Aftershock seismicity and fault structure of the 2005 West Off Fukuoka Prefecture Earthquake (MJMA7.0) derived from urgent joint observations

Abstract

On March 20, 2005, a large M JMA 7.0 earthquake occurred in the offshore area, west of Fukuoka prefecture, northern Kyushu, Japan. A series of joint observations were carried out by teams from several universities in Japan with the aim of investigating the aftershock activity. Six online telemetered and 17 offline recording seismic stations were installed on land around the aftershock area immediately followed the occurrence of the mainshock. Because aftershocks were located mainly in offshore regions, we also installed 11 ocean bottom seismometers (OBSs) just above the aftershock region and its vicinity in order to obtain accurate locations of hypocenters. The OBS observation was carried out from March 27 to April 13, 2005. We further conducted temporary GPS observations in which ten GPS receivers were deployed around the aftershock region. The aftershocks were mainly aligned along an approximately 25-km-long NW-SE trend, and the hypocenters of the main aftershock region were distributed on a nearly vertical plane at depths of 2-16 km. The mainshock was located near the central part of the main aftershock region at a depth of approximately 10 km. The largest aftershock of M JMA 5.8 occurred near the southeastern edge of the main aftershock region, and the aftershock region subsequently extended about 5 km in the SE direction as defined by secondary aftershock activity. Enlargement of the aftershock region did not occur after the peak in aftershock activity, and the aftershock activity gradually declined. The distribution of hypocenters and seismogenic stress as defined by aftershocks suggest that the 2005 West Off Fukuoka Prefecture Earthquake occurred on the fault that is the NW extension of the Kego fault, which extends NW-SE through the Fukuoka metropolitan area, and that the largest aftershock occurred at the northwestern tip of the Kego fault.

Spatial heterogeneities in tectonic stress in Kyushu, Japan and their relation to a major shear zone

“…1, the main areas with high seismicity of microearthquakes are the B-U area and the northern offshore area of the island. The high activity observed in the northern part corresponds to the aftershock area of the M7.0 2005 Fukuoka earthquake (Shimizu et al 2006). The seismic activity in the central part of Kyushu Island remains stable and high, thus requiring several intrinsic conditions of stress and/or strength of the medium to explain the occurrence of earthquakes.…”

Spatial distribution of static stress drops for aftershocks of the 2005 West Off Fukuoka Prefecture earthquake

“…The 2005 West Off Fukuoka Prefecture earthquake (M j = 7.0) occurred on March 20, 2005 in the GenkaiNada region of the Japan Sea, with aftershocks distributed over about 30 km in the NW-SE direction (Shimizu et al, 2006). The main shock initiated at 33.739 • N, 130.176…”

“…from six temporary online telemetry stations installed in and around the aftershock region (Shimizu et al, 2006), in order to assess the possibility of a second large earthquake occurring on the SE extension. Our ultimate goal is to estimate absolute stress around the earthquake fault.…”

“…The stations were installed immediately above or around the aftershock region. The waveform data, digitized at a rate of 100 Hz or 200 Hz, were sent through satellite telemetry systems or telephone line networks (Shimizu et al, 2006).…”

Imaging S-wave scatterer distribution in southeast part of the focal area of the 2005 West Off Fukuoka Prefecture Earthquake (MJMA 7.0) by dense seismic array

“…Here, we adopt a depth-dependent velocity model expressed by v(z) = v 0 + kz, where z is depth in kilometers, and v 0 and k are 3.06 km/s and 0.033 1/s, respectively. This velocity structure is obtained by smoothing the model adopted by Shimizu et al (2006) to determine the hypocenters in this region. We also assume that multiple-scattered waves can be ignored.…”