Spatio-temporal distribution of magma intrusions inducing repeated earthquake swarms off the east coast of the Izu peninsula, central Japan, for 1973–1998

Abstract

A time-dependent model for magma intrusion associated with repeated earthquake swarm activities off the east coast of the Izu peninsula was developed from precise levelling, electronic distance measurements (EDM) and global positioning system (GPS), and sea level observation data for the period 1973–1998. Significant deformations were observed in the inter-swarm period, suggesting the existence of a preparatory process for repeated earthquake swarm events. The geometry of a dyke-like tensile crack at a depth of 13 km from deformation detected was estimated by precise levelling and GPS for the inter-swarm period, using a genetic algorithm. A large crack plane was considered; it had a combination of estimated deep tensile crack with a shallow dyke detected using the hypocentre distribution and calculated time-dependent inversion, which was carried out to estimate the temporal changes in the volumes of the crack in the period 1973–1998. Akaike's Bayesian information criterion (ABIC) was used to choose the smoothing hyperparameters for the inversion. The characteristics of the estimated temporal change in magma volume were as follows. A rapid and large magma intrusion occurred at a depth of about 15 km between 1976 and 1977. Despite a decrease in its expansion rate, a tensile crack at a depth greater than 8 km continued to inflate between 1978 and 1988. The swarm activity started just one year after the rapid and large intrusion around the 15 km depth. In addition, the tensile crack at the depth of about 15 km beneath the internal region of the eastern Izu peninsula expanded in the inter-swarm period and deflated in the swarm period. This result suggests that magma was temporarily reserved in a dyke-like body beneath the eastern part of the Izu peninsula at a depth of 15 km in a preparatory process for the earthquake swarm. These features strongly show that estimating the spatio-temporal distribution of this deep crack provides useful information for long-range predictions of volcanic earthquake swarms off the east coast of the Izu peninsula.