The January 17, 1995 Hyogo Ken Nanbu (Kobe) Earthquake

Geoscience and Geotechnical Aspects

Figure 1. Mainshock epicenter (JMA), aftershock zone, and peak ground motions (cm/s/s) of the 1995 Kobe earthquake, superimposed on a map of active faults (Research Group for Active Faults, 1991). The aftershock zone is based on locations by the Disaster Prevention Research Institute, Kyoto University, using data from the microearthquake networks of Kyoto, Tokyo, and Nagoya Universities. The strong motion data are from the Committee of Earthquake Observation and Research in the Kansai Area, JR, Osaka Gas, and JMA, and represent different measures of ground motion, as described in the text. The active faults are from Research Group for Active Faults (1980). Italicized values indicate inexact station locations. Modified from K. Koketsu, Earthquake Research Institute, University of Tokyo. (P. Somerville, Woodward-Clyde, Pasadena, 01/31/95)

The earthquake was assigned a JMA magnitude of 7.2 by the Japan Meteorological Agency (JMA). Seismological analyses indicate a seismic moment of about 3 x 10**26 dyne- cm, corresponding to a moment magnitude of 6.9 (Kikuchi, 1995). The hypocenter of the earthquake (34.6 N, 135.0 E, focal depth =10 km, origin time 5:46:52 JST; JMA) was located about 20 km southwest of downtown Kobe between the northeast tip of Awaji Island and the mainland (Figure 1). Based on the distribution of aftershocks (Disaster Prevention Research Institute, Kyoto University; Figure 2) and teleseismic waveform modeling (Kikuchi, 1995; Figure 3), the rupture length of the 1995 earthquake is inferred to have been in the range of 30 to 50 km, produced by bilateral rupture from the hypocenter. The rupture of this strike-slip earthquake directly into downtown Kobe, as inferred from the aftershock distribution and the waveform modeling of teleseismic and strong motion seismograms, appears to have contributed to the high level of destruction that occurred.

Figure 2. Locations of aftershocks. Source: Disaster Prevention Research Institute, Kyoto University.

Figure 3. Focal mechanism and source process of the earthquake. The earthquake consisted of three subevents, whose relative locations are shown in the upper left, time functions in the lower left, and focal mechanisms and moment magnitudes to the right. The largest subevent was located at the epicenter. Source: Kikuchi, 1995.

The earthquake occurred in a region where a complex system of active faults had been previously mapped (Research Group for Active Faults in Japan, 1980; Figure 2). The focal mechanism of the earthquake indicates right-lateral strike-slip faulting on a vertical fault striking slightly east of northeast, parallel to the strike of the mapped faults (Kikuchi, 1995; Figure 3). The earthquake produced surface rupture with an average horizontal displacement of 1 to 1.5 meters on the Nojima fault, which runs along the northwest shore of Awaji Island (Nakata, personal communication, as shown in Figure 4). Marine seismic surveys have found a 300 meter long offshore extension of this rupture (Japan Maritime Safety Agency). The surveys also found two fault rupture segments that span a length of about 7 km in the region offshore from the northeast tip of Awaji island, parallel to the Nojima fault but offset from it by about 5 km. Near the onshore projection of these underwater rupture segments, a releveling survey after the earthquake found a change in elevation of 26 cm over a distance of 6 km between Tarumi and Suma Wards in western Kobe. This change in elevation is inferred to mark the location of subsurface faulting.

Figure 4. Scarp of the Nojima fault on Awaji Island showing both vertical and horizontal offset. Source: Newsweek, February 1, 1995, Japan edition.

The earthquake mechanism is compatible with the tectonic environment of western Japan as revealed by historical seismicity. This seismicity contains a sequence of earthquakes between 1891 and 1948 that includes the magnitude 8 Nobi earthquake of 1891, the magnitude 7.3 Tango earthquake of 1927, the magnitude 7.2 Tottori earthquake of 1943, and the magnitude 7.1 Fukui earthquake of 1948 (Kanamori, 1973). All of these earthquakes, as well as the 1995 earthquake, had strike-slip mechanisms that accommodated east-west shortening of the Eurasian plate due to its collision with the North American plate along the Izu-Itoigawa line to the east in central Honshu (Huzita, 1980), as illustrated in Figure 5. In summary, the 1995 Kobe earthquake occurred on a mapped system of active faults, and had a mechanism that is compatible with the tectonics of western Japan as inferred from similar earthquakes that occurred during the past century. Currently available evidence does not suggest any difference between the source characteristics of the Kobe earthquake and those of crustal earthquakes that occur in California.

Figure 5. Collision of the North American plate with the Eurasian plate in central Japan. Plate boundary faults (green) and active crustal faults (red) in Japan. Epicenters of large earthquakes during the past century are shown. Source: Newsweek, February 1, 1995, Japan edition.

Strong ground motions were recorded by several organizations, including the Committee on Earthquake Observation and Research in the Kansai Area, Japan Rail, Osaka Gas, JMA, Hankyu Railroads, Japan Highways, Building Research Institute, and Port and Harbor Research Institute. Selected recording sites and their peak accelerations are shown in Figure 1. The various contributing organizations present different measures of peak acceleration. The Kansai and JMA values are the largest of three orthogonal components; the Osaka Gas values are the vector combination of the two horizontal components; and the JR values are vector combinations of the two horizontal components after they have been highcut filtered at 5 Hz.

This is the first large set of strong motion data including near-fault records from a crustal earthquake in Japan, and will be very useful for evaluating the criteria that are currently used in the seismic resistant design of structures in Japan. The near-fault ground velocity time histories have large, brief pulses of ground motion (Figure 6) that are indicative of rupture directivity effects and are potentially damaging to multi-story buildings and other long-period structures such as bridges. The near-fault horizontal/peak velocities were 55 cm/sec on rock at Kobe University, and went off scale at soil sites at levels of 40 cm/sec and 100 cm/sec in central Kobe. These values are similar to those recorded close to comparable earthquakes in California.

Figure 6. Velocity waveforms (horizontal east-west component) recorded by the Committee on Earthquake Observation and Research in the Kansai area.The records at Kobe Univ. and Kobe show brief pulses indicative of rupture directivity effects. Source: GeoResearch Institute, 1995.

Peak accelerations as large as 0.8 g were recorded in the near-fault region on alluvial sites in Kobe and Nishinomiya. To make a preliminary comparison of the recorded values with those predicted by empirical attenuation relations used in California, we have adjusted the Kansai and Osaka Gas values to approximate the average of the two horizontal components. The resulting adjusted values are comparable to those predicted for a strike-slip earthquake using empirical attenuation relations for soil based mainly on California data (Idriss 1991), as shown in Figure 7. Although it is known that most of the data are from ground level sites, some may be in or near buildings, and a few may be from above ground level in structures. Also, instrument corrections at the sites have not been reviewed, so further information is required before definitive conclusions can be drawn from these data.

Figure 7. Attenuation of recorded peak acceleration at soil sites, adjusted to approximate the average of 2 horizontal components, compared with an empirical relation for soil based mainly on strike-slip data from California (Idriss, 1991). Strong motion data are from the Committee of Earthquake Observation and Research in the Kansai Area, Osaka Gas, and JMA.

Widespread ground failure was observed throughout the strongly shaken region along the margin of Osaka Bay. On the islands of Rokko and Portopia, which are reclaimed land in Osaka Bay near Kobe, liquefaction caused subsidence in the range of 50 to 300 cm, and large volumes of silt were ejected. Local lateral spreading of soils occurred along quay walls in many parts of the extensive port facilities in Kobe (Figure 8), rendering some of them inoperative, and causing the disruption and collapse of cranes. Approximately 30% of Japan's commercial shipping passes through the Port of Kobe.

In the Sannomiya district of downtown Kobe, large deformations of road pavements and of the ground around building foundations were observed. These deformations were typically on the order of tens of centimeters (Figure 9), and may have been responsible in part for the severe damage including tilting, collapse of individual stories, or collapse of the entire structure experienced by many multi-story buildings in the downtown area. Along the route of the elevated Hanshin Expressway, there was widespread evidence of ground failure as manifested by disruption of the road pavement, subsidence of the pavement around manholes, and ejected silt (Figure 10).

Since the 1933 Long Beach earthquake, California has not experienced a strike-slip earthquake that ruptured directly into a heavily populated urban region, and has no experience of a strike-slip earthquake rupturing into the downtown region of a major city. Although the 1994 Northridge earthquake occurred within an urban region, almost all of the slip occurred at depths greater than 10 km, and the great majority of the multi-story buildings in the San Fernando Valley were at least 20 km from the closest part of the fault because they are mostly located along the southern margin of the valley. However, numerous urban regions in California and other states contain strike-slip faults which can rupture all the way to or close to the ground surface, as occurred in Kobe during the 1995 earthquake. There is no doubt that these faults will produce earthquakes at some time in the future. The urgent questions for earthquake scientists and engineers are whether the ground motions from these earthquakes will be as severe as those experienced in Kobe, and whether these ground motions will cause the tragic loss of life and disastrous damage to Californian cities that they brought to the city of Kobe.

Figure 8. Collapse of a quay in the Port of Kobe. Photograph by Paul Somerville.

Figure 9. Disruption of the road in the Sannomiya district of downtown Kobe. Photograph by Paul Somerville.

Figure 10. Ejected silt along a side street abutting the Hanshin Expressway in Nishinomiya. Photograph by Paul Somerville.

(Unfortunately figures 8-10 are currently unavailable at this site. Please return at a later point for these items. Thank you.)

Acknowledgments

References

Kikuchi, M., Teleseismic analysis of the Southern Hyogo (Kobe), Japan, earthquake of January 17, 1995, Yokohama City University Seismological Note #38, 1995.

Kanamori, H. (1973). Mode of strain release associated with major earthquakes in Japan, Ann. Rev. Earth and Planet. Sci. 1, 213-239.

Research Group for Active Faults in Japan (1980). Maps of Active Faults in Japan with and Explanatory Text, University of Tokyo Press, Hongo, Bunkyo-ku, Tokyo 113, Japan.