Earthquake swarms and dike intrusions off the east coast of Izu Peninsula, central Japan

Abstract

Relationships between earthquake swarms and dike intrusions are investigated in the monogenetic volcano region off the east coast of the Izu Peninsula, central Japan. Hypocentral distribution of an earthquake swarm associated with the 1989 Ito-oki submarine eruption and those of eight major earthquake swarms for 7 years before the eruption in the same region are precisely relocated. Comparison of the hypocentral distribution with a dike model obtained from crustal movement measurements by Okada and Yamamoto (1991) ensures that hypocentral areas of the earthquake swarms can be used as reliable indicators of intruded dikes. The epicentral area of each earthquake swarm shows an elliptic shape with a 3–5-km-long axis and a 2–4-km-short axis, the long axis trending from a NW-SE to WNW-ESE direction. The total hypocentral distribution of the nine swarms forms a seismic zone of about 20 km long and about 5 km wide, with each of the swarm areas overlapping. Focal migrations from deep (∼ 10 km) to shallow (∼ 3 km) depths are found in the two largest events in 1988 and 1989, indicating that dikes propagated from deep to shallow areas in the upper crust. These features of the hypocentral distributions suggest that the seismic swarms were induced by dike intrusions which ascended from a deep-seated magma reservoir with dike-like geometry located beneath the seismic zone. Solidification time of a dike with thickness comparable to that of the dike model of Okada and Yamamoto (1.45 m at the bottom of the dike) is several days, indicating that the dikes inducing the earthquake swarms solidified almost during the swarm period. The physical mechanisms of the dike-induced earthquake swarms are investigated for the Izu region. The focal depth migration and fault type of the earthquake swarm suggest that stress change in the region surrounding the dike due to dike emplacement is the most important factor for the cause of the earthquakes.