Temporal slip change based on curved slickenlines on fault scarps along Itozawa fault caused by 2011 Iwaki earthquake, northeast Japan

Abstract

We observed co-seismic slickenlines caused by the 2011 Mw 6.6 Iwaki earthquake along the Itozawa fault co-seismic surface rupture zone. The structures are characterized by co-seismic curved slickenlines and slickenlines with cross-cutting relationships on the fault scarps. The curved slickenlines indicate that the direction of fault motion during the rupture of the 2011 Iwaki earthquake shifted from normal faulting with a left-lateral component to that with a right-lateral component. The angular misfits between the slip direction predicted from the NW–SE trending extensional stress before the 2011 Iwaki earthquake and that predicted from each component of the curved slickenlines on the fault scarps are ~ 33° to 65° and ~ 2° to 17°, respectively. The misfit changes show that the co-seismic slip direction, discordant with the extensional stress, shifted to normal faulting and is explained by the stress during the faulting. These results suggest that co-seismic rupture processes near the surface are a key in revealing the detailed dynamic processes of a seismic event. • 2011 Mw6.6 Iwaki earthquake was a triggered event after 2011 Mw9.0 Tohoku earthquake. • We observed curved slickenlines along the Itozawa fault co-seismic surface ruptures. • Slickenlines indicate a change in the direction of fault motion during rupture. • Fault motion shifted from that with a left- to a right-lateral component. • Fault motion shift is explained by the local stress perturbation during faulting.