High frequencies are a critical component of aftershock triggering at <100-150 km (Invited)

Triggered earthquakes at large distances from the mainshock have been observed to closely follow the arrival of ~0.03-0.6 Hz surface waves (Hill, 2008). Triggering by body waves at these distances is generally not observed. At distances closer than 50-100 km, however, surface waves are not well developed and have minimal amplitude. Thus triggering at these distances is presumably accomplished by static stress change and/or by body waves via a mechanism that does not work at further distances. Pollitz (2006) demonstrated that slow slip events on the San Andreas fault do not trigger many aftershocks, suggesting that static stresses alone are not effective triggers, while Felzer and Brodsky (2006) demonstrated that dynamic stresses alone do appear to trigger aftershocks at least in the 10--50 km range. Yet Parsons and Velasco (2009) found that underground nuclear tests, which are essentially dynamic-only sources, do not produce aftershocks at regional distances. Here we demonstrate that Southern California quarry blasts also fail to produce aftershocks. Both nuclear tests and quarry blasts are depleted in high frequency energy in comparison to tectonic earthquakes (Su et al. 1991; Allman et al. 2008). Therefore the observation that both slow slip events and blasts fail to trigger many aftershocks suggests that the missing ingredient of high frequency body wave energy plays a critical role in the triggering process. Quarry blast spectra data and scaling considerations allow the critical triggering frequency to be constrained to > 20-60 Hz. Energy in this frequency band may be expected to persist at depth at least out to 100 km (Leary, 1995). Huc and Main (2003) found that aftershock triggering by global earthquakes follows a continuous decay curve out to ~150 km, suggesting that triggering by high frequency body waves might extend this far. At much further distances the high frequencies are likely attenuated, explaining why only low frequency surface wave triggering persists.