STRIATIONS FORMED ON THE SURFACE RUPTURE DUE TO THE EARTHQUAKE IN THE FUKUSHIMA PREFECTURE ON 11th OF APRIL, 2011, JAPAN

After the occurrence of the 2011 Mw 9.0 Off the Pacific Coast of Tohoku Earthquake, a Mj 7.0 (Mw 6.8) earthquake on April 11, 2011 at east Fukushima Prefecture, NE Honshu, Japan caused normal-fault surface ruptures along the Idosawa and Yunotake faults. The earthquake on April 11 produced surface rupture with a maximum NW-down displacement of 2.2 m on the Idosawa fault (e.g. Ishiyama et al., 2011). We measured the orientation of striations formed on the surface rupture along the Idosawa fault, to reveal the fault motion during the earthquake on April 11, 2011.

The curved or cross-cutting fault striations are observed at 8 localities along Idosawa fault. It is noteworthy that the dogleg shaped striations are clearly developed on the fault planes at 2 localities. The striations plunging to south turn to north toward the lower part of the fault plane. In other localities, the cross-cutting fault striations that the striations plunging to south overprinted the striations plunging to north are observed, while the no dogleg shaped striations observed. And, the curved or cross-cutting fault striations overprint the horizontal striation is recognized with difficulty at some localities.

We interpret that the instantaneous slip vector changed from southward to northward patterns through the rupture process of the earthquake on April 11. The curved striations formed on the surface rupture caused by intraplate earthquakes have been reported from other area (e.g. 1995 Hyogo-ken Nanbu (Kobe) earthquake; Otsuki et al., 1997). While the striations caused by the earthquake on April 11 are clearly developed on the earthquake fault plane, the older striation (horizontal striation) which are overprinted by striations of this time are found. The older striation may have been formed during the Quaternary period, because they develop in clayish fault gouge zones, suggesting a possibility that directions of fault motion of past earthquakes are different.

References: Ishiyama, T. et al. (2011) http://outreach.eri.u-tokyo.ac.jp/eqvolc/201103_tohoku/eng/#hamadori; Otsuki, K. et al. (1997) Zisin 49, 451-460.

*Main part of this research project has been conducted under the research contract with the Nuclear and Industrial Safety Agency (NISA).