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Analysis of three-component data from IRIS/IDA stations in the USSR

CENTER FOR SEISMIC STUDIES, 1300 NORTH 17TH STREET, ARLINGTON, VIRGINIA 22209

Abstract

This paper presents results of backazimuth, incidence angle, and phase identification studies based on the three-component polarization of 0- to 5-Hz seismic signals recorded at the IRIS/IDA stations within the USSR. A data set was built from the analysis of three-component (3-C) recordings from seismic stations at Arti (ARU), Garm (GAR), Kislovodsk (KIV), and Obninsk (OBN) for the month of March 1989. Approximately 260 events were studied. At stations ARU, KIV, and OBN, teleseisms dominated the sample of recordings. Station GAR, however, recorded a larger set of events that included a significant number of local and regional events.

Except for station OBN, all of the IRIS/IDA stations are located near major geologic structural boundaries that are characterized by significant lateral variation in the deep velocity structure. These stations near boundaries show system-atic deviations in estimates of backazimuth and angle of incidence. At station GAR, both of these measurements were influenced by the structure.

Polarization characteristics of seismic phases were studied individually for each station. At stations OBN and KIV, the initial teleseismic P wave was characterized, for the most part, by prograde elliptical motion. At ARU and GAR, the motion was rectilinear. Regional S phases from Hindu-Kush earthquakes recorded at GAR were dominated by transverse horizontally polarized motion (SH). At KIV and ARU, Lg motion was predominantly in a near-horizontal plane.

A phase identification study was performed using discriminant analysis based on polarization parameters and frequency content. Two discriminants, linear combinations of polarization and frequency measurements, were able to separate arrival data into one of three groups of phases: teleseismic P, regional P, and regional S. At GAR, the first discriminant is correlated with polarization and separates arrivals with P-wave characteristics from arrivals with S-wave characteristics. The second discriminant accentuates the differences in the frequency content of the arrivals and provides a good separation between regional and teleseismic phases.

Backazimuth estimates were made for local, regional, and teleseismic events at GAR and for teleseismic events at ARU, KIV, and OBN. The unique structural environment of each station required that custom processing parameters be developed for each station. The assumed particle motion (rectilinear or elliptical) used in the polarization computation, the window length, and the frequency band (characteristic for each station) were determinant factors. Backazimuth estimates within ±25° of the theoretical value were obtained for about 70% of the events at most of the stations. At GAR, systematic backazimuth errors caused by large lateral velocity discontinuities degraded the results. Corrections could be made to reduce these errors.

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