The causative plate action.
Southwestern Japan is located on the southeastern margin of the Eurasian Plate, where the Philippine Sea Plate is being thrust (subducted) beneath the Eurasian Plate in a northwest direction along the Nankai Trough. A portion of this relative plate motion is taken up by right-lateral strike-slip faulting along a major east-northeast-trending fault known as the Median Tectonic Line (MTL), located immediately south of Awaji Island and Osaka Bay.
The main shock occurred along a northwest-trending branch of the MTL called the Arima-Takatsuki Tectonic Line (ATTL). This fault system, like the MTL, has a predominantly right-lateral strike-slip sense of displacement. Historically, this region has seen somewhat lesser seismicity than in the Tokyo area and some other parts of Japan, but has had magnitude 7 or greater events in historical times (e.g., in 1596). In 1916, a magnitude 6.1 earthquake occurred at almost the same epicentral location as the 1995 event.
In the Kobe area, cretaceous granites are overlain by a relatively thick Plio-Pleistocene sedimentary unit called the Osaka group, which consists of alluvium interbedded with marine clays. Relatively thin terrace deposits and recent alluvium overlie the Osaka group. Fill material has been placed along much of the waterfront and comprises human-made islands, such as Port and Rokko islands.
Preliminary reports from the Japanese Earthquake Research Institute indicate that the hypocenter of the Mj7.2 (equivalent to Mw6.9) main shock occurred at a depth of approximately 15 to 20 kilometers. The main shock's focal mechanism indicates predominantly strike-slip movement along a plane that dips 80° to 90° to the southwest. The aftershock sequence (and, by inference, the faulting below the surface) is approximately 60 kilometers long, extending from the northern part of Awaji Island along the Nojima Fault to northeast of Kobe along the Rokko Fault zone.
Japanese earthquakes, 1961-1994.
An approximately 9-kilometer-long surface fault rupture was identified along the Nojima Fault, which is on the northwestern coast of Awaji Island and southwest of Kobe. The fault strikes N40°E, dips steeply to the southeast, and has a predominantly right-lateral strike-slip sense of displacement consistent with the mechanism of the main shock and the trend of the aftershocks. Geomatrix Consultants (a geotechnical firm) measured local displacements at two locations along the northern part of the fault from the recent earthquake: Vertical displacements were 1.2 meters, and right-lateral displacements were 1.5 meters. These displacements are ...
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Japanese earthquakes, 1961-1994.
An approximately 9-kilometer-long surface fault rupture was identified along the Nojima Fault, which is on the northwestern coast of Awaji Island and southwest of Kobe. The fault strikes N40°E, dips steeply to the southeast, and has a predominantly right-lateral strike-slip sense of displacement consistent with the mechanism of the main shock and the trend of the aftershocks. Geomatrix Consultants (a geotechnical firm) measured local displacements at two locations along the northern part of the fault from the recent earthquake: Vertical displacements were 1.2 meters, and right-lateral displacements were 1.5 meters. These displacements are in good agreement with measurements by others, who reported maximum vertical displacements of about 1.2 meters and right-lateral displacements of 2.1 meters. Past surface-faulting events, which are probably similar to the most recent event, were evidenced by the 6- to 7-meter-high fault scarp along the fault. Given a long-term slip rate of 1 millimeter per year for the ATTL, as listed in Active Faults in Japan: Sheet Maps and Inventory by the Research Group of Active Faults," and an average displacement of about 1 to 1.5 meters, as suggested from observed displacement on the Nojima Fault, it appears that an earthquake roughly the size of the Kobe shock occurs on average once every 1,000 to 1,500 years along this portion of the ATTL.
It is unknown whether the surface fault rupture extended to the northeast across the Akashi Strait and onland to connect with faults in the Kobe-Nishinomiya area. Equivocal evidence of surface faulting has been described in this area and apparently is consistent with the aftershock sequence, which is approximately 60 kilometers long and extends northeast of Kobe. Based on empirical data of earthquake magnitude versus surface fault length, a 9-kilometer-long surface rupture should yield only an Mw6.2 earthquake, whereas a 60-kilometer-long rupture should yield an Mw7.1 earthquake, which is more consistent with the observed magnitude for this earthquake.
Ground motion map.
A shaking intensity of up to 7 on the JMA intensity scale [equivalent to X to XI on the Modified Mercalli Intensity (MMI) scale] has been assigned to the coastal strip extending from the Suma Ward to Nishi-nomiya and in the Ichinomiya area on Awaji Island; JMA 5 (MMI VII to VIII) to Iwakuni, Hikone, Kyoto, and Toyooka; and JMA 4 (MMI VI) to Nara, Okayama, Osaka, Takamatsu, Shikoku, and Wakayama. The distribution of maximum horizontal ground accelerations and velocities recorded in the Kansai area is shown on page 8. This figure was modified from a map provided by the Earthquake Research Institute, University of Tokyo. The map has been augmented with additional acceleration and velocity recordings reported by the Committee of Earthquake Observation and Research in the Kansai Area. The maximum horizontal accelerations are those reported by several different agencies and represent either the maximum of the two peak horizontal accelerations or the vectoral combination of the two horizontal components. A maximum acceleration of 0.84g (g equals 981 cm/s/s) was reported in central Kobe, and several recordings in the range of 0.5g to 0.8g were reported in the heavily damaged Kobe-Ashiya-Nishinomiya area.
A preliminary estimate of the 250 cm/s/s (0.25g) and 500 cm/s/s (0.51g) iso-acceleration contours is overlain on the map on page 8. The contours show a distinct bulge toward the northeast, indicating that ground motions were higher northeast of the epicenter in the direction of rupture propagation principally because of source directivity (i.e., focusing). The 250 cm/s/s contour does not extend as far as Osaka, which is consistent with the lower intensity (JMA 4) reported for this area. It is interesting to note that the maximum accelerations in the Kyoto area are similar to those in the Osaka area, even though the former was reported to have a JMA intensity of 1 unit higher.
Left: Comparison of Kobe Earthquake strong ground motion data with predictions from Campbell and Bozorgnia (1994) indicates that the Kobe strong motion was typical.
Right: Generalized Modified Mercalli Intensity (MMI) map for the January 17 event.
A comparison of the recorded maximum accelerations with predictions for an Mw6.9 strike-slip earthquake (page 9) indicates that the accelerations recorded during the earthquake are generally consistent with, though possibly slightly higher than, those recorded worldwide during other major strike-slip earthquakes of similar magnitude. The maximum accelerations are also similar on average to those recorded during the 1994 Mw6.7 Northridge, California, Earthquake. This comparison, along with other structural and geotechnical information that is available, would seem to suggest that the greater damage and the larger numbers of deaths, casualties, and homeless sustained during the Kobe Earthquake were likely caused by the aggregated effects of an extremely dense population, an older building stock, and the predominance of poor soils in the strongly shaken area.
