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.

We estimate stress drops for 148 shallow (<6 km) earthquakes in the complex 2008 Mogul, Nevada, swarm using empirical Green's function‐derived spectral ratios. Near‐source, temporary broadband seismometers deployed before the Mw4.9 main shock provide high‐quality records of many foreshocks and aftershocks, and an ideal opportunity to investigate uncertainties in corner frequency measurement as well as stress drop (Δσ) variation related to space, time, depth, mechanism, and magnitude. We explore uncertainties related to source model, measurement approach, cross‐correlation limit, and frequency bandwidth. P (S) wave Δσ results range from 0.2 ± 0.15 (0.3 ± 0.15) to 36±20 (58±7) MPa, a variation greater than the error range of each individual estimate. Although this variation is not explained simply by any one parameter, spatiotemporal variation along the main shock fault plane is distinct: coherent clusters of high and low Δσ earthquakes are seen, and high‐Δσ foreshocks correlate with an area of reduced aftershock productivity. These observations are best explained by a difference in rheology along the fault plane. Average Δσs of 3.9±1.1 (4.0±1.1) MPa using P (S) are similar to those found for earthquakes in a variety of settings, implying that these shallow, potentially fluid‐driven earthquakes do not have systematically lower Δσ than average tectonic earthquakes (~4 MPa) and, therefore, have similar (or higher, due to proximity to the surface) expected ground motions compared to typical earthquakes. The unprecedented detail achieved for these shallow, small‐magnitude earthquakes confirms that Δσ, when measured precisely, is a valuable observation of physically meaningful fault zone properties and earthquake behavior.

Global Navigation Satellite Systems (GNSS)‐based earthquake early warning (EEW) algorithms estimate fault finiteness and unsaturated moment magnitude for the largest, most damaging earthquakes. Because large events are infrequent, algorithms are not regularly exercised and insufficiently tested on few available data sets. We use 1300 realistic, time‐dependent, synthetic earthquakes on the Cascadia megathrust to rigorously test the Geodetic Alarm System. Solutions are reliable once six GNSS stations report static offsets, which we require for a “first alert.” Median magnitude and length errors are −0.15 ± 0.24 units and −31 ± 40% for the first alert, and −0.04 ± 0.11 units and +7 ± 31% for the final solution. We perform a coupled test of a seismic‐geodetic EEW system using synthetic waveforms for a Mw8.7 scenario. Seismic point‐source solutions result in severely underestimated peak ground acceleration. Geodetic finite‐fault solutions provide more accurate predictions at larger distances, thus increasing warning times. Hence, GNSS observations are essential in EEW to accurately characterize large (out‐of‐network) events and correctly predict ground motion.

Displacement waveforms derived from Global Navigation Satellite System (GNSS) data have become more commonly used by seismologists in the past 15 yrs. Unlike strong-motion accelerometer recordings that are affected by baseline offsets during very strong shaking, GNSS data record displacement with fidelity down to 0 Hz. Unfortunately, fully processed GNSS waveform data are still scarce because of limited public availability and the highly technical nature of GNSS processing. In an effort to further the use and adoption of high-rate (HR) GNSS for earthquake seismology, ground-motion studies, and structural monitoring applications, we describe and make available a database of fully curated HR-GNSS displacement waveforms for significant earthquakes. We include data from HR-GNSS networks at near-source to regional distances (1-1000 km) for 29 earthquakes between M w 6.0 and 9.0 worldwide. As a demonstration of the utility of this dataset, we model the magnitude scaling properties of peak ground displacements (PGDs) for these events. In addition to tripling the number of earthquakes used in previous PGD scaling studies, the number of data points over a range of distances and magnitudes is dramatically increased. The data are made available as a compressed archive with the article.

Analysis of a small earthquake swarm near Virginia City, NV, reveals complex structural features, including an interplay of both fluid‐driven and aseismic‐driven earthquake migration within a naturally occurring tectonic sequence. The Virginia City earthquake sequence occurred over ~10 days in January 2014. We relocate 305 events to reveal three separate, well‐defined planar structures. The earthquakes initially migrate at a rate consistent with pore fluid diffusion, outlining a moderately dipping plane. The earthquakes then jump to a vertical plane and migrate at a higher rate; the sequence continues to migrate rapidly onto a third, shallowly dipping plane, consistent with rates observed elsewhere associated with aseismic creep. Focal mechanisms indicate right‐lateral strike slip on the vertical plane and both normal and left‐lateral strike slip movement on the other planes, and the newly imaged structures illuminate the orientation of active faults at depth in the Walker Lane tectonic region.