Timing of recent surface faulting events on the Shigenobu fault of the Median Tectonic Line Active Fault zone, Southwest Japan

Abstract

The Median Tectonic Line (MTL) active fault zone is one of the most active and longest active fault zones onshore in Japan, and is a potential source of destructive large earthquakes. The history is a key data to evaluate the long-term seismic risks of a fault zone. However, the timing of the most recent event on the MTL in west Shikoku is not well defined yet. In this paper, we studied coseismic deformation on the walls and floor of an archaeological trench across the fault scarp of the Shigenobu fault, in order to date the recent surface faulting events on the Shigenobu fault. The Late Holocene sediments including cultivated soil contain evidence for two faulting events in the past 5315 years. The recurrence interval is estimated to be less than 2200 to 2600 years. This interval is much shorter than the previous estimates of 3500 to 4000 years. The latest event occurred during the Medieval Period of the Japanese history. The penultimate event was estimated to have occurred between 5315 cal yrs BP and AD 6th century.