Spatial and temporal seismicity patterns from dynamic trigger by great earthquakes

Questions remain regarding the dynamic triggering of large earthquakes at global distances and the role of earthquake nucleation times following dynamic stress perturbations on active faults. At distances beyond 2-3 fault lengths, transient stress changes as a result of seismic waves have been shown to trigger microseismicity in regions of geothermal activity, non-volcanic tremor, and small tectonic earthquakes, i.e. M<3, during the passage of seismic waves and subsequent days. Less frequently, immediate triggering of earthquakes ranging from 3 > M < 5 is detected during the passage of the surface wave train, whereas events M>5 are shown to experience no rate increase outside the aftershock zone of large-magnitude events. An exception to this magnitude threshold for dynamically triggered activity is the MW 8.6 2012 east Indian Ocean earthquake that resulted in above-average global seismicity for events >M5.5 in the 6 days following the mainshock (Pollitz et al., 2012 Nature) followed by a 95 day period of global quiescence for M>=6.5 events (Pollitz et al., 2013 submitted to BSSA). The activity observed following the 2012 event is an example of delayed dynamic triggering which suggests that the stress perturbations experienced during the teleseismic surface waves were large enough to advance the earthquake cycle of multiple >M5 events. Conversely, the same stress changes could suppress activity while earthquake rates return to equilibrium. Here we investigate the idea of global 'dynamic shadowing' in order to determine if a spatial and temporal relationship to large-magnitude events exists for periods of reduced global seismicity. We build on the previous work by Parsons and Velasco (2011 Nature Geoscience) who concluded that increases in M>5 seismicity are confined to within 2-3 fault lengths of the rupture and activity returned to background rates within 36 hours following a M>7 event. Using a similar methodology, we explore the suppression of global activity with respect to background earthquake activity following large magnitude events using 30 years of earthquake catalog data obtained from the Advanced National Seismic System. Our goal is to determine if a dynamic shadow effect results in a reduction of seismicity at a distinguishable level below the background seismicity over various temporal and spatial ranges following the largest cataloged earthquakes. We utilize spatial ranges consistent with maximum transient stress changes for different fault mechanisms in order to document the temporal characteristics of seismicity patterns.