Geodetic inversion for the fault model of the 1994 Shikotan earthquake

The fault parameters of the 1994 Shikotan earthquake were determined by inverting all available geodetic data using an iterative linearized least-squares method. A one-fault inversion shows that a shallow-dip fault-plane model does not reproduce the minor deformation observed on Shikotan island, while a steep-dip model can fit all the data well. Multi-fault inversion shows that a fault-plane divided into two subfaults lengthwise is the best fitting model based on Akaike's Information Criteria [AIC] for shallow-dip faulting mechanism. This model yields a very high stress drop, 40 MPa, on the deeper subfault. For a steep-dip model, a fault plane divided into two subfaults sideways gives the minimum AIC. All the computations for models consisting of two to eight subfaults show that the chi-square values of the shallow-dip models are about 10% higher than those of the steep-dip models. Considering the high stress drop, the larger chi-square value, and the requirements of a complex subfault system for the shallow-dip model, we reach the conclusion that the steep-dip model is more suitable to explain the geodetic observations.