Variability Among Tsunami Sources in the 17th–21st Centuries Along the Soutehrn Kuril Trench

Abstract

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 historic documents. 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. 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. This evidence for variable rupture mode complicates the task of forecasting future earthquakes and tsunamis in eastern Hokkaido. According to a long-term forecast, issued six months before the 2003 earthquake, probability of an M∼8 earthquake, similar to the one in 1952, was 60 % by 2033. The forecast was correct for the timing but overestimated the earthquake size.