Development of ocean bottom strong-motion seismograph with multi-seismic sensors

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。虽然小型三分量加速度计可能只安装在传统的长期OBS上，但为各种分析扩大记录的动态范围是重要的。在这项研究中，我们开发了一种既有加速度计又有普通地震仪的OBS，在海底既获得了高灵敏度的地震记录，又获得了低灵敏度(强震)的加速度记录。2006年9月，我们在伊巴拉吉海域部署了5500个加速计和普通地震仪，并于2007年10月成功回收。从5个OBSS中，我们获得了从大地震到小震的许多记录，这些记录没有饱和。从而奠定了一套OBS多台地震仪的基础技术。

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. Although it becomes possible that a small three-component accelerometer was only installed to a conventional long-term OBS, it is important to enlarge a dynamic range of records for various analyses. In this study, we developed an OBS with both accelerometer and ordinary seismometer, and obtained both high-sensitivity seismogram and low-sensitivity (strong motion) accelerograms on the sea floor. In September 2006, we deployed 5OBSs with both accelerometer and ordinary seismometer off lbaragi, and successfully recovered in October, 2007. From five OBSs, we obtained many records from large earthquakes to small earthquake without saturation of the records. It can be concluded that fundamental technology of multi-seismometers in one OBS package is established.