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2 THE EARTHQUAKE AND THE TSUNAMI The epicenter of the Japan Sea central region earthquake was located at o o 40.4 N latitude, l38.9 E longitude, approximately l00 km off the northwest coast of Honshu, Japan. The geographic location of the earthquake is shown in Figure l. Figure l also shows the location of historical earthquakes that have occurred offshore of Japan in the Japan Sea. The Japan Sea central region earthquake is one of the largest earthquakes that have been recorded in that region. The earthquake occurred at a focal depth of approximately 20 km along a fault line running north by northeast to south by southwest. The orientation of the fault line is indicated by the major axis of the aftershock ellipse, which is also shown in Figure l. The measured area of the aftershock has a length of approximately l50 km along the major axis of the ellipse and a width of about l00 km. According to Kinjuro Kajiura of the University of Tokyo, the fault plane appears to dip to o o the east at an angle of about 20 to 30 . The seismic moment (M ) for this earthquake is estimated to be in 28 the range 0.8 to 2.0 x l0 dyne-cm. Assuming static and seismic FIGURE l The epicenter and aftershock area of the Japan Sea central region earthquake are shown in cross-section area. Also shown are other historical earthquakes that have occurred in the region. The figures accompanying each earthquake give the year, the month, and the date, respectively. Recent earthquakes are also listed with their measured magnitude (M). Source: Japanese Tsunami Committee, l983.
Longitude 0) T3 3 4J â¢H -P (0 9/251-U S39-5X I l-l) 38*
moments to be equal, a permanent seafloor offset, Â£ , along the fault plane is found from Â£0 * Mo/yS where S is the source area and u is the shear modulus of the fractured 2 ll -2 material. With S *â¢ l5,000 km and y = 5 x l0 dynes-cm , we find the ground offset to be 5 to l0 m. For a fault plane inclination o of 30 , this yields a vertical offset component of 2 to 5 m. The displacement was positive over most of the aftershock area, with slight subsidence occurring only in the eastern portions of the aftershock ellipse. At Kyuroku Island (Kyurokujima in Figure 2), the observed subsidence was approximately 30 cm. Figure 2 also shows the size of the earthquake's bottom displacement area. This area was delineated by reverse wave propagation based on records of wave arrival from surrounding tide gauge stations.
Longitude 129-E I 4i â¢H 4J JAPAN _S E'A - ''<â¢Â«â¢â¢ 'â¢ 'Jff-OSHIMA. "- FIGURE 2 The earthquake area based on tsunami arrival times. Source: Shuto, l983.