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Foreshocks are the most obvious signature of the earthquake nucleation stage and could, in principle, forewarn of an impending earthquake. However, foreshocks are only sometimes observed, and we have a limited understanding of the physics that controls their occurrence. In this work, we use high-resolution earthquake catalogs and estimates of source properties to understand the spatiotemporal evolution of a sequence of 11 foreshocks that occurred ~ 6.5 hours before the 2020 Mw 4.8 Mentone earthquake in west Texas.  Elevated pore-pressure and poroelastic stressing from subsurface fluid injection from oil-gas operations is often invoked to explain seismicity in west Texas and the surrounding region. However, here we show that static stresses induced from the initial ML 4.0 foreshock significantly perturbed the local shear stress along the fault and could have triggered the Mentone mainshock. The majority (9/11) of the earthquakes leading up to the Mentone mainshock nucleated in areas where the static shear stresses were increased from the initial ML 4.0 foreshock. The spatiotemporal properties of the 11 earthquakes that preceded the mainshock cannot easily be explained in the context of a preslip or cascade nucleation model. We show that at least 6/11 events are better classified as aftershocks of the initial ML 4.0.  Together, our results suggest that a combination of physical mechanisms contributed to the occurrence of the 11 earthquakes that preceded the mainshock, including static-stressing from earthquake-earthquake interactions, aseismic creep, and stress perturbations induced from fluid injection.  Our work highlights the role of earthquake-earthquake triggering in induced earthquake sequences, and suggests that such triggering could help sustain seismic activity following initial stressing perturbations from fluid injection. 

Abstract On 5 April 2024, 10:23 a.m. local time, a moment magnitude 4.8 earthquake struck Tewksbury Township, New Jersey, about 65 km west of New York City. Millions of people from Virginia to Maine and beyond felt the ground shaking, resulting in the largest number (>180,000) of U.S. Geological Survey (USGS) “Did You Feel It?” reports of any earthquake. A team deployed by the Geotechnical Extreme Events Reconnaissance Association and the National Institute of Standards and Technology documented structural and nonstructural damage, including substantial damage to a historic masonry building in Lebanon, New Jersey. The USGS National Earthquake Information Center reported a focal depth of about 5 km, consistent with a lack of signal in Interferometric Synthetic Aperture Radar data. The focal mechanism solution is strike slip with a substantial thrust component. Neither mechanism’s nodal plane is parallel to the primary northeast trend of geologic discontinuities and mapped faults in the region, including the Ramapo fault. However, many of the relocated aftershocks, for which locations were augmented by temporary seismic deployments, form a cluster that parallels the general northeast trend of the faults. The aftershocks lie near the Tewksbury fault, north of the Ramapo fault. 

Abstract Relative relocation techniques are widely used to improve the resolution of earthquake hypocenter positions. Here, we present GrowClust3D.jl—an open-source software package written in the programming language Julia that builds and improves upon the original GrowClust algorithm, an established relative relocation technique based on cluster analysis instead of a more traditional matrix inversion approach. The adoption of Julia’s modern programming environment allows for greater flexibility in GrowClust3D.jl’s algorithm design and its computational implementation. Notable additions to the GrowClust3D.jl package include (1) several parallel processing options to improve efficiency in uncertainty quantification routines, (2) incorporation of geographic map projections and station elevations during the relocation process, and (3) the ability to use travel-time tables derived from 3D velocity models. We demonstrate the new features of the software package on relocation problems of different scales in Nevada, California, Texas, and New Zealand, where in the latter two cases the use of a 3D velocity model helps resolve structures that remain obscure with earlier versions of GrowClust. We expect that the new GrowClust3D.jl software package will become a valuable public resource for the earthquake science community.