Seismic radiation by magma injection: An anomalous seismic event near Tori Shima, Japan

The earthquake with a bodywave magnitude m b =5.5, which occurred near Tori Shima, Japan, on June 13, 1984 (origin time: 0229:25.3 UT, 31.448°N, 140.036°E, depth of 10 km, m b =5.5, M S =5.5) is anomalous because it generated tsunamis which are disproportionately large for the magnitude of the earthquake. At Hachijo Island, 150 km from the epicenter, tsunamis were visually observed with peak‐to‐peak amplitude of 130 to 150 cm. Long‐period seismic radiation is also anomalous. Love waves are almost absent, and Rayleigh waves are radiated with equal amplitude and phase in all directions. A simple double‐couple model cannot explain these observations. With the assumption of no net volume change at the source, these data can be best explained with a compensated linear vector dipole (CLVD) with the principal tensional dipole in the vertical direction. The scalar moment of this dipole is 4×10 24 dyn cm. Moment tensor inversions of long‐period body waves and surface waves yield an almost identical solution. This CLVD source can be interpreted as horizontal fluid injection. The location of the event is in the Smith depression which is one of the nascent back arc basins just behind the Bonin arc. These basins are filled with thick sediments, and numerous young volcanoes are found near this site. Magmatic injection is most likely to occur in this tectonic environment. However, the time scale of the seismic event seems too short for magma injection to occur. A more likely mechanism involves water‐magma interaction. The injection may be viewed as hydrofracturing driven by supercritical water heated by injected magma. The estimated volume of injected water is about 0.018 km 3 and that of basaltic magma is about 10% of this. This type of deformation is more efficient for tsunami generation than faulting with the same scalar moment.