A study of Amplitude Variations and m sub b Bias at LASA Subarrays
TELEDYNE GEOTECH ALEXANDRIA VA SEISMIC DATA ANALYSIS CENTER
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LASA subarray amplitude anomalies are investigated using 395 medium- sized events, distributed in ten azimuthally divided sectors. Although LASA magnitudes, when averaged over all azimuths, are only slightly biased relative to NEIS magnitudes, the amount of bias varies with azimuth and subarray, suggesting that a simple station correction for m sub b bias is not adequate. In addition, fluctuations among LASA subarrays are about 0.15 in standard deviation, even when these magnitudes are calibrated in sectors. Details of such fluctuations are explained in part by local crustal and upper-mantle heterogeneities under LASA. The amplitude anomalies are linearly related to the amount of travel-time anomalies in each sector, implying that both effects are due to crustal focussing.