Richard W. Briggs

The 7 January 2025 Mw 7.1 Tingri earthquake ruptured the southern Xainza–Dinggye rift in the Tibetan Plateau, ∼200 km north of the Himalayan frontal thrust system. This rift lies in a network of discrete normal and strike-slip faults that accommodate gravitational collapse and lateral extrusion of the Plateau with overall India–Eurasia convergence. Subpixel image correlation and coseismically shifted digital elevation model differencing of high-resolution (∼0.5 m) satellite optical imagery reveal the full 3D coseismic displacement field of the earthquake. Rupture occurred on two west-dipping faults: the Lagoi fault in the north, cutting through mountainous terrain at relatively high elevation (∼5500 m) within granitic–gneissic rocks, and the Dingmu Co fault in the south, bounding a lower elevation (∼4200 m) basin within Tethyan metasedimentary rocks. Northern deformation consists of coherent, block-like oblique hanging wall subsidence with left-lateral slip (>2 m). Southern deformation is more distributed, with comparable magnitudes of subsidence and northwestward translation accommodated along a series of hanging wall faults. Footwall rebound is minimal and occurs over shorter distances than observed in the long-term landscape record. Between these domains, a ∼5–10 km wide east–west transition zone absorbs diffuse north–south contraction (∼1 m) and right-lateral motion (∼1.5 m), above a concave-up along-strike curved fault imaged by aftershocks that may direct slip toward its deepest center. Collectively, these observations indicate the control of topographic stresses, lithology, fault geometry, and a weak lower crust in partitioning discrete and diffuse deformation during a single earthquake, reproducing Plateau-wide kinematic patterns typically inferred from geodetic and geomorphic observations over much longer timescales.

On 28 March 2025, the Mw 7.7 Mandalay earthquake ruptured ∼475 km of the Sagaing fault. The surface rupture was mapped using high-resolution satellite optical images guided by displacements from subpixel optical image correlation. The surface rupture was generally continuous where remotely observable (i.e., not tree-covered) and very simple without large steps or bends along strike. We analyze remote surface rupture mapping from pixel tracking and optical images and determine that ∼10% of the surface rupture passed through potentially saturated ground. The ground saturation observed on high-resolution satellite images was compared with Sentinel-2 normalized differential water index and generally agrees spatially. We test for correlations between ground saturation observations and the presence or absence of surface rupture and of strike-slip geomorphology. We infer that transient near-surface conditions, such as ground saturation, may influence the geomorphic and paleoseismic preservation of surface rupture, as well as infrastructure (dam) performance, both in this earthquake and globally.

We present evidence of a previously unrecognized east‐west‐trending normal fault, named the Parguera Fault, in southwest Puerto Rico. The fault is located at the northwestern edge of seismicity from the southwest Puerto Rico seismic sequence that began in 2019, near the coastal town of La Parguera. We integrated geomorphic and neotectonic mapping from lidar‐derived topography, field observations, and luminescence geochronology of natural exposures of the fault with observations from subsurface data sets (ground penetrating radar (GPR), seismic reflection, and aeromagnetic data) to better characterize the fault at depth. We mapped a set of east‐west oriented scarps that cross multiple generations of alluvial fans. The scarps mapped on the lidar‐derived topography are discontinuous, cross elevation contours, and extend at least 12 km laterally, with up to a 2 m, primarily down‐to‐the‐south vertical offset of correlative geomorphic surfaces. In some places, the scarp coincides with a pre‐existing unnamed bedrock fault that separates Cretaceous basalts and Parguera Limestone or juxtaposes different Cretaceous basalt units. The scarp location is coincident with weak discontinuities in seismic reflection and GPR data and magnetic lineaments in the aeromagnetic data, supporting the presence of a fault at depth. Luminescence samples from alluvial fan deposits date to ∼32–7 ka. We interpret that the scarps crossing late Pleistocene and Holocene fans indicate potential Quaternary reactivation of a previously unrecognized normal or normal‐oblique fault. Deformation may be partitioned between the Punta Montalva and Parguera Faults in a distributed transtensional zone in southwest Puerto Rico.

How much tectonic geodetic strain is accommodated as earthquakes? Can estimates of tectonic geodetic strain be used in probabilistic seismic hazard analyses? Understanding the balance of moment accumulation (i.e., geodetic moment) and accommodation (i.e., geologic and seismologic moment) is a necessary step toward leveraging corrected geodetic strain rates to quantify earthquake budgets. We investigate these questions using updated geodetic, geologic, and seismologic datasets compiled for the western United States (WUS) portion of the 2023 U.S. National Seismic Hazard Model (NSHM23-WUS) to provide new views of moment rates across the WUS. We compare moment rates using a floating footprint approach across the extent of the NSHM23-WUS fault sections database. We make these comparisons using “moment rate deviation (MRD),” which is defined as a surplus or deficit of geodetic moment rate relative to the sum of geologic and seismologic moment rates. We find that, in general, higher strain rate areas of the WUS (i.e., plate boundary) have smaller values of MRD, meaning that geodetic moment rates approximate the earthquake record preserved by geologic and seismologic moment rates well. In contrast, the lower strain rate, eastern portions of our analysis domain (i.e., intraplate) have very high values of MRD, with geodetic moment rates exceeding measured geologic and seismologic moment rates. We find that geodetic moment rates appear to be representative of the earthquake budget in relatively well-characterized, interplate regions, and geodetic estimates of off-fault deformation could be considered in seismic hazard analyses where MRD is minimized.

Splay, or branch, faults are a common geometric feature of earthquake surface ruptures and may provide constraints on the rupture behavior of an earthquake. The 2023 Mw 7.8 Pazarcık and Mw 7.5 Elbistan, Türkiye, earthquakes are examples of ruptures with multiple small splays, and the Pazarcık earthquake nucleated on a splay fault, the Narlı fault, before rupturing bilaterally on the East Anatolian fault (EAF). Here, we present 3-m-resolution surface displacement from subpixel correlation of Planet Dove optical images for the entirety of both ruptures with corresponding surface slip distributions. For a 30-km-long study region spanning the Narlı-EAF intersection, we compare surface slip derived from five data sets with different resolutions (on-the-ground, WorldView, Planet Dove, Sentinel-2, and Sentinel-1) to elucidate complementary information. In addition, we integrate information from the surface expression of faulting with published dynamic rupture simulations and rupture process studies to constrain a rupture evolution for the Pazarcık earthquake that is consistent across data sets. This work highlights the complementary nature of disparate surface slip data sets and the role that high-resolution surface displacement information, including from fault splays, can play in constraining nonunique rupture models and refining understanding of the earthquake rupture process.

The 2025 Mw 7.7 Mandalay, Burma (Myanmar), earthquake ruptured 475 km of the central Sagaing fault and is the longest continental strike-slip rupture on record. The observed rupture length is 1.6–4.7 times the value expected (100–300 km) from existing length–magnitude scaling relations for strike-slip earthquakes. The earthquake resulted from shallow dextral faulting and ruptured bilaterally with supershear speeds south of the epicenter, rupturing close to three major cities in Myanmar and exposing more than six million people to violent or extreme shaking. We report on the surface rupture character, length, and slip distribution based on subpixel correlation of Sentinel-2 (10 m) and Planet Dove (3 m) optical images and visual analysis of SkySat and WorldView (0.3–0.5 m) optical images. The earthquake had moderate surface slip (average = 3.3 m, maximum = 5.6 m, 25%–75%, range = 3.0–4.0 m), narrow deformation zone width (1–10 pixels in subpixel correlation and ≤190 m for the detailed surface rupture mapping), and simple fault geometry (no stepovers or large changes in strike, 87% of the rupture that was mapped in detail is single stranded). We attribute the extreme length of the Mandalay earthquake to supershear rupture speed, simple fault geometry, narrow down-dip width, and moderate surface slip. Based on a compilation of 25 supershear strike-slip earthquakes (Mw 6.5–8.6; 1979–2025), we find that the rupture length of supershear earthquakes does not fit empirical scaling relationships for strike-slip earthquakes that predict length from magnitude. A length–magnitude scaling relationship based on supershear earthquakes has a best fit of log10 (surface rupture length) = 0.89 Mw – 4.44, indicating that supershear earthquakes tend to be longer than their subshear counterparts for any given magnitude and thus may expose a greater population to shaking.

A key use-case of geologic slip rates is within deformation models used in probabilistic seismic hazard analyses. Field-derived geologic slip rates have formed the cornerstone of deformation models in such applications for decades. Recent advancements in seismic hazard analyses have expanded the use of faults for which geologic slip rates are not well constrained using categorical slip rate estimates. Because of these advancements, application of a geologic deformation model for use in 2025 U.S. Geological Survey National Seismic Hazard Model Puerto Rico-U.S. Virgin Islands (NSHM25-PRVI) proved challenging due to: (1) a lack of field-based geologic slip rates, and (2) a lack of epistemic uncertainty distributions within a broad range of estimated slip rates. Preliminary versions of the NSHM25-PRVI model sampled these slip rate bins in a coincident manner along preferred and extreme value branches, which yielded untenable correlations in mean hazard results. To minimize the influence of correlated uncertainties amid these challenges, we develop a synthetic epistemic uncertainty distribution for deformation rate on each crustal fault. Each fault has a weighting schema across four possible distribution shapes: uniform, normal, triangular favoring local minima, and triangular favoring local maxima. The synthetic distributions are then sampled several times for each logic tree branch. The results provide a more realistic distribution of rates across the study region as compared with using correlated extrema sampling. This exploration of our method in a small region like PRVI can pave the way for larger-scale, more complicated applications (e.g., western United States).

We present the 2025 U.S. Geological Survey Puerto Rico and U.S. Virgin Islands (PRVI) time-independent earthquake rupture forecast (ERF), developed for the 2025 update to the National Seismic Hazard Model (NSHM) for PRVI. The updated ERF improves upon a prior model from 2003, including an expanded fault inventory with slip-rate estimates, updated seismicity catalogs, and refined subduction zone geometries and deformation models. It applies the fault-system inversion methodology to solve for rates of ruptures on modeled faults, adapted from the 2023 NSHM (NSHM23) for the western United States, including the first application of the inversion to model rates on a U.S. subduction interface. Off-fault and intraslab seismicity are constrained by observed seismicity and use updated methods developed for NSHM23. Uncertainties in model components are substantial, and the ERF represents epistemic uncertainties through a comprehensive logic tree consisting of 1.7 billion logic-tree branches combined across all sources.

Several onshore faults in southern Puerto Rico have recently been recognized as Quaternary active. However, the kinematics of these faults, particularly any lateral component, remain largely unconstrained. It is difficult to characterize low strain-rate faults, partially due to extensive erosional and anthropogenic landscape modification, steep relief, and frequent landsliding, limiting the preservation of geomorphic features that could serve as recorders of fault motion. Here, we constrain the kinematics along sections of the Great Southern Puerto Rico Fault Zone (GSPRFZ) on the southern coastal plain of Puerto Rico. We integrate ∼1-m-resolution light detection and ranging (lidar)-derived topography, historical air photos, and field mapping to identify a series of ∼50–1200-m-long fault scarps and lineaments that trend northwest–southeast and extend for ≥25 km across the southern coastal plain. Fault scarps are primarily south facing, cut across topography, and displace Quaternary deposits and landforms. We document multiple offset geomorphic markers, including channel thalwegs and interfluves formed in deposits previously mapped as Quaternary piedmont alluvial plain. We observe both vertical (south-side-down) and right-lateral meter-scale displacements, which indicate that the GSPRFZ accommodates right-lateral oblique motion in the late Pleistocene, consistent with northeast motion of the Puerto Rico and the Virgin Islands microplate away from the Hispaniola block.

We articulate a scientific vision and roadmap for the development of improved Earthquake Rupture Forecast (ERF) models, which are one of the two main modeling components used in modern seismic hazard and risk analysis. One primary future objective is to provide fully time-dependent models that include both elastic rebound and spatiotemporal clustering nationwide, which can be particularly important for shorter-term hazard and risk considerations (e.g., earthquake insurance products). We also discuss the importance and perennial challenges associated with quantifying epistemic uncertainties, including those associated with deformation-model slip rates, unquantified sampling errors with respect to off-fault seismicity, and any spatial covariances. The need for more physics-based approaches is also emphasized, as is the benefit of adding model valuation (quantifying usefulness) to our verification and validation protocols. Given the multidisciplinary and system-level nature of this activity, modular design is critical. Future updates will also draw from best-available science by both the U.S. Geological Survey and the external community. The primary goal of this article is to highlight plans that guide research and facilitate community engagement with model development, especially with respect to lowering the entry barrier for early-career scientists and engineers. The article is written so readers can focus on the sections that interest them most (see Manuscript Organization section), with the Introduction and the Discussion section providing a stand-alone overview and summary, respectively.