Assessing Holocene Fault Movement along the Hinagu Fault Zone using Paleo-Sea Level Extents at Kawata Machinishi, Kumamoto, Japan

Kumamoto Prefecture, located in Kyushu, Japan, has previously been assessed as relatively tectonically inactive (National Astronomical Observatory, 1992), with historically little vertical fault movement occurring in this region (The Research Group for Active Tectonics in Kyushu, 1980). Thus, studies have used this area to analyze subtle geological phenomena observable in eustatic sea level changes, such as isostatic crustal tilting (Yokoyama et al., 1996; Nakada et al., 1994). Despite this, the occurrence of the 2016 Kumamoto earthquake saw a MJ 7.3 (Japan Meteorological Agency magnitude) mainshock on April 16th, with movement along the Futagawa and Hinagu fault zones (Shirahama et al., 2016). Horizontal displacement was observed in the northern part of the Hinagu Fault Zone (the Takano-Shirahata segment), but fault slip did not occur along the more southern sections (Shirahama et al., 2016).

The occurrence of the 2016 Kumamoto earthquake has called into question the degree to which the Kumamoto Prefecture can be called tectonically inactive. To investigate the extent of vertical tectonic movement that has occurred along the southern portions of Hinagu Fault Zone, this study assessed the vertical tectonic component observable in sea level changes from over the last 10 ka.

Sediment cores from Kawata Machinishi were analyzed with CNS analyses, radiocarbon dating, and diatom assemblage analyses, and cumulatively were used to identify former marine limits at elevations between -0.56 to 2.24 m relative to modern sea level, occurring from ~1.7 cal ka BP to 7.1 cal ka BP. Identified marine limits were adjusted with local Marine Isotope Stage (MIS) 5e uplift rates and compared to geophysically modelled relative sea level (RSL) curves, and large differences in elevation for similarly aged marine limits were assessed to examine vertical movement that occurred along the Hinagu Fault Zone. Vertically offset tephra layers representing the historic Akahoya eruption (ca. 7.3 cal ka BP) were also observed within the sediment cores. Uplift rates calculated from elevation differences of marine limits and observed tephra ranged from -0.12±0.02 mm/yr to -0.34±0.06 mm/yr, and comparisons to uplift rates from the northern sections of the Hinagu Fault Zone suggest a decrease in vertical fault displacement proceeding southwards.