Development of free-fall pop-up type ocean bottom strong motion meter for study of asperites

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To understand characteristics of large earthquakes which occur 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 which 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 to a conventional long-term OBS and obtained both high-sensitivity seismogram and low-sensitivity (strong motion) accelerograms on the sea floor. 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, and recovered on August 3. 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=8.5) whose epicentral distance is about 470km was recorded and the records of this aftershock is not saturated. We continue to improve the system including re-selection of a sensor for better quality data