Eric Beaucé

We invert horizontal S-wave Fourier acceleration spectra between 0.5 and 25 Hz from 7,637 earthquakes in the 2010 Mw 8.8 Maule aftershock sequence to estimate source, path, and site terms in south-central Chile using a nonparametric generalized inversion technique. The dataset contains 95,515 event-station records from 117 stations during the first 10 months after the mainshock. To account for propagation heterogeneity, we divide crustal and slab-related earthquakes separately into spatially coherent clusters and invert each one independently. We use 100 randomized clustering iterations to evaluate the variance of the recovered terms. Relative site responses remain stable across these iterations, with site amplification above a factor of 10 at 2–4 Hz at some stations. Effective attenuation varies markedly across the rupture area, with Q0 values of 36–159 and frequency exponents of 0.35–1.4. Crustal clusters generally have lower Q0 and stronger frequency dependence than slab-related ones. We fit the recovered source spectra with an ω² model and obtain well-resolved source parameters for 6,279 events. Stress-drop distributions overlap between crustal and slab-related earthquakes, with median values of 2.93 and 2.76 MPa, respectively. For Mw ≥ 3, corner frequency scales approximately as M0^(−1/3), broadly consistent with self-similarity. Stress drops are systematically higher in a slab-related seismicity band between 42.5 and 52.5 km depth, with a median of 6.3 MPa. These results show that accounting for spatially variable propagation is important for interpreting source variability. The empirical model provides a basis for regional ground-motion characterization and for tracking postseismic fault evolution across the Maule rupture zone.

We studied the tidal modulation of the rate of seismicity in the Ridgecrest, California area between 2008 and 2024, thus including the 2019 Mw 7.1 earthquake. We combined machine-learning-enhanced earthquake detection and location with a novel methodology to characterize tidal modulation. We identified significant modulation of seismicity in the Ridgecrest fault zone and in the neighboring regions, the Coso volcanic field and the Garlock fault, but tidal modulation in the fault zone exhibits a unique temporal evolution. Modulation in the fault zone shifts, around 2014–2015, from a stress-controlled regime where peak seismicity occurs at peak tidal stress to a stress-rate-controlled regime where peak seismicity occurs at peak tidal stress rate. In contrast, peak seismicity in Coso and along the Garlock fault consistently occurs at peak tidal stress throughout time. Moreover, the strength of tidal modulation in the fault zone increases before the 2019 Mw 7.1 earthquake, unlike in its surroundings. A possible interpretation based on experimental and theoretical work suggests that some characteristic frictional time scale (the nucleation time or the critical period of the fault) becomes shorter than the dominant tidal period after 2015. This change implies that the fault loading rate increases, which may be accompanied by a decrease in effective normal stress. Reduced effective stress, fault zone resonance, or increased mechanical compliance could cause the increase in modulation strength. We propose that these changes manifest a preparation phase before a large earthquake.

We invert horizontal 𝑺-wave Fourier acceleration spectra between 0.5 and 25 Hz from 7,452 earthquakes in the 2010 𝑴𝐰 8.8 Maule aftershock sequence to estimate source, path, and site terms in south-central Chile using a nonparametric generalized inversion technique. The dataset contains 101,566 event-station records from 2,257 crustal and 5,195 slab-related earthquakes recorded by 114 stations during the first 10 months after the mainshock. To account for propagation heterogeneity, we perform separate inversions for spatially coherent event clusters within both seismotectonic regimes, assign cluster-specific shear-wave velocity from a 3D velocity model, and use 100 randomized clustering iterations to evaluate the variance of the recovered terms. We find that relative site responses remain stable between inversion strategies. At some stations, relative amplification approaches a factor of 10 at 2–3 Hz. Effective attenuation varies markedly across the rupture area, with 𝑸𝟎 values of approximately 30–160 and frequency exponents of 0.4–1.5. Crustal clusters generally have lower 𝑸𝟎 and stronger frequency dependence than slab-related clusters. We fit the recovered source spectra with an 𝝎𝟐 model. Stress-drop distributions overlap between crustal and slab-related earthquakes, with median values of 0.95 and 0.78 MPa, respectively. Source scaling is close to self-similar above approximately 𝑴𝐰 3.5, where the weak dependence of stress drop on seismic moment becomes nearly flat. A slab-related seismicity band between 42.5 and 52.5 km depth contains systematically higher stress-drop values, with a median value of 1.75 MPa. These results show that resolving spatially variable propagation is necessary for interpreting source variability. The resulting empirical model provides a basis for regional ground-motion characterization and for tracking postseismic fault evolution across the Maule rupture zone.

Intraplate regions commonly host energetic earthquakes on less‐prominent faults, raising questions on how fault structure may influence intraplate seismogenesis. We investigate the causative fault of the 2024 Mw4.8 New Jersey earthquake which occurred near the misoriented 300‐km long Ramapo Fault, by integrating seismicity‐guided field geological mapping with friction experiments and slip tendency analysis. We find that the up‐dip projection of relocated aftershocks aligns with a zone of NNE‐striking topographic (LiDAR) traces of fault scarps, marking a previously unmapped fault zone in the New Jersey Highlands, henceforth designated as “Mountainville Fault (MF).” In basement outcrops along the topographic lineaments, we observe a predominance of NNE‐striking/E‐dipping, slickensided, gougeless fracture clusters with horsetail tip‐splays, indicating a brittle shear zone origin. The fractures are poorly coalesced, and comprise steeply‐dipping sets that are crosscut by gently‐dipping sets. At the surface, MF lacks a break‐through principal slip surface (core), and the geomorphic scarps and fractures strike at ∼15–25° obliquity to the mainshock's nodal plane, suggesting that the field‐mapped structures represent the Riedel‐shears of a deeper blind fault core, characteristic of an “immature” fault zone. Laboratory‐constrained frictional behavior of sampled representative fault rocks, and slip tendency computation from focal mechanism‐resolved stress field predict frictional instability of the MF. Additionally, the friction experiment results suggest a possible contribution of concealed fluid‐rock alterations at the depth of the mainshock. Altogether, these MF characteristics fit those of other structurally investigated earthquake sources in the region, demonstrating the seismic hazard of elusive, critically‐oriented immature intraplate faults.

We re‐examine the aftershock sequence of the M _w 8.8 Maule earthquake in south‐central Chile to understand how seismicity, magnitude‐frequency distribution, and fault structure vary along the rupture zone. Using the International Maule Aftershock Deployment (IMAD) data set, we analyze 10 months of continuous data from approximately 156 temporary stations and build a high‐resolution aftershock catalog for the Maule rupture zone. We apply the BackProjection and Matched‐Filtering (BPMF) workflow, which integrates a deep‐learning phase picker with backprojection‐based association, relative relocation, and template matching. We initially detect and relocate 130,575 earthquakes, then use a subset of high‐quality events as templates to identify smaller earthquakes missed by initial detection. The final catalog contains 537,387 earthquakes, nearly 13 times more events than in previous studies, with a completeness magnitude of ≈ M _w 1.8 and magnitudes ranging from M _w 0.2 to M _w 6.2. A local magnitude () calibration provides a homogeneous magnitude scale across the network. The dense catalog reveals detailed seismotectonic features along the rupture. In the Pichilemu region, aftershocks delineate a shallow normal fault system with L‐shaped geometry, whereas the Concepción area contains aseismic patches. Using the classical maximum likelihood and ‐more‐incomplete methods, we find that temporal ‐values range between 1.2 and 1.6 early in the sequence and converge toward ≈1.0. Meanwhile, ‐values vary strongly along strike, with higher values in the north and lower values in the south. These contrasts are consistent with along‐strike variations in effective stress and pore fluid pressure on the plate interface, in line with previous studies.

The Ramapo Seismic Zone (RSZ) in the Northeastern United States hosts frequent but poorly understood intraplate earthquakes, potentially posing a significant hazard to the nearby New York metropolitan area. The 5 April 2024, 4.8, Tewksbury, New Jersey earthquake, provides a rare opportunity to study the RSZ seismicity. We applied machine‐learning‐enhanced backprojection, matched‐filtering, correlation‐timing and double‐difference methods to continuous waveforms recorded at local and regional stations to detect and locate about 2,000 aftershocks ( 0.0) within the 74 days following the mainshock. They reveal a single, 51° east‐southeast dipping fault plane possibly abutting the Ramapo fault at depth to the north. Aftershock locations are consistent with a shallow (4 km) mainshock hypocenter with rupture propagating downward and terminating at a depth of about 6 km. A relatively high Gutenberg‐Richter b ‐value and a low aftershock spatial fall‐off rate suggest that the Tewksbury sequence activated a rough, immature fault.

Intraplate regions commonly host energetic earthquakes on less-prominent fault zones, raising questions on how fault structure may influence intraplate seismogenesis. Here, we investigate the causative fault of the strongly-felt April 5, 2024 Mw4.8 New Jersey Earthquake which occurred near the misoriented 300-km long Ramapo Fault. Field mapping of ancient fracture networks reveals that the up-dip projection of relocated aftershocks is co-located with a previously unmapped fault zone striking parallel to the mainshock’s nodal plane, henceforth designated as “Mountainville Fault (MF)”. MF hosts an irregular ‘stair-step’ internal structure with dominantly NNE-striking/E-dipping gougeless fracture clusters, and steeply-dipping sets cross-cut by gently-dipping slickensided surfaces, indicating an immature, rough fault. Laboratory friction experiments suggest possible shallow stress perturbations and/or concealed fault rock alteration, and with slip tendency analysis, predict the fault’s instability. These MF characteristics fit other structurally investigated earthquake sources in the region, demonstrating the seismic hazard of elusive, critically-oriented immature intraplate faults.

In subduction zones, along‐strike and downdip variations in megathrust slip behavior are linked to changes in properties of the subducting and overriding plates. Although marine geophysical methods provide insights into subduction zone structures, most surveys consist of sparse 2D profiles, limiting our understanding of first‐order controls. Here, we use active‐source seismic data to derive a 3D crustal‐scale P‐wave velocity model of the Alaska Peninsula subduction zone that encompasses both plates and spans the Semidi segment and SW Kodiak asperity. Our results reveal modest variations within the incoming plate, attributed to a series of fracture zones, seamounts and their associated basement swell, collectively contributing to plate hydration. Basement swell appears to modulate the distribution and type of sediment entering the trench, likely impacting observed variations in slip behavior. The overriding plate exhibits significant heterogeneity. The updip limit and width of the dynamic backstop are similar between the SW Kodiak asperity and eastern Semidi segment, but differ significantly from the Western Semidi segment. These distinctions likely account for differences in earthquake rupture patterns and interseismic coupling among these segments. Additionally, high‐velocities in the mid‐lower forearc crust coincide with the location of megathrust slip during the Mw 8.2 2021 Chignik event. We interpret these velocities as intracrustal intrusions that contributed to the deep rupture of the 2021 event. Our findings suggest that the contrasting structural and material properties of both the incoming and overriding plates influence the spatially complex and semi‐persistent segmentation of the megathrust offshore the Alaska Peninsula.