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

•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.

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.