Study of amplitude anomaly and m/sub b/ bias at LASA subarrays
Large-aperture seismic array (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 in relation National Earthquake Information Services (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 crustasl focusing. Using a fixed effects model, the authors attempt to separate the cause of m/sub b/ bias into sector (azimuth) effect, subarray effect, and subarray-sector interaction. However, even this detailed modeling could not, with confidence, explain m/sub b/ bias. Using the reciprocity principle, even with a well-calibrated station, a factor of 2 in uncertainty results for predictions of a station's amplitude for an event only 50 km away from a calibration event in a region as complex as LASA. The event magnitude uncertainty would probably be reduced by network averaging, or by a nonstatistical detailed crust and mantle structure which could be analyzed by ray tracing to remove receiver effects.
- Research Organization:
- Teledyne Geotech, Alexandria, Virginia 22314
- OSTI ID:
- 7077530
- Journal Information:
- J. Geophys. Res.; (United States), Journal Name: J. Geophys. Res.; (United States) Vol. 85:B9; ISSN JGREA
- Country of Publication:
- United States
- Language:
- English
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