Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures to mitigate the COVID-19 pandemic caused widespread changes in human activity, leading to a months-long reduction in seismic noise of up to 50%. The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic noise reduction on record. While the reduction is strongest at surface seismometers in populated areas, this seismic quiescence extends for many kilometers radially and hundreds of meters in depth. This provides an opportunity to detect subtle signals from subsurface seismic sources that would have been concealed in noisier times and to benchmark sources of anthropogenic noise. A strong correlation between seismic noise and independent measurements of human mobility suggests that seismology provides an absolute, real-time estimate of population dynamics.

[1] Over the past several decades, the Yellowstone caldera has experienced frequent earthquake swarms and repeated cycles of uplift and subsidence, reflecting dynamic volcanic and tectonic processes. Here we examine the detailed spatial-temporal evolution of the 2010 Madison Plateau swarm, which occurred near the northwest boundary of the Yellowstone caldera. To fully explore the evolution of the swarm, we integrated procedures for seismic waveform-based earthquake detection with precise double-difference relative relocation. Using cross correlation of continuous seismic data and waveform templates constructed from cataloged events, we detected and precisely located 8710 earthquakes during the 3 week swarm, nearly 4 times the number of events included in the standard catalog. This high-resolution analysis reveals distinct migration of earthquake activity over the course of the swarm. The swarm initiated abruptly on 17 January 2010 at about 10 km depth and expanded dramatically outward (both shallower and deeper) over time, primarily along a NNW striking,~55°ENE dipping structure. To explain these characteristics, we hypothesize that the swarm was triggered by the rupture of a zone of confined high-pressure aqueous fluids into a preexisting crustal fault system, prompting release of accumulated stress. The high-pressure fluid injection may have been accommodated by hybrid shear and dilatational failure, as is commonly observed in exhumed hydrothermally affected fault zones. This process has likely occurred repeatedly in Yellowstone as aqueous fluids exsolved from magma migrate into the brittle crust, and it may be a key element in the observed cycles of caldera uplift and subsidence.

ABSTRACT The 2019 Ridgecrest earthquake sequence culminated in the largest seismic event in California since the 1999 Mw 7.1 Hector Mine earthquake. Here, we combine geodetic and seismic data to study the rupture process of both the 4 July Mw 6.4 foreshock and the 6 July Mw 7.1 mainshock. The results show that the Mw 6.4 foreshock rupture started on a northwest-striking right-lateral fault, and then continued on a southwest-striking fault with mainly left-lateral slip. Although most moment release during the Mw 6.4 foreshock was along the southwest-striking fault, slip on the northwest-striking fault seems to have played a more important role in triggering the Mw 7.1 mainshock that happened ∼34  hr later. Rupture of the Mw 7.1 mainshock was characterized by dominantly right-lateral slip on a series of overall northwest-striking fault strands, including the one that had already been activated during the nucleation of the Mw 6.4 foreshock. The maximum slip of the 2019 Ridgecrest earthquake was ∼5  m, located at a depth range of 3–8 km near the Mw 7.1 epicenter, corresponding to a shallow slip deficit of ∼20%–30%. Both the foreshock and mainshock had a relatively low-rupture velocity of ∼2  km/s, which is possibly related to the geometric complexity and immaturity of the eastern California shear zone faults. The 2019 Ridgecrest earthquake produced significant stress perturbations on nearby fault networks, especially along the Garlock fault segment immediately southwest of the 2019 Ridgecrest rupture, in which the coulomb stress increase was up to ∼0.5  MPa. Despite the good coverage of both geodetic and seismic observations, published coseismic slip models of the 2019 Ridgecrest earthquake sequence show large variations, which highlight the uncertainty of routinely performed earthquake rupture inversions and their interpretation for underlying rupture processes.

Faulting and fluid transport in the subsurface are highly coupled processes, which may manifest seismically as earthquake swarms. A swarm in February 2014 beneath densely monitored Mammoth Mountain, California, provides an opportunity to witness these interactions in high resolution. Toward this goal, we employ massive waveform‐correlation‐based event detection and relative relocation, which quadruples the swarm catalog to more than 6000 earthquakes and produces high‐precision locations even for very small events. The swarm's main seismic zone forms a distributed fracture mesh, with individual faults activated in short earthquake bursts. The largest event of the sequence, M 3.1, apparently acted as a fault valve and was followed by a distinct wave of earthquakes propagating ~1 km westward from the updip edge of rupture, 1–2 h later. Late in the swarm, multiple small, shallower subsidiary faults activated with pronounced hypocenter migration, suggesting that a broader fluid pressure pulse propagated through the subsurface.

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