The rupture process of the 1989 offshore ito earthquakes preceding a submarine volcanic eruption

Abstract

An earthquake swarm at the eastern offshore region of Ito, the Izu peninsula, started on June 30, 1989, and became more active on and after July 4. The largest earthquake among the swarm (MJMA= 5.5) occurred at 1109 JST (0209 UT) on July 9, 1989, and a second significant event followed 45 s later. A remarkable volcanic tremor started on July 11, and a submarine eruption occurred just above the source region of the largest earthquake on July 13. In this paper we present a new inversion method to derive the source time function of rupture process. The source time function is divided into several sections following an automatic algorithm and is interpolated with a cubic spline function. The parameters, which are the values and first derivatives at knots of the spline function, are determined by minimizing a squared error between the observed and synthetic seismograms. To demonstrate the performance of this method, some numerical experiments (simulations) are carried out on near‐field seismic records. Applying this method to analysis of the two large earthquakes, the detailed rupture processes are obtained. A heterogeneous crustal structure near the volcanic area affects the rupture processes of these earthquakes. The large stress drop due to the largest earthquake just beneath the submarine eruption point would assist a magma ascending and may have triggered the eruption 4 days after these earthquakes.