A Trial for Improvement in Reproducibility of Spatial Distribution of Afterslip in Geodetic Data Inversion

Abstract:

A Trial for Improvement in Reproducibility of Spatial Distribution of Afterslip in Geodetic Data InversionAfterslips of plate boundary earthquakes in subduction zones sometimes show doughnut-shaped slip distribution. For example, little afterslip occurred in the coseismic slip area of the 2003 Tokachi-oki earthquake of magnitude (M) 8.0 while large slip occurred in its surrounding area (ex. Miyazaki et al., 2004). However, it is difficult to resolve such heterogeneous distribution for M7 class earthquakes especially in offshore region. We demonstrated the reproducibility of spatial distribution of afterslip following a M7 class earthquake through numerical experiments to estimate slip distribution on the plate interface beneath the Hyuga-nada offshore region, southwest Japan (Nakata et al., 2013). We calculated synthetic displacement data from the result of numerical simulation conducted for the afterslip following a M 6.8 earthquake, for existing global navigation satellite system stations on land and planned ocean floor pressure gauge network stations. The spatial distribution of fault slip is estimated using a Kalman filter-based inversion. The slip distribution estimated by using ocean floor stations demonstrates that the heterogeneity of plate coupling, which roughly corresponds to the coseismic area of the M 6.8 earthquake with a radius of 10 km. The estimated slip amount in the coseismic area is nearly half of the peak one around it, although no slip is the true answer. This discrepancy is caused by the smoothness constraint in the inversion. To improve the reproducibility of the slip distribution, it is necessary to introduce different type of constraints. Based on a Bayesian approach, we introduce an evaluation function that can treat both discontinuity and smoothness in slip distribution.