[SCG67-P01] Practical method to determine relative orientation of horizontal components of ocean bottom seismometer in an array
Keywords:Ocean Bottom Seismometer, array, orientation of ocean bottom seismometer, beamform, offshore observation, slow earthquake
We have processed data with beamforming, which is a method allows us to reveal azimuth and apparent velocity of incoming wave observed by an array. It has been used to detect tremors in Cascadia subduction zone [Ghosh et al., 2009; 2012] and San Jacinto Fault [Hutchison and Ghosh, 2017]. Beamforming was applied to OBS array dataset in previous study [Ohta et al., 2017, EGU]. However, only waveforms of vertical components had been used, because of unknown orientation of horizontal components due to free-fall deployment of the OBSs. Definite orientation of horizontal components can be obtained using particle motion of regional or teleseismic earthquakes [e.g., Scholz et al., 2016]. However, associated uncertainty is too large to satisfy required condition of beamforming, which is horizontal components of all seismometers in an array must oriented in a same direction. Here, we align the orientation of horizontal components of all OBSs within each array by cross correlating observed waveforms. On each array, we pick one OBS as a reference. Horizontal components of the other OBSs in an array are rotated every 1 degree, and cross correlated with the horizontal components of the reference OBS using first several seconds of P-wave radiated from regional and local earthquakes. As a result, we obtained relative orientation where waveforms of horizontal components of the other OBSs correlate for the best with the reference OBS. We verified these orientation angles are accurate enough for beamforming through beamform of local, regional and teleseismic earthquakes.
Our method can determine relative orientation of horizontal components of OBSs in the array accurately, which is a crucial condition for any array analysis technique. Analysis of waveforms of horizontal components would be a huge advantage for study of low frequency tremors, since S-wave dominates its waveform [Obara, 2002; La Rocca et al., 2005] and thus expected to be more energetic in horizontal components rather than vertical.
Acknowledgement: This study is supported by JSPS KAKENHI (26000002).