Abstract
AbstractEarthquake triggering by seismic waves has been recognized as a phenomenon for nearly 30 years. However, our ability to study dynamic triggering has been limited by our ability to capture the triggering stresses accurately and record the resultant earthquakes. Here we use full waveforms from a dense seismic network and a modern, high‐resolution seismic catalog to measure triggering in Southern California from 2008 to 2017 based on interevent time ratios. We find that the fractional seismicity rate change, which we term triggering intensity or triggerability, as a function of peak strain change for the period of ∼20 s due to distant earthquakes is monotonically increasing and compatible with earlier measurements made with a disjoint data set from 1984 to 2008. A triggering strain of 1 microstrain is equivalent to the local productivity generated by an M1.8 earthquakes. This result implies that a prediction of seismicity rate changes can be made based on recorded ground shaking using the same formalism as currently used for aftershock prediction. For a teleseismic event, this small level of triggering occurs throughout the region and thus aggregates to a regional effect. We find that the triggering rate decays after the triggerer follows an Omori‐Utsu law, but at a much slower rate than a typical aftershock sequence. The slow decay rate suggests that an ancillary process such as creep or fluid flow must be part of dynamic triggering. The prevalence of triggering in areas of creep or fluid involvement reinforces this inference. A triggering cascade of secondary earthquakes is insufficient to explain the data.
Highlights
The n-value for low dynamic strain change, or peak ground velocity (PGV), with 90% (+45% and −45%) confidence strain change shows a small value with small errors, indicating no signifinterval. (a) n-values on a log scale and (b) n-values on a linear scale
Icant seismicity rate change associated with small strain perturbations
We successfully reproduced and refined the relationship between the peak dynamic strain and the triggering intensity in Southern California by using new datasets that are independent of previous studies
Summary
2015; Hill et al, 1993; Kilb et al, 2000; Miyazawa, 2011; Parsons et al, 2014; Velasco et al, 2008) It is one of the few situations where a known, measurable, natural stress can be identified as the immediate cause of an earthquake. Triggered earthquakes often occur subsequent to the passage of the perturbing stress in the seismic waves. This delay is hard to understand in a simple Coulomb failure model and is a fundamental feature of dynamic triggering that makes it a physically mystifying process (Belardinelli et al, 2003; Gomberg, 2001; Parsons, 2005). Datasets that can approach this problem have been few (Brodsky, 2006; Shelly et al, 2011)
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