Spatiotemporal Changes in Fault Displacements Associated with Seismovolcanic Events in and Around Miyakejima and Kozushima in 2000 Inferred from GNSS Data

We describe crustal deformation associated with seismovolcanic events in the Izu Islands, central Japan that began at the end of June 2000; three faults were assumed and modeled for inversion analyses using GNSS data provided by the Geospatial Information Authority of Japan. After removing steps caused by antenna exchanges and coseismic steps associated with five large earthquakes, common mode errors, and a linear trend from the raw data, we divided the processed time series into five successive nearly equal 15-day time spans. As a first step, three fault models with homogeneous dislocations that best explained the observed GNSS data for the entire period were randomly searched for in an area where microearthquakes had occurred. As a result, the following three faults were identified: Fault 1, located to the northwest near Niijima; Fault 2, which extended from Miyakejima volcano to Kozushima northwestward; and Fault 3, located beneath Miyakejima volcano. As a second step, we divided Faults 2 and 3 into several subfaults with the same area to perform more detailed inversion analyses to estimate the spatiotemporal dislocation field on the three faults. The inversion results show that Fault 3 deflated by 0.57 km 3 , and the change in volume of Fault 2 was estimated to be 0.74 km 3 by opening. Fault 1 slipped significantly right-lateral by 18.9 m, and the change in volume was estimated as 0.40 km 3 by opening. Since the total increase in volume for Faults 1 and 2 was greater than 0.57 km 3 , which was the total deflation for Fault 3, a magma supply from greater depths to Faults 1 and 2 was required.