earthquakes 0.001 I for soil sites that are in current use 10 100 in the United States (Cohee et al., Closest Distance (km) 1991; Crouse, 1991; Youngs et al., 1988). The peak accelerations Figure 4 -Very preliminary uncorrected peak horizontal acceleration recorded during the Hokkaido- ~ ~' lotted ~gainst c~osest distance t,o the rupture surface, , compare.d with nansei-oki earthquake are generally :1 ttenuat/on relat/ons for subduct/on earthquakes used /n the UnIted larger in the distance range of 70 States.