Earthquake swarms on transform faults

Abstract

S U M M A R YSwarm-like earthquake sequences are commonly observed in a diverse range of geological settings including volcanic and geothermal regions as well as along transform plate boundaries. They typically lack a clear mainshock, cover an unusually large spatial area relative to their total seismic moment release, and fail to decay in time according to standard aftershock scaling laws. Swarms often result from a clear driving phenomenon, such as a magma intrusion, but most lack the necessary geophysical data to constrain their driving process. To identify the mechanisms that cause swarms on strike-slip faults, we use relative earthquake locations to quantify the spatial and temporal characteristics of swarms along Southern California and East Pacific Rise transform faults. Swarms in these regions exhibit distinctive characteristics, including a relatively narrow range of hypocentral migration velocities, on the order of a kilometre per hour. This rate corresponds to the rupture propagation velocity of shallow creep transients that are sometimes observed geodetically in conjunction with swarms, and is significantly faster than the earthquake migration rates typically associated with fluid diffusion. The uniformity of migration rates and low effective stress drops observed here suggest that shallow aseismic creep transients are the primary process driving swarms on strike-slip faults. Moreover, the migration rates are consistent with laboratory values of the rate-state friction parameter b (0.01) as long as the Salton Trough faults fail under hydrostatic conditions.

Aftershock seismicity and tectonic setting of the 2015 September 16 Mw 8.3 Illapel earthquake, Central Chile

“…The associated pressure pulses are suggested to trigger aftershocks by reducing the effective normal stress of incipient slip at Bibliothek des Wissenschaftsparks Albert Einstein on September 29, 2016 http://gji.oxfordjournals.org/ Downloaded from applies. However, aftershock migration related to fluid-driven processes was reported to be significantly smaller (<1 km s −1 ) (Roland & McGuire 2009, and references therein) than the observed migration velocity flowing the first day after the Illapel 2015 earthquake. A further test of the viability of these or other ideas will require indepth modelling, for example, of diffusion processes and Coulomb stress changes, beyond the scope of this study.…”

Contemporary Seismicity in and around the Yakima Fold-and-Thrust Belt in Eastern Washington

“…Blakely et al (2011) suggested that the Wooded Island earthquakes and aseismic slip were caused by flexing of shallow strata associated with movement on underlying, larger structures that form the concealed Yakima Ridge. Notably the geodetic moment of the inferred deformation sources was significantly larger (eight times) than the cumulative seismic moment of the swarm, a feature observed in swarms elsewhere (Roland and McGuire, 2009).…”

The 2015 Fillmore Earthquake Swarm and Possible Crustal Deformation Mechanisms near the Bottom of the Eastern Ventura Basin, California

“…We assumed a faulting volume of 1:5 km × 0:75 km × 0:2 km and used an elasticity constant of 30 GPa to determine an effective seismic stress drop of 0.6 MPa. Although this value should not be compared to the stress drop of individual events, it is close to the median values obtained by Roland and McGuire (2009) and Chen et al (2012) for other swarms, such as the Brawley, Obsidian Buttes, and Salton trough sequences in the Imperial Valley of southern California. Creep events have been shown to have similar effective stress drops (Brodsky and Mori, 2007).…”

“…The simplest clusters are mainshock-aftershock sequences that exhibit rapid increase in moment release, follow Båth's law (Richter, 1958) with the mainshock magnitude ∼1:5 units larger than the largest aftershock, have high-effective stress drop, and exhibit a temporal decay following the Omori law (Omori, 1907). In contrast, swarm sequences exhibit a gradual increase of moment release, deviate from Båth's law, have low effective stress drop, and, in some cases, exhibit slow Omori-like temporal decay (Roland and McGuire, 2009). The occurrence of swarms is usually explained with two different mechanisms, pressure front stress increase associated with fluid flow or aseismic creep on a slip surface, which both facilitate cascading failures of nearby asperities (Chen et al, 2012).…”

“…Tectonic swarms are often interpreted as being either related to movement of fluids in the crust or, in some cases, related to aseismic creep events causing localized changes in stress (Roland and McGuire, 2009).…”