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

An earthquake with hypocenter located in the sea near the Kozu-shima occurred around 4 p.m. on July 1. JMA intensity scale 6-lower in the Kozu-shima, and 5-lower in the Nii-jima. The earthquake was also felt in broad areas of southern Kanto and Tokai regions. It has been reported that one was dead in washout. JMA quick information of hypocenter are as follows.

time/date	epicenter		depth	M
16:01(JT)	34.2N	139.2E	10 km	6.4

We processed far-field broadband P wave seismograms collected at nine IRIS-DMC stations.

Results are shown in Figure 1. Source parameters are as follows.

(Strike, Dip, Rake)		(352, 76, -15) / (86, 76, -166)
Seismic moment Mo		2.0 x 10**18 Nm (Mw = 6.1)
Rupture time		T = 4.5 s
Main rupture depth		H = 7 km
Fault area(supposing bilateral rupture propagation)
	L	3(s)x 2.5(km/s) x 2 = 15 km
	W	L /2
	S	15 x 7. 5 km**2
Dislocation		D = Mo/mS = 0.44m (m = 40GPa)
Stress drop Ds		2.5 Mo / S**1.5 = 4.2 MPa

Mechanism solution is strike-slip-fault with a NE-SW tension. The mechanism is generally consistent with the regional stress field. Aftershocks are distributed from east to west, and that indicates right-lateral-slip on a fault of L=15km with E-W strike. (Figure 2) We believe that was induced by stress due to magma intrusion around the Miyake-jima. The magnitude of this earthquake belongs to the largest scale level around this area.