Orientation Corrections of a Borehole Seismometer Network in Taiwan Using Teleseismic Earthquakes

The Central Weather Bureau, Taiwan, has deployed a borehole seismic network of 62 stations nationwide in Taiwan since 2008. Each borehole station includes a force-balance accelerometer on the surface, and a combination of an accelerometer and a broadband seismometer in the deep hole. The noise analyses have shown significant improvement in the performance of the borehole instruments compared with the instruments deployed on the surface. We propose a method to obtain robust orientations, as well as the uncertainty for both the borehole accelerometer and broadband seismometer using long-period waves (>2 s) after the S-wave arrival time of three mid-large teleseismic events and one local event. Overall, we successfully calibrated the orientations of the borehole sensors and found that the significant azimuth change for some borehole seismometers was always due to sensor maintenance. Based on the results of this study, we conclude that the long-period waveforms generated by a teleseismic event, wavelength of which is four times longer than the borehole depth, are useful for borehole orientation calibration. The correlation coefficients of the three components for both surface and borehole sensors larger than 0.9 are appropriate. The magnitude and epicenter distance of the event and on-site noise level are the factors that affect the quality of calibration. In addition, we conclude that the method of orientation calibration we propose is helpful for data quality control, including data timing and sensor failure. With the supplement of this azimuth estimation information, we expect that these high-quality borehole data in Taiwan will improve seismological research and will benefit the society.