Noto Peninsula (Japan) Earthquake

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Posting Date: March 26, 2007, 8:00:00 AM

A strong earthquake struck off the Noto peninsula on the west coast of Japan’s largest island, Honshu, yesterday, March 25, at 9:42 a.m. local time. The Japan Meteorological Agency (JMA) estimates the earthquake’s magnitude at 6.9, while the USGS has assigned a moment magnitude of 6.7. The JMA intensity was 6 on a scale of 7. The epicenter was located about 80 km north of Kanazawa in Ishikawa prefecture, and about 335 km west northwest of Tokyo. The USGS estimates focal depth at 5 km. The JMA's current estimate is 11 km. At least one person was killed and more than 200 injured by Sunday’s quake. Japan television showed footage of buildings in Ishikawa prefecture swaying. According to the latest report from Japan’s Fire and Disaster Management Agency, 57 buildings collapsed, including a lumber mill. Another 151 buildings suffered severe damage and 392 suffered slight damage. Up to 1,000 people were evacuated to temporary shelters while damage to their homes was assessed and as a precaution against possible aftershocks, which officials warn may continue for up to a week. The vertical component of the ground motion was significant, with reports of paving stones having flipped upside down. Most of the damage occurred in, a fishing town with a population of about 34,000. The most severe damage was to older, traditional structures of shinkabe and okabe construction, which are woodframe buildings with little or no bracing, poor roof-to-wall and wall-to-foundation connections, and heavy tiled roofs—all characteristics that make them particularly vulnerable to the lateral loads imposed by ground shaking. Train service in Ishikawa and Toyama prefectures was suspended and air traffic into the region was delayed. Water, power and telephone service were interrupted. Some villages in the affected region have been cut off as a result of damage to roads caused by liquefaction. Several landslides were also reported. There were no reports of damage to nuclear power facilities and operations were not affected. A tsunami warning was issued following the event and a small swell of 20 cm was reported along the coast some 40 minutes later. Sunday’s earthquake occurred in the Sea of Japan within an area that can be defined by either the back arc basin of the Nankai subduction zone, where the Philippine plate subducts beneath the Eurasian plate, or by the subduction of the Eurasia plate beneath the Okhotsk plate. Historically, this region has not been very seismically active. Known active faults lie primarily south of the epicenter, where two earthquakes of magnitude 6.3 and 6.4 occurred in 1892. Since 1973, only four earthquakes of magnitude 5.0 to 5.9 the have been experienced within 50 km of the epicenter of yesterday’s event. The largest earthquakes within a 100 km radius were two of magnitude 6.9 in 1729 and 1815. The reported focal mechanism of the earthquake indicates reverse faulting with a small strike slip component. A preliminary report from the Earthquake Information Center of Japan extends the earthquake’s rupture plane onshore. 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. Because the quake occurred in a relatively sparsely populated region, and because insurance take-up rates are low in the area, AIR does not expect significant insured losses.

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Posting Date: March 26, 2007, 8:00:00 AM