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，有必要对多次发生大地震的凹凸体进行研究。海底地震仪（OBS）是研究海底凹凸体的重要手段，它是海底凹凸体附近地震波观测的基础。由于传统的OBS是为高灵敏度观测而设计的，所以在OBS附近发生的大地震的OBS记录往往是饱和的。为了记录大振幅的地震波，伺服型加速度计是合适的。然而，由于大的电力消耗，OBS难以使用加速度计。最近，一个伺服型加速度计具有大的动态范围和低功耗正在开发中。另外，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。虽然它成为可能，一个小的三分量加速度计只安装到一个传统的长期OBS，重要的是要扩大动态范围的记录进行各种分析。在这项研究中，我们开发了一个OBS与加速度计和普通地震计，并获得了高灵敏度的地震记录和低灵敏度（强震）加速度记录在海底。2006年9月，我们在lbaragi附近部署了50个带有加速度计和普通地震仪的obs，并于2007年10月成功回收。从五个观测站中，我们得到了从大震到小震的大量记录，而且记录没有饱和。由此可以得出结论，在一个OBS封装中的多个地震计的基本技术是建立的。

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.