Calibration of broadband seismograph responses based on the spectral ratio of teleseismic records

Abstract:

Broadband seismometer data are essential for the development of seismological studies such as those investigating earthquake sources and the Earth’s structure. However, previous studies [e.g., Kimura et al., 2015] have revealed that the metadata describing these data can possibly be contaminated by instrumentation response errors that are often difficult to recognize from visual waveform checks. Herein, we report on the development of a method to calibrate the instrumentation responses from recorded ground motions in observation networks whose station intervals are smaller than 200 km.

The method is based on spectral ratios between teleseismic surface wave records at a target station and those at multiple surrounding reference stations, from which we estimate the corner period, damping factor and sensitivity in the instrumentation response at the target station. By stacking the spectral ratios, we could estimate the response stably, and Bootstrap technique enabled us to evaluate the estimation errors. We applied the method to F-net broadband seismometers in Japan.

For STS-1 seismometer of vertical component at TMR station, we could not recognize obvious errors in the corner period and sensitivity. However, the damping factor had increased gradually since 2005 and reached about 2.0 in 2011. After the leveling of the seismometer in November 2011, the damping factor returned to about 0.7. For STS-2 seismometer of vertical component at SHR station, the sensitivity had been about half of the normal value from start of the observation to September 2012. After the replacement of the host-box, the sensitivity settled at normal value.

By systematic evaluation of the seismometer responses, the obtained calibration values will be useful for the various research using the observation data.