HR: 0800h
AN: U11A-0813    [Abstracts]
TI: Low-sensitivity acceleration seismic records obtained on sea floor in the source region of the 2004 Sumatra-Andaman Earthquake
AU: * Shinohara, M
EM: mshino@eri.u-tokyo.ac.jp
AF: ERI, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Yamada, T
EM: yamada@eri.u-tokyo.ac.jp
AF: ERI, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Kanazawa, T
EM: kanazawa@eri.u-tokyo.ac.jp
AF: ERI, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: To understand characteristics of large earthquakes occurred in a subduction zone, it is necessary to study an asperity where large earthquakes occur repeatedly. Because observation near an asperity is needed for studies of asperities, ocean bottom seismometer (OBS) is essential to observe seismic waves from earthquakes in subduction area. Since a conventional OBS is designed for high-sensitivity observation, OBS records of large earthquake occurred near OBS are often saturated. To record large amplitude seismic waves, a servo-type accelerometer is suitable. However it was difficult for OBS to use an accelerometer due to large electric power consumption. Recently a servo-type accelerometer with a large dynamic range and low-power consumption is being developed. In addition, a pressure vessel of OBS can contain much more batteries by using a large size titanium sphere. For the long-term sea floor observation of aftershock of the 2004 Sumatra-Andaman earthquake, we installed a small three-component accelerometer in a conventional long-term OBS and obtained both high-sensitivity seismogram and low-sensitivity (strong motion) accelerograms on the sea floor. We used a compact three-component servo-type accelerometer whose weight is 85 grams as a seismic sensor. Measurement range and resolution of the sensor are 3 G and 10-5 G. The sensor was directly attached to the inside of the pressure vessel. Signals from the accelerometer were digitally recorded to Compact Flash memory with 16 bit resolution and a sampling frequency of 100 Hz. The OBS with the accelerometer was deployed on February 24, 2005 in a southern part of the source region of the 2004 Sumatra-Andaman earthquake by R/V Natsushima belonging to JAMSTEC, and recovered on August 3 by R/V Baruna Jaya I belonging to BPPT, Indonesia. The accelerograms were obtained from the deployment to April 13 when the CF memory became full. Although there are some small troubles for the recording, we could obtain low-sensitivity accelerograms on sea floor with a good S/N ratio. The largest aftershock (2005/3/28 Mw=8.7) whose epicentral distance is about 470 km was recorded and the records of this aftershock are not saturated. We continue to improve the system including re-selection of a sensor for better quality data.
DE: 3050 Ocean observatories and experiments
DE: 3094 Instruments and techniques
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7294 Seismic instruments and networks (0935, 3025)
SC: Union [U]
MN: Fall Meeting 2005