Delayed dynamic triggering: Local seismicity leading up to three remote M ≥ 6 aftershocks of the 11 April 2012 M8.6 Indian Ocean earthquake

PUBLICATIONS Journal of Geophysical Research: Solid Earth RESEARCH ARTICLE 10.1002/2015JB012243 Key Points: • Delayed dynamic triggering after multiple hours of ±1–10 kPa transient loading • Earthquakes observed after more than 2 h of surface wave passage • Seismicity rate increases observed in transtensional tectonic environments Delayed dynamic triggering: Local seismicity leading up to three remote M ≥ 6 aftershocks of the 11 April 2012 M8.6 Indian Ocean earthquake Christopher W. Johnson 1 and Roland Burgmann 1 Berkeley Seismological Laboratory and Department of Earth and Planetary Science, University of California, Berkeley, California, USA Abstract The 11 April 2012 M8.6 strike-slip Indian Ocean earthquake (IOE) was followed by an increase in Supporting Information: • Figures S1–S9 • Movie S1 • Movie S2 • Movie S3 • Figure S10 • Figure S11 • Figure S12 Correspondence to: C. W. Johnson, cwj@seismo.berkeley.edu Citation: Johnson, C. W., and R. Burgmann (2016), Delayed dynamic triggering: Local seismicity leading up to three remote M ≥ 6 aftershocks of the 11 April 2012 M8.6 Indian Ocean earthquake, J. Geophys. Res. Solid Earth, 121, doi:10.1002/2015JB012243. 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 Tiburon Fault Zone, Mexico, each host a strike-slip event, and the Michoacan 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. 1. Introduction Received 29 MAY 2015 Accepted 11 DEC 2015 Accepted article online 15 DEC 2015 ©2015. American Geophysical Union. All Rights Reserved. JOHNSON AND BURGMANN Seismic waves from large earthquakes are capable of transferring energy to remote distances, i.e., many fault lengths from the source, which is a well-documented phenomenon and can potentially increase seismic activity in far-ﬁeld regions [Freed, 2005; Gomberg, 2001; Gomberg et al., 2004; Gonzalez-Huizar et al., 2012; Hill and Prejean, 2015; Hill et al., 1993; Pollitz et al., 2012; Prejean and Hill, 2009; Velasco et al., 2008; West et al., 2005]. The 11 April 2012 M8.6 Indian Ocean earthquake (IOE; 08:38:36 UTC; 93.06°, 2.32°, 40 km) is the largest strike- slip earthquake observed in the modern era that ruptured a series of intraplate conjugate faults and was followed by a M8.2 strike-slip aftershock 2 h after the initial rupture [McGuire and Beroza, 2012]. The IOE is inter- esting with regard to earthquake communication because of the number of large-magnitude (M ≥ 5.5) events that occurred in the ensuing days. Pollitz et al. [2012] quantify the signiﬁcance of the 6 day increase in remote M ≥ 5.5 earthquakes following the IOE, and Johnson et al. [2015] document the rare occurrence of M ≥ 5.5 events triggered by global remote M ≥ 7.5 main shocks, indicating the uniqueness of the large-magnitude remote aftershocks of the IOE. Interestingly, M > 5 triggering candidates appear to only occur after a delay period on the order of hours to weeks following the stress perturbation [Gomberg, 2013; Gomberg and Bodin, 1994; Gonzalez-Huizar et al., 2012; Parsons et al., 2014; Pollitz et al., 2012] and physical evidence to connect these M > 5 events to a transient stress via a seismic or aseismic process is lacking. The delay period coupled with a limited number of observations of M > 5 triggered events [Parsons et al., 2014] seems to contradict the ubiquity of M 5 earthquakes to the point of failure. Case studies quantifying seismicity rate changes DELAYED TRIGGERING OF REMOTE M ≥ 6 EVENTS