by M.Kikuchi & Y.Yamanaka
(EIC: Earthquake Information Center)

A M6.2 earthquake occurred off the southwest coast of the Miyake-jima on the night of July 30. JMA intensity scale 6-lower was recorded in the Miyake-jima and 5-lower in the Kozu-shima. JMA quick information of hypocenter is as follows.

time/date	epicenter		depth	M
07/30 21:25(JT)	34.0 N	139.4 E	10km	6.2=>6.4

Broadband body wave (vertical P wave) at eight IRIS-DMC stations were collected. Azimuthal coverage is good in general. Waveform shows the feature of multiple shock (two successive shocks).

Results are shown in Figure 1. Mechanism solution shows almost pure strike-slip-fault. Figure left below shows distribution of subevents on the north-south fault plane. The right direction points the south, and the left points the north. It seems likely that rupture propagated from south to north. Source parameters are as follows.

(Strike, Dip, Rake)	(198, 83, 0 ) / (288, 90, -173)
Seismo moment: Mo	6.3 x 10**18Nm (Nm = 6.5)
Rupture time	T = 6 s x 2
Centroid depth	H = 10 - 15 km
Fault area (supposed; unilateral rupture, W = L/2)	15 x 7.5 km**2
Dislocation	Mo /mS = 1.7 m (m = 33 GPa)
Stress drop Ds	2.5 Mo/ S**1.5 = 13 MPa

Hypocenter is different from those of a series of M6 earthquakes since July 1. It is similar to the location of M6 earthquake which occurred during eruption of Miyake-jima volcano in 1983. Mechanism is strike-slip-fault with ENE-WSW tension. Waveform match is better for N-S fault-strike than E-W fault-strike, but the difference is not significant. Moment magnitude (Mw) of this event is the largest of the series of earthquakes. Stress drop is rather large and waveform shows the feature of multiple shock. Close attention to the influence on magma near the Kozushima Island should be kept.