Depth-dependent mode of tremor migration beneath Kii Peninsula, Nankai subduction zone

We investigated the migration mode of deep non‐volcanic tremor activity beneath Kii Peninsula, southwest Japan. Major tremor episodes are characterized by long‐term migration with a velocity of about 10 km/day, propagating along the strike of the subducting plate. Similar tremor migration in Cascadia is accompanied by reverse propagation at speeds on the order of 100 km/day and much faster slip‐parallel migration at speeds on the order of 1000 km/day. We systematically searched for migrating tremor with clear linearity in space and time. As a result, we found tremor migrations at speeds ranging from 1 to 60 km/hr depending on the along‐dip position in the tremor zone. The observed decrease in migration speed with increasing measurement time scale suggests that migration is controlled by a diffusion process. The along‐strike migration at lower speeds, including both forward and backward directions relative to the long‐term migration episode, is concentrated at the updip side of the tremor zone, whereas the faster slip‐parallel migration is distributed over the entire zone. The long‐term migration seems to consist of and be excited by the propagation of along‐strike creep at the updip part. The concentration of along‐strike migrating tremor sequences at the updip side may reflect the existence of abundant fluid that accumulates at the corner of the mantle wedge. The faster slip‐parallel migrations represent projections of along‐strike fluctuations in slip pulse propagation controlled by striations along the plate interface. Key Points Tremor migration is classified into two modes depending on the depth in the zone The updip‐most along‐strike tremor migration boosts the long‐term propagation The faster slip‐parallel migration and RTR reflect fluctuation in slip pulse