Development and Evaluation of Compact Long-Term Broadband Ocean Bottom Seismometer

Abstract

Recently low frequency tremors and very low frequency earthquakes were discovered along plate boundaries near trenches. These events are estimated to relate to coupling of plates. For observation of the events, we have developed a compact long-term broad-band ocean bottom seismometer by replacement of the seismic sensor of the existing long-term ocean bottom seismometer to a broadband seismometer. We adopted a seismic sensor with a natural period of 20 seconds. Because a tilt of equipment on seafloor cannot be controlled due to free-fall deployment, a levelling system for a seismic sensor is necessary. We have developed a new levelling system for the adopted seismic sensor, which has the same size as the existing levelling system. Levelling can be performed up to 20 degrees by using micro-processor, and accuracy of levelling is less than one degree. The developed compact long-term broadband ocean bottom seismometer is useful for observation of slow earthquakes on seafloor. In addition, seafloor observations of teleseismic events and deep earthquakes to estimate seismic structure of deep regions and observations of submarine volcanoes are expected.