[SCG63-P03] Occurrence of normal-type earthquakes and stress loading model under Boso Peninsula, Japan
Keywords:Boso Peninsula, Stress loading, Coulomb stress, Plate subduction, Philippine Sea Plate
In our model, the subduction of the Pacific and Philippine Sea plate is expressed by the steady slip (dislocation) on the plate interfaces. The collision of the Izu Peninsula is expressed by slip-rate deficit. The area of the slip-rate deficit is determined to satisfy the long-term deformation data obtained by geological and geomorphological methods. Then, slip-rate deficit turned out to be distributed on the northwest side of the Izu Peninsula.
Using this optimum model, we obtained all-around horizontal extensional stress rates under Boso Peninsula both above and below the Eurasian-Philippine Sea plate interface. In broader view, the stress rates are northwest-southeast compression in the collision zone around Izu Peninsula, strike-slip type in the Philippine Sea plate to the south, and horizontal extension in the Eurasian plate north of Boso Peninsula.
We apply our calculated stress rates to the nodal planes of the observed earthquakes to calculate the Coulomb failure function (ΔCFF). These calculated ΔCFFs are generally positive on normal-type earthquakes under Boso. This result indicates that modeled intra-plate stress pattern is consistent with earthquake occurrence as the stress release process. The ΔCFFs are also consistent with earthquakes in adjacent seismically active areas: the Izu collision zone, the interior of the Philippine Sea plate, and in seismic swarms in the Eurasian plate northeast of Boso Peninsula, which further supports our stress loading model.
To reveal the individual contributions of the Philippine Sea and Pacific subduction and the Izu collision, we decomposed the calculated stress rates. The collision effect is negligible on Boso Peninsula. The subduction of the Pacific plate creates east-west extension, while that of the Philippine sea plate creates north-south extension. Both subduction together creates the characteristic all-around extension under Boso Peninsula.