Japan Geoscience Union Meeting 2018

Presentation information

[JJ] Oral

S (Solid Earth Sciences) » S-CG Complex & General

[S-CG67] Ocean area observation to detect crustal activity under the seafloor: Present and future

Thu. May 24, 2018 3:30 PM - 5:00 PM 302 (3F International Conference Hall, Makuhari Messe)

convener:Kazuro Hirahara(Department of Geophysics, Earth and Planetary Sciences, Graduate School of Sciences, Kyoto University), Ryota Hino(Graduate School of Science, Tohoku University), Takane Hori(独立行政法人海洋研究開発機構・地震津波海域観測研究開発センター), Chairperson:Hino Ryota(Graduate School of Science, Tohoku University)

3:45 PM - 4:00 PM

[SCG67-08] Compact long-term broadband ocean bottom seismometer for slow earthquakes observation

*Masanao Shinohara1, Tomoaki Yamada1, Hajime Shiobara1, Yusuke Yamashita2 (1.Earthquake Research Institute, The University of Tokyo, 2.Disaster Prevention Research Institute, Kyoto University)

Keywords:Compact broadband ocean bottom seismometer, Slow earthquakes, Levelling system for seismic sensor

It is important to understand coupling between plates in a subduction zone for studies of earthquake generation. Recently low frequency tremor and very low frequency earthquake (VLFE) were discovered in plate boundary near a trench. These events (slow earthquakes) in shallow plate boundary should be related to slow slip on a plate boundary. For observation of slow earthquakes, Broad Band Ocean Bottom Seismometer (BBOBS) is useful, however a number of BBOBSs are limited due to cost. On the other hand, a number of Long-term OBSs (LT-OBSs) with recording period of one year are available. However, the LT-OBS has seismometer with a natural period of 1 second. Therefore frequency band of observation is slightly narrow for slow earthquakes. Therefore we developed a compact long-term broad-band OBS by replacement of the seismic sensor of the LT-OBSs to broadband seismometer.
We adopted seismic sensor with natural period of 20 seconds (Trillium Compact Broadband Seismometer, Nanometrics). Because tilt of OBS on seafloor can not be controlled due to free-fall, leveling system for seismic sensor is necessary (Figure). The broadband seismic senor has cylinder shape with diameter of 90 mm and height of 100 mm, and the developed levelling system can mount the seismic sensor with no modification of shape. The levelling system has diameter of 160 mm and height of 110 mm, which is the same size as existing levelling system of the LT-OBS. The levelling system has two horizontal axes and each axis is driven by motor. Leveling can be performed up to 20 degrees by using micro-processor (Arduino). Resolution of levelling is less than one degree. The system immediately starts leveling by the power-on of controller. After levelling, the the seismic senor is powered and the controller records angles of levelling to SD RAM. Then the controller is shut down to consume no power.
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.

Figure. Installation of developed levelling system of seismometer to a pressure vessel.