Co-Seismic Displacements of the 1995 Hyogo-Ken Nanbu Earthquake Manabu Hashimoto,* Takeshi Sagiya, Hiromichi Tsuji, Yuki Hatanak
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J. Phys. Earth, 44, 255-279, 1996 Co-seismic Displacements of the 1995 Hyogo-ken Nanbu Earthquake Manabu Hashimoto,* Takeshi Sagiya, Hiromichi Tsuji, Yuki Hatanaka, and Takashi Tada GeographicalSurvey Institute, Tsukuba 305, Japan We presentco-seismic displacements of the Hyogo-kenNanbu earthquake of January 17, 1995,detected by continuousGPS (Global Positioning System) observation, campaign type GPS survey and leveling. Continuous GPS observationgives a consistentpattern of displacementswith thoseexpected from a right lateralslip on a NE-SWtrending vertical fault in far field:stations about 50km east and west of the epicentermoved toward the epicenterby about4 cm,while stations north and south movedaway from the epicenter.By comparing with line lengthsobtained by geodoliteabout 10 yearsago, the campaigntype GPS revealed most controlpoints on AwajiIsland moved to the southwestor south,which may be attributedto the movementof the NojimaFault whichcut the surface.On the other hand, controlpoints northwest of the Rokkofault systemmoved toward the northeastand thoseon theother side moved slightly to the west,in and aroundKobe. Leveling data revealed upliftof 19cm on the northwesternside of theSuma Fault, a memberof the Rokko faultsystem, and subsidence of 7 cmjust east of this fault. Furthermore,uplift of about 5 cmwas observedin the centralpart of Kobe City, and subsidenceof 5 cm was detectedeast of Kobe. There is no significantgap in horizontaland vertical displacementsaround the northernextension of the NojimaFault, which implies a complicatedrupture process of this event.Leveling on the east coastof AwajiIsland revealed a significantuplift of about 20 cm with slight subsidenceat both edgesof this upliftregion during the past 20 years. By fittingthe abovegeodetic data, we searchedfor an optimalset of parametersof a dislocationmodel. We assumedsix nearlyvertical faults trending NE-SWfrom Kobe to AwajiIsland on the basisof aftershockdistribution and focal mechanism.About 250cm of the right lateral slip for the fault on AwajiIsland is derivedfrom large horizontaldisplacements near the NojimaFault. The fault in Kobe maybe dividedinto two segmentswith 100-200cm slip by a slip-freezone whichroughly corresponds to the clusterof aftershocks.The southernpart of the NojimaFault, segments near theAkashi Strait, and northof centralKobe may have significantly large thrustcomponents of 100cm. We also examinedthe possibilityof buriedfaults beneath the zonesof severedamage. Since this model cannotexplain the observedgeodetic data and the estimatedslips are inconsistentwith focal mechanism,these possibleburied faultsmay not play a significantrole, if any. possibility of the commencement of new activity 1. Introduction (e.g. Ando, 1994). On January 17, 1995, an earthquake of mag- The Hyogo-ken Nanbu earthquake was a typical nitude = 7.2 (Japan Meteorological Agency, 1995) intraplate event from the seismological point of struck the Kinki district, in southwest Japan (Fig. view. Its focal mechanism was of a strike-slip type 1). This event claimed more than 5,500 lives and and its hypocenter was about 14 km deep beneath caused severe structural damage. Since the Fukui the Akashi Strait (Japan Meteorological Agency, earthquake of 1948, there had been no earthquakes 1995). Its aftershocks were aligned NE-SW from the of magnitude larger than 7 in southwest Japan. north of Kobe City to Awaji Island (Hirata, 1995; Nearly 50 years had passed since the last activity Fig. 2). Therefore, the rupture of right lateral strike- of major earthquakes along the Nankai trough. slip may have started beneath the Akashi Strait and Therefore, some seismologists had inferred the propagated bilaterally. On the basis of the inversion Received July 3, 1995; Accepted November 28, 1995 * To whom correspondence should be addressed . 255 256 M. Hashimoto et al. (a) (b) Fig. 1 J. Phys. Earth Co-seismic Displacement 257 Fig. 2. Aftershock distribution of the 1995 Hyogo-ken Nanbu earthquake after Hirata (1995) with the zone of intensity 7 by the JMA scale (Japan Meteorological Agency, 1995). of seismograms, Kikuchi (1995) showed there are candidate. at least three rupture events aligned along the The Geographical Survey Institute (hereafter aftershock distribution and they occurred sub- GSI) has been conducting several geodetic observa- sequently from SW to NE at very short intervals. tions in and around the Kinki district. GSI has Some geologists have presented a different view made repeated precise distance measurements by of this event on the basis of fieldsurveys. On Awaji geodolite (electro-optical distance meter; hereafter Island, the Nojima Fault ruptured the Earth's EDM) and leveling. Right after the Hyogo-ken surface (Nakata et al., 1995).However, there are no Nanbu earthquake, GSI conducted a revision survey notable co-seismic surface ruptures in and around of horizontal control points and leveling bench- Kobe, which led them to try to derive the location marks in and around the epicentral region in order of buried faults from the distribution of damage. to detect their co-seismic displacements and revise Since the damage to construction in and around their coordinates or heights. Furthermore, GSI Kobe is concentrated away from the aftershock established a nationwide array of continuous distribution (Fig. 2), they consider that there may monitoring stations of GPS called GRAPES (GPS be other unknown buried faults responsible for the Array for Precise Surveys) in 1994 and estimated damage (Shimamoto et al., 1995;Ikeda et al., 1995). co-seismic displacements of the stations around the Thus, the geological view is significantly different epicentral region. from that derived from seismologicaldata. In order In this paper, we present the results of these to resolvethis discrepancy in interpretation between geodetic observations and give our view of this seismologists and geologists,there should be con- earthquake on the basis of elastic dislocation theory. tributions from other fields. Geodesy is a possible Fig. 1. Index maps. (a) Tectonic setting of the Japanese Islands. EU, NA, PA and PH indicate the Eurasian, North American, Pacific and Philippine Sea plates, respectively. Lines with solid triangles show major plate boundaries, while dashed line with open triangle indicates a former plate boundary. Centered line shows the aseismic front (AF). (b) Active faults in the Kinki district (solid lines) modified after Geological Survey of Japan (1995). SMF and SWF indicate Suma and Suwayama Faults, respectively. Asterisks in each map show the epicenter of the Hyogo-ken Nanbu earthquake. Vol. 44, No. 4, 1996 258 M. Hashimoto et al. exceptvertical component for baselines longer than 2. Co-seismic Displacements Derived from Geodetic 100km. In this analysis, we used 24 h phase data, Data except on January 16, because the earthquake As mentioned previously, GSI conducted several occurred at 5:46 (UTC) on that day. We can kinds of geodetic surveys and observations after the obviouslysee co-seismicsteps on January 17 (JST). occurrence of the Hyogo-ken Nanbu earthquake. We can see no clear anomalous movementsbefore In this chapter, we present outline of surveys, the earthquake, when we take the repeatability into observations and co-seismic displacements derived consideration. There are no clear post-seismic from these efforts. movements, either. These results may be attribut- ed to the distance of these stations from the 2.1 Continuous monitoring by GPS epicenter. GSI established a nationwide array of 210 con- In order to obtain co-seismic displacements of tinuous monitoring stations of GPS in 1994 (Abe stations, we fitted regression lines with co-seismic and Tsuji, 1994). In southwestern Japan, stations steps to all displacementcomponents of 17 stations are distributed every 120km. Observations are relative to the Tohaku station. Solid arrows in Fig. being conducted 24 h per day with 30 s sampling. 4 show the co-seismicdisplacement vectors of 17 All phase data are transferred to GSI's main office stations. The Mitsu station moved eastward by in Tsukuba and analyzed within several days to 4 cm and the Minoo and Yoshino stations moved obtain displacements of stations. westward by 3 to 4 cm. On the other hand, the We calculated coordinates of 18 stations (Table Kainan and Anan stations moved southwestward Al) surrounding the epicenter for several days be- and the Miyazu station moved northward by 2 to fore and after the event, using GPS orbits from the 3 cm. Qualitatively,this pattern of displacementsis International GPS Service for Geodynamics. Figure consistentwith displacementsexpected from a right 3 indicates time variations in baseline lengths and lateral slip on a NE-SW trending vertical fault. their three components of the Minoo, Yoshino and Mitsu stations, relative to the Tohaku station (Fig. 2.2 Campaigntype GPS survey of controlpoints 4). Repeatability of each component is less than 1 cm GSI has repeated precise distance measurements (a) J. Phys. Earth Co-seismic Displacement 259 (b) (c) Fig. 3. Daily variations in components of the continuous GPS monitoring stations relative to Tohaku. (a) Minoo, (b) Mitsu, and (c) Yoshino. Rate and offset are shown for each component with regression lines. Vol. 44, No. 4, 1996 260 M. Hashimoto et al. Fig. 4. Co-seismic displacements of the continuous GPS monitoring stations in and around the Kinki district relative to the Tohaku station. Thick arrows show the observed displacement, while white and thin arrows show those calculated for