Abstract
P-VELOCITY decreases before earthquakes1–4 can be observed by monitoring the average P-residual at seismic stations near the focus5–7. The P-residual is the difference between the observed travel time from an epicentre to a station and that computed theoretically on the basis of some Earth model and a least squares estimate of the earthquake location and origin time. If the P-velocity in the crust underneath a seismic station is decreased temporarily, each seismic wave will arrive somewhat later during the premonitory period. Before three thrust5,6,18 and one strike-slip event7 the delay was found to be approximately 0.4 s.
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WYSS, M. Will there be a large earthquake in central California during the next two decades?. Nature 251, 126–128 (1974). https://doi.org/10.1038/251126b0
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DOI: https://doi.org/10.1038/251126b0
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