Complex ruptures of the 11 April 2011 Mw 6.6 Iwaki earthquake triggered by the 11 march 2011 M_w 9.0 Tohoku earthquake, Japan

After the 11 March 2011 M_w 9.0 Tohoku earthquake, Japan, a swarm of shallow normal-faulting earthquakes was triggered in a localized region beneath the border between Fukushima and Ibaraki prefectures in eastern Honshu. Here we examine the coseismic displacement field and a fault model of the largest event of the swarm, the 11 April 2011 M_w 6.6 Iwaki earthquake. Radar interferometry applied to data of the Japanese ALOS satellite reveals complex ruptures associated with the Iwaki earthquake. In particular, the interferogram clearly shows multiple surface ruptures along each of the subparallel Yunodake and Itozawa faults. The slip distributions on these faults, as estimated by inverting the interferogram, suggest that the faults dip 60°-70° westward and that slip on the Itozawa fault extended to a splay fault at depths shallower than about 5 km. Our results indicate that the highland area west of the Itozawa fault was downthrown by up to 2.4 m during the Iwaki earthquake.