A Comprehensive Study of Precursory Quiescence Before Great and Large Earthquakes in the Central Aleutians and Chile

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Four large and great earthquakes have occurred in the Central Aleutians and Valparaiso, Chile regions since 1971. All of these events have had precursory quiescence reported prior to them. Several observations sugget that the reported precursors to events in these regions during 1971 reflect systematic changes in teleseismic magnitude estimates rather than real changes in seismicity rates. These include: 1) Both seismicity rate decreases can be observed in regions which are much larger than the rupture zones of the events; 2) the distribution of the rate changes in the Chile region in the magnitude domain can be well modeled as being due to changes in magnitudes; and 3) the change in the Aleutians is strong in sets with a magnitude cutoff and a weak in all events, a characteristic expected for changes in magnitudes. The average number of stations used for teleseismic magnitude estimates in Chile is less than 10, which is probably too small to provide magnitudes which are independent of station biases. We propose to redetermine the teleseismic magnitudes for these regions using station magnitude corrections and to re-examine the seismicity patterns prior to the large events to determine whether the reported precursors are real or due to systematic changes in magnitudes. We plan to integrate these observations with local seismic network data from each region to increase our understanding of the magnitude distribution of real precursory rate variations if they exist. This work is crucial for understanding what precursors exist prior to these events. More importantly, it is critical for understanding the strengths and weaknesses of seismicity data and avoiding making serious errors in interpreting these data for earthquake predictions.