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Kobe Earthquake Essay, Research Paper

The January 17, 1995 Kobe Earthquake

An EQE Summary Report, April 1995

Executive Summary

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On the first anniversary of the moment magnitude (MW) 6.7 1994 Northridge Earthquake, Kobe, Japan was struck by an MW6.9 earthquake. Both earthquakes struck in the pre-dawn hours, both ruptured beneath densely populated areas, and both caused horrible damage. Yet in Kobe there were many more deaths, financial losses dwarfed those in Northridge, and the amount of destroyed building stock and infrastructure was far worse in Kobe than in Northridge.

The reasons for these differences are many, but it would be incorrect to issue a blanket condemnation of current Japanese seismic engineering practice. While engineered structures did fail due to design flaws, they were predominantly older structures built before the current Japanese building code became effective; or they frequently failed due to problems revealed to be deficiencies in California design practices by the Northridge Earthquake. Japanese seismic engineering expertise has justifiably been considered among the best in the world, and a careful examination of the damage in Kobe does not change that conclusion.

Despite differences in design and construction practices, the same general principles frequently came into play: highway collapses were often primarily due to insufficient lateral ties in the concrete columns, nonductile concrete frame buildings did much worse than ductile design, shear walls typically helped to lessen catastrophic damage, and soft soils resulted in greater damage to the structures constructed on them.

The most important lesson in both earthquakes is that the knowledge to significantly improve structures to resist earthquake damage and thereby avoid most of the deaths and financial losses exists; what is lacking is a consistent willingness to marshall the resources necessary to put that knowledge to work on the scale necessary to prevent disasters. It is an odd paradox, for time and time again it is demonstrated that it usually costs less to prepare for earthquakes in advance than to repair the damage afterwards.

Differences in Kobe and Northridge

While there are more similarities than differences in structural performance in the Kobe and Northridge earthquakes, there are important differences that explain why the Kobe Earthquake was so much more damaging. Some of the lessons from these differences apply only to Japan, others apply to all areas of the world at risk from earthquakes.

The vast majority of deaths in Kobe occurred in the collapse of housing built using traditional Japanese methods. Traditional Japanese housing construction is based on a post-and-beam method with little lateral resistance. Exacerbating the problem is the practice of using thick mud and heavy tile for roofing, resulting in a structure with a very heavy roof and little resistance to the horizontal forces of earthquakes. U.S.-style frame housing with light-weight roofs is now coming into use in Japan and newer housing constructed using these methods had little or no damage from the earthquake.

Another significant difference between the Kobe area and the Northridge area is the quality of the soils. Because of a severe shortage of available land, much of modern urban Japan, including Tokyo, is built on the worst soil possible for earthquakes. Much of the newer construction in Kobe, particularly larger buildings, is built on very soft, recent alluvial soil and on recently constructed near-shore islands. Most of the serious damage to larger commercial and industrial buildings and infrastructure occurred in areas of soft soils and reclaimed land. The worst industrial damage occurred at or near the waterfront due to ground failures-liquefaction, lateral spreading, and settlement.

The Port of Kobe was an extreme example of the problems associated with poor soils in areas prone to earthquakes. The port is built almost entirely on fill. The engineering profession has tried hard to develop methods for strengthening filled areas to resist failures during earthquakes, but most of these methods have been put into practice without the benefit of being adequately tested in strong earthquakes. The results were decidedly mixed, but the failures costly_most retaining walls along the port failed, and the related ground settlement pulled buildings and other structures apart.

Buildings

The large commercial and industrial buildings in the Kobe area, particularly those built with steel or concrete framing, are similar to buildings of the same vintage in California. The Japanese building code had a major revision for concrete-frame buildings and a more limited revision for steel-frame buildings in 1981. The Uniform Building Code, as used in California, had major changes in 1973, 1975, and several times since then. The current Japanese code requires that buildings in Japan be designed for somewhat higher force levels than does the Uniform Building Code. Both areas require design for much higher forces than most other earthquake regions of the world.

Typically, pre-1981 concrete-frame buildings performed very poorly in Kobe, with many collapses. Post-1981 buildings performed much better_some were extensively damaged, but most had light damage. The buildings that fared best, and those without significant damage, had extensive concrete shear walls.

As in other earthquakes, large commercial and industrial steel-frame buildings performed better than any other type. However, major damage and a few collapses were observed. Pre-1981 steel buildings had most of the serious known damage. Certain innovative types of steel buildings, including high-rises, had very serious damage, and collapses might have occurred if the duration of the earthquake had been a few seconds longer.

Building owners usually do not understand that the earthquake provisions of even the strictest building codes do not necessarily have reasonable performance criteria for larger and stronger earthquakes. The current regulations, including those for all of California, are typically written with the expectation that in a strong earthquake a building will be severely damaged_in fact, it is assumed the building may need to be torn down, but it should not collapse. In California, higher performance criteria are mandated for certain types of structures_schools, hospitals, police and emergency response buildings, and certain power facilities. An informed building owner can choose to use these higher criteria, and thus avoid having their high-value, heavily occupied commercial building designed, in effect, to the same earthquake performance level as a low-value farm building.

Transportation

A number of major expressways, rail lines, and bridges, some of very modern design, were severely damaged. There are no significant new lessons from the collapse and damage of the older unretrofitted bridges and elevated structures. The structural and foundation details that typically caused damage to the expressways and rail lines have been observed in numerous earthquakes, and the damage was predictable. Some of the upgrade details observed in older retrofitted structures, such as steel column jacketing, are now widely used in California for strengthening. The apparent good performance of these details in Kobe is important to ongoing U.S. programs and needs to be studied in detail.

Many bridges and bridge approaches were severely damaged. The performance of large new bridges, including cable-tied arch, braced arch, and cable-stayed bridges, should be studied extensively because this is the strongest earthquake to affect such bridges.

The Port of Kobe, much of which was new, was devastated by widespread and severe liquefaction and/or permanent ground deformation, which destroyed more than 90% of the port’s 187 berths and damaged or destroyed most large cranes. Shipping will be disrupted for many months, and some shipping business will probably never return to Kobe, resulting in significant losses to the local economy.

Other Infrastructure

The electrical and telecommunications systems in Kobe and surrounding areas performed as expected based on experience from previous earthquakes. Long term power outages were isolated to the most heavily damaged areas. Facilities near the epicenter sustained damage while resiliency of the systems prevented widespread service interruption. Most of the major transmission lines skirt the heavily damaged region of Kobe the results may have been substantially different if the epicenter was located closer to the 500 kV transmission system. There were substantial financial losses to the electrical utilities, however, because expensive specialized equipment must be replaced and the distribution network must essentially be rebuilt within heavily damaged areas of Kobe.

During the earthquake, Kobe’s water system sustained approximately 2,000 breaks. Generally, ground or building failure was the cause of the severe damage to Kobe’s water systems. The resulting lack of water contributed significantly to the fire problem and will be a major hardship on the population for several months. The gas system had major damage, generally caused by ground or building failure, which also contributed significantly to the fire problem.

Fire

More than 150 fires occurred in Kobe and surrounding areas in the hours after the earthquake. These resulted in several large fires, and fire fighters were for the most part unable to combat them because of streets being blocked by collapsed buildings and building debris, traffic congestion, and severe water system damage. Calm wind conditions prevented conflagrations. The United States and Japan have both sustained the largest peacetime urban conflagrations in this century’s history_because of earthquakes. Fire following earthquake is a potential major agent of damage, and needs to be recognized as such by planners.

Conclusion

The Kobe Earthquake dramatically illustrates the damage that can be expected from earthquakes to modern industrialized society. Most of what happened could have been predicted and much of the damage was preventable. Hopefully, the disaster will spur building owners to continue, and to increase where needed, their efforts to improve the earthquake resistance of their properties.

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Return to The January 17, 1995 Kobe Earthquake Contents Page.

Go to the Next Chapter.

Go to EQE International’s Home Page.

The January 17, 1995 Kobe Earthquake

An EQE Summary Report, April 1995

Introduction

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An entire city block destroyed by fire, Chuo Ward.

On Tuesday, January 17, at 5:46 a.m. local time, an earthquake of magnitude 7.2 (Mj)1 struck the region of Kobe and Osaka in south-central Japan. This region is Japan s second-most populated and industrialized area, after Tokyo, with a total population of about 10 million. The shock occurred at a shallow depth on a fault running from Awaji Island through the city of Kobe, which in itself has a population of about 1.5 million. Strong ground shaking lasted for about 20 seconds and caused severe damage over a large area.

Nearly 5,500 deaths have been confirmed, with the number of injured people reaching about 35,000. Nearly 180,000 buildings were badly damaged or destroyed, and officials estimate that more than 300,000 people were homeless on the night of the earthquake.

The life loss caused by the earthquake was the worst in Japan since the 1923 Great Kanto Earthquake, when about 140,000 people were killed, mostly by the post-earthquake conflagration. The economic loss from the 1995 earthquake may be the largest ever caused by a natural disaster in modern times. The direct damage caused by the shaking is estimated at over 13 trillion (about U.S.$147 billion). This does not include indirect economic effects from loss of life, business interruption, and loss of production.

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Left: Map of the Kobe area.

Right: One of hundreds of collapsed buildings throughout central Kobe.

Damage was recorded over a 100-kilometer radius from the epicenter, including the cities of Kobe, Osaka, and Kyoto, but Kobe and its immediate region were the areas most severely affected. Damage was particularly severe in central Kobe, in an area roughly 5 kilometers by 20 kilometers parallel to the Port of Kobe. This coastal area is composed primarily of soft alluvial soils and artificial fills. Severe damage extended well northeast and east of Kobe into the outskirts of Osaka and its port.

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Left: Collapsed portion of the Hanshin Expressway.

Right: Search party investigating a collapsed residential wood-frame building, Nada Ward.

Our experience with many past earthquakes in developed, industrial areas is that the media, particularly television, can present an exaggerated image of the damage by concentrating on the few spectacular collapses that occurred. The actual damage in Kobe and the surrounding region, however, was much worse than the media could convey, because it is very difficult to show more than local damage at one time. For example, images of the main, 550-meter-long collapsed section of Kobe s elevated Hanshin Expressway were ever-present throughout the media, but that collapse was only a small fraction of the losses to the area s highway system.

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Self-defense troops performing a search and rescue operation at a collapse site.

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Left: Many streets were blocked by collapsed buildings, hindering emergency response.

Right: This man is hauling water. Nine days after the earthquake, 367,000 households were still without water.

Central Kobe, according to many older residents and our investigators, presented the image of a war zone, with a large percentage of both commercial and residential buildings destroyed.

All of this happened in about 20 seconds.

Return to The January 17, 1995 Kobe Earthquake Contents Page.

Go to the Next Chapter.

Go to EQE International’s Home Page.

The January 17, 1995 Kobe Earthquake

An EQE Summary Report, April 1995

Earth Science

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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.

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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.W, 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.

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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.

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