Monitoring temporal variations in instrument responses in regional broadband seismic network using ambient seismic noise

High-quality broadband data are required to promote the development of seismology research. Instrument response errors that affect data quality are often difficult to detect from visual waveform inspection alone. Here, we propose a method that uses ambient noise data in the period range of 5-25 s to monitor instrument performance and check data quality in situ. Amplitude information of coda waves and travel time of surface waves extracted from cross-correlations of ambient noise are used to assess temporal variations in the sensitivity and poles-zeros of instrument responses. The method is based on an analysis of amplitude and phase index parameters calculated from pairwise cross-correlations of three stations, which provides multiple references for reliable error estimates. Index parameters calculated daily during a two-year observation period are evaluated to identify stations with instrument response errors in real time. During data processing, initial instrument responses are used in place of available instrument responses to simulate instrument response errors, which are then used to verify our results. The coda waves of noise cross-correlations help mitigate the effects of a non-isotropic field and make the amplitude measurements quite stable. Additionally, effects of instrument response errors that experience pole-zero variations on monitoring temporal variations in crustal properties appear statistically significant of velocity perturbation and larger than the standard deviation. Monitoring seismic instrument performance helps eliminate data pollution before analysis begins.