Calibration of the Sonseca array with large magnitude regional and teleseismic events

Abstract

In order to calibrate the Sonseca station, a 19-element short-period seismic array with a 9 km diameter circular aperture located in central Spain (39.68N, 3.96W), wavefield measurements made on observed seismic phases are compared with expected values. Thirty-five well-recorded regional and teleseismic events are used to study bearing and phase velocity estimation properties. Preliminary results indicate that in general the Sonseca array performs well for both regional and teleseismic events for frequencies less than 5 Hz using standard array signal processing techniques. Main findings of this study are: (1) A systematic bias is observed in bearing estimates; the bias is a function of the true bearing for events from the easterly directions of the array and can be mitigated with a simple bias correction. Using a least-squares quadratic polynomial fit, the bearing estimation error can be reduced to less than two or three degrees. (2) Measured signal and noise coherence functions and beamforming suggest that for regional events improved SNR is obtained by beamforming in the frequency band of 0.5 to 4 Hz with a resulting array gain as high as 10 dB. (3) Because the element spacing of Sonseca array corresponds to that of a sparse regional array, spatial aliasing can be observed in narrowband f-K analysis at the higher frequencies. We compare performance of narrowband and broadband frequency-wavenumber (f-k) analysis and suggest preliminary recipes for f-k and beamforming analysis.