Chapter Eight: Summary and Conclusions
The Hyogoken-Nanbu Earthquake of January 17, 1995, with a JMA magnitude of 7.2 and epicenter about 20 km southwest of downtown Kobe, devastated the city of Kobe, its port, and adjacent areas. As of March 31, 1995, the toll from the earthquake has reached over 5,500 dead and over 26,000 injured. More than 200,000 houses, about 10% of all houses in the Hyogo prefecture, were damaged, including more than 80,000 collapsed, 70,000 severely damaged, and 7,000 consumed by fire (AIJ, 1995). Current estimates of losses in Kobe are about 20 trillion yen (200 billion dollars), one order of magnitude larger than that from the January 17, 1994, Northridge Earthquake.
In addition, the damage to the infrastructure, lifelines, and the port had a large economic impact extending well beyond the immediately affected area. Before the earthquake, Kobe was Japan's busiest and the second largest port after Yokohama. About 39% of the city's gross industrial product was derived from port related industries, accounting for more than 17% of the city work force. The earthquake damage caused essentially total shutdown of all facilities in the port area for several weeks following the earthquake and the full recovery of the port is still in the future.
A combination of factors contributed significantly to the severity of much of the damage: the area had been previously considered to have relatively low seismic risk, the projected location of the release of energy along the earthquake fault was almost immediately below a densely developed urban area, and the geologic setting of the region, on the shores of a large embayment, provided for a substantial thickness and areal distribution of liquefiable sediments and fills. Most importantly, however, the area affected by the 1995 Hyogoken-Nanbu Earthquake has many similarities, in terms of geologic setting and the level of development, to other locations around the world. Therefore, much can be learned about the type and the extent of damage that can occur when a major fault ruptures through an urban environment.
8.1 Geoscience and Strong Motions
The earthquake occurred in a region with a complex system of previously mapped active faults. The focal mechanism of the earthquake indicates right-lateral strike-slip faulting on a nearly vertical fault striking slightly east of northeast, parallel to the strike of the mapped faults. The earthquake produced a surface rupture with an average right-lateral horizontal displacement of 1 to 1.5 m and a maximum recorded horizontal displacement of 1.7 m on the Nojima fault, which runs along the northwest shore of Awaji Island. Vertical displacement across the observed surface rupture of the Nojima fault varied considerably with an average vertical displacement of 0.5 to 1 m (east side up) and a maximum vertical displacement of 1.3 m (east side up). At the southern end of the exposed ground rupture, the observed vertical displacement decreased and was actually in the opposite sense, with the west side moving up slightly (<0.2 m) with respect to the east side. Currently available evidence does not suggest any major differences between the source characteristics of the Kobe earthquake and those of crustal earthquakes that occur in California.
Peak ground accelerations as large as 0.8 g were recorded in the near-fault region on alluvial sites in Kobe and Nishinomiya. The recordings at near-field rock sites typically had significantly lower magnitudes of peak ground acceleration (i.e. around 0.3 g). The vertical peak ground accelerations were generally about two-thirds as large as the horizontal accelerations in the near-field, and significantly lower at more distant alluvial sites. The near-fault ground velocity time histories had large, brief pulses of ground motion which are indicative of rupture directivity effects and are potentially damaging to multi-story buildings and other long-period structures such as bridges. The recorded peak velocities were as large as 175 cm/sec at Takatori in western Kobe, and the largest values occurred in the densely populated urban region. 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.
The recorded peak horizontal accelerations from the Kobe earthquake are comparable to those predicted for a strike-slip earthquake using the Abrahamson and Silva (1995) empirical attenuation relation for soil based mainly on California data. The recorded peak horizontal velocities are slightly higher (about one standard deviation higher) than those predicted for a similar event using the Campbell (1990) empirical attenuation relation for soil based mainly on California data. Thus, it appears that one important factor contributing to the much larger level of damage that occurred in Kobe, compared with that of recent earthquakes in California, may have been rupture directivity effects.
8.2 Liquefaction and Related Effects
Extensive liquefaction of natural and artificial fill deposits occurred along much of the shoreline on the north side of the Osaka Bay. Probably the most notable were the liquefaction failures of relatively modern fills on the Rokko and Port Islands. On the Kobe mainland, evidence of liquefaction extended along the entire length of the waterfront, east and west of Kobe, for a distance of about 20 km. Overall, liquefaction was a principal factor in the extensive damage experienced by the port facilities in the affected region.
Most of the liquefied fills were constructed of poorly compacted decomposed granite soil. This material was transported to the fill sites and loosely dumped in water. Compaction was generally only applied to materials placed above water level. As a result, liquefaction occurred within the underwater segments of these poorly compacted fills.
Typically, liquefaction led to pervasive eruption of sand boils and, on the islands, to ground settlements on the order of as much as 0.5 m. The ground settlement caused surprisingly little damage to high- and low-rise buildings, bridges, tanks and other structures supported on deep foundations. These foundations, including piles and shafts, performed very well in supporting superstructures where ground settlement was the principal effect of liquefaction. Where liquefaction generated lateral ground displacements, such as near island edges and in other waterfront areas, foundation performance was typically poor. Lateral displacements fractured piles and displaced pile caps, causing structural distress to several bridges. In a few instances, such as the Port Island Ferry Terminal, strong foundations withstood the lateral ground displacement with little damage to the foundation or the superstructure.
Shallow foundations consisting of a grid of interconnected perimeter-wall footings and grade beams performed well in several areas subjected to liquefaction. Where foundation elements were not well tied together, differential ground displacements pulled apart overlying structures at points of weakness, such as joints and doorways. More importantly, the differential ground movements invariably caused breaks in underground utilities at their entry points into the buildings.
8.3 Performance of Improved Ground Sites
The earthquake provided a severe test for several improved ground sites in the Kobe area. All cases involved loosely dumped hydraulic fill placed over soft alluvial clay. The treatment method was to construct sand compaction piles in-situ by vibro-rod probes or using the casing method with introduction of additional material from above ground. The post-treatment SPT N-values were typically 20 to 25 blows per 30 cm as opposed to about 10 to 15 blows per 30 cm before treatment. The treatment itself resulted in ground surface settlements of 20 to 40 cm, with the largest values being reported for the Portopialand Park area. The liquefaction-induced surface settlements of adjacent untreated ground were on the order of 50 cm. Overall, the observations show that improved ground sites on land sustained significantly less deformation and damage than did the untreated ground.
8.4 Performance of Dams and Levees
In general, most dams performed well with little or no damage noted. The principal exception was the failure of the relatively small Upper and Middle Niteko Dams, together with significant damage sustained at the Lower Niteko Dam. The earthquake was estimated to have induced peak ground accelerations of approximately 0.3 to 0.5g in this area. All three dams are reported to have been constructed over 100 years ago with minor additions and modifications made to them in more recent times. Of the three dams, the Lower Niteko Dam performed the best. Nevertheless, the dam sustained major cracking and slumping in the middle of the embankment, losing as much as 2 meters of height. The Upper and Middle Niteko Dams experienced flow failures with the material traveling as much as 70 m downstream. This mode of failure strongly suggests that liquefaction was responsible, despite the fact that sediment boils were not observed.
The rivers in the area are commonly confined by gravity and cantilever retaining walls. In some places, armored earth embankments are also used as levees. In several locations major damage to these structures occurred, apparently as a result of liquefaction and associated settlement and lateral spreading. While as much as 3 m of settlement was observed in the most seriously affected area, much of the damage consisted of vertical and lateral deformations in the range of 10 to 20 cm.
8.5 Lifeline Systems
Extensive damage to lifeline systems occurred throughout the epicentral area of the earthquake. Liquefaction appears to have been a major factor involved in the failures of lifeline systems due to geotechnical causes, such as the damage to port facilities, underground transit, and underground utilities. There was pervasive disruption of underground utilities caused by ground deformations. In some locations pipe joints were simply pulled apart due to ground displacement. However, most commonly failures occurred due to differential movements between foundation elements and the surrounding soil at points of entry to buildings and other structures.
Particularly notable was the damage to several underground stations of the Kobe Rapid Transit Railway. These stations were constructed using the cut and cover method of construction and soil-structure interaction appears to have been responsible for the observed failures and distress.
8.6 Slopes, Retaining Structures, and Landfills
Seismically-induced landslides were generally limited to shallow slips and raveling of boulders, with the exception of one large flow slide which killed 34 people. In addition to landsliding on natural slopes, structural fills for roads and house pads experienced cracking and lateral deformations in the hills above Kobe. In many cases, this form of distress caused disruption of underground utilities as well as structural damage to houses and retaining walls. Large retaining structures for roads and railroads generally performed well. In particular, mechanically stabilized walls uniformly performed very well.
Waste fills located on reclaimed land experienced distress which can be directly attributed to the liquefaction of the loosely dumped fill. Liquefaction-induced lateral spreading resulted in cracking in the covers and in lateral displacements of the side slopes. The overall impact of these deformations on the integrity of the containment has yet to be established.
8.7 Conclusions
In conclusion, it is important to note that many of the observed effects of the January 17, 1995, Hyogoken-Nanbu Earthquake are similar to those observed in the most recent urban earthquakes such as the Northridge and Loma Prieta Earthquakes in California. Moreover, while many of the observations will provide an opportunity to further improve and refine our methods of analysis and design, much of the observed phenomena and effects could have been predicted with currently available methodologies. This recognition that seismically-induced damage can be predicted and minimized or even avoided, if appropriate analyses are performed and if appropriate designs are employed, has to be effectively communicated to the profession and to the public. Ultimately, we hope that this knowledge will help shape policies and decision making toward the development and implementation of successful earthquake hazard reduction strategies.
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