ALBERT

Description: We developed an inversion method to estimate unbiased interseismic slip-deficit rates at plate interfaces from observed GPS velocity data with an elastic dislocation model. In this method, first, we subtract theoretical surface velocities due to known steady relative plate motion from the observed GPS data, and presume the residuals to be caused by slip deficit at plate interfaces. However, the observed GPS data always include rigid block translation and rotation, which cannot be explained by the elastic dislocation model. We treated the rigid block translation and rotation as systematic errors in the analysis, and removed them by transforming the velocity data into the average strain rates of triangle elements composed of adjacent GPS stations. By this transformation, original information about intrinsic deformation is preserved. Applying a unified Bayesian inversion formula to the GPS strain data, we can obtain unbiased slip-deficit rate distribution. We demonstrated the applicability of the method of GPS strain data inversion through the analysis of interseismic GPS velocity data (1996–2000) in the Kanto region, central Japan, where the North American (NAM), Pacific (PAC) and Philippine Sea (PHS) plates are interacting with each other in a complicated way. From this analysis we found a broad and high slip-deficit rate zone on the NAM-PHS plate interface, extending from southeast off the Boso peninsula to the Tokai region through the Izu-Mainland collision zone. Two high slip-deficit rate zones along the Sagami and Suruga troughs correspond to the source regions of the 1923 Kanto earthquake ( M 7.9) and a potential Tokai earthquake. On the PHS-PAC plate interface, though the estimation errors are large, we found a moderate slip-deficit rate zone far southeast off the Boso peninsula, where an M 7.4 earthquake has occurred in 1953.