Honshu Earthquake

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Posting Date: August 11, 2009, 11:30:00 AM

The second seismic event to hit Japan in two days occurred off the central coast of the Japanese island of Honshu today at 5:15 a.m. local time (20:15 UTC on Monday). The US Geological Survey (USGS) and the Japan Meteorological Agency (JMA) have reported preliminary magnitudes of 6.4 and 6.5 for the event, respectively. Depth has been estimated at 20 km (12 mi). The epicenter was located off the coast of Shizuoka prefecture, 170 km (105 mi) southwest of Tokyo. A tsunami watch had been issued by the JMA but was rescinded two hours later after it was determined that the threat had passed. A magnitude 6.9¹ earthquake struck near Tokyo on Sunday, 187 km (116 mi) south of today’s tremor, but much deeper (340 km); that event caused no significant damage. Today’s earthquake occurred in the Suruga trough west of the Izu-Bonin Peninsula at the southeastern tip of central Honshu Island. The epicenter is located in a complex tectonic region where the Philippine Sea, the North America, and the Eurasia Plates converge. The Philippine Sea Plate moves west-northwest at a rate of about 3-4 cm/year toward the central Honshu Island (on the North American Plate) to the north. In the west, the Philippine Sea Plate subducts under the Shikoku and southern Honshu Islands (on the Eurasia Plate) at the Nankai and Tokai trough. The M7.9 Taisho Kanto earthquake struck this region in 1923, killing nearly 100,000 people and destroying over 570,000 buildings. That quake took place approximately 80 km (50 mi) to the east of the Izu Peninsula in the Sagami trough. While ground shaking rattled houses and knocked goods off shelves in the area of the epicenter, there have been no immediate reports of property damage. The quake halted trains, disrupted transportation throughout Shizuoka prefecture, and shut down two nuclear reactors for safety checks. A large landslide closed a major highway connecting Tokyo and Nagoya, Japan's third-largest city. About 2,000 homes were left without electricity. Heavy rains from recent typhoons in east Asia have increased the risk of landslides and local authorities have warned residents to take precautions. One person has been reported killed; more than 100 people have been injured. In Japan, about 70% of all residential construction is estimated to be of wood and about 25% of concrete. Commercial construction consists of more than 50% concrete, about one third light metal or steel, and less than 10% wood. In light of Japan’s stringent seismic codes, the fact that Shizuoka prefecture is largely agricultural, and because take-up rates in the affected region are relatively low, insured losses resulting from this event are expected to be minimal. AIR will continue to monitor the situation and will provide updates as warranted. Meanwhile, only a few minutes before the earthquake in Japan, a magnitude 7.6 earthquake struck off the Andaman Islands, in the Bay of Bengal. According to the USGS, the quake occurred 262 km (163 mi) north of Port Blair at a depth of 33 km (20.6 mi). No immediate reports of damage or casualties have been reported. The Pacific Tsunami Warning Centre issued a tsunami watch for India, Myanmar, Indonesia, Thailand and Bangladesh, but later lifted it. ¹The JMA reported a magnitude of 6.9 while the US Geological Survey issued a magnitude of 7.1.

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Posting Date: August 11, 2009, 11:30:00 AM