SCEC Award Number 08138 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Triggerabilty: A tool to connect aftershocks and long-range triggering
Investigator(s)
Name Organization
Emily E. Brodsky University of California, Santa Cruz
Other Participants Nicholas van der Elst
SCEC Priorities A6, A4 SCEC Groups Seismology, EFP, FARM
Report Due Date 02/28/2009 Date Report Submitted N/A
Project Abstract
 Mechanistic studies of short-term earthquake interactions have taken two separate tracks. One is to study farfield triggering (Hill et al., 1993; Brodsky et al., 2000; Gomberg et al., 2001; Prejean et al., 2004). The other is to study aftershocks (Stein, 1999; Kilb et al., 2002; Felzer and Brodsky, 2006). The long-range triggering studies have the advantages of cleanly separating out
the stresses and thus having a well-defined triggering stress. The clear role of the dynamic stress is well-illustrated by phase-locked triggering that tracks the surface waves (West et al., 2005). The long-range approach has the disadvantages that the number of triggered events is often too small for statistical study and the phenomenon may not be representative of common processes. Aftershock studies have the advantages that enormous numbers of triggered events have been recorded and it is clear that aftershock generation is a common, major process that occurs throughout the crust. The disadvantage of studying aftershocks is that the nearfield stress field is complex with potentially significant static and dynamic components.

In this project we connect observationally these two types of triggering. We exploit the understanding that dynamic strain is the dominant triggering agent at large distances and relatively short time scales in order to constrain the contribution of additional triggering agents in the near field. We first determine an empirical relationship between a measure of triggering intensity and peak dynamic strain in the far field based on the waiting time to early triggered earthquakes. Then we compare this far‐field relationship to near‐field observations to assess the proportion of near‐field earth- quakes that can be explained by the far‐field proportionality. We ultimately find that dynamic triggering can account for a significant portion of near‐field aftershocks, but that there is an additional triggering component in the near field. Whether this reflects additional triggering agents (e.g., static strain, afterslip) or the effect of second‐order aspects of the dynamic strain (e.g., duration, frequency) we cannot resolve.
Intellectual Merit The SCEC project funded a very early stage of this work. Ultimately the preliminary results were developed enough to warrant a full NSF proposal and culminate in the critical result of van der Elst and Brodsky (2010).
Broader Impacts This work supported the early education of graduate student Nicholas van der Elst, who is now poised to begin a successful research career as an NSF Geoscience Postdoctoral Fellow at Lamont-Doherty Observatory. The work was also reported on by NPR and formed a key component of the National Science Board Voices from the Future online video.
Exemplary Figure Figure 1. Triggered Seismicity Rate Scales with Dynamic Strain. (a) The mean interevent time ratio , in terms of the deviation from the value in the absence of triggering ( ). Compare to Figure 3. Long-range (>800 km) triggers are in red and short-range triggers (<6 km) are in blue. Vertical and horizontal error bars are 95% confidence limits. The green point corresponds to the Denali earthquake. The red horizontal bar shows the 2σ uncertainty associated with the farfield peak ground motion estimates. (b) Triggering intensity n ( transformed via Equation 2) as a function of peak dynamic strain in California and (c) Japan. The black dashed line in panels (a) and (b) shows the weighted least squares fit to the California farfield data, along with 95% confidence levels. The best-fit curve for California is also shown in (c) for comparison with Japan.