Simultaneous Reactivation of Two, Subparallel, Inland Normal Faults during the Mw 6.6 11 April 2011 Iwaki Earthquake Triggered by the Mw 9.0 Tohoku-oki, Japan, Earthquake

The gigantic M-w 9.0 11 March 2011 Tohoku-oki earthquake suddenly changed the overriding inland area to an extensional stress regime and triggered massive seismic swarms in the coastal region. The largest earthquake of M-w 6.6 struck southern Fukushima on 11 April 2011 and ruptured two previously mapped faults, the northwest (NW)-trending Yunodake fault and the north-northwest (NNW)-trending Itozawa fault. Clear 15-km-long and 14-km-long surface ruptures appeared along both faults, respectively, exhibiting a predominantly normal sense of slip, down to the west. The maximum vertical offset on the Yunodake fault is similar to 0.8 m, whereas that on the Itozawa fault is similar to 2.1 m. The Itozawa fault rupture is in part marked by uphill-facing scarps, which is discordant with the large-scale topography but consistent with saddled ridges and ponded alluvium. The Yunodake fault, which bounds a Neogene half-graben structure juxtaposing Mesozoic metamorphic rocks against Neogene sedimentary rocks, shows left-lateral deflections of streams. Seismological data and interviews of local residents revealed that the two subparallel faults ruptured simultaneously. Based on the location of its hypocenter, we, however, interpret that the Itozawa rupture was primary and then triggered normal faulting on the Yunodake fault under the heightened Coulomb stress caused by the preceding Tohoku-oki earthquake. Our paleoseismic trench across the Itozawa fault exposed evidence for a penultimate earthquake that occurred sometime between 12,620 and 17,410 cal yr B.P. There is no evidence that the Itozawa fault ruptured during or immediately after the A.D. 869 Jogan earthquake, which is believed to be the penultimate giant megathrust earthquake along the Japan trench.