Complex spatiotemporal evolution of the 2008 M_w 4.9 Mogul earthquake swarm (Reno, Nevada): Interplay of fluid and faulting

Abstract

After approximately 2 months of swarm‐like earthquakes in the Mogul neighborhood of west Reno, NV, seismicity rates and event magnitudes increased over several days culminating in an Mw 4.9 dextral strike‐slip earthquake on 26 April 2008. Although very shallow, the Mw 4.9 main shock had a different sense of slip than locally mapped dip‐slip surface faults. We relocate 7549 earthquakes, calculate 1082 focal mechanisms, and statistically cluster the relocated earthquake catalog to understand the character and interaction of active structures throughout the Mogul, NV earthquake sequence. Rapid temporary instrument deployment provides high‐resolution coverage of microseismicity, enabling a detailed analysis of swarm behavior and faulting geometry. Relocations reveal an internally clustered sequence in which foreshocks evolved on multiple structures surrounding the eventual main shock rupture. The relocated seismicity defines a fault‐fracture mesh and detailed fault structure from approximately 2–6 km depth on the previously unknown Mogul fault that may be an evolving incipient strike‐slip fault zone. The seismicity volume expands before the main shock, consistent with pore pressure diffusion, and the aftershock volume is much larger than is typical for an Mw 4.9 earthquake. We group events into clusters using space‐time‐magnitude nearest‐neighbor distances between events and develop a cluster criterion through randomization of the relocated catalog. Identified clusters are largely main shock‐aftershock sequences, without evidence for migration, occurring within the diffuse background seismicity. The migration rate of the largest foreshock cluster and simultaneous background events is consistent with it having triggered, or having been triggered by, an aseismic slip event.

Spatiotemporal Variation of Stress Drop During the 2008 Mogul, Nevada, Earthquake Swarm

“…The results are also not significantly different from intraplate earthquakes (e.g., Viegas et al, ), earthquakes observed near major fault zones (e.g., Abercrombie, ), or reverse faulting earthquakes (e.g., Abercrombie et al, ; Cocco & Rovelli, ). Fluid‐driven pore pressure diffusion inferred from seismicity migration likely drove the Mogul earthquake swarm (Ruhl et al, ) but did not facilitate rupture at lower stress drops as some studies suggest for induced geothermal events near an injection point (e.g., Goertz‐Allmann et al, ). This suggests that shallow, fluid‐driven earthquakes do not have systematically lower stress drops and, therefore, have similar (or higher, due to proximity to the surface) expected ground motions compared to typical tectonic earthquakes.…”

“…Combining moment tensors from Ruhl et al () with eight additional moment tensors in the Reno region, we calculate the following relationship between local magnitude and seismic moment for the Reno area: …”

“…Small‐magnitude seismicity began affecting the Mogul neighborhood of west Reno, NV, in late February 2008 and continued to build in intensity leading to an M w 4.9 main shock on 26 April 2008. Ruhl et al () relocated and analyzed the sequence and found that statistically dependent subclusters behaving like main shock‐aftershock sequences evolved on multiple distinct fault planes transiently defining an optimally oriented fault‐fracture mesh just southwest of the main fault plane. During the 2 month foreshock period, seismicity migrated away from the initial relocated events at a rate consistent with fluid diffusion (Ruhl et al, ).…”

“…Ruhl et al () relocated and analyzed the sequence and found that statistically dependent subclusters behaving like main shock‐aftershock sequences evolved on multiple distinct fault planes transiently defining an optimally oriented fault‐fracture mesh just southwest of the main fault plane. During the 2 month foreshock period, seismicity migrated away from the initial relocated events at a rate consistent with fluid diffusion (Ruhl et al, ). Foreshocks also highlight the main shock fault plane allowing us to access earthquake behavior on the primary fault before and after main shock rupture.…”

“…The waveform‐based double‐difference catalog of Ruhl et al () includes 7,549 earthquakes ( M L ‐0.75 to M w 4.9) occurring above 6 km depth; these depths are unusually shallow for the Reno‐Tahoe region (Ruhl, Seaman, et al, ). The relatively dense regional seismic network combined with a very near‐source, temporary deployment of 13 broadband seismometers provide high‐quality records of many foreshocks and aftershocks and enabled calculation of over 1,000 focal mechanisms and 11 moment tensors (Figure b).…”