UC Berkeley

The 11 April 2012 M8.6 strike-slip Indian Ocean earthquake (IOE) was followed by an increase in global seismic activity, with three remote M ≥ 6.0 earthquakes within 24 h. We investigate delayed dynamic triggering by systematically examining three offshore regions hosting these events for changes in microseismic activity preceding the IOE, and during the hours between the IOE surface-wave arrival and the triggered-event candidate. The Blanco Fault Zone, USA, and the Tiburõn Fault Zone, Mexico, each host a strike-slip event, and the Michoacán Subduction Zone, Mexico, hosts a reverse event. At these locations we estimate transient Coulomb stresses of ±1-10 kPa during the IOE. Each study area contains a regional seismic network allowing us to examine continuous waveforms at one or more nearby stations. We implement a short-/long-term-average algorithm and template matching to detect events and assess the seismicity with the β-statistic. Our results indicate low-magnitude seismicity in the days prior to the IOE and the occurrence of earthquakes during the surface-wave passage after more than 2 h of transient loading. We find both transtensional tectonic environments respond to the transient stresses with a substantial increase observed in the seismicity rates during the hours after the passage of surface waves. In contrast, seismicity rates remain constant in the subduction zone we investigate during the 14 h delay between the IOE and the large-magnitude earthquake. The seismicity rate increases we observe occur after many hours of dynamic stresses and suggest the long duration of transient loading initiated failure processes leading up to these M ≥ 6.0 events.