Preliminary Reports from the Hyogo-ken Nambu Earthquake of January 17, 1995

Summary of the Earthquake

by Masanobu Shinozuka

The Hyogo-ken Nambu (Hyogo Prefecture, South Part) earthquake struck at 5:47 a.m. on January 17, 1995 (January 16, 3:47 p.m. EST in the U.S.). The Northridge earthquake occurred on exactly the same calendar day one year earlier. The earthquake occurred on a northeast trending fault extending from Awaji Island to the City of Kobe, on the main island of Honshu.

The Japanese Meteorological Agency (JMA) indicates a magnitude 7.2 (M_w 6.8) with approximately 40 km of bilateral rupture from an epicenter centered at about the northern tip of Awaji Island. According to Scawthorn, Geomatrix associates reported surface faulting has been found on Awaji Island with a horizontal displacement of about 1 m and a vertical offset also of about 1 m. According to JMA, the seismic intensity scale was VII (equivalent to MMI = X~XII) in the narrow corridor of 2 to 4 km width, stretching 40 km or so along the coast of the Osaka Bay in the general east-west direction. In this narrow strip, Kobe's major business, industrial and port facilities, and residential houses and buildings form a bustling city of approximately 1.5 million people. The ground motion recorded at the Kobe Oceanic Meteorological Observatory (shown in table 1) represents one of the strongest ground motion records observed. The duration of the earthquake was measured at the Observatory at less than 15 seconds. Velocity time histories of rock sites elsewhere in the Kobe-Osaka region are consistent with this observation. Some soft soil sites, however, show durations of strong shaking as long as 100 seconds.

As far as residential and office buildings are concerned, there is a clear indication that the extent of the damage sustained by these buildings depended upon when, and under which design code, they were constructed. A recent study by Ohbayashi Corporation on the performance of the residential and office buildings it constructed is summarized in table 2, as reported by the Nikkei Shinbun newspaper (2/4/95 edition).

Table 2 clearly indicates the effect of upgrading the seismic design code. For example, green tagged buildings counted only 42% among the buildings of pre-1971 vintage, while they counted 84% among those constructed under the new seismic design code. Such a global evaluation of seismic performance of buildings must in turn be used to evaluate the effectiveness of the design code. Older wooden residential houses in Kobe's less affluent district, Nagata-ku, appear to have sustained the most damage, first by extremely severe ground shaking and then by fire. The valuable lesson here is the need to retrofit older buildings designed below the current seismic standard and, in particular, the many older wooden houses that were built without any seismic provisions.

Liquefaction was particularly destructive for Kobe's port facilities including those located on Port Island and Rokko Island, both built on reclaimed land. Derricks and cranes were either severely damaged or made unusable due to liquefaction-induced ground deformation. The Kobe area port facilities were constructed under the Ministry of Transportation's Construction Standard B (as opposed to more stringent A and Super A Standards). After the fact, and considering the importance of Kobe's facilities in that they handle 30% of Japan's container freight traffic, this is now considered substandard. Current Japanese seismic codes do not consider importance factors.

Flexural failure and sheared monorail column. (Photo courtesy of C. scawthorn.)

Collapse of a 500 m segment of the Hanshin expressway truly surprised the Japanese civil engineering community. However, this elevated segment was constructed in 1968-1969 under older seismic provisions and slated for retrofitting in the future by the Hanshin Expressway Public Corporation. The elevated segment had a structural configuration and followed construction procedures representative of concrete bridges popular in Europe, particularly in Germany. The elevated highway was acclaimed for its aesthetic harmony with Kobe's coastal environment at the time of construction. Also, some of the welded box type steel columns were crushed in compression under the high level of vertical ground acceleration due to brittle fracture of welded parts, separating the four sides of the box column into four independent plates. This is reminiscent of brittle fracture of welded joints of steel frame buildings in California under the Northridge earthquake.

Detail of flexural damage to Monorail column. (Photo courtesy of C. Scawthorn.)

Several spans of the structures supporting bullet train tracks near Shin-Kobe station collapsed and a number of bridge piers sustained severe damage apparently through concrete joints. These structures were constructed around 1965 and were reported to have been designed under older design codes. Because of these collapses bullet train operation between Osaka and Okayama will not resume until sometime in May this year, resulting in huge restoration costs and revenue loss to Japan Rail, and causing significant inconvenience to passengers.

A few reinforced concrete bridge columns were irreparably damaged even though they were designed under the most current seismic design provisions, indicating the severity of the seismic ground motion and the influence of liquefaction-induced ground deformation. Indeed, these columns tended to be located at the boundaries between Kobe's original coastline and the two man-made islands. Four collapsed spans of elevated Portliner (automated train) connecting Kobe and Port Island occurred at such a location.

Electric power was restored very quickly and telecommunications relatively quickly considering the extent of the devastation. Gas and water supply are a different story. As of February 6,725,000 customers are still without gas supply. Restoration of service is expected to be completed by the middle of March. Also, 29,000 customers are without water. The expected date of total restoration of water service is February 20. These numbers include the cities of Kobe, Nishinomiya and Ashiya, and are in spite of the fact that both the Kobe Water Department and Osaka Gas Company made a sincere effort to implement antiseismic measures to prevent such service interruptions from occurring.

Some of the material reported herein is based upon work supported in whole or in part by the National Science Foundation, tbe New York State Science and Technology Foundation, the U.S. Department of Transportation and other sponsors. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of MCEER or its sponsors.