2009 Aug 09 South Off Tokaido, Japan, Earthquake

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August 12, 2009
Strong Motion Seismology Group
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An earthquake of Mw 7.1 (Global CMT Project) occurred far south of the Tokaido region, off the south coast of Honshu, Japan at 10:55:52.13 UTC (19:55:52.13 local time) on August 9, 2009. According to the hypocentral location reported by JMA and the USGS preliminary report, the earthquake is located within the Izu-Bonin subduction system extending from southern Honshu to the Mariana Island and constituting the part of the subduction boundary between the Pacific and the Philippine Sea plates. The subduction of the Pacific plate beneath the Philippine Sea plate along the Izu-Bonin trench generates a large amount of seismicity extending to the depth of 500 km. The August 9 event is classified as a deep-focus earthquake due to it focal depth of 303 km (USGS). It occurred as the result of the thrusting mechanism within the Pacific plate. The reviewed location of the hypocenter determined by JMA is lat = 33.128 N, lon = 138.404 E, and depth = 332.9 km.

To investigate the source process of this earthquake, we have performed the teleseismic waveform inversion of Kikuchi and Kanamori (1991) using displacement records observed at FDSN stations from IRIS DMC. The 52 vertical components of the body-wave data at epicentral distances of 35-100 deg. were used in the inversion (Fig. 1). We fixed the epicenter determined by JMA and search for the focal depth to minimize the waveform fit. We inverted 135 seconds of the records to include both the P-waves and the depth phases (pP and sP). The use of the later ones gives a better constrain on the focal depth estimation. The residuals of the finite fault inversion prefer a low-angle southwest-dipping fault plane (strike = 105 deg., dip = 25 deg.). The focal depth is estimated to be 300 km. In the obtained slip distribution over the fault plane, we can find a single main asperity. The peak slip of around 1.4 m appears on the upper part of the fault plane. The total rupture duration is 17 s. The results of the inversion are summarized in Fig. 3.

Fig. 1. Stations used for the teleseismic waveform inversion.

Fig. 2. Location of the mainshock, aftershocks, and historical Global GMT moment tensor solutions.

The location of the August 09, 2009 Tokaido earthquake (red star) and the aftershocks (orange circles) as reported by JMA. Moment tensors correspond to the focal mechanisms of the deep events (>300 km) with Mw > 6.5 from the Global CMT catalog during the period of 1976-2009. Gray lines represent the plate boundaries of Pacific (PA), Philippine Sea (PS), Okhotsk (OK), and Amur (AM) plates, after Bird (2003).

Fig. 3. Source time function, point source solution, slip distribution, and comparison of the obsereved and synthetic displacement waveforms.

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