Global Catalog Analysis Shows That Dynamic Triggering Or Shadowing Of Remote M≥5.5 Earthquakes Is Rare

Tuesday, 16 December 2014: 11:20 AM
Christopher W Johnson1, Roland Burgmann1 and Frederick F Pollitz2, (1)Univ California Berkeley, Seismological Laboratory, Berkeley, CA, United States, (2)USGS, Menlo Park, CA, United States
Probing the effects of a dynamic stress perturbation and the timing of earthquake nucleation is necessary for understanding the remote interaction of large magnitude events and the possibility of a remote hazard increase. Catalog data containing 35 years of M≥5.5 earthquakes allows us to explore the records for periods of enhanced or suppressed seismic activity. We consider 113 M≥7.5 mainshocks between 1977-2012 as sources of a transient stress on receiver faults at remote distances. Our study focuses on seismic activity on time scales from days to months. One objective is to search for evidence of delayed dynamic triggering of large magnitude events; i.e. an increase in M≥5.5 remote earthquakes following a M≥7.5 mainshock, similar to the global increase following the 2012 M8.6 Indian Ocean earthquake [Pollitz et al., 2012]. We restrict the data to regions of elevated strain during the passage of surface waves by calculating the strain field for the M≥7.5 mainshocks. The strain field is determined using empirical scaling laws and through synthetic seismograms. Using a threshold of 0.1 µstrain and temporal window of ±1-year we stack daily rate curves for M≥5.5 pre- and postshocks in order to resolve rate deviations from the background rate. Our results do not indicate an observable change in activity when considering the collective set of 113 mainshocks used in the study. Additionally, our method enables us to look for periods of reduced seismic activity in order to test the hypothesis of dynamic shadowing; i.e. a seismic quiescent period following a transient stress. The results suggest a subtle decrease in daily rates of M≥5.5 earthquakes following the mainshocks, but activity changes are not below the confidence intervals for the background rate and cannot significantly support the hypothesis. We conclude that a global increase in M≥5.5 earthquakes following a transient stress is a rare occurrence. To continue investigating delayed dynamic triggering a detailed analysis of any large mainshock that results in this type of anomalous behavior would be required in order to advance the understanding of earthquake nucleation and the seismic cycle.