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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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.
Liquefaction and Other Ground Failures
The earthquake caused extensive ground failures, which affected buildings, underground infrastructure, the port, highways, all types of other facilities on soft or filled ground, and recovery efforts.
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Parking lot on reclaimed land near Ashiya. Sand covering the lot is evidence of large-scale liquefaction and sand ejection.
Ground failures occurred primarily because of liquefaction, the result of loose, water-saturated sand being shaken during an earthquake and assuming a semiliquid state. The areas affected by liquefaction were more heavily developed than any other earthquake-stricken region to date. Therefore, the lessons are valuable and will enhance our knowledge of liquefaction for both natural soils and reclaimed lands with high water tables.
The affected areas were located primarily along the coastline and the numerous watercourses in the general area of Kobe and the valleys between Kobe and Osaka. Widespread liquefaction, over many square kilometers, occurred around Kobe, Ashiya, Nishinomiya, Amagasaki, Osaka, Sakai, Izumiotsu, Kishiwada, and other areas around Osaka Bay. Massive liquefaction and lateral spreading took place in areas of reclaimed land and on the many artificial islands in the city of Kobe and Nishinomiya. Ejected sand from liquefaction covered much of the islands and interfered with rescue and recovery operations.
Similar effects were observed throughout the Kobe mainland along the coast, including parts of downtown. Typically, as in downtown Kobe, settlement and liquefaction of less than 50 centimeters were observed. That increased to as much as 3 meters along the coastline. The settlement caused severe damage to underground utilities, severing all services (gas, water, sewage) to large parts of the mainland and to all reclaimed islands, including the largest islands Rokko and Port. A month after the earthquake, these services had largely been restored to Rokko and Port islands.
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Failed quay wall in Nishinomiya. Lateral spreading and settlement of fill material have pushed the wall to the right. Note the backhoe for scale.
The most obvious and destructive liquefaction and related lateral spreading of soils and settlement occurred along the dozens of kilometers of seawalls along the port. Lateral spreading on the order of 3 (or more) meters and vertical settlement of 2 to 3 meters were observed along the seawalls of numerous islands, including Port and Rokko islands, and throughout the Port of Kobe. The largest settlements, and worst damage, seemed to be associated with the older reclaimed lands, such as the older parts of the port. The newer, engineered fills performed somewhat better than did the old fills, but with less than adequate results.
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Human-made island in Nishinomiya showing evidence of large-scale liquefaction, settlement, and lateral spreading
Numerous buildings on reclaimed land tilted because of ground settlement. These were primarily older, heavy concrete, industrial buildings, probably on mat foundations. The majority of industrial and other buildings on fill were supported on piles (most of these were lighter steel buildings). Most pile-supported buildings appeared to perform well; many multistory or large pile-supported buildings in areas where extensive liquefaction (and limited lateral spreading) occurred had little or no damage. Typically, the sidewalks of such buildings would settle 50 centimeters or more, but there would be no apparent damage to the buildings themselves. The same was generally true for newer highway structures supported on piles. However, the strong shaking may have exceeded the capacity of many pile foundations supporting elevated expressway and bridge piers, causing tilting or lateral movements (observed to be as much as 2 meters) of the piers. This often contributed to damage or collapse of the superstructures.
References
1. Pacheco, J. F., and L. R. Sykes. 1992. Seismic Moment Catalog of Large Shallow Earthquakes, 1900 to 1989. Bulletin of the Seismological Society of America, Vol. 82: 1306-1349.
2. Ellsworth, W. L. 1990. Earthquake History, 1769 – 1989. In The San Andreas Fault System, California. R. E. Wallace, ed. U.S. Geological Survey Professional Paper 1515: 153-187.
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The January 17, 1995 Kobe Earthquake
An EQE Summary Report, April 1995
Buildings
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This collapsed concrete building in Kobe completely blocked the street.
The number of buildings destroyed by the earthquake exceeds 100,000, or approximately one in five buildings in the strongly shaken area. An additional 80,000 buildings were badly damaged. The large numbers of damaged traditional-style Japanese residences and small, traditional commercial buildings of three stories or less account for a great deal of the damage. In sections where these buildings were concentrated in the outlying areas of Kobe, entire blocks of collapsed buildings were common. Several thousand buildings were also destroyed by the fires following the earthquake.
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Left: Badly damaged concrete shear wall building.
Right: Ground settlement in central Kobe.
Mid-rise commercial buildings, generally 6 to 12 stories high, make up a substantial portion of the buildings in the Kobe business district. The highest concentration of damaged mid-rise buildings was observed in the Sannomiya area of Kobe’s central business district. In this area, most of the commercial buildings had some structural damage, and a large number of buildings collapsed on virtually every block. Most collapses were toward the north, which was evidently the result of a long-period velocity pulse perpendicular to the fault. This effect has also been observed in other earthquakes. Failures of major commercial and residential buildings were noted as far away as Ashiya, Nishinomiya, and Takarazuka. In general, many newer structures performed quite well and withstood the earthquake with little or no damage.
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Mid-height collapse of a mixed-use building (built circa 1977) in Nishinomiya. This type of collapse was very common in this earthquake.
In the heavily damaged central sections of downtown Kobe, approximately 60% of the buildings had significant structural damage, and about 20% completely or partially collapsed. One survey of a 120,000-square-meter area in downtown Kobe (the Sannomiya area) found that 21 out of 116 buildings, or 18%, were visibly destroyed. Another report indicated that 22% of office buildings in a portion of the Kobe city center were unusable, while an additional 66% may need more than six months for complete restoration. City inspectors declared approximately 50% of the multifamily dwellings in Kobe as unsafe to enter or unfit for habitation, leaving more than 300,000 people homeless.
Age of construction, soil and foundation condition, proximity to the fault, and type of structural system were major determining factors in the performance of structures. Damage was worst in the areas bordering the port or streams and rivers-where soils were either poorly consolidated alluvial deposits or fill-and tended to be relatively minor in the foothills of Rokko Mountain, where either soils are very shallow or there are rock outcroppings. Loose and soft soils amplify ground motions in comparison to bedrock, especially ground motions within a certain frequency range. The duration of shaking also tends to be longer on such soils.
Structural damage directly resulting from soil failures was observed for smaller buildings without pile-supported foundations, but it did not appear to be the dominant problem for mid- and high-rise structures supported on piles that extended into dense soils or rock. Although hidden damage may be discovered at a later date, the performance of piles appeared to be good as long as substantial lateral soil displacement did not occur.
A survey of 24 commercial buildings being demolished in the central Sannomiya area of Kobe two months after the earthquake found the following breakdown of building types: 70% were frame type, 20% were shear wall type, and 10% were braced frame type. The breakdown of the frame-type structures included 50% nonductile concrete frame, 35% steel reinforced concrete (SRC) frame, 10% moment-resisting steel frame, and 5% steel frame with masonry infill. Of the shear wall buildings being demolished, 75% were concrete and one was unreinforced masonry. Several of the buildings being demolished were of multiple construction types.
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The ruins of the Ginza after the 1923 Great Kanto (Tokyo) Earthquake and fire.
Building Code
The first building code in Japan was introduced in 1926 after the 1923 Great Kanto Earthquake and ensuing fire devastated Tokyo. The regulations have been reviewed and amended several times over the years as the result of damage during subsequent strong-motion earthquakes. Bridge codes and codes for civil-engineering-type structures (e.g., quay walls) have undergone similar changes over the years.
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Buildings in central Kobe (Chuo Ward). In the foreground is the complete collapse of a two- or three-story traditional Japanese wood-frame building with a heavy tile roof. On the right is a six- or seven-story office building of 1960s’ or 1970s’ vintage. This reinforced concrete building is a typical example of a mid-height story collapse. The high rise to the left is a post-1981 office building that has no apparent damage. Ground settlement in the vicinity of these buildings was between 30 and 60 centimeters.
Since the 1926 code, Japan’s seismic codes have typically been as advanced as any in the world. Japanese engineers upgraded their standards after the 1968 Tokachi-oki Earthquake in northern Japan and California’s 1971 San Fernando Earthquake. In the early 1980s, laws and orders concerning seismic design methods for buildings were extensively revised. The current Japanese seismic provisions are specified in the Building Standard Law Enforcement Order by the Ministry of Construction (1981), and in the Standards for Seismic Civil Engineering Construction in Japan (1980). During the period between 1971 and 1980, some lessons learned in previous earthquakes were included in the design of major buildings, even though the requirements were not yet codified.
In the last several years, U.S. and Japanese professionals have been working together to understand seismic performance and to upgrade codes. Direct comparison of the codes for the two countries is difficult because of their different formats; however, comparative studies have suggested that newer Japanese mid- and high-rise buildings are comparable to or somewhat stronger than their counterparts in the United States.
The current design philosophy in Japan is to keep seismic stresses within the elastic (non-damaging) range for earthquakes that can be expected to occur once or twice (moderate earthquakes) during a building’s life span, and to prevent collapse for larger, less frequent earthquakes. This means that for a moderate earthquake, the building is expected to have little or no damage. A similar philosophy is used in the United States, although, in general, more damage is considered acceptable for moderate-sized earthquakes.
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Concrete-frame structure with a mid-story collapse (Flower Road, Kobe).
Buildings are divided into four general types in the current Japanese code. In general, the guideline is: the larger the building, the more engineering and attention to quality of the seismic-force-resisting system required.
· Small buildings
For small, one- or two-story wood buildings and one-story buildings of other construction types, prescriptive construction requirements apply, and no explicit design is required. A similar practice is applied to wood-frame houses in the United States.
· Buildings less than approximately 30 meters high
For buildings with a regular configuration, prescriptive requirements apply. Additionally, a comparison of calculated and permissible stresses for the loads associated with a moderate earthquake (0.2g peak ground acceleration) must be made.
Irregularly shaped buildings are checked using the same requirements as for regular buildings. In addition, calculated drift (horizontal deflections) must be compared with allowable drifts, and the engineer must either (1) limit configurational irregularities, and meet minimum member size and detailing requirements that vary with construction material type, or (2) check the ultimate strength at each floor level versus the demands for a severe earthquake (1.0g peak ground acceleration). The demands for a severe earthquake are amplified for structures with large configurational irregularities, and reductions in demand are made to account for the ductility of the construction type. Note: Many Japanese buildings are quite irregular in their configurations when compared to U.S. buildings, which makes them much more difficult to design for earthquakes.
Steel buildings less than 13 meters high can be checked as regular buildings if the assumed moderate earthquake forces are amplified by 50%, and if the connections for the braces and the frames are designed to be stronger than the braces, columns, and beams.