邑知潟平野南東縁・石動山断層の活動時期と地震時変位量

Abstract

The Sekidosan fault is a northeast-trending active reverse fault that extends for about 33 km along the southeastern margin of the Ouchigata Plain, central Japan. We constrained the age and offset of the recent faulting events of the fault by re-interpreting the results of trenching and drilling surveys at three sites, namely Mijiro, Hongo, and Nakagawa-Udono sites. We identified three surface-rupturing events that had occurred during the last 5000 years: 4830-3580, 3690-1990, and 850-250 cal yrBP, and estimated the amounts of net slip during individual events at Mijiro and Nakagawa-Udono sites to be about 1.9-2.2 m and 3.6-3.8 m, respectively. The amount of vertical displacement estimated at Mijiro site is discordant with the slip distribution previously reconstructed based on tectonic geomorphological investigations. This is probably due to the lack of a tectonic scarp associated with only the latest event around the site. On the other hand, the amount of vertical displacement estimated at Nakagawa-Udono site is consistent with the slip distribution. So, we suggest that the previous slip distribution is appropriate to a certain extent. The length of surface rupture calculated from the amounts of the net slip using empirical relationships is at least 25-30 km, indicating that at least three quarters of the entire length of the Sekidosan fault rupture during individual events. Recurrence interval and slip rate of the fault are estimated at 1400-2300 years and 1-3 mm/yr, respectively.