Modeling the updip migration of the 2018 Boso slow slip event with geodesy and seismicity observations

Abstract:

While most subduction slow slip events are deep-seated, at the down-dip edge of seismogenic zones, some are also observed at shallower depths. Among them are the well-studied Boso slow slip events located on the Sagami trough, between 10 and 20 km depth. Occurring every ~5 years since 1996, they are always accompanied by swarms of Mw 1 to 5 earthquakes on their northern and western flanks. Being located right beneath the Boso peninsula coastline, the dynamics of these slow slip events is particularly well imaged by dense GPS and tiltmeter networks. During the two-weeks-long June 2018 event, both geodetic slip inversions and double difference relocated seismicity indicate updip migrations, at a speed of a few km/day. In this study, we model the migrating mixed seismic-aseismic transient event as a single process that reconciles both geodetic and seismicity observations. The interaction between slip and seismicity is modelled by Coulomb stress changes due to transient slow slip that explains the nucleation of seismic events in the surrounding volume of the transient slip. The nucleation of the earthquakes is governed by rate and state friction. We compare a wide range of models including time-dependent slip models based on geodetic observations only and slip models based on the joint seismicity-geodesy analysis, exploring the effect of all parameters of the physical model (in particular, the constitutive parameter that relates changes in stress to logarithmic changes of slip velocity, the effective normal stress and the tectonic stressing rates) on the results. We finally investigate possible stress relationships between this slow slip event and a Mw 6.0 intraplate strike-slip earthquake that happened down below, within the Pacific plate, less than a month later.