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

为了了解俯冲带中大地震的特征，有必要研究反复发生大地震的凹凸体。由于研究凹凸体需要在凹凸体附近进行观测，因此海底地震仪(OBS)是观测俯冲区地震波必不可少的仪器。由于常规的OBS是为高灵敏度观测而设计的，在OBS附近发生的大地震的OBS记录往往是饱和的。对于记录大幅度地震波，伺服式加速度计是合适的。然而，由于耗电量大，OBS很难使用加速度计。近年来，一种大动态范围、低功耗的伺服加速度计正在研制中。此外，通过使用大尺寸的钛球，OBS的压力容器可以容纳更多的电池。为了对2004年苏门答腊岛-安达曼地震的余震进行海底长期观测，我们在常规的长期观测平台上安装了小型三分量加速度计，得到了海底高灵敏度地震记录和低灵敏度(强震)加速度记录。带有加速度计的OBS于2005年2月24日部署在2004年苏门答腊-安达曼地震震源区南部，并于8月3日恢复。从部署到4月13日CF内存充满时，获得了加速度计记录。虽然记录有一些小故障，但在S/N比良好的情况下，我们可以在海底获得低灵敏度的加速度记录。记录到了震中距约470公里的最大余震(2005/3/28 MW=8.5)，该余震的记录尚未饱和。我们继续改进系统，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