Craig dePolo’s research on foreshock occurrence was published in the May/June 2014 issue of Seismological Research Letters.
High Probability of Foreshock Occurrence and Significant Probability of Multiple Events Associated with Magnitude ≥6 Earthquakes in Nevada, U.S.A., by Craig M. dePolo
Considering 17 earthquakes in Nevada with magnitude ≥6 and adequate records, 88%–94% were preceded by reported foreshock activity within 120 days before the mainshock. Considering a 30-day timeframe, 82%–88% of the events had reported foreshock activity. For a five-day window, 76% of events had reported foreshocks; for three-day window, 71% of events had reported foreshocks; and for a one-day window, 47% of events had reported foreshocks. The foreshocks were commonly small- and moderate-size events, and measured magnitudes ranged from M 1.4 to M 6.1. Eight of these 17 earthquakes had foreshock sequences, and three of these were escalating foreshock sequences. One of the 17 events (1887 Carson Valley earthquake) had no reported foreshocks. The 1887 earthquake is an important reminder that even with the encouragingly high number of events with foreshocks in Nevada, damaging earthquakes will still occur without any apparent foreshock activity. Additionally, four of the 17 events only had reported foreshock activity within the hour before the mainshock; if this was all the foreshock activity for these events, it is a very short lead time. Twenty-two percent of larger earthquakes in Nevada were multiple events, and 17% were followed by a second M ≥6 earthquake within 12 hours. These observations have important emergency response and earthquake-preparedness safety implications.
The response of Nevadans to foreshock activity suggests the broader possibility that moderate events near communities can be used as windows of opportunity for earthquake preparedness. Such preparedness can have great short-term benefits should a strong earthquake occur and long-term benefits for future earthquakes.
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