Niigata, Japan Earthquake

First Posting | Summary
Posting Date: October 25, 2004, 1:00:00 PM

A major earthquake and several significant aftershocks struck the west coast of Honshu, Japan on Saturday, October 23, 2004. The Japan Meteorological Agency, using its own intensity scale, estimated a magnitude of 6.8 for the mainshock, while the U.S. Geological Survey’s current estimate of moment magnitude is 6.4. Several significant aftershocks followed, ranging from magnitude 5.5 to 6.2 and local officials have warned that these may continue for the next week. The USGS has estimated the focal depth at 19 km. Yesterday's earthquake was centered in Niigata prefecture near Ojiya, a small city with a population of about 41,000. Ojiya is about 85 km south southwest of the city of Niigata and about 256 km northwest of Tokyo, where buildings swayed. Niigata is Japan's largest rice-producing region. A magnitude 7.5 quake destroyed some 2,000 buildings there in 1964. The epicentral area is located within the convergent plate margin between the Pacific Plate and Amurian Plate, about 350 km east of Japan Trench where the Pacific plate subducts westward under Honshu Island. Many seismologists believe that the Japan Sea is subducting under the Honshu Island, making the western coastal and offshore areas of the Niigata Prefecture an active plate boundary. Some of the great historical earthquakes, including the 1891 Nobi M8.0 earthquake and the 1964 Niigata earthquake mentioned above, all occurred within this seismic belt. Many active reverse faults and folds have been mapped along the western coastal area of Niigata prefecture, but no mapped surface faults can be specifically identified as the causative fault of Saturday’s quake. It was most likely generated by a buried fault under the active folds in this region. Sewage and water mains burst as a result of Saturday's event and several fires broke out, possibly exacerbated by the fact that the quake occurred around dinner time when people were cooking on gas stoves. Some 280,000 households were left without power. Roads buckled and one bullet train derailed before an automatic trigger halted service, the first time this has happened since the trains were introduced 40 years ago. This earthquake is large enough to cause significant damage and even the collapse of buildings in the epicentral region, and there are reports of more than 1400 buildings destroyed. Twenty-five deaths have been reported and several hundred more were injured. However, the fact that the earthquake occurred in a relatively sparsely-populated, mountainous region will keep insured losses lower than might otherwise be expected from an earthquake of this magnitude in Japan. The AIR earthquake team has assembled the available information in the aftermath of this event and has run simulations based on estimates of epicentral location and magnitude using the AIR Japan earthquake model. Five scenarios are now posted on the Scenarios page of this web site where CATRADER and CLASIC/2 clients can view maps of total insured loss (in JPY millions) and intensity. Clients can also download event sets representing insurable losses for all scenarios. When running an analysis in CATRADER, please apply appropriate take-up rates. Please note that there is still uncertainty regarding the actual parameters of this event.

First Posting | Downloads
Posting Date: October 25, 2004, 1:00:00 PM