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Far‐field detection of slow precursors to fast seismic ruptures
Author(s) -
Jordan Thomas H.
Publication year - 1991
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/91gl02414
Subject(s) - seismology , geology , moment (physics) , seismic moment , dimensionless quantity , centroid , skewness , nucleation , detector , term (time) , field (mathematics) , geodesy , physics , mathematics , mechanics , geometry , optics , statistics , fault (geology) , classical mechanics , quantum mechanics , pure mathematics , thermodynamics
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 t 0 . 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 (≲ 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 ≥ t * . Any moment‐release function for which t 0 = t * (no precursor) must then satisfy the inequality is the centroid time shift, τ c is the characteristic duration, and α 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.