Late Pleistocene-Holocene activity and paleoseismicity of the Nojima Fault in the northern Awaji Island, southwest Japan

Abstract

Field investigations and analyses of tectonic landforms show that the topographical markers of terrace risers and alluvial fans formed in the late Pleistocene-Holocene have been systematically offset by ~40–62 m along the Nojima Fault on which the 1995 Mw 6.9 Kobe earthquake occurred. Analysis of tectonic landforms, trench excavations, and radiocarbon dating results reveal that i) at least two large earthquakes prior to the 1995 Mw 6.9 Kobe earthquake occurred in the past ~1800 years; ii) the most recent event occurred in ~1000 CE, and the penultimate large earthquake event took place in ~200 CE, suggesting an average recurrence interval of ~900 yr for large earthquakes on the Nojima Fault. Based on the deformation features of tectonic landforms and radiocarbon dating ages, the late Pleistocene-Holocene strike-slip right-lateral rate is estimated to be ~2.0–3.0 mm/yr with an average of 2.5 mm/yr for the Nojima Fault. These findings show that previous studies had significantly overestimated the recurrence interval and underestimated the slip rate of the Nojima Fault. Accordingly, the Nojima Fault poses a much greater seismic hazard than previously believed, and therefore, it is necessary to reassess the potential seismic hazard for the densely-populated Awaji-Kobe regions, southwest Japan.