abstract

Anomalous variations in three different seismic processes have been observed before an earthquake of magnitude 4 on May 14, 1971 in the Fairview Peak region of central Nevada. The data used are the three-component seismograms from Tonopah (TNP) and Battle Mountain (BMN) as well as the vertical-component seismograms from several other seismographic stations. The observed precursory phenomena are (1) reorientation of the compressive stress axis: evidence for this is based on clear reversals of first motion of Pg at certain stations together with reversal of wave form of the SV component of Sg at BMN; (2) vertical migration of hypocenters: the PmP phase is often distinctly observed in central Nevada and the observed temporal variations in t(PmP)−t(Pg) at TNP indicate generally upward migration of foci before the main event; (3) changes in the extent of S-wave splitting: a large precursory increase is observed along the tensile stress direction and a small premonitory decrease along the compression direction. Similar but unidentical results have been obtained before another earthquake of magnitude 4 whereas all events of magnitude 3.5 or larger have been preceded by anomalous S-wave splitting along the tension direction. The various observed pre-earthquake processes appear to be interrelated and may be explained in terms of recent laboratory and theoretical results when applied to the tectonics of central Nevada. It seems highly desirable to attempt simultaneous observations of anomalous changes in more than one seismic process.

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