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

On August 18,10:52, a M6.0 earthquake occurred near the Kozu-shima where seismicity had been active. JMA intensify scale 6-lower was recorded at the Shikine-jima and 5-upper at the Nii-jima. At 12:49 a M4.9 earthquake occurred near the Shikine-jima. JMA intensify scale 6-lower was recorded at the Shikine-jima Island and 4 at the Nii-jima. (Figure) JMA quick information of hypocenter is as follows.

time/date	epicenter		depth	M
00/08/18 10:52(JT)	34.2N	139.3E	10 km	6.0
00/08/18 12:49(JT)	34.3N	139.2E	10 km	4.9

Regarding the first earthquake, teleseismic P waves at four stations were collected from IRSI-DMC. Regarding the second one, broadband seismographs data of Tateyama(TYM), Nakaizu(JIZ), and Tsuru(SGN) were collected from FREESIA seismic stations of NIED. The data were bandpass filtered (0.01-1Hz) and transformed into displacement.

Results are shown in Figure 1 and Figure 2. The earthquake near the Kozu-shima is normal fault with a slight strike-slip component (NE-SW tension). The earthquake near the Shikine-jima is strike-slip with a slight normal fault component (E-W tension). Main source parameters are as follows:

	M6.0 earthquake near Kozu-shima	M4.9 earthquake near the Shikine-jima
Origin time	10:52:22 (JT)	12:49:14 (JT)
(Strike, Dip, Rake)	(150, 59, -48) / (270 , 51, -138)	(325, 68, -164) / (229, 76, -22)
Seismic moment Mo	5.8 x 10**17 Nm (Mw = 5.8)	2.0 x 10**16 Nm (Mw = 4.8)
Rupture time	4 s	5 s
Centroid depth	5 km	6 km
Fault area	10 x 5 km**2	4 x 2 km**2
Dislocation	D = Mo/mS = 0.39 (m = 30GPa)	0.08 m
Stress drop Ds	4.1 MPa	2.2 MPa

The hypocenter of the first earthquake is off east coast of the Kozu-shima. It is close to the hypocenters of a series of M6 earthquakes in July. The hypocenter is shallower than those previous earthquakes. The mechanism is normal fault with a slight strike-slip component, and it is similar to that of the M5.6 earthquake on July 24. It is noticed that seisimicity has been less active since August 16 around the present hypocenter. Anyhow, magma must have continued to push up. Monitoring crustal activity with GPS data and seismic data is still indispensable from now on. Determination of focal depth is rather reliable because reflection phases such as pP and sP are used in the teleseismic body wave inversion. The earthquake near the Shikine-jima Island was not very large, but JMA intensity scale was rather large because the source was very close to the Shikine-jima. The tension axis of this earthquake is different from those of a series of earthquakes. Regional stress field must have been locally disturbed by magma intrusion.