[S-SS12] Active faults and paleoseismology

[SSS12-P01] Tectonic geomorphology and slip rates along the Asahi-Semiguchi Fault, western margin of the Niu Mountains, Fukui Prefecture

*Masayoshi Tajikara¹, Tomoo Echigo² (1.Association for the Development of Earthquake Prediction, 2.Kankyo Chishitsu Co.,Ltd)

Keywords:Asahi-Semiguchi Fault, active fault, slip rate, Niu Mountains

The Niu Mountains are located in northern Fukui Prefecture, and are located between the Sea of Japan and the Fukui Plain. The Asahi-Semiguchi Fault is distributed along the eastern margin of the southern Niu Mountains (Research Group for Active Faults, 1991). Although the distribution of the Asahi-Semiguchi Fault was mapped by Tajikara et al. (2019), details of tectonic landforms, deformation style, and activities of the Asahi-Semiguchi Fault are not fully understood. In this study, we aimed to clarify these issues through more detailed air photo interpretation, observation of the tectonic landforms by field survey, construction of cross-sections, sampling and analysis of the cover layer of the fluvial terrace surfaces. Fluvial terraces formed by the Tenno River and the Wada River and their tributaries are well developed in the study area. We classified these terraces into six surfaces (T1 to T6) based on continuity, elevation, and morphology (degree of dissection, shape of terrace scarp, etc.) of these terraces. The Asahi-Semiguchi Fault displaces these terraces. Tectonic landforms are particularly clear in the Asahi to Iwakai, Sasou, Semiguchi areas, and the Faults deform T1 terraces in Asahi to Iwakai area, and T5 terraces in Sasou and Semiguchi areas. We conducted geological observations at one outcrop and two boreholes, and topographic cross-sectional surveys on five survey lines. We identified some tephras, including Daisen-hoki, DKP, K-Tz, Mihama tephra from outcrop samples (T1 terrace). Based on ages of these tephras, we estimated T1 terraces are formed at about 230 kyr ago. We identified AT tephra from overbank deposits covering terrace deposits on T5 terrace. Based on the age of AT tephra, we estimated that T5 terraces formed about 30 kyr ago. The vertical slip rates of the Asahi-Semiguchi Fault were estimated from the ages of each terraces described above and the vertical displacement measured on the topographic cross-sections. The vertical sip rate at Iwakai area was calculated to be 0.13 mm/yr from the vertical displacement of T1 terraces (about 30 m). The vertical slip rates at Sasou to Semiguchi areas were calculated to be 0.06-0.20 mm/yr from the vertical displacement of T5 terraces (1.7-5.9 m). All the vertical slip rates estimated above are similar, suggesting that uniform displacement may have accumulated for at least the past 0.2 million years.