Earthquake Triggering due to Volcanic Deformation Sources in Areas East off Ito and around the Mt. Iwate Volcano

Abstract

A change of Coulomb Failure Stress (CFS) is commonly used to explain a mechanism of earthquake triggering due to both earthquake faulting and volcanic sources. First, we introduced crustal deformation and source model for five episodes of volcanic activities off Ito, central Japan and around the Mt. Iwate volcano, northern Japan based on dense geodetic measurements. And then, we calculated spatial distribution of a CFS change due to the estimated source models and compared the observed seismic activities.
In the case of the 1998 earthquake swarm off Ito, the region where opening of dike and the largest earthquake (M5.7) increase CFS was concordant with the hypocenter distribution observed after the largest earthquake. However, the largest earthquake occurred in the area where CFS decreased.
In the case of the Iwate volcano, a volcanic inflation source increased CFS by 0.33 MPa at the hypocenter of the M6.1 earthquake that occurred on September 3, 1998. The calculated CFS change is 7 % of the coseismic stress drop. The M6.1 earthquake occurred at the northern edge of the Nishine Fault found by geographical and geological studies. It suggests that the inflation promoted the rupture of a known Quaternary fault. Stress shadow caused by the M6.1 earthquake explains the observed quiescence of the seismic swarm activity west of E140.95° after the occurrence of the earthquake.