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

为了了解发生在俯冲带的大地震的特征，有必要研究反复发生大地震的陡岩。海底地震仪（OBS）是观测潜没区地震地震波的必要设备，因为潜没区地震地震波的观测需要在潜没区附近进行。由于传统的海底地震仪是为高灵敏度观测而设计的，在海底地震仪附近发生的大地震的海底地震仪记录往往是饱和的。为了记录大振幅地震波，伺服式加速度计是合适的。然而，由于电力消耗大，OBS很难使用加速度计。近年来，人们正在研制一种动态范围大、功耗低的伺服式加速度计。此外，OBS的压力容器使用了大尺寸的钛球，可以容纳更多的电池。为了对2004年苏门答腊-安达曼地震的余震进行长期海底观测，我们在传统的长期海底地震仪上安装了一个小型三分量加速度计，获得了海底高灵敏度地震记录和低灵敏度（强震）加速度记录。装有加速度计的OBS于2005年2月24日在2004年苏门答腊-安达曼地震震源区的南部部署，并于8月3日恢复。从部署到CF内存满的4月13日为止，获得了加速度图。虽然在记录过程中存在一些小问题，但我们可以在海底获得低灵敏度的加速度信号，并且信噪比良好。最大的一次余震（2005/3/28 Mw=8.5），震源距离约470km，余震记录不饱和。我们将继续改进系统，包括重新选择传感器以获得更高质量的数据

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