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Detecting Vertical Faulting Event Horizons from Holocene Synfaulting in Shallow Marine Sediments on the Western Margin of the Nobi Plain, Central Japan

Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8563, Japan

Osamu Fujiwara and Yasuo Awata

Active Fault Research Center, National Institute of Advanced Industrial Science and Technology, Site 7, 1-1-1Higashi, Tsukuba-shi, Ibaraki 305-8567, Japan

Holocene activity of the Kuwana fault was reconstructed on the basis of 82 accelerator mass spectrometry (AMS) dates and stepwise changes in the depositional rates of shallow marine sediments in four cores drilled to examine the regularity of the recurrence interval of ruptures of this active intraplate fault. In our model, depositional rates on the footwall and hanging wall are nearly equal during interseismic periods, but depositional rates on the footwall become markedly larger than those on the hanging wall just after vertical displacement of the fault. To detect the timing of each faulting event more precisely, we compared changes in the depositional rates in cores from both the downthrown (footwall) and uplifted (hanging-wall) sides of the fault. As a result, six probable and one possible paleoseismic events, including the last two historical earthquakes, were inferred to have occurred during the last 7000 yr on the basis of stepwise changes in the depositional rate on the two sides of the fault. The timing of the faulting events was estimated from depositional rate curves of four cores. Probable ages of seismic events were approximately 6200, 5700, 4000, 3600, and 2100 calendar years before present and A.D. 745 and 1586. The average earthquake recurrence interval on the Kuwana fault calculated from these data is about 1000 yr. The average vertical slip rate of the fault has been at least 1 mm/yr during the last 7000 yr.