[SSS17-P02] Accuracy improvement of DONET seismometer orientations estimated from seismic waveforms
Keywords:Ocean-bottom seismometer, Nankai trough, DONET, Seismometer orientation
In 2012, Nakano et al. estimated DONET1 seismometer orientations by the 3 types of analysis method as follows: 
(1) correlations of long-period seismic waveforms with land seismometer observation
(2) direction of P-wave first motion for distant earthquakes
(3) particle motions of airgun signals
This study shows that the orientation difference compared to the measurement data of ROV video is up to 10 degrees for the most seismometers, while the difference comes up about 50 degrees for some others.
Further, Nakano reinforced the information of estimated orientations of seismometers after the construction of DONET2 in 2017 using the method (1) and (2) described in Nakano et al. (2012) as above. Nakano also re-estimated the seismometer orientations of DONET1. 
In this study, Nakano shows that the obtained standard errors of estimated seismometer orientations is around 1 to 2 degrees. However, for some seismometers, estimated orientation figures are considered to have a few degrees of error, because of insufficient distant earthquake event that were available during the study. Since DONET2 construction completed in the middle of this study, enough observation period could not be available for these seismometers.
Therefore, it has been expected to re-estimate the DONET seismometer orientations with longer observation period and with sufficient distant earthquake events that can be used for the analysis.
We re-estimated DONET seismometer orientations by the method (2) described in Nakano et al. (2012) with 9 years of DONET data and obtained the latest figures of estimated orientation after the replacement of seismometer at observation point KMC11 in June 2018. We confirmed that the obtained orientations are basically consistent with the previous studies, e.g. Nakano et al. (2012) and Nakano (2017). Also, 9 years of DONET data enables us to obtain the standard errors of estimated orientation with the order of 0.1 degree for most seismometers. We hope that the most up to date and more accurate figures of estimated DONET seismometer orientations will help the concerned scientists and researchers for their studies.
 Masaru Nakano, Takashi Tonegawa, and Yoshiyuki Kaneda. Orientations of DONET seismometers estimated from seismic waveforms. JAMSTEC Rep. Res. Dev., Volume 15, September 2012, 77-89
 Masaru Nakano, Orientations of DONET2 seismometers estimated from seismic waveforms, JpGU-AGU Joint Meeting 2017, STT59-P03