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

Abstract

The earthquake with a bodywave magnitude mb=5.5 , which occurred near Tori Shima, Japan, on June 13, 1984 (origin time: 0229:25.3 UT, 31.448øN, 140.036øE, depth of 10 km, mb=5.5, MS=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 ( Except for the anomalous tsunamis, no obviously unusual observations have been reported. In view of the importance of this event in relation to magmatic processes and tsunami generation, we made a detailed analysis of global seismic data observed for this earthquake. We will show that under the assumption of no net volume change, this earthquake can be best represented by a compensated linear vector dipole (CLVD) rather than the conventional double force couple.lshimoto [1932]

New Methods and Future Trends in Seismological Volcano Monitoring

“…A similar CLVD mechanism was also proposed by Julian and Sipkin (1985) to explain nondouble couple mechanisms observed for earthquakes in the Long Valley Caldera area, California. It is clear from these examples that various combinations of feeder and receiver geometries can produce the same type of CL VD mechanism, although the magnitude of the tensor components will differ (see Kanamori et al 1993, for a more complete discussion).…”

“…Assuming A = Il, a crack has eigenvalues in the ratios (3,1,1), (1,3,1), or (1,1,3), a pipe is determined by the ratios (1,2,2), (2,1,2), or (2,2,1), and a sphere has the ratios (1,1,1). For composite sources, such as a CL VD or the hybrid sources discussed above, the interpretation of source geometry may be nonunique (Kanamori et al 1993;Shimizu et al 1987). In those cases, additional information is required to develop a complete image of the source geometry and dynamics.…”

“…(16) for (J = 0 as (22) so that the sum of the two tensors yields ~1 D ' (23) which again represents a CLVD. This particular CLVD mechanism was investigated by Kanamori et al (1993) in their analysis of the source mechanism of the Tori Shima earthquake, Japan. A similar CLVD mechanism was also proposed by Julian and Sipkin (1985) to explain nondouble couple mechanisms observed for earthquakes in the Long Valley Caldera area, California.…”

Seismically and geodetically determined nondouble‐couple source mechanisms from the 2000 Miyakejima volcanic earthquake swarm

“…However, this flow may not occur very rapidly in viscous magma and, on the timescale of seismic radiation, a tensile crack model with net volume change may be more appropriate. Also, it is possible that injection may involve net volume change as is postulated by Kanamori et al [1993] for the 1984 Torishima, Japan, earthquake.…”

“…[9] Kanamori et al [1993] argue that the duration of the source time function is too short for magma to propagate across the length of the source region. Thus the earthquake must have resulted from the interaction of magma with water rather than the injection of magma alone.…”