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Meteorologists Predict Week-Long Aftershocks From Strong Japan Quake

Seismic News: Japan
March 25, 2007

The Meteorological Agency on Sunday predicted that the Hokuriku region will experience at least one week of aftershocks after the major 6.9 quake struck the Sea of Japan coast Sunday morning. The JMA advised residents in the region to stay away from damaged buildings and other infrastructures as the aftershocks might cause the collapse of these damaged buildings. As of Sunday evening, over 100 aftershocks have been recorded by the agency in areas affected by the powerful quake which rocked the area around the Ishikawa prefecture. The aftershocks could reach as high as intensity 5 on the Japanese seis
mic scale of 7.

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Aftershocks jolting the west coast of central Japan are keeping residents on edge. A 5.3 magnitude tremor, one of more than 175 aftershocks, struck early Monday, and a 4.8 magnitude quake jolted the area in midafternoon. About 2,600 people spent the night in evacuation shelters and many other residents slept in their cars. The government's disaster agency put the total number of damaged houses at 564 and more than 10,000 households lack running water. An emergency relief team of firefighters that had been searching the rubble of collapsed houses confirmed that no one was trapped. The Meteorological Agency, using an early warning system that detects smaller tremors before a main quake hits, issued a tsunami alert on Sunday about 100 seconds after the quake, about two minutes faster than previously. The agency was also able to send an "emergency earthquake flash" to monitors about 50 km (30 miles) from the focus about five seconds before the strong quake rattled the region. But hard-hit Wajima failed to receive the warning before the tremor struck because it was too close to the focus.

The powerful earthquake that hit the Noto Peninsula on Sunday was similar to the 1995 Great Hanshin Earthquake and the 2004 Niigata Prefecture Chuetsu Earthquake where an earthquake fault moved across a very shallow part of the earth. The frequency of the quake's tremor was also similar to the Great Hanshin Earthquake, and was particularly damaging to houses. As the fault is located off the coast, its existence was until now unknown. However, many such undiscovered faults are believed to exist in the coastal regions of the country, suggesting areas that recorded low-intensity tremors in Sunday's earthquake should be prepared for stronger quakes in the future. The government's Geographical Survey Institute estimated the fault, about 21 kilometers long and 14 kilometers wide, shifted by about 1.4 meters based on analysis using the global positioning system. "According to an analysis of seismic waves, the earthquake was caused when one fault slipped over another. This is called a 'reverse fault type,' and we also observed a large horizontal sway." Despite the intensity of the quake reaching an upper 6 on the Japanese intensity scale of 7, the resulting tsunami was only about 20 centimeters high, partly because the force pushing up the seabed was comparatively small.

Simultaneous quakes Sunday - Three earthquakes - the two strong ones and one moderate - hit the Pacific region Sunday morning, with the biggest quake, measuring 6.4 on the Richter scale, occurring near West Coast of Honshu Island, Japan. The second strong quake, registering 6.3 on the Richter scale, struck Loyalty Islands, an archipelago in the Pacific. The third quake, a moderate one, recording 5.6 on the Richter scale, hit Talaud Island in south Philippines, about 883km East of Lahad Datu, Sabah. The earthquake in Japan and the Philippines struck simultaneously at 8.46am.
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