HR: 1340h
AN: T13C-1373    [Abstracts]
TI: Variability Among Tsunami Sources in the 17th-21st Centuries Along the Southern Kuril Trench
AU: * Satake, K
EM: kenji.satake@aist.go.jp
AF: Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Nanayama, F
EM: nanayama-f@aist.go.jp
AF: Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Yamaki, S
EM: yamaki@kc4.so-net.ne.jp
AF: Seamus Co. Ltd, 2235 Kizaki, Toyosaka, 950-3304 Japan
AU: Tanioka, Y
EM: tanioka@eos.hokudai.ac.jp
AF: Hokkaido University, N10 W8, Kita-ku, Sapporo, 060-0810 Japan
AU: Hirata, K
EM: hiratak@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsuhima-cho, Yokosuka, 237-0061 Japan
AB: Instrumental, historical, and geological records of tsunamis show that successive plate-boundary ruptures differ in size along the southern Kuril trench off eastern Hokkaido. Tsunami source area of the 2003 Tokachi-oki earthquake (M 8.0), the most recent and the best-measured earthquake, is only about 2/3 of that of the predecessor, the 1952 Tokachi-oki earthquake (M 8.2). This difference is apparent from tsunami waveform inversions of the two events. The inversion of the 1952 event, redone with the clock corrections estimated from comparison of the 1952 and 2003 tsunami waveforms, confirms that the 1952 tsunami source extended about 100 km to the east of the 2003 source. The coastal tsunami runup heights were also different; the maximum height in 1952 was recorded by more than 100 km east of that in 2003. An earthquake in 1843 may have resembled the 1952 event, based on tsunami damage distribution recorded in historical documents. For the earthquakes in the 19th century, macroseismic data, i.e., distribution of tsunami heights and seismic intensity, are the only available data to estimate the source areas. As in 1952, the largest tsunami (4-6 m) was estimated around Akkeshi, where the 2003 tsunami was about 2 m. However, historical documents for 1843 are limited around the population center of that time, and no information is available from the Toakchi coast where the tsunami was highest in 2003. Prehistoric tsunami deposits have shown that larger tsunamis occurred in the eastern Hokkaido in an approximately 500 year interval with the last event in the 17th century. In Kiritappu, prehistoric sand sheets extend as much as 3 km inland across a beach-ridge plain. The 1952 tsunami penetrated about 1 km from the coast and the 2003 tsunami did not penetrate at all. Diatom analysis indicates that the sand sheets are of marine origin. Volcanic ash layers aid in the correlation and dating of sand sheets. The youngest of the prehistoric sand sheets shortly predates ash layers from 1663 (Usu volcano) and 1667 (Tarumai). A second, earlier sand sheet postdates a 10th century ash that erupted from Baitoushan on the border of China and North Korea. Three to five sand sheets are commonly found between the 10th century ash and a Tarumai ash about 2500 years old. These deposits are best explained by earthquakes that broke not only the area of the 1952 event but also the adjoining Nemuro-oki segment to the east. The above evidence for variable rupture mode complicates the task of forecasting future earthquakes and tsunamis in eastern Hokkaido. According to a long-term forecast that was issued six months before the 2003 earthquake, probability of an M~8 earthquake was 60 % by 2033. The forecast was correct for the timing but overestimated the earthquake size.
DE: 7221 Paleoseismology
DE: 7230 Seismicity and seismotectonics
DE: 8123 Dynamics, seismotectonics
DE: 4564 Tsunamis and storm surges
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting