Abstract Distributed acoustic sensing (DAS) on seafloor telecommunications cables is emerging as a powerful approach for observing geophysical processes. However, instrumental noise increases steeply below 10 millihertz (mHz), limiting the resolution of long-period signals. Here, we overcome this limitation by isolating low-frequency common-mode noise using channels with minimal signal amplitude, enabling the observation of low-frequency spectral peaks consistent with Earth’s normal-mode oscillations in the mHz band along a submarine fiber-optic cable in the western Mediterranean Sea. Following the 2025 Mw 8.8 Kamchatka earthquake, we observe systematic variations in modal amplitudes with cable azimuth over 120 km, consistent with predictions from a normal-mode summation model. The amplitude is predicted above the noise level, but the signal is only observed after noise removal. This is explained by an event-independent response coefficient (C ∼ 0.13), consistent with a flat instrumental response. Normal modes are also detected for smaller earthquakes down to Mw 7.7. The variably oriented cable resolves the radial and transverse horizontal components of motion. The large-scale signal’s amplitude is locally modulated by variable cable azimuth, demonstrating a sensitivity for cable relocation. Our results demonstrate that DAS can resolve subtle solid-Earth deformation at mHz frequencies, opening new opportunities for large-scale, ocean-based monitoring of Earth structure and dynamics.
Abstract Seismicity in the interrift zone between the Upper Rhine graben and the Lower Rhine Embayment has historically been dominated by the Ochtendung seismic zone near the Laacher See Volcano in the East Eifel. However, in recent years, persistent activity has emerged farther south. A seismic swarm initiated in 2018 in the Taunus region near Bad Schwalbach, followed in 2024 by a second swarm 50 km to the west in the Hunsrück region near Mörsdorf. Both sequences are still active. Relocated hypocenters reveal northwest–southeast elongated clusters that migrate systematically from ∼15 km depth toward a structural transition near 10 km, with apparent diffusivities of 0.04 m2/s (Mörsdorf) and 0.01 m2/s (Bad Schwalbach). Moment tensor solutions indicate normal faulting on planes rotated by ∼30° relative to the larger-scale hypocentral trend, suggesting seismic slip on secondary Riedel-type fractures within the damage zones of inherited Variscan faults. The combination of multiyear duration, slow migration, and a structural barrier locally breached by ML 1.5 events distinguishes these sequences from typical tectonic swarms and points to a sustained, deep-seated fluid supply, plausibly connected to the broader mantle-fluid system documented for the Eifel region.
The 28 March 2025 Mw 7.7 Mandalay earthquake produced long-period ground motions that propagated throughout mainland Southeast Asia. Although the supershear rupture along the Sagaing fault has been documented, its influence on regional shaking has not yet been quantified. Earlier work by Ornthammarath et al. (2025) focused mainly on long-period basin amplification in Bangkok. The present study expands the analysis to quantify amplitude and duration characteristics using 30 three-component strong-motion recordings from Myanmar and Thailand. We analyze amplitude and duration residuals relative to Next Generation Attenuation-West2 ground-motion and duration models. The results reveal a pronounced azimuthal pattern: stations located between 110 and 155 azimuth degrees from the rupture fault aligned with the Mach fronts of the southward-propagating supershear rupture exhibit large positive spectral acceleration residuals and strongly negative D5–95 duration residuals. In contrast, stations southwest and west of the rupture (e.g., Yangon and Nyaung U, respectively) show negative amplitude residuals and positive duration residuals. This bilateral contrast closely matches the expected Mach cone radiation pattern of a supershear rupture, demonstrating that regional ground motions (150–600 km) may retain the geometric imprint of the rupture-front speed. These findings suggest that supershear-related directional effects may persist beyond the near-fault region and contribute to far-field amplification patterns, with potential implications for incorporating rupture dynamics and a fault-to-site azimuth parameter into ground-motion models used in seismic hazard assessments of large strike-slip faults in similar tectonic environments.
Strong-motion record processing provides the necessary inputs for seismic hazard analysis, structural design, and risk assessment; however, low-frequency noise and baseline offsets can introduce drift in the displacement obtained through integrating acceleration. Therefore, selecting an appropriate high-pass corner frequency (fcHP) is critical. Existing automated routines can be efficient; however, they remain sensitive to the pre-event noise window, which depends on accurate identification of the P-wave arrival time (tP). This study reformulates the joint estimation of tP and fcHP as a single segmentation-based localization task. For each component, the records were processed over candidate fcHP values from 0.005 to 1.28 Hz, resampled to 1024 points, and stacked into a 256 & times; 1024 displacement-frequency matrix. The dataset is composed of 2380 three-component records from the Kyoshin network and Kiban-Kyoshin network and uses an earthquake-level split. Reference tP and fcHP values were obtained by visual review following a fixed protocol using a web-based tool and displacement stability criteria in the pre-event and tail windows. DeepLabv3+ with an EfficientNet-B3 encoder achieved the lowest mean localization error (5.17 pixels). For independent events, tP predictions achieved R2 = 0.985 for the vertical component, and fcHP predictions achieved R2 = 0.825 across the components. This approach has been applied to recent earthquakes in Japan.
The tectonics of south-central Alaska are dominated by subduction of the Pacific plate, the Yakutat oceanic plateau, and evolution of the major continental fault system, the Denali fault. Our new structural images show the razor-sharp edge and extent of the Yakutat slab as it subducts beneath the North American plate with remarkable detail. Thousands of small, previously undetected earthquakes form a prominent linear cluster that illuminates the precise edge of the subducted Yakutat microplate and defines the location of changing slab morphology reflecting the change in stress state. Our new seismic images and refined locations of down-dipping seismicity further elucidate the lack of mantle wedge below the Denali volcanic gap and the northeastern margin of the subducted Yakutat microplate, imaged here to be directly below the curved section of the Denali fault. We suggest these features have controlled the nucleation of the 2002 Mw 7.9 Denali fault earthquake and the position of recently onset volcanic fields.
Seismic instruments are commonly installed at depth to reduce ambient noise, yet lower noise levels do not necessarily translate into improved event detectability if the signal amplitudes also vary with depth. Evaluating borehole performance, therefore, requires considering both background noise and signal behavior. We analyze a vertical borehole array at the Glasgow Observatory, Scotland, consisting of five identical broadband accelerometers deployed between 29 and 198 m depth within a single borehole, allowing depth-dependent effects to be isolated from site and instrumentation differences. We combine long-term power spectral density (PSD) analysis with event-based short-term average/long-term average and root mean square measures to assess how instrument depth influences seismic signal quality. PSD results show a reduction in background noise with increasing depth at frequencies above 1 Hz, with the deepest instrument exhibiting a noise reduction on the order of 10 dB in the 1-10 Hz band. Analysis of 39 regional earthquakes shows that seismic signal amplitudes also vary with depth and can partially offset noise reductions at intermediate depths, such that improvements in detection are observed only where noise attenuation exceeds depth-dependent changes in signal amplitude. These results demonstrate that deeper instrument installation does not guarantee improved detection performance. Instead, detectability reflects the balance between noise attenuation, depth-dependent signal behavior, and site-specific structure.
MyShake is a free smartphone application that delivers earthquake early warning alerts to users in California, Washington, and Oregon. It also provides earthquake information and can gather community science earthquake data, such as earthquake shaking waveforms and felt report data, from users. In this study, we focus on the felt reports collected by the app: users can qualitatively report their experience of earthquake shaking on a simple five-point shaking scale. We leverage a MyShake felt report database of 314,999 records to evaluate the utility of MyShake felt reports for intensity mapping. We determine a linear relationship between the MyShake shaking scale and modified Mercalli intensity (MMI) values derived from the U.S. Geological Survey's "Did You Feel It?" (DYFI) felt reports and show that the MyShake-reported shaking correlates well with MMI values from DYFI reports. We then demonstrate the performance of the MyShake shaking scale in shaking intensity maps for small-to-moderate earthquakes, highlighting the potential for MyShake felt reports to contribute to further intensity and ground-motion analyses. At higher intensity levels, damage corresponds to MMI better than felt shaking, and as such, our experience-based MyShake-to-MMI mapped values might present shortcomings at those levels.
A handful of credible accounts, both recent and historical, describe trees being broken during earthquakes. Apart from landslides and tsunamis, such accounts are uncommon, suggesting that trees break only when ground acceleration is extreme or when trees are especially vulnerable to breakage. Here, I consider the question: what ground acceleration is required to break a living tree? I present simplified first-principles calculations, considering the properties of common trees, focusing on tall, thin trees that can be approximated as uniform vertical cantilevers. The results suggest that, possibly excepting tall trees with heavy canopies, standing trees can only be broken by shaking approaching or exceeding 1g, as anticipated by the rarity of documented accounts of snapped trees. I briefly consider several well-documented instances of tree damage during strong earthquakes, drawing inferences about local ground motions.
On 15 December 2024, seismic networks detected signals that the media reported as being associated with a large surface explosion near the city of Tartus in Syria. Arrival times of seismic signals from this event show that the epicenter is near a surface crater identified in satellite imagery, and observed P/S ratios are different from those observed for a nearby presumed earthquake. At seismometer stations in eastern Cyprus, the largest amplitude arrivals are air-to-ground coupled waves, and the observation of further such arrivals allows us to identify and estimate origin times for 10 smaller events in a 40 min time window, many of which were not directly detected seismically. Trinitrotoluene equivalent explosive yields were estimated for the seismically detected events by modeling the amplitude of Pg, and estimates for the largest event are consistent with infrasound observations and the crater diameter. A linear relationship between the seismically estimated explosive yields and the amplitudes of air-to-ground coupled waves allowed us to estimate yields for the remaining events and highlighted the importance of making use of air-to-ground coupled waves when characterizing sequences of small, weakly coupled explosive events.
The 10 September 2025 Mw 4.1 earthquake in northeastern Utah, United States, had a focal depth 68 km beneath sea level, which is 20-25 km greater than estimates of local crustal thickness, making it a rare example of a continental mantle earthquake (CME). The focal depth is well resolved from arrival-time inversion (nearest station 13 km away) and moment tensor inversion of regional waveforms. Similar to other CMEs in the Intermountain West, there were no obvious aftershocks or foreshocks, and the waveforms were enriched in high-frequency energy. Spectral modeling gives a stress drop of 80 MPa and a radiation efficiency of 0.08, albeit with large uncertainties. The high stress drop and low radiation efficiency are consistent with a dissipative source process such as thermal runaway. Also similar to previous Intermountain West CMEs, the event occurred along the boundary of the Archean Wyoming craton, where pressure-temperature conditions favor ductile deformation. We hypothesize that edge-driven or regional-scale mantle convection produces increased strain rates near the craton boundary that make either conventional brittle failure or thermal runaway feasible at relatively high pressure-temperature conditions. High conductivity inferred around the edge of the craton may suggest that fluids also contribute to CME occurrence.
Three multinode infrasound station arrays were deployed along the perimeter of a prescribed burn at Eglin Air Force Base, Florida. One six-node station and two two-node stations monitored the fire progression while a helicopter encircled the burning area, generating significant acoustic signals. Fundamental-mode power analysis and beamforming search were used to extract estimated infrasound wave propagation direction and wavespeed. The three stations triangulated azimuth angles to estimate helicopter location. Elevation was estimated using variations of spectrum power, and apparent source speed was extracted via Doppler shift. Publicly available Global Positioning System locations provided true helicopter location, speed, and flight orientation compared here with acoustically derived estimates, thus providing uncertainty limits. For the six-node, hexagonal station array, the median angle error was 9 degrees with an interquartile distance of 9.8 degrees. Overall, helicopter location errors, using three station arrays, ranged from 10s to 100s of meters. Spatially distributed multinode stations can be effective at tracking moving sources such as helicopters, especially if each station array has three or more distributed sensor nodes.
Seismicity in the Eagle Ford Shale (EFS) of south-central Texas has increased over the past decade, largely because of intensified oil and gas activities. Since 2017, the Texas Seismological Network (TexNet) has improved monitoring in this region, increasing cataloged earthquakes from 33 events with M >= 2.3 (1982-2017) to 3882 events with M >= 0.4 (2017-2026). However, routine TexNet locations in southern Texas relied on the global IASP91 1D velocity model, which cannot best constrain the focal depths because of the overly simplified crustal structure that ignores local geologic features. Here, we present a relocated EFS earthquake catalog for TexNet operations using a regional 1D velocity model (EF1D). Compared with the original IASP91-based catalog, travel-time residuals decreased, and hypocenters tightened into northeast-southwest lineations that coincide with mapped normal faults. Most events were relocated to 6-9 km depth and near the basin-basement interface, indicating predominantly basement-rooted seismogenic faults. We relocated all 2461 EFS events in the TexNet catalog from 2017 through May 2025 with EF1D, and events detected after July 2025 in southern Texas are now routinely reported with this model. The relocated catalog and metadata are publicly available through the TexNet Earthquake Catalog and the U.S. Geological Survey ComCat and International Federation of Digital Seismograph Networks webservices, superseding earlier EFS solutions and improving the basis for induced seismicity and hazard studies in southern Texas.
Higher-frequency waveform simulation and processing could improve seismic monitoring of low-magnitude events at local to regional distances, but it is unclear when the investment is worthwhile. Earth model uncertainty (EU) and background noise create information-theoretic limits on how informative a waveform can be. We introduce a Bayesian experimental design framework to rigorously predict the benefit of incorporating waveform data or extracted waveform features into seismic monitoring for source-parameter inference. Using a synthetic, model-based study, we leverage this framework to answer questions about the value of high frequencies for constraining event location and source parameters. We identify the minimum frequency requirements for inference and a maximum frequency at which there are diminishing returns under different EU and background noise assumptions. Ultimately, this informs how we should invest research and development efforts across Earth model refinement, higher frequency computational simulation, and reducing background noise.
The olivine-to-wadsleyite transition near 400 km depth (d400) is sensitive to the thermochemical structure of the upper mantle. We estimate the depth of d400 using three seismic phases with different sensitivities to d400 and to the P-wave and S-wave velocities (VP and VS) above d400. Sv400s and P400s are the teleseismic reflection and conversion at d400. Stacks of cross-correlated late coda from 83 large earthquakes recorded by the Southern California Seismic Network (SCSN) contain a unique signal of the topside p400p reflection off the d400. The joint analysis of the p400p lag time and the travel times of Sv400s and P400s helps mitigate the trade-offs and uncertainties in the estimate of the depth of d400. The three travel times are longer than expected in the preliminary reference Earth model due to low wave speeds in the upper mantle beneath the SCSN and/or a d400 deeper than 400 km. A unique combination of OVP/VP and OVS/VS can be found if d400 is shallower than about 430 km. A depth of 410-420 km requires concomitant reductions in OVP/VP and OVS/VS that are similar to the velocity reductions found in the tomographic models REVEAL and GLAD-M35.
In Earth’s deepest mantle (D″), shear wave speeds can vary with propagation and vibration direction of the wave because of convection-driven mantle deformation, a phenomenon known as seismic anisotropy. Although D″ anisotropy has been mapped globally at long wavelengths (>4000 km laterally) in tomographic inversions, models exhibit disagreements at smaller scales (100s to 1000s of kilometers). Here, we present 70,000 differential splitting SKS-SKKS and PKS–SKKS measurements from a global seismic dataset that samples nearly 75% of Earth’s D″ layer, predominantly in the highest seismic wave speed regions. Anisotropy is found in about two-thirds of our sampled area, almost tripling the area where seismic anisotropy has been investigated at short wavelengths. Although the overall distribution is complex, lowermost mantle deformation is frequently detected in locations associated with putative deeply subducted slabs, thereby linking D″ deformation processes to the subduction of tectonic plates. In these colder, slab-dominated regions, the observed pervasive seismic anisotropy is consistent with the crystallographic preferred orientation of postperovskite as a significant contributor.
The Seattle basin is a deep sedimentary basin in the Seattle-Bellevue, Washington metropolitan area within the Puget Lowland of Washington State. We determine the structure of a portion of the basin and the underlying basement using analysis of P waves converted from direct S incident from below. A deep local crustal event beneath Monroe, about 35 km northeast of Seattle, was recorded by a 100-station nodal array deployed in 2019. The event produced a variety of coherent seismic phases, including converted waves from the sediment -basement boundary, internal structure within the basin, and additional crustal discontinuities. Using observed Sp converted waves, we apply an adjoint-based full waveform inversion (FWI) method to determine the amplitude and extent of seismic discontinuities at depth. We find the strongest source of converted waves for this event lies 6 to 7 km depth below northern Lake Washington, interpreted to be the local depth to basement rock. The newly imaged shallow basement structure may be part of a deformation zone associated with the Siletzia eastern boundary. Our results highlight the utility of converted seismic waves recorded by a dense array, combined with an FWI method, to illuminate crustal structure.
The recent observation of neighboring earthquakes with anti-correlated waveforms, referred to as anti-repeating earthquakes, or anti-repeaters, suggests that almost opposite fault slip may occur consecutively along the same or nearby structures in Earth. These earthquakes have so far been identified in a few areas only, although involving different seismogenic contexts, like tectonic, volcanic, and induced seismicity, so they could be a more common phenomenon than previously thought. Here, we analyze large and moderate-sized earthquakes worldwide in a systematic manner, scanning the Global Centroid Moment Tensor catalog over 48 yr in search of pairs of earthquakes with neighboring centroid locations and nearly opposite moment tensors. The results are somewhat surprising: thousands of nearby earthquake pairs with opposite source orientations can be found, clustering at hundreds of locations on Earth, occurring at different depths, in different tectonic settings, as well as in volcanic environments. We validate the detections by their waveform anti-correlation and report a global catalog of opposite faulting, as a first atlas for this new type of seismological observation.
Determining why earthquake ruptures stop is a central challenge in earthquake science and seismic hazard assessment. The P & uuml;t & uuml;rge segment of the East Anatolian Fault Zone, T & uuml;rkiye, exhibits shallow creep ( 6.5 earthquake ruptures at greater depth. Here, we evaluate whether variations in frictional stability along this segment aided arrest of the 2020 M 6.7 Elazi & gbreve; and 2023 M 7.8 Pazarc & imath;k earthquake ruptures. Analysis of Sentinel-1 Synthetic Aperture Radar imagery indicates the 2023 M 7.8 rupture propagated laterally into a metamorphic massif within the P & uuml;t & uuml;rge segment, where slip rapidly decayed below detection limits. Creepmeters along this segment recorded no significant surface afterslip (<3 mm) in the following year. To investigate this fault-slip behavior, we conducted triaxial friction experiments on P & uuml;t & uuml;rge fault gouge sampled from an outcrop exposure. The gouge, composed primarily of muscovite, quartz, and calcite, is velocity strengthening at conditions approximating 0-2.5 km depth and velocity weakening at 4-5 km depth. This transition to velocity-weakening friction is associated with enhanced comminution and shear localization observed microstructurally. Our results suggest that depth-dependent frictional stability of the P & uuml;t & uuml;rge fault segment facilitates rupture nucleation and propagation at depth while maintaining rupture-arresting behavior near Earth's surface.
Earthquake swarms are often linked to variations in subsurface fluid pressures, driven by natural processes or anthropogenic activity. Here, we investigate whether an earthquake swarm in Nordland, a region in northern Norway, was possibly related to the construction of a hydropower tunnel. Using Interferometric Synthetic Aperture Radar, we observe subsidence following the tunnel's construction, which can be explained by significant water leakage encountered during the work. Three years later, an earthquake swarm occurred about 3 km from the tunnel, in an area without previous swarm activity. We build a catalog of the swarm and through analysis of its spatio-temporal distribution propose that changes in pore pressure and stress, potentially linked to the leakage, may have played a role in triggering the swarm. Fault valving or pumping are possible mechanisms to explain the cascading nature of the earthquakes in the swarm.
We document, for the first time, the dynamic triggering of earthquakes in Costa Rica due to the wavefield from teleseismic events. Dynamically triggered failure is typically observed in volcanic or geothermal settings, characterized by the presence of high pore pressures weakening fault settings by reducing the normal effective stress, and is less common in continental or local faults. Using the local network in Costa Rica, we analyzed the seismic records from 25 teleseismic events from 2010 until February 2023. From these, two events were identified as prime examples of triggering during the surface-wave passage: the 2018 Mw 7.5 North of Honduras earthquake and the 2023 Mw 7.8 Türkiye earthquake. Both earthquakes share similarities as they have a distinctive strike-slip focal mechanism and are supershear events. Our results reveal that the triggered seismicity primarily happened in the northern volcanic areas of Costa Rica. However, we also observed triggering along the subduction zone in the southern Pacific coast and in crustal shallow faults in central Costa Rica, a shear zone with a high population density. Indicating that dynamically triggered earthquakes highlight the presence of weakened faults in Costa Rica.
