Abstract

The 11 April 2011 Hamadoori, Fukushima prefecture, northeast Japan, earthquake occurred in a seismically active area, one month after the 11 March 2011 Off the Pacific Coast of Tohoku earthquake. The Hamadoori earthquake resulted in a 14‐km‐long down‐to‐the‐west normal‐slip rupture on the north‐northwest–south‐southeast‐striking Itozawa fault, as well as a 16‐km‐long down‐to‐the‐south normal‐slip rupture on the west‐northwest–east‐southeast‐striking Yunodake fault. The slip along the Itozawa fault increases abruptly from the southern end to the central part of the fault, where the measured maximum displacement is close to 2 m; the slip decreases gradually from the central section to the northwestern end of the fault. The Yunodake fault also has an asymmetric slip distribution, with a peak of about 0.9 m near the northwestern section of the fault. Considering the direction of rupture propagation along the two faults, it is apparent that the measured surface offset is asymmetric along the strike, with the largest offsets observed nearer the end of the fault where the rupture began, and showing a gradual decrease in the direction of rupture termination. The surface observations of the slip distribution and the epicenter location are consistent with the general features of earthquake ruptures derived from slip‐inversion models. On more than 60% of the rupture length of the Hamadoori earthquake faults, as well as on other historic normal‐fault earthquakes, slip displacements are less than or equal to 30% of the maximum slip. The moment magnitude (Mw) of the earthquake, estimated from ruptures length, maximum and average displacements, is between 6.5 and 6.8, in accordance with the seismologically determined value of 6.6.

Online Material: Supplementary data of the surface slip distributions of the 11 April 2011 Hamadoori earthquake along the Itozawa and Yunodake faults.

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