An earthquake is defined as a shaking or trembling of the earth that is volcanic or tectonic in origin. In the case of the Great Hyogo (Hanshin) earthquake of Kobe, Japan it is tectonic in origin. This devastating earthquake which occurred on the 17th of January at 5:46 a.m measured at a whopping 7 (7.2) on the Richter scale. This was the largest recorded earthquake to ever hit Japan.
According to the Japan Meteorological its epicentre was in 34.6? north and 135.0? east in the northern tip of Awaji Island with a focal depth of 14 km. According to the results of the recorded in 24 observation centre, the fracture which triggered the earthquake was happened inside the Nojima Fault. Also investigation in the sea bed by the Ocean observatory Centre shows that the cracks occurred in Nojima Fault. This fault forms a part of a group of faults known as the “Rokko Fault’, which extends from Awaji Island through Kobe City to the foot of Mount Rokko. The movement of the fault can be explained by the Tectonic characteristic of west Japan. The main reason for the big earthquakes in west part of Japan are the tectonic activities of the Eurasian, the Pacific, the North America and the Philippine plates. The collision between these plates in the central part of Honshu (Japan main island) is the main source of strain accumulation in the crust of western part of Japan.
Casualty figures quickly mounted by the hour and when it was all said and done 5,472 people had lost their lives and 415,000 people were left injured. By the fourth day after the earthquake about 310,000 residents spent the night at 1,077 refugee centres. More than 350 fires broke out wiping out around 100 hectares of densely populated area. Fire fighting was virtually ineffective due to an inadequate water supply and access to fires. Lifeline facilities were severely damaged over a large area and following the earthquake, 900,000 households were left without electricity, 850,000 households were without gas, and water supply cuts affected about 2.5 million people. Nippon Telegraph and Telephone (NTT) Corp.’s 160,000 lines out of 800,000 lines were out of order.
The damage to major infrastructure, especially to the elevated expressway and shinkansen (bullet train) tracks, shocked engineers, planners and the general public. According to news reports its tracks were damaged at 42 locations. The first Shinkansen train was to run 30 minutes after the time of the earthquake. Had the disaster occurred few hours later, with an average capacity of about 1,600 passengers/train, casualty rate could have been much higher. Sections of elevated Hanshin expressway toppled virtually cutting off major transport lines to the affected areas. A contiguous section supported by eighteen single columns had fallen sideways in Higashi Nada-ku. Modern buildings — and structures retrofitted with up-to-date engineering techniques — fared well in the Great Hyogo quake. Much of the damage occurred in traditionally built older homes and in areas near the coast where liquefaction of the soil caused instability in structures. A large number of reinforced concrete structures were completely devastated. The economic damage estimates vary from around 100 – 150 billion dollars US.
The most extensively and severely damaged structures were smaller commercial buildings (often with residences upstairs) constructed with limited engineering design and traditional homes. The smaller commercial and mixed occupancy buildings are typically framed with wood or light steel and have walls of stucco over wood slats. Many of these buildings have a large shop window in the front and lack interior walls, factors which weaken the first floor. Traditional homes, typically those built before the 1970s, have heavy tile roofs with tiles set in a thick clay and mud mortar, few partitions, and are not waterproofed which causes widespread dry rot and water damage. Little nailing is used; wood joinery is more common. Many casualties were found in damaged and collapsed traditional homes. The heavy tile roofs stressed the walls, which cracked, crumbled and often collapsed, triggering fires from broken gas pipes.
The Kobe earthquake exposed more modern and engineered buildings to stronger forces than any previous earthquake. The preliminary report indicates more studies are needed to evaluate seismic codes, design practice and construction methods; and to make improvements based on the studies. The failure of transportation structures produced dramatic and frightening images flashed across the world following the quake. Perhaps the most memorable image was a bridge on the Hanshin expressway which “rolled over.” Most of the damage to bridges occurred to older structures designed before modern earthquake engineering. The damage was typically column shear and structures that broke instead of bending. Other more modern structures suffered extensive damage due to liquefiable soils along the bay. The Akashi Suspension Bridge bore the brunt of the earthquake with essentially no damage — an example of how engineering can prevent damage during earthquakes. Rail facilities were hard hit — and more casualties and fatalities would have resulted if the quake had occurred during commute times. Railway structures failed because of shear failures in support structures, inadequate restraint between spans at critical joints, and large ground movements causing spans to fall off supports; also many cars rolled because of the ground movement. The Port of Kobe suffered extensive damage, mainly due to liquefaction. Modern design criteria are more stringent and liquefiable soils can now be identified and the effects mitigated. Had the port conformed to modern design standards, the damage would likely have still been severe — but less than the damage that occurred. Electric power and telecommunications systems performed remarkably well during the earthquake, with little or no disruption to service. Water pipelines sustained severe damage, causing a general lack of service in Kobe, Ashiya and Nishinomiya. Some residents we reinformed to expect no water service for two months. The lack of water also inhibited firefighting efforts. It is expected that the sewer system suffered similar damage. The gas system sustained numerous breaks, which will interrupt service to residents for several months.
The Great Hanshin (Hyogo) Earthquake has had a profound influence on Japan’s earthquake preparedness. Voices are heard urging more emphasis on relief and non structural measures from a disaster mitigation strategy heavily depended on structural measures. Bitter criticism is levelled at government for slow response and inadequate preparedness. The earthquake, which proved to be a combination of a number of worst case scenarios, again reminded us that fight against natural disasters is an endeavour where one cannot afford to relax.