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

An earthquake with hypocenter located in the sea near the Kozu-shima occurred around 4 a.m. on July 9. JMA intensity scale 6-lower in the Kozu-shima, and 4 in the Nii-jima. The earthquake was also felt in broad areas of southern Kanto and Tokai regions. Some washouts and damages of roads have been reported so far. JMA quick information of hypocenter is as follows.

time/date	epicenter		depth	M
07/09/03:57 (JT)	34.1N	139.3E	10 km	6.0

We processed far-field broadband body wave (P , S) seismograms collected at five IRIS-DMC stations. Though the spatical coverage of seismic stations is poor, mechanism solution is reliable.

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

(Strike, Dip, Rake angle)		( 94, 71, -163) / (358, 74, -20)
Seismic moment		6.1 x 110**17Nm (Mw = 5.8)
Rupture time		T = 3.0 s
Main rupture depth		H = 6 km
Fault area(supposing bilateral rupture propagation)
	L	2.0(s) x 2.5(km/s) x 2 = 10 km
	W	L /2
	S	10 x 5 km**2
Dislocation		D = Mo /mS = 0.41m ( m = 30GPa)
Stress drop (Ds)		2.5 Mo / S**1.5 = 4.3 MPa

Mechanism solution, which is almost the same with that of the earthquake on July 1, is strike-slip-fault with a NE-SW tension with a slight normal-fault component. That is either right-lateral-slip on a plane with E-W strike or left-lateral-slip on a plane with S-N strike. Though a precise epicenter has not been determined yet it is unlikely that the same fault with the previous earthquake slipped. It is more probable to suppose the epicenter is in the neighboring area at a shallower depth. The scale of this earthquake is estimated to be one-third as much as the previous one in terms of energy, smaller by 0.3 in terms of magnitude.