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

为了了解俯冲带发生的大地震的特征，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。虽然有可能只将一个小的三分量加速度计安装到传统的长期OBS上，但扩大动态记录范围以进行各种分析是很重要的。在本研究中，我们研制了一个同时具有加速度计和普通地震仪的海底地震仪，并获得了海底高灵敏度地震记录和低灵敏度（强运动）加速度记录。2006年9月，我们在伊巴拉吉岛部署了5台地震仪，其中包括加速度计和普通地震仪，并于2007年10月成功回收。从5个地震仪中，我们获得了许多从大地震到小地震的记录，但没有记录饱和。由此可见，建立了一个地震仪包内多台地震仪的基本技术。

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.