2010 Volume 119 Issue 5 Pages 775-793
Episodic subsidence events at the east coast of Lake Mikata, Fukui Prefecture, suggesting fault activity in the Mikata fault zone, are recognized from two drilling cores, MK09 core (60-m long) and NEXCO core (100-m long).
Detailed lithology including widespread tephra horizons is described for the MK09 core, with 18 AMS-14C dates. The radiocarbon ages and tephrochronology of the core sediments indicate the continuous sedimentary history of Lake Mikata back to about 130 ka. We recognize eight depositional units ranging in thickness from 5 to 10 m in the MK09 core, each of which shows a clear coarsening-upward sequence from clay to gravel beds. These units are also recognized in the NEXCO core drilled 50 m from the MK09 core site by the correlation of tephras and lithofacies.
Analysis of sedimentary features suggests that each unit and its boundary reflect a rapid rise of lake level and subsequent progradation of alluvial fans. The mean sedimentary interval of individual units is equivalent to about 10.6 kyr, which is meaningfully shorter than astronomical cycles of climate change (∼20, 41 and ∼100 kyrs). Thus, we interpret these cyclic units to be the results of recurrent co-seismic subsidence due to Mikata fault zone activity. According to this interpretation, we could identify at least seven subsidence events besides the 1662 Kanbun Earthquake. The mean recurrence interval of these events is estimated to be about 7.7 kyr.
