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, 305,

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 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 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 (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.

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(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

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J. Phys. Earth Co-seismic Displacement 259

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(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 the optimal and "Belt-of-Damage" models, respectively. The locations of upper margins of modeled faults are indicated with dark and light thick lines for the optimal and "Belt-of-Damage" models, respectively.

between control points (triangulation points) with A control point near the Nojima Fault, Ezaki- EDM since the 1970's. The EDM survey was yama (EZK), on Awaji Island, moved south- conducted twice (1975, 1984-1985) in the Kinki westward by 1 m. Kamaguchiyama (KMG) also district (GSI, 1985, 1986, 1987). After the Hyogo- moved southward by 30 cm. These displacements ken Nanbu earthquake, GSI conducted a campaign are attributed to the right lateral slip on the Nojima type GPS survey in and around the epicentral region Fault. In and around Kobe, control points north- and revised the coordinates of 20 control points. west of the Rokko fault system (ROK, KTG, FDO, Comparing the revised coordinates of control points SUM and others) moved to the northeast by 30 cm, with those determined using EDM data in 1975 and while three stations on the other side moved to the 1984-1985, we can obtain horizontal displacements west by less than 10 cm. This pattern of displace- of control points. Figure 5 shows an inner coordinate ments suggests that there is a right lateral slip on solution (a so-called free-network adjustment) for the Rokko fault system. The displacement vector of these data, where we assume no translation or Kanagasaki (KNG) in Akashi is parallel to those rotation over the whole network. We can consider of control points just northwest of the Rokko fault that the displacements of inner coordinate solution system (ROK, KTG, FDO and SUM). A straight are approximately relative to the center of mass of extension of the Nojima Fault in Awaji Island is the network. Since the present network covers a very located between KNG and SUM. This pattern of wide area with dense distribution around the displacement indicates that the Nojima Fault does epicenter and the displacements of control points not reach the opposite shore. on the periphery are small, this inner coordinate solution may represent a real displacement field 2.3 Leveling which we cannot determine exactly. GSI also conducted leveling around the epicentral

J, Phys. Earth Co-seismic Displacement 261

Fig. 5. Horizontal displacements of control points around the source region during the period from 1984-1985 to 1995. Thick arrows show observed displacements (inner coordinate solution), while white and thin arrows show those calculated for the optimal and "Belt-of-Damage" models, respectively. See also Table Al to refer to code of control points. The locations of upper margins of modeled faults are indicated with dark and light thick lines for the optimal and "Belt-of-Damage" models, respectively. Arrows with open circles denote calculated displacements at Kobe University (KOBE-U) for both models.

region during the period from the later half of of Kobe City (benchmarks 451-453). Aftershocks January to March, 1995. Figure 6a shows the routes are concentrated near this uplift zone and the of leveling. structure of faults is complicated (Fig. 2). These Comparing the data with those in 1990, we observations suggest the existence of a barrier on obtained vertical displacements of benchmarks the source fault in this vicinity. along the route from to Nishinomiya via There is a wide region of subsidence in Kobe during the past 5 years (Fig. 6b). In this figure, Nishinomiya, east of Kobe. In this region, leveling benchmark J423 in Himeji is fixed. An uplift of 19 cm has been repeated once per year by GSI and the is detected west of Kobe, where the Suma Fault is and Hyogo Prefectures in order to monitor located, with the strike of NE-SW. This uplift land subsidence. Leveling was last conducted in monotonously decreases westward and there is no October-December, 1994. We obtained vertical gap or jump in height changes around the northern movements of benchmarks, assuming both bench- extension of the Nojima Fault, which might be marks J229 and J472 in Osaka to be fixed (Fig. 6c between benchmarks 441 and 444. On the other and d). The amount of the subsidence during this hand, this uplift rapidly changes to subsidence period was 4-5 cm in Nishinomiya (benchmarks between 445 and 002-043 where the leveling route J460-10700 and J460-462). However, there was no crosses the Suma Fault. This subsidence reaches 7 cm large subsidence east of Nishinomiya during this at 448. These results also suggest that the Nojima period. Fault may not extend to Island, and that Leveling on Awaji Island revealed a remarkable the Suma Fault is responsible for these movements. movement as well (Fig. 6e). Because the previous There is a region of uplift (< 5 cm) in the center leveling was made in the early 1970's, this move-

Vol. 44, No. 4, 1996 262 M. Hashimoto et al.

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J. Phys. Earth Co-seismic Displacement 263

(d)

(e)

Fig. 6. Observed and calculated height changes after the Hyogo-ken Nanbu earthquake along several leveling routes. Obs., denotes observed data. Optimal and "Damage" show calculated displacements for the optimal and "Belt-of-Damage" models, respectively. (a) Leveling routes which were surveyed. (b) Vertical movements along the route from Himeji to Osaka via Kobe during the period from July-November, 1990 to January-March, 1995. (c) Vertical movements along the route from Nishinomiya to Osaka via during the period from October-November, 1994 to January-February, 1995. (d) Vertical movements along the route from Nishinomiya to Osaka via during the period from December, 1994 to March, 1995. (e) Vertical movements along the route from Naruto to Kobe via Sumoto during the period from August, 1970-May, 1973 to February-March, 1995. ment may be contaminated with artificial effects and Himeji. Therefore, it is reasonable to assume that other unknown sources. We fixed benchmark 300 in movements of benchmarks along this route are Naruto considering the distance from the source mainly co-seismic. Large uplift was detected in the region. However, the uplift of the benchmark northeastern part of the island (benchmarks 442 in Kobe is as large as that observed at J423 in 028-029—SF2001). This uplift area is located several

Vol. 44, No. 4, 1996 264 M. Hashimoto et al. km east of the Nojima Fault. Since the movement each fault which gives the least square sum of of the Nojima Fault may have a significant thrust residuals. component on a southeastward dipping plane on Let us assume the unknown parameter vector of the basis of geomorphological observations (Nakata m components to be m, and the data vector of n et al., 1995; Ota et al., 1995; Awata et al., 1995), it components to be d. Observation equation is is reasonable to attribute this uplift to the movement d=Am+s ,(1) of the Nojima Fault. Although we consider the movement of the where A is a n •~ m design matrix, which consists of Nojima Fault is dominant on Awaji Island, we wish Green's functions for changes in line length, to point out a couple of possible sources of the uplift. coordinate and height, and s is observation error There are other faults such as the Kusumoto and vector. We use Okada's formula (Okada, 1985) to Kariya Faults in this region and some aftershocks calculate Green's functions for the half space of are observed along these faults (Fig. 2). Further- Poissonian solid. We adopt 0.25 for Poisson's ratio. more, a significant amount of soil had been removed In general this least squares fitting usually be- there before the Hyogo-ken Nanbu earthquake. This comes underdetermined due to shortage of the removal of soil may have also caused a local uplift, number of available data and their sparse or uneven

It should be noted that there is a region of distribution. In order to avoid this problem, we use subsidence or small uplift at the northern tip of the least square method with a priori information

Awaji Island, though there are only a few bench- (Jackson, 1979; Jackson and Matsu'ura, 1985). In marks (028-013 and 028-011). GPS observation by this preliminary analysis we assume a slip derived Tabei et al. (1996) revealed subsidence of about from seismological and geological data as a priori 2.5 cm at Iwaya, relative to Usuda, central Japan, information. Assuming a priori information vector when comparing the result of February, 1995 with of m components to be p, the observation equation that of November, 1994. Since independent observa- for a priori information is tions with two different techniques detected similar movements around Iwaya, we take this subsidence (2) and small uplift of benchmarks into consideration where e is error in a priori information. Then, we in the following inversion analysis. try to search for a solution which minimizes 3. Fault Model for Co-seismic Displacements 3.1 Modeling method and assumptions (3) We presented co-seismic deformations of the 1995 Hyogo-ken Nanbu earthquake in the preceding where Wand V are weight matrices for observations section. In this section, we present a preliminary and a priori information, respectively. In this fault model on the basis of elastic dislocation theory, analysis, we assume there is no correlation between by fitting these geodetic data. Results of seismologi- observed and a priori information and adopt cal and geological research and geodetic data in- reciprocals of square of standard error of each data dicate that a fault with uniform slip cannot explain as weight. the observations. Therefore, it is necessary to es- The solution will be obtained by timate many fault parameters. However, theoreti- (4) cal displacement field depends non-linearly on fault parameters, and except for slip components, we must Partial resolution matrix for observed data. make some assumptions. To overcome the non- (5) linearity, we assume fault geometry based on distribution of aftershock, focal mechanism and gives the proportion of contribution of observed active faults, and estimate only slip (strike and data to the solution (Jackson, 1979). The trace of dip-slip components) on each fault segment. Since this matrix gives the number of resolved parameters. the geometry of the source fault is not clearly We assign the estimated standard error in the least resolved, we compute a displacement field for squares fitting in Fig. 3 for changes of components various sets of fault parameters and adopt the of continuous GPS monitoring stations. We adopt optimal set of strike- and dip-slip components of conventional threshold as standard error for EDM

J. Phys. Earth Co-seismic Displacement 265

and leveling in the present analysis: (a2 + b2D2)1/2 hardly affects the results. For example, square sums

for distance measurements, where a= 0.5 cm, of residuals for the same fault model were 862.99,

b= 1.2 •~ 10-6, D is line length between control 697.51, 672.66, 660.69 and 657.53 with standard

points in km (Komaki, 1993), and 2.5S1/2 (S: error of 10, 30, 50, 100 and 200 cm, respectively. distance along the leveling route from the reference Then, we assume 100 cm for the standard error of

benchmark in km) for leveling. We obtain line length in the followings.

changes by comparing EDM data before the earth-

quake with subsequent GPS data. The standard 3.2 Results of fitting error should be the combination of errors of these We fitted 34 coordinate changes, 53 line length different measurements. However, because the pre- and 105 height changes, as stated in the previous cision of the GPS survey may be of the order of sections. Since all the data are in APPENDIX 0.1-0.2 ppm, it is reasonable to adopt the same (Tables Al and A2), interested readers should refer standard error for the EDM survey as that for line to them. As for the aftershock distribution and active length change. We obtained height changes by faults (Figs. 1b and 2), we assumed that these comparing the two leveling data before and after modeled fault segments were aligned along the

the earthquake. Therefore, standard error for height Rokko fault system in Kobe and along the Nojima

change is •ã2 •~ 2.5S1/2. Fault on Awaji Island (Fig. 7). In this inversion, for

Next, we set standard error for a priori in- simplicity we did not take into consideration faults

formation. In this analysis, we tried to find a on the east coast of Awaji Island or faults crossing

suitable value that would not affect the results so the Akashi Strait from east to west. Therefore, our

much, instead of searching for a model with fault model is separated into two parts at the Akashi

minimum ABIC (Yabuki and, Matsu'ura, 1992). As Strait. We made calculations using several models

the number of parameters (12 in the present analy- with different geometry. In these calculations, we

sis) are much smaller than that of observation and found several characteristics on the source fault. The

the data points that are distributed around the depth of upper margin of faults in Kobe may be

modeled faults, the present problem is well-posed. larger than 2 km, which comes from the profile of

We set least squares fitting with several values of uplift and the distance between maxima of uplift

standard error ranging from 10 to 200 cm, and found and subsidence across the Suma Fault. The width that the standard error values larger than 50 cm of faults may be as large as 10 km. This may have

Fig. 7. The geometry of modeled fault segments for the optimal model; lower right: plane view; upper left: sectional view from the southeast (direction shown in the plane view by a thick arrow). Rectangles in plane view show projections of modeled faults onto the horizontal plane. The side of the solid line shows the upper margin of each fault. Arrows in the sectional view show the estimated slip of each segment. Movement of the southeastern side is shown relative to the northwestern side.

Vol. 44, No. 4, 1996 266 M. Hashimoto et al. caused the pattern of displacement variation with component of segment 5 is slightly smaller than the

distance from faults, such as vertical movement observed slip there. Since the leveling data are between Nishinomiya and Osaka. The north- distributed not so close to the Nojima Fault, the eastward extension of the easternmost segment, if dip-slip components of segments 5 and 6 may not the slip is right lateral, should run north of the be well resolved by the present data. Most segments

Minoo GPS station in order to explain its westward are estimated to be almost right lateral faulting, but displacement. segments 2 and 4 have large thrust components

As we change the geometry of the modeled fault relative to those of strike slip component. A slight and adopt the optimal model by mainly examining uplift was observed around segment 2, which may be the fitness to geodetic data, the hypocenter of the attributed to this thrusting. Right-lateral slip of main shock may not be located on our modeled segment 2 is appreciably smaller than that of seg- faults. The hypocenter is located beneath the Akashi ment 1. Segment 4 is the southernmost among the

Strait, but the geometry of the fault there has not 4 segments on the Kobe side. The thrusting of necessarily been clarified yet. We assume the source segment 4 may be responsible for the large verti- fault is separated into two parts beneath the Akashi cal displacement near the Suma Fault. It should be

Strait. It should be discussed how both parts are noted that the slip of segment 3 is significantly connected with each other. There are several models smaller than the neighboring segments. After on this problem. Ando (1995) suggested a smooth all,there are three areas of large slip; the Nojima and straight fault beneath the Akashi. Strait on the Fault on Awaji Island, and both ends of the fault basis of precise aftershock distribution. Mizoue et in Kobe. al. (1995) and Tada et al. (1995) proposed E-W trending faults beneath Akashi Strait. Umeda et al. 3.3 Notable discrepancies between observed and

(1995) proposed a bright spot, which consists of modeled displacements multiple segments of fault, existing beneath Akashi The optimal model explains well the observed

Strait. displacements with several exceptions. We suppose

Finally, we chose the model in Table 1 and Fig. that it is valuable to mention defects of the present

7 as the optimal model, which gives the smallest optimal model for the detailed discussion on the square sum of residuals among the models under nature of the source fault. consideration. Table 1 indicates estimated slips and First, the calculated horizontal displacement their errors for each fault segment of the optimal vectors of some control points are significantly model as well as their resolution. All the estimated smaller than that observed (Fig. 5). We may at- parameters (slips of segments) have almost unity tribute this discrepancy to the adjustment method resolution which means that these parameters are of line length data. As in Fig. 8, our optimal model well resolved by the observed data. The trace of can fit most of the observed line length changes the resolution matrix is 11.92 for 12 parameters in within one standard error. Observed displacements total, which also means almost all the param- in Fig. 5 are an inner coordinate solution. Because eters are resolved. Total seismic moment is esti- the mass center of network depends on the dis- mated to be 2.2 •~ 1019 N•Em, which is comparable tribution of control points, there may be still with the seismological estimate of 2.5 x 1019 N•Em unknown systematic offset from the real displace- by Kikuchi (1995), with the rigidity of 40 GPa. ment field. Another possible source of this dis-

Figure 7 illustrates the geometry of the modeled crepancy might be due to the contraction of the faults and their slips. Figures 4-6 and 9 show the network during the period between 1985 and 1995. estimated horizontal and vertical displacements for An east-west contraction is prevailing in the Kinki the optimal model. Figure 8 shows the fitness of es- district on the basis of a century-old geodetic sur- timated line length changes to that observed. vey (Tada, 1991; GSI, 1985, 1986, 1987) and the

The estimated slips of segments 5 and 6 on Awaji average strain rate is estimated to be of the order of

Island which correspond to the Nojima Fault are 1 •~ 10-7/year (Hashimoto, 1990). Therefore, we can as large as or larger than 2 m. The magnitude of the expect about 1 •~ 10-6 contraction (10 cm for 100 horizontal slips are comparable to the observed km) for 10 years. It should be noted that the observ- surface slips along the Nojima Fault (Nakata et al., ed displacement field obtained by EDM and cam-

1995; Ota et al., 1995). However the dip-slip paign type GPS contains this aseismic movement.

J. Phys. Earth Co-seismic Displacement 267

Table 1. The estimated fault parameters for the optimal and the Belt-of-Damage models.

Vol. 44, No. 4, 1996 268 M. Hashimoto et al.

(a) (b) Fig. 8. Observed vs. calculated line length changes for the optimal model. (a) Whole dataset. (b) Line lengths with observed change less than 10cm. Error bars show one standard error for the observed line length changes (see text).

Fig. 9. Calculated vertical displacement for the optimal model. Solid, centered and dashed lines indicate uplift, zero and subsidence, respectively, with an interval of 5 cm. Thick solid lines show the leveling route, and solid squares show joint benchmarks connecting leveling routes or terminal benchmarks .

Second, the optimal model cannot explain some coast. Uchide is located on the alluvial plane in line length changes, especially related to Wadamisaki front of the Rokko range. Therefore, we suspect the and Uchide (Fig. 8, Table A2). Wadamisaki is lo- displacements of these points are partly affected by cated east of the Rokko fault system and near the land liquefaction or other phenomena in alluviums.

J. Phys. Earth Co-seismic Displacement 269

Third, the observed uplift along the east coast of 4. Discussion Awaji Island has a shorter wavelength than that One of the most controversial problems on the predicted (Fig. 6e). The predicted uplift is produced Hyogo-ken Nanbu earthquake is whether there are by the faulting of segments 5 and 6, which represent unknown buried faults beneath the zones of severe the Nojima Fault. Since the length of segments 5 damage in Kobe (Shimamoto et al., 1995; Ikeda et and 6 are longer than the wavelength of this uplift, al., 1995). The present analysis suggests that the it may be hard to explain this deformation pattern faulting along the known active faults or along the with our model. However we must point out the aftershock distribution is enough to explain the possibility that there were some motions on the observedco-seismic displacement field, though there active faults on the east coast of Awaji Island and are several exceptions as stated above. In order to removal of a significant amount of soil before the clarifythe possibility of existenceof unknown buried Hyogo-ken Nanbu earthquake. faults, we made the same inversionanalysis by using Fourth, the model predicts slight subsidence in a fault model, "Belt-of-Damage" model, in which the west of Akashi, where uplift was observed three eastern segments are located under the zones (Fig. 6b). Figure 9 suggests that this subsidence of severe damage (Table 1 and Fig. 10). It is clear is generated by thrusting of the fault segments on that the fitness to observed geodetic data is worse Awaji Island. This displacement field strongly than that of the optimal model although this model depends on their geometry. Since the length of these can reproduce a similar displacement field to the segments are constrained by the field observation, optimal model. Moreover, the "Belt-of-Damage" we should examine the widths of the faults. In our model has two fatal defects. First, the estimated slip model, segments 5 and 6 are 10 km wide. This of the easternmost segment is left lateral, which is discrepancy implies that these segments are too wide. physicallyunacceptable since the other segments are Fifth, the predicted uplift of benchmark F22 is right lateral. This left lateral slip is required to fit twice as large as that observed (Fig. 6b). Since this the displacement of the Minoo GPS station, since benchmark is located off the main leveling route continuous GPS data are of much larger importance (Fig. 6a), only this benchmark represents vertical in this analysis than the other data because of their displacements in its vicinity. The vertical displace- accuracy. CMT solutions based on seismological ment profile may be constrained by the data of the data showed that the mainshock is a right lateral main leveling route, and F22 affects the inverted event (e.g., Kikuchi, 1995).The second defectis that result less than the benchmarks along the main route the "Belt-of-Damage" model cannot explain dis- in this inversion. Another possible source of this placements at several observation sites such as the discrepancy is the subsidence at the northern tip of movement of the Kobe University, where displace- Awaji Island. By having subsidence and uplift occur ment as large as 6 cm was observed ( along the main leveling route together, we observe University et al., 1995). The "Belt-of-Damage" a large gradient of vertical movement in the N—S model has a rather large displacement at Kobe direction. When we fit this gradient with the present University, while the optimal model has not (Fig. fault model, it may result in a larger uplift of 5). Furthermore, the "Belt-of-Damage" model benchmark F22 than those on the main leveling predicts uplift in Kobe and Nishinomiya, where route. In fact, there is no discrepancy between the subsidencewas observed (Fig. 6b and c). As shown observed and predicted uplift of F22 in the anal- in Fig. 11, the zero line of vertical movements runs ysis if data from Awaji Island is not included between the two leveling routes from Nishinomiya (Hashimoto, 1995). The vertical movement around to Osaka. In this case, left lateral strike-slip motion F22 may be largely affected by the movement of the of the easternmost segment causes uplift along the segment beneath Akashi Strait. However, the Nishinomiya-Amagasaki-Osaka route and sub- geometry of the fault there has not been clarified sidence along the Nishinomiya—Toyonaka-Osaka clearly. In order to solve this discrepancy, we need route. In the case of right lateral strike-slip, not only more vertical displacement data off the movements along each route simply change their main leveling route but also information of the fault signs.Therefore it may be hard to explain subsidence around. Akashi Strait. along the both routes as long as strike-slip motion is considered for the segment beneath the zone of

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Fig. 10. The geometry of modeled fault segments for the "Belt-of-Damage" model; lower right: plane view; upper left: sectional view from the southeast. See also legend in Fig. 7.

Fig. 11. Calculated vertical displacement for the "Belt-of-Damage" model. See also legend of Fig. 9. severe damage. In conclusion, we can say that second event has a large thrust component. The faulting along the aftershock zone, or the known optimal model is qualitatively consistent with Rokko fault system, plays the main role in Kikuchi's model. However Kikuchi predicted a generating co-seismic displacements. Unknown much longer segment for the first rupture event than buried faults may have only secondary effects on a ours, since he considered bilateral rupture for the rather small scale, if any. first event. Kikuchi assumed bilateral rupture for Kikuchi (1995) suggested that this earthquake the Nojima Fault in order to range the rupture consists of three subsequent rupture events and the velocity below an acceptable limit. Because the

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duration of the rupture of this earthquake is rather preliminary one, and a more detailed analysis will short (about 10 s), a very large rupture velocity is be conductedwith a more denselydistributed dataset required under the assumption of unilateral rupture. when it becomes available. Also, faults on Awaji Island and Kobe are smoothly The Rokko range has been considered to have connected in Kikuchi's model, while our optimal been uplifted by the activity of the Rokko fault model is separated at the Akashi Strait. system (Huzita, 1962, 1985). The 1995 earthquake As is mentioned above, the hypocenter of the is the result of faulting on this fault system. Though mainshock is not located on our modeled fault. The vertical movements of peaks in the Rokko range hypocenter is the initiating point of rupture, but our have not been observed yet, we do not expect an geodetic fault model suggests main rupture zones. uplift as large as 1 m. According to our optimal Since our model assumes a uniform slip on each model, uplift in the Rokko range is no larger than segment, estimated slips tend to be confined to a 40 cm. Utilizinga new spacetechnique with synthetic narrower zone than the real rupture zone. There are aperture radar, Murakami et al. (1995) detected several examples showing that the hypocenter of the changesin distance between the Earth's surface and main shock is not in the main rupture zone, such a satellite which revealed small movements of the as the 1992 Landers earthquake (Wald and Heaton, Rokko range. Because the Rokko range is con- 1994). Hudnut et al. (1994a, b) also suggested that sidered to have been uplifted by about 1,000m the hypocenter of the 1994 Northridge earthquake during the last 1 Ma, an uplift of 40 cm is considered is located much deeper than the main rupture zone to be too small as an uplift of the Rokko ranges estimated on the GPS data. Our optimal model every 1,000 years. Therefore, there should be other implies that the Hyogo-ken Nanbu earthquake is types of faulting which uplift the Rokko range. The one such example. Off Southwestern Hokkaido earthquake in 1993, There are some examples showing that after- and the Northridge earthquake in 1994, are similar shocks occur in the region of small slips (e.g. Bakun examples in which co-seismic movements are in- et al., 1986). In the 1992 Landers earthquake, the consistent with those observed during the Quater- estimated slip is smaller near the jogs or steps of nary (Hashimoto et al., 1994; Hudnut et al., the faults than on the planar parts (Hudnut et al., 1994a,b). The faults may be so deep that future 1994a, b; Wald and Heaton, 1994; Freymueller et excavationof the Rokko fault system will not reveal al., 1994). Thus, slip distribution on a fault may the event of 1995. These are serious problems be closely related to the aftershock distribution and from the viewpoint of countermeasures to seismic geometry of faults, and reflects the strength hazards because the limited geological knowledge distribution on the fault. Our optimal model suggests may mislead hazard analysis. the existence of a small slip zone (segment 3) in Kobe. Segments 2 and 3 of our model correspond 5. Conclusions to the cluster of aftershocks in central Kobe. We analyzed geodetic data obtained so far and Furthermore, the boundary between segments 2 and presented a six segment fault model. We hereby 3 corresponds to the jog of the Rokko fault system. summarize all the conclusions derived from this Therefore, the present results suggest that there is a preliminary analysis. region of high strength located here. Segment 2 has 1) Continuous GPS observation revealed a an appreciably small right lateral movement. If the pattern of displacements consistent with that from uplift in central Kobe is of shallow origin, the thrust a right lateral slip on a NE-SW trending vertical component of segment 2 will be much smaller and fault: western and eastern stations moved toward segment 2 will be a slip-free zone as well. In order the epicenter and northern and southern ones moved to clarify the relation among the slip distribution, away. aftershock distribution, and geometry of the faults, 2) Campaign type GPS survey revealed a com- we need to resolve the slip variation on the fault plicated displacement field: the control points on plane. Because we assume uniform slip in each Awaji Island moved southwest to south, while the segment, we cannot resolve variation in slip at control points north of the Rokko fault system depth. Unfortunately, we have insufficient data moved northeast, and those on the other side of owing to the existence of ocean and the relatively the fault had no notable displacements in Kobe. sparse geodetic network. The present analysis is a 3) Levelingrevealed about 19cm uplift just west

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of the Suma Fault and abrupt subsidence of 7 cm Freymueller, J., N. E. King, and P. Segall, The co-seismic on the other side of the fault. Around the northern slip distribution of the Landers earthquake, Bull. extension of the Nojima Fault no discontinuity was Seismol. Soc. Am., 84, 646-659, 1994. seen in the height changes. Slight uplift and sub- Geographical Survey Institute, Crustal movement in the sidence were observed in central Kobe and east of Kinki district, Rep. Coord. Comm. Earthq. Predict., 34, Kobe, respectively. Remarkable uplift of 20 cm was 346-357, 1985 (in Japanese). seen on the east coast of Awaji Island with rapid Geographical Survey Institute, Crustal movement in the subsidence at the northern tip of the island. Kinki district, Rep. Coord. Comm. Earthq. Predict., 36, 4) Fitting these geodetic data, we obtained a 333-354, 1986 (in Japanese). Geographical Survey Institute, Horizontal crustal strain optimal set of parameters for the six segment fault in Japan: 1985-1883, Technical Report of the model which align along the aftershocks or known Geographical Survey Institute, p. 133, 1987. active faults. A slip of about 250 cm was estimated Geological Survey of Japan, Presented at the 111th for the segments on Awaji Island. There are two Meeting of the Coordinating Committee for Earthquake parts of relatively large slip of 100-200 cm in Kobe, Prediction on January 18, 1995, 1995. between which a nearly slip-free zone exists. Among Hashimoto, M., Horizontal strain rates in the Japanese the segments, the one in the southern part of the islands during interseismic period deduced from geodetic Nojima Fault and those near the Akashi Strait and surveys (Part I): Honshu, Shikoku and Kyushu, J. in central Kobe may have significantly large thrust Seismol. Soc. Jpn., 43, 13-26, 1990 (in Japanese with components. English abstract). Hashimoto, M., Coseismic crustal deformations due to the The authors would like to express sincere appreciation 1995 Kobe earthquake and its fault model, Chishitsu to the staff members of the GeographicalSurvey Institute News, 490, 33-40, 1995 (in Japanese). who conducted several kinds of geodetic surveys and Hashimoto, M., T. Sagiya, S. Ozawa, and T. Tada, Fault compiledthese data. We also thank anonymousreviewers models for the crustal movements associated with the for helpful comments on improvingmanuscript. 1993 Off Kushiro earthquake and the 1993 South- western Off Hokkaido earthquake and their tectonic REFERENCES significance, Proceedings of the CRCM '93, Kobe, December 6-11, 1993, 57-64, 1994. Abe, Y. and H. Tsuji, A nationwide GPS array in Japan Hirata, N., Urgent research on the crustal activity for geodynamicsand surveying,Geode. Inf. Mag., 8, associated with the 1995 Hyogoken-nanbu earthquake, 29-31, 1994. News Lett. Seismol. Soc. Jpn., 6 (6), 8-9, 1995 (in Ando, M., Summary of observations for earthquake Japanese). predictionin Japan (Part 3)-Kinki, Chugoku, Shikoku, Hudnut, K. W., M. H. Murray, A. Donnellan, Y. Bock, Kyushu and Okinawa areas, Special Report of the P. Fang, M. Cline, Y. Feng, Z. Shen, B. Hager, T. Regional Subcommitteesof the CoordinatingCommit- Herring, and R. King, Coseismic displacements of the tee for Earthquake Prediction,Vol. 6, 303-306, 1994 (in 1994 Northridge, California earthquake, Program for Japanese). Northridge Abstracts, The 89th Annual Meeting of the Ando, M., Foreshocks, mainshock, aftershocks and Seismol. Soc. Am., 40, 1994a. induced earthquakes of the 1995 Hyogo-ken Nanbu Hudnut, K. W., Y. Bock, M. Cline, P. Fang, Y. Feng, J. earthquake, Chikyu Monthly(Suppl.), 13, 18-29, 1995 Freymueller, X. Ge, W. K. Gross, D. Jackson, M. Kim, (in Japanese). N. E. King, J. Langbein, S. C. Larsen, M. Lisowski, Awata, Y., K. Mizuno, Y. Sugiyama,S. Shimokawa, R. Z.-K. Shen, J. Svarc, and J. Zhang, Co-seismic dis- Imamura, and K. Kimura, Surfacefaults associatedwith placements of the 1992 Landers earthquake sequence, the Hyogoken-nanbu earthquake of 1995, Chishitsu Bull, Seismol. Soc. Am., 84, 625-645, 1994b. News, 486, 16-20, 1995 (in Japanese). Huzita, K., Tectonic development of the median zone Bakun, W. H., G. C. P. King,and R.S. Cockerham,Seismic (Setouchi) of southwest Japan since Miocene, J. Geosci. slip, aseismic slip, and the mechanics of repeating Osaka City Univ., 6, 103-144, 1962. earthquakes on the Calaveras fault, California, in Huzita, K., Evolving Japanese Islands, Iwanami-Shinsho, Earthquake SourceMechanics, AGU Geophys.Monogr., Iwanami Shoten, , p. 228, 1985 (in Japanese). Vol. 37, ed. S. Das, C. Sholz, and J. Boatwright, Ikeda, Y., M. Togo, S. Sawa, S. Kato, and T. , American Geophysical Union, , D.C., Distribution of first motion derived from the move- pp. 195-207, 1986. ments of gravestones and the motion of buried earth-

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quake faults, Record of the Urgent Research Conference of the Urgent Research Conference on the Hyogoken- on the Hyogoken-nanbu Earthquake of January 17, nanbu Earthquake of January 17, 1995,37-38, 1995 (in 1995, 45-46, 1995 (in Japanese). Japanese). Jackson, A.D., The use of a priori data to resolve Shimamoto, T., A. Tsutsumi, Y. Ohtomo, and E. non-uniqueness in linear inversion, Geophys. J. R. Kawamoto, Earthquake disasters in Kobe, Ashiya and Astron. Soc., 57, 137-157, 1979. Nishinomiya and presumed earthquake-generating Jackson, D. D. and M. Matsu'ura, A Bayesian approach faults, Record of the Urgent Research Conference on to nonlinear inversion, J. Geophys. Res., 90, 581-591, the Hyogoken-nanbu Earthquake of January 17, 1995, 1985. 41-42, 1995 (in Japanese). Japan Meteorological Agency, The 1995 Hyogoken- Tabei, T., T. Kato, J. P. L. Catane, T. Chachin, K. Nanbu earthquake and its aftershocks, Rep. Coord. Fujimori, K. Hirahara, A. Kubo, T. Matsushima, T. Comm. Earthq. Predict., 54, 584-592,1995 (in Japanese). Nakano, S. Nakao, S. Otsuka, T. Terashima, and T. Kikuchi, M., Source process of the Hyogo-ken Nanbu Yamamoto, Crustal deformation associated with the earthquake of January 17, 1995, Chishitsu News, 1995Hyogo-ken Nanbu earthquake, Japan and derived 486, 12-15, 1995 (in Japanese). from GPS measurements, J. Phys. Earth, 44, 281-286, Komaki, K., Horizontal crustal movements revealed by 1996. geodetic measurements-on the methods for estimating Tada, T., Crustal deformation and tectonics in the Kinki horizontal crustal movements, Bull. Geogr. Survey Inst., triangle region (1), Prog. Abstr. Fall Meeting Seismol. 39, 1-41, 1993. Soc. Jpn., No. 2, 318, 1991 (in Japanese). Mizoue, M., M. Nakamura, and N. Seto, Fault system Tada, T., M. Hashimoto, T. Sagiya, and S. Ozawa, accompanied by coincidental seismicity around the Geodetic fault model for the 1995 Hyogo-ken Nanbu source region of the 1995 Hyogo-ken Nanbu earth- earthquake, Chikyu Monthly (Suppl.), 13, 136-140, quake, Chikyu Monthly (Suppl.), 13, 38-46, 1995 (in 1995 (in Japanese). Japanese). Umeda, Y., T. Yamashita, K. Ito, and H. Horikawa, The Murakami, M., S. Fujiwara, and T. Saito, Detection of bright spot of the 1995 Hyogo-ken Nanbu earthquake, crustal deformations associated with the 1995 Hyogo- Prog. Abstr. Fall Meeting Seismol. Soc. Jpn., No. 2, ken-Nanbu earthquake by interferometric SAR, Koku- A78, 1995 (in Japanese). dochiriin Jiho, 83, 24-27, 1995 (in Japanese). Wald, D. J. and T. H. Heaton, Spatial and temporal Nagoya University, Kobe University, and Geographical distribution of slip for the Landers, California, Survey Institute, Coseismic displacement at Kobe earthquake, Bull.Seismol. Soc. Am., 84,668-691,1994. University in the Hyogoken-nanbu earthquake (Jan. 17, Yabuki, T. and M. Matsu'ura, Geodetic data inversion 1995) observed by GPS measurements, Rep. Coord. using a Bayesian information criterion for spatial Comm. Earthq. Predict., 54, 691-694,1995 (in Japanese). distribution of fault slip, Geophys.J. Int., 109, 363-375, Nakata, T., K. Yomogida, J. Odaka, K. Asahi, A. 1992. Sakamoto, and N. Chida, Earthquake fault which appeared at the Hyogoken-nanbu earthquake, Record APPENDIX DATA USED IN THIS STUDY of the Urgent Research Conference on the Hyogoken- nanbu Earthquake of January 17, 1995, 29-30, 1995 (in All the geodetic data used in this study are listed Japanese). in the following tables. Table Al lists coordinates Okada, Y., Surface deformation due to shear and tensile (longitude and latitude) of GPS stations, control faults in a half-space, Bull. Seismol. Soc. Am., 75, points and benchmarks. Table A2 lists position 1135-1154, 1985. changes of GPS stations, line length changes between Ota, Y., M. Horino, and the Geographical Survey Team control points and height changes between bench- from the Geographical Survey Institute, The Nojima earthquake fault which appeared at the Hyogoken- marks with the predicted changes for the optimal nanbu earthquake and damages in its vicinity, Record model (Table 1 and Fig. 7).

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Table A1. Coordinates of continuous GPS stations, control points and benchmarks.

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Table A1. (continued)

Table A2. Observed and modeled displacements.

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Table A2. (continued)

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Table A2. (continued)

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Table A2, (continued)

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Table A2. Observed and modeled displacements.

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