HR: 10:20h
AN: G22A-01 INVITED    [Abstracts]
TI: Discovery of slow earthquakes by dense high-sensitivity broadband borehole seismic observation network in Japan, NIED Hi-net
AU: * Obara, K
EM: obara@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, Ibaraki, 305-0006, Japan
AB: National Research Institute for Earth Science and Disaster Prevention (NIED) has constructed the nation-wide high-sensitivity seismograph network (Hi-net) with 700 borehole stations, which uniformly covers the Japanese Islands with a spacing of 20–30 km. The borehole sensor is composed of three-component high-sensitivity velocity seismometer, three-component strong motion accelerometer and horizontal component high-sensitivity accelerometer which is available to measure the ground tilt and long-period seismic waves. The sensor vessel is placed at the bottom of the borehole deeper than 100m. There are four advantages of Hi-net; high sensitivity, high signal-to-noise ratio, broadband property of sensors and high density of stations. As a result, detection capability for micro earthquakes has been dramatically improved and some new geophysical phenomena have been discovered. Most remarkable discovery by Hi-net is wide variety of slow earthquakes; non-volcanic deep low- frequency tremors (LFT) [Obara, 2002], short-term slow slip events (SSE) [Obara et al., 2004], and very low- frequency (VLF) earthquakes [Ito et al., 2007]. These slow earthquakes occur simultaneously with a certain recurrence interval at the transition zone on the deeper plate interface along the strike of the subducting Philippine Sea plate, southwest Japan. The tremor is characterized by randomly wave trains lasting for hours to weeks with a predominant frequency of around 2 Hz. The tremor activity is clustered spatially and temporally within the narrow belt-like zone. Peak of tremor activity recurs with a time interval of six months accompanying to the short-term SSE lasting for a several days. VLF earthquake has a predominant period of 20s and occurs coincident with peak of tremors. During the active stage, the source of LFT migrates with a propagation velocity of around 10km/day along the strike of the plate geometry at the downdip side of the locked seismogenic zone. The space-time property of SSE and VLF earthquakes is also coincident with the migration of tremor. On the other hand, another kind of VLF earthquake with a predominant period of 10s has been detected at the updip side of the seismogenic zone [Obara and Ito, 2005]. This shallow VLF earthquake mainly occurs inside the accretionary prism on the landward side of the Nankai trough. We have succeeded to detect some slow earthquakes at updip and downdip sides of the seismogenic zone on the subducting plate interface. These slow earthquakes reflect the stress relaxation process around the plate boundary. Therefore, it is very important to watch these slow earthquakes with high accuracy by using the Hi-net in order to monitor the status of stress accumulation at the seismogenic zone on the plate interface.
DE: 1209 Tectonic deformation (6924)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7294 Seismic instruments and networks (0935, 3025)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Geodesy [G]
MN: 2007 Fall Meeting