FAR-FIELD DETECTION OF SLOW PRECURSORS TO FAST SEISMIC RUPTURES

An earthquake is said to have a short-term slow precursor when there is significant moment released in an interval immediately before its high-frequency origin time t0. A method is formulated for detecting slow precursors from observations of the earth's free oscillations. It assumes that the far-field radiation at low frequencies (less-than-or-similar-to 10 mHz) is due to a compact source of constant orientation having a moment-release function which is zero prior to some start time t* and nondecreasing for t greater-than-or-equal-to t*. Any moment-release function for which t0 = t* (no precursor) must then satisfy the inequality tau-c < DELTA-t1 square-root 2 (1 + square-root 1 + 8-alpha) where DELTA-t1 = t1-t0 is the centroid time shift, tau-c is the characteristic duration, and alpha is a dimensionless measure of the temporal skewness. A detector based on this inequality is applied to two events. Data from the shallow-focus Loma Prieta earthquake of 18 Oct 89 are consistent with no moment release prior to the high-frequency origin time, whereas the intermediate-focus Peru-Ecuador earthquake of 12 Apr 83 appears to have a significant slow precursor. The systematic analysis of large earthquakes using this detector may elucidate earthquake nucleation processes and help to assess, on a global scale, the feasibility of short-term earthquake prediction from near-field measurements.