Migration process of very low - frequency events based on a chain - reaction model 1 and its application to the detection of preseismic slip for megathrust earthquakes 2 3

15 In order to reproduce slow earthquakes with short duration such as very low 16 frequency events (VLFs) migrating along the trench direction as swarms, we apply a 17 3-D subduction plate boundary model based on the slowness law of rate-and 18 2 state-dependent friction, introducing close-set numerous small asperities 1 (rate-weakening regions) at a depth of 30 km under high pore pressure condition, in 2 addition to a large asperity. Our simulation indicates that swarms of slip events occur 3 repeatedly at the small asperities, and these events are similar to the observed slow 4 earthquake group, especially to VLF, on the basis of the relation between characteristic 5 duration and seismic moment. No slip events occur there without the small asperities, 6 which mean that the close-set numerous small asperities may be one of the necessary 7 conditions for generating the short-duration slow earthquakes such as VLFs. In the 8 preseismic stage of the megathrust earthquakes that occur at the large asperity, the 9 swarms of VLFs have higher migration speeds and higher moment release rate as well 10 as shorter recurrence interval. Thus, monitoring the migration of slow earthquakes may 11 be useful in imaging the preseismic slip of megathrust earthquakes.