Geologic and Geophysical Evidence of Repeated Normal Faulting on the Itozawa Fault—the Source of the Mw 6.6 Iwaki Earthquake Triggered by the Mw 9.0 Tohoku-Oki Megathrust Earthquake, Northeast Japan

Abstract

ABSTRACT The 2011 Mw 9.0 Tohoku-Oki earthquake ruptured the 500 km long and 200 km wide convergent plate margin between the North American and Pacific plates, and changed the crustal stress field and triggered widespread seismic activity in northeast Japan. In particular, many crustal earthquakes struck the southern Fukushima area. The largest inland normal-faulting earthquake with Mw 6.6 occurred on the northwest-trending Yunodake fault and the north-northwest-trending Itozawa fault on 11 April 2011. The coseismic surface ruptures appeared along the previously identified active and presumed active faults. To investigate the near-surface structure of the Itozawa fault, we conducted ground-penetrating radar (GPR) profiling across the fault, and two drilling surveys on its hanging-wall and footwall blocks. The GPR survey line about 50 m long was located near the previous trenching site. The GPR two-way travel-time sections were collected by common-offset modes by 50 and 100 MHz GPR systems. We acquired common midpoint ensembles on both sides of the surface rupture for depth conversion of the time sections. The resulting GPR sections show detailed subsurface structures to a depth of about 7 m. We identified event horizons for the 2011, penultimate, antepenultimate, and preantepenultimate earthquakes, indicating that the Itozawa fault has ruptured repeatedly as a normal fault. The vertical displacements during these earthquake events, obtained from the GPR sections, were about 0.8, 0.3, 0.4, and 0.7 m, respectively. These observations suggest that the coseismic surface offset amount on the Itozawa fault varies greatly. The recurrence intervals of surface-rupturing earthquakes on the Itozawa fault, calculated from the previous research and this study, are much longer than those of giant interplate earthquakes along the Japan trench, such as the 2011 Tohoku-Oki earthquake. We conclude that not every giant earthquake along the Japan trench triggers a rupture of the Itozawa fault.