A comparison of regional-phase amplitude ratio measurement techniques

Abstract

Abstract We compare several procedures for measuring regional-phase amplitude ratios commonly used in discrimination studies. Amplitude measurements are made on both raw velocity-proportional broadband seismograms and instrument-deconvolved displacement seismograms in the time and frequency domains. Pn/Lg and Pg/Lg ratios are measured on vertical-component waveforms for regional earthquakes observed at the Global Seismic Network (GSN) station ABKT (Alibek, Turkmenistan). Time-domain amplitude measurements are made on narrow-band filtered wave-forms using signal energy measures given by the absolute mean, rms, and envelope mean, and peak measures given by the absolute maximum and envelope maximum. Frequency-domain measurements are made by computing the log10-mean amplitude within a narrow band from the broadband spectrum of each windowed phase. Time-domain amplitude ratios for raw velocity and instrument-deconvolved displacement seismograms are very similar (linear correlations ≧0.97), indicating that the instrument response does not affect the time-domain amplitude ratio measurements for the broadband GSN data at the frequencies studied (0.75 to 9.0 Hz). Time-domain amplitude ratios made using the energy or peak measures are well correlated among themselves (linear correlations ≧0.97). However, the energy and peak measures are slightly less well correlated with each other (linear correlations between 0.87 and 0.99). For the time-domain measures, the correlations generally degrade, and the scatter increases for the higher-frequency bands. Time- and frequency-domain measurements are well correlated for the lowest-frequency band (0.75 to 1.5 Hz). However, as the frequency band increases, the correlations decrease and the slopes deviate from 1.0. Time-domain amplitude ratios (for both Pn/Lg and Pg/Lg) are consistently larger than the frequency-domain amplitude ratios. Investigation of the frequency spectra of broadband and narrow-band filtered phases revealed that significant energy from outside the passband, particularly from lower frequencies, possibly biases the time-domain measurements. Log averaging the spectral amplitudes in a given passband before forming the amplitude ratios does not bias the measurements to the low end of passband. Better agreement between time- and frequency-domain measurements is found when linear averaging of the spectrum is used. These observations suggest that with care, time- and frequency-domain measures can be made to agree. We suggest that log-averaged frequency measures look promising for discriminant measures because (1) frequency-domain measurements offer complete control of the frequencies that contribute to the measurement, and (2) log averaging of the spectra does not overweigh spectral amplitudes associated with lower frequencies within the passband. However, because we consider only earthquake data, we cannot evaluate the discrimination performance of the various measurement techniques.