Earthquake Focal Mechanisms Research Articles

Accelerating low-frequency ground motion simulation for finite fault sources using neural networks

SUMMARYIn the context of early emergency response to moderate and large earthquake shaking, we present a simulation based low-frequency ground motion estimation workflow that expedites an existing simulation method while taking into account simplified source process information. We focus on using source information that can be expected to be available shortly after an impacting earthquake, for example moment-tensor and simple finite-fault parameters. We utilize physics-based simulations which can include effects based on source orientation or finite faults, like rupture directivity. In order to keep the computational effort within feasible bounds and to apply the approach on global scale, we restrict ourselves to a low-frequency setup (standard 1-D layered earth model and 2 Hz sampling frequency) for either a moment tensor or a simple kinematic finite fault model. From the simulated records we then extract ground motion parameters of interest for arbitrary locations within the area of expected impact and display the expected spatial patterns of ground motion. Although simulations are kept simple, the results from this low-frequency ground motion parameter simulation (e.g. for peak-ground displacement) are in good agreement with observations from two well-studied earthquakes and partially more accurate than traditional, more empirical approaches (standard deviation <0.3 log10 units). However, waveform calculation and subsequent ground motion parameter extraction is computationally expensive. For a significant computational speedup in the context of rapid ground motion assessment, we directly train neural network (NN) models from large sets of source model information and their corresponding spatial ground motion distribution. We show that the trained NNs are able to reproduce the earthquake source related effects, like directivity and focal mechanism patterns, of the ground motion in any case. Given a set of source parameters, we obtain prediction errors smaller than 0.05 log10 units (ca. 11 per cent) and a magnitude dependent increase in computational speed of more than 1000 times compared to the initial waveform modelling. The proposed procedure enables thus to immediately compute probabilistic ground motion maps related to uncertainties in source parameters estimates, for example by sampling distributions based on parameter uncertainties or directly from an existing ensemble of focal parameter solutions.

Updated stress dataset of the Circum-Pannonian region: Implications for regional tectonics and geo-energy applications

In this paper we present the updated stress database of the Circum-Pannonian region in Central-Eastern Europe, containing stress indicators from earthquake focal mechanism solutions, borehole breakout analysis, overcoring measurements and geologic fault-slip data. Recent improvements in seismic network density in the framework of the AlpArray project, as well as methodological advancements, facilitated the calculation of focal mechanism solutions of small magnitude events, contributing to the more detailed reconstruction of the stress field in the area. The new database consists of 368 A – C quality stress indicators that we interpolate to a regular grid to produce stress orientation and relative stress magnitude maps. Interpolated maximum horizontal stress (SH) orientations are in general marked by low uncertainties (mostly between 10° – 30°), indicating that the interpolation results can be considered reliable for the majority of the study area. Inferred SH directions show a good fit with previous studies, confirming earlier findings that the Pannonian basin is under compression and transpression, controlled by the combination of its landlocked position with the N-NE directed motion of the Adriatic microplate. The new tectonic regime map highlights several new features, such as the complete absence of extension in the entire Pannonian basin. The determined stress regimes and trajectories calculated from interpolated SH directions generally agree well with the orientation and kinematics of neotectonic structures in the Pannonian region. Additionally, measured and interpolated SH directions and faulting styles can provide boundary conditions for geomechanical studies, serving as valuable input for geo-energy applications.

Estimation of crustal strain in Kashmir Himalayan region of north India using continuous GPS measurements

Global positioning system (GPS) measurements of crustal deformation provide insight into the crustal plate velocity and strain rate variation in the actively deforming Earth's crust. GPS measurements in the Kashmir Himalaya indicate that most of the crustal motion between the Indian Plate and southern China is accommodated along the Himalayan arc, resulting in the accumulation of crustal strain and locking of the Main Himalayan Thrust (MHT). Here, we present geodetically estimated spatial and temporal crustal strain rates in the Kashmir Himalaya from GPS measurements to understand the nature of crustal deformation. We processed and analysed GPS data from 21 sites across Kashmir Himalaya along with several IGS sites. We find that the average deformation rate of the GPS sites in Kashmir Himalaya with reference to the Indian Plate range between 2.8 and 15.5 ± 1 mm/year. The GPS results illustrate that the strain rate field is dominated by a compression component with its rate as 7.2 × 10−8 strain/year and the maximum shear strain, γmax, of 1.9 × 10−7 strain/year. The earthquake focal mechanisms suggest that earthquake activity in the Kashmir Himalaya is predominantly manifested in the form of thrust motion. The maximum principal shear strain value reflects that the Kashmir Himalaya is under NE–SW compressional regime. The spatial distribution of crustal strain indicates that the region north‐east of Kashmir Valley is accumulating maximum strain, resulting in the stress build‐up and could be the region of a future large earthquake.

Anomalous Crustal Stress in the Eastern Tennessee Seismic Zone

AbstractThe eastern Tennessee seismic zone (ETSZ) experiences the second highest rates of natural seismicity in the central and eastern United States (CEUS), following the New Madrid area, yet the cause of elevated earthquake rates is unknown. We probe the origin of ETSZ seismicity using geomechanically constrained stress inversions of earthquake focal mechanisms from 57 earthquakes, including 24 newly derived here and five from the recent events not used in the previous stress studies. Highly oblique northwest–southeast (NW–SE) extension that is unique in the CEUS dominates the ETSZ—central Alabama to southeastern Kentucky—and preferentially reactivates normal to strike-slip faults in the northeast (NE) and southwest (SW) quadrants (strikes 018°–086° or 196°–272° and dips 55°–90°). This extension cannot be explained by the compressive tectonic plate-boundary tractions that cause oblique NE–SW contraction elsewhere in the CEUS. Although our analyses do not uniquely determine the origin of the anomalous stress, we favor isostatic disequilibrium, due to anything from surface processes to crust–mantle interactions, as the possible cause. Increased long-term seismic hazard in the ETSZ may be controlled by and confined to the spatial extent of this anomalous seismotectonic state.

DiTingMotion: A deep-learning first-motion-polarity classifier and its application to focal mechanism inversion

Accurate P-wave first-motion-polarity (FMP) information can contribute to solving earthquake focal mechanisms, especially for small earthquakes, to which waveform-based methods are generally inapplicable due to the computationally expensive high-frequency waveform simulations and inaccurate velocity models. In this paper, we propose a deep-learning-based method for the automatic determination of the FMPs, named “DiTingMotion”. DiTingMotion was trained with the P-wave FMP labels from the “DiTing” and SCSN-FMP datasets, and it achieved ∼97.8% accuracy on both datasets. The model maintains ∼83% accuracy on data labeled as “Emergent”, of which the FMP labels are challenging to identify for seismic analysts. Integrated with HASH, we developed a workflow for automated focal mechanism inversion using the FMPs identified by DiTingMotion and applied it to the 2019 M 6.4 Ridgecrest earthquake sequence for performance evaluation. In this case, DiTingMotion yields comparable focal mechanism results to that using manually determined FMPs by SCSN on the same data. The results proved that the DiTingMotion has a good generalization ability and broad application prospect in rapid earthquake focal mechanism inversion.

Stress orientations from hydraulic fracturing tests in the Ruhr area in comparison to stress orientations from borehole observations and earthquake focal mechanisms

Stress orientations from hydraulic fracturing tests in the Ruhr area in comparison to stress orientations from borehole observations and earthquake focal mechanisms

Indications of transtensional strike slip fault in the Mount Anak Krakatau based on earthquake focal mechanism data

Anak Krakatau volcano in the Sunda Strait was through pyroclastic materials landslide phenomenon that caused by a tsunami wave at the end of December 2018. Tsunami wave hit the west coast of Pandeglang and part of the coast of Lampung. Recent tectonic activity produces an epicenter around Anak Krakatau volcano, a shallow earthquake depth of 10 - 15 km below the surface. These data were collected by the interpretation of remote sensing DEM SRTM images, focal mechanism, bathymetry, and hillslope analysis. Quantitative analysis in determining the movement mechanism of strike slip faults and the damage zone as the trigger for landslides on the southwestern volcano sidewall. The main fault has N153°E / 74°E rake 9° is related to the surface of stress and strain results of synthetic and antithetic faults. It is suspected that the post-eruption caldera of Krakatau Volcano is the remaining surface openings in the 1883 while erupted. Knowledge of the causes of landslides to be prepared to anticipate broken infrastructure, victims, and tourism along the coast of Banten and Lampung.

A New Focal Mechanism Calculation Algorithm (REFOC) Using Inter‐Event Relative Radiation Patterns: Application to the Earthquakes in the Parkfield Area

AbstractAccurate earthquake focal mechanisms are essential for solving fault zone structure, estimating stress variations, and assessing seismic hazards. Small earthquakes' focal mechanisms are usually solved using P‐wave first‐motion polarities and/or S‐/P‐wave amplitude ratios, which are limited due to the low signal‐to‐noise ratio of small‐earthquake waveforms and a limited number of three‐component seismograms. To increase the number of high‐quality focal mechanisms, we develop a method that utilizes the inter‐event relative radiation patterns to perform a joint focal mechanism inversion of numerous clustered events (called REFOC). The method first uses P‐wave polarities and S‐/P‐wave amplitude ratios to constrain the initial solutions and then combines these solutions and the inter‐event P‐/P‐wave and S‐/S‐wave amplitude ratios to refine solutions. For example, we apply the method to 38,413 earthquakes in the Parkfield region, California. The REFOC outperforms traditional methods with 57% more solutions with <55° uncertainties (>70% of the catalog events) and 126% more solutions with <25° uncertainties (>40% of the catalog events), illuminating unprecedented fine‐scale rupture processes. Instead of rupturing along the main fault, many M < 2 focal mechanisms show >45° angular differences from the 2004 Mw 6.0 Parkfield earthquake. The variation of focal mechanism properties is spatially related to the variation of fault strength and geometry, and temporally correlated with the shear stress variations before, during, and after the 2004 Mw 6.0 earthquake. The observations highlight the potential of applying REFOC to monitor unprecedented details of fault zone structure and stress field, providing new insights into fault rupture physics, seismotectonic processes, and seismic hazards.

Subduction transition and relation to upper plate faults revealed by the 2019 Mw 6.0 and 6.2 Costa Rica-Panama border earthquakes

We study the May 12, 2019, Mw 6.0 (hereafter Laurel earthquake) and a June 26, 2019, Mw 6.2 (hereafter Manaca earthquake) earthquakes located in a complex strike-slip to subduction transition zone in the Costa Rica-Panama border using a dense local network of broadband and strong-motion instruments, concluding that the Laurel mainshock data (hypocenter, centroid, and focal mechanism), concurrently with its aftershock distribution, indicate an almost vertical north-south oriented, dextral strike-slip fault. This fault ruptured at depths between 10 and 30 km in the lower crust of Panama Microplate and the geometry implies a shallow influence of the Panama fracture zone. Meanwhile, for the Manaca earthquake, the aftershocks are concentrated on a 25 km long NW-SE trend, with a depth between 20 and 45 km. One of the nodal planes of the focal mechanism of the Manaca earthquake, as well as the centroid location coincides with the aftershock locations trend, supporting the hypothesis that a nearby steeply dipping sinistral strike-slip fault originated this event. Earthquake locations and focal mechanism analyses of collected data from global and local earthquake catalogs suggest that the strong coupling caused by the subduction of the Cocos Ridge and Nazca plate is largely accommodated by outer arc crustal block migration with a combination of NW-SE sinistral and N-S dextral strike-slip faults deforming the overriding Panama Microplate. Sinistral motion of crustal blocks, on the Costa Rica side, are consistent with observed motions along with the Azuero-Sona fault system and Coiba Faults on the Panamanian side.

DEFORMATION REGIMES AND LOW-RANK PALEOTECTONIC STRESSES OF THE WESTERN TIAN SHAN IN THE MESOZOIC – CENOZOIC

There is a description of the results of studies of deformation regimes and reconstruction of stress fields of certain time intervals of the Mesozoic – Cenozoic history of the western part of the Tien Shan based on the geological and structural data and earthquake focal mechanisms. The relevance of research is determined by the existing difficulties in the age reference of paleostress fields, which is associated with the retrospective nature of the initial data and the features of geological evolution, as well as the existence of controversial, unresolved issues regarding the development of the western periclinal part of the Tien Shan in the Mesozoic – Cenozoic. The aim of the research is to reconstruct the rank components of paleostresses in the earth’s crust in the western part of the Tien Shan at separate stages of the Mesozoic – Cenozoic development. The time intervals for which the stress field diagrams were obtained represent cyclically manifested phases of the first-rank rhythms, which are characterized by alternation of different deformation modes. The method for reconstructing paleostresses was based on the concept of the formation of superimposed systems of cracks of various generations, which were sequentially manifested at various stages of the development of deformation processes. Analysis of the data showed that with increasing age of rocks the number of different fracture systems actually increases – the earlier systems are superimposed by the later. At the same time, the characteristic elements in Quaternary rocks manifest themselves in different ways in the older rocks. This allows them to be used for the reconstruction of the Late Cenozoic stress field based on mass measurements of fracturing in rocks of earlier Mesozoic – Cenozoic ages. Based on the step-by-step identification of the heterochronous cracks system and determination of a pair of maxima of the fracture density with asymmetric scattering, region-based diagrams of the first-rank stress fields were obtained for different stages of manifestation of Mesozoic – Cenozoic deformation regimes: 1 – from the Permian-Triassic to the Middle Jurassic; 2 – Middle-Late Jurassic; 3 – Late Jurassic – Early Cretaceous; 4 – Late Cretaceous – Paleocene – Eocene; 5 – Oligocene – Quaternary. The results obtained indicate the alternation of compression and extension phases, first-rank for the region, and give an idea of the main trends in its geodynamic evolution. They can be used in the development of a tectodynamic model of the region.

FocMech-Flow: Automatic Determination of P-Wave First-Motion Polarity and Focal Mechanism Inversion and Application to the 2021 Yangbi Earthquake Sequence

P-wave first-motion polarity is important for the inversion of earthquake focal mechanism solutions. The focal mechanism solution can further contribute to our understanding of the source rupture process, the fault structure, and the regional stress field characteristics. By using the abundant focal mechanism solutions of small and moderate earthquakes, we can deepen our understanding of fault geometry and the seismogenic environment. In this paper, we propose an automatic workflow, FocMech-Flow (Focal Mechanism-Flow), for identifying P-wave first-motion polarity and focal mechanism inversion with deep learning and applied it to the 2021 Yangbi earthquake sequence. We use a deep learning model named DiTingMotion to detect the P-wave first-motion polarity of 2389 waveforms, resulting in 98.49% accuracy of polarity discrimination compared with human experts. The focal mechanisms of 112 earthquakes are obtained by using the CHNYTX program, which is 3.7 times more than that of the waveform inversion method, and the results are highly consistent. The analysis shows that the focal mechanisms of the foreshock sequence of the Yangbi earthquake are highly consistent and are all of the strike-slip type; the focal mechanisms of the aftershock sequence are complex, mainly the strike-slip type, but there are also reverse and normal fault types. This study shows that the deep learning method has high reliability in determining the P-wave first-motion polarity, and FocMech-Flow can obtain a large number of focal mechanism solutions from small and moderate earthquakes, having promising application in fine-scale stress inversion.

Using Corrected and Imputed Polarity Measurements to Improve Focal Mechanisms in a Regional Earthquake Catalog Near the Mt. Lewis Fault Zone, California

AbstractWe utilized relative polarity measurements and machine learning techniques to better resolve focal mechanisms and stress orientations considering a catalog of ∼29,000 relocated earthquakes that occurred during 1984–2021 in the southeastern San Francisco Bay Area. Earthquake focal mechanisms are commonly produced using P wave first motion polarities, which traditionally requires events to be well‐recorded across a seismic network with good focal sphere coverage. We adapted recently developed approaches that are less dependent on high signal‐to‐noise records and exploit similar waveforms to produce relative polarity and amplitude measurements between earthquake pairs. These techniques were previously only applied on localized earthquake sequences, and we further developed these approaches so that they can be utilized for regional catalogs. We validated or corrected manually identified polarities by performing polarity consensuses using earthquake pairs. Missing and unreliable polarity measurements were imputed using iterative random forests, an unsupervised ensemble machine learning method. Relative P and S wave amplitude measurements were made between earthquakes, constraining S/P ratios for low signal‐to‐noise waveforms. Using these techniques, we were able to reduce focal mechanism uncertainties by an average of ∼13° and produced well‐constrained focal mechanisms for ∼6 times as many earthquakes than those produced using only the traditionally derived polarities. We performed stress inversions using the focal mechanisms by grouping the focal mechanism results into a quadtree structure. Our stress results are consistent with previous work, albeit at a higher spatial resolution, and demonstrate these techniques can aid our understanding of fault structures and kinematics in more detail than was previously possible.

Refined Earthquake Focal Mechanism Catalog for Southern California Derived With Deep Learning Algorithms

AbstractEarthquake focal mechanisms, determined with P‐wave polarities and S/P amplitude ratios, are primary data for analyzing fault zone geometry, sense of slip, and the crustal stress field. Solving for the focal mechanisms of small earthquakes is often challenging because phase arrivals and first‐motion polarities are hard to be separated from noise. To overcome this challenge, we implement convolutional‐neural‐network algorithms (Ross, Meier, & Hauksson, 2018, Ross, Meier, Hauksson, & Heaton, 2018, https://doi.org/10.1029/2017jb015251, https://doi.org/10.1785/0120180080) to detect additional phases and polarities. Using both existing and these new data, we build a high‐quality focal mechanism catalog of 297,478 events that occurred from 1981 to 2021 in southern California with the HASH method of Hardebeck and Shearer (2002), https://doi.org/10.1785/0120010200, Hardebeck and Shearer (2003), https://doi.org/10.1785/0120020236. The new focal mechanism catalog is overall consistent with the standard catalog (Yang et al., 2012, https://doi.org/10.1785/0120110311) but includes 40% more focal mechanisms, and is more consistent with moment tensor solutions derived using waveform‐fitting methods. We apply the new catalog to identify changes in focal mechanism properties caused by the occurrences of large mainshocks such as the 2010 Mw7.2 El Mayor‐Cucapah and 2019 Mw7.1 Ridgecrest earthquakes. Such changes may be associated with co‐seismic stress drops, post‐seismic deformation processes, and static stress changes on a regional scale. The new high‐resolution catalog will contribute to improved understanding of the crustal stress field, earthquake triggering mechanisms, fault zone geometry, and sense of slip on the faults in southern California.

A Cautionary Tale: examples of the mis-location of small earthquakes beneath the Tibetan plateau by routine approaches

SUMMARYEarthquake moment tensors and centroid locations in the catalogue of the Global CMT (gCMT) project, formerly the Harvard CMT project, have become an essential resource for studying active global tectonics, used by many solid-Earth researchers. The catalogue’s quality, long duration (1976–present), ease of access and global coverage of earthquakes larger than about Mw 5.5 have transformed our ability to study regional patterns of earthquake locations and focal mechanisms. It also allows researchers to easily identify earthquakes with anomalous mechanisms and depths that stand out from the global or regional patterns, some of which require us to look more closely at accepted interpretations of geodynamics, tectonics or rheology. But, as in all catalogues that are, to some extent and necessarily, produced in a semi-routine fashion, the catalogue may contain anomalies that are in fact errors. Thus, before re-assessing geodynamic, tectonic or rheological understanding on the basis of anomalous earthquake locations or mechanisms in the gCMT catalogue, it is first prudent to check those anomalies are real. The purpose of this paper is to illustrate that necessity in the eastern Himalayas and SE Tibet, where two earthquakes that would otherwise require a radical revision of current geodynamic understanding are shown, in fact, to have gCMT depths (and, in one case, also focal mechanism) that are incorrect.

The present-day stress field along the Northwest Pacific Wadati-Benioff zone constrained by focal mechanisms of moderate earthquakes

The present-day stress field in the northwest Pacific Wadati-Benioff Zone (WBZ) has been investigated on the basis of earthquake focal mechanism catalog covering the period from 2012 to 2021. Our catalog contains 201 focal mechanism solutions (Mw 4.0–5.9) compiled from the F-net network along three profiles (southern Kurile trench, central Japan trench and northern Izu-Bonin trench). According to the spatial variations in the stress regimes, we outlined 12 subvolumes for three profiles. Our results present that the maximum compressive stress is, slab-normal in trench-outer rise regions and nearly strike-normal and sub-horizontal in the depth range 0–70 km. At intermediate depths, a two-planar stress pattern is observed (70–200 km) in southern Kurile and central Japan trench WBZ, with the maximum compressive stress closely parallel to the slab in upper plane and slab-normal in lower plane, respectively. However, the southern Kurile trench WBZ exposes an extension stress regime in 200–300 km depth range. At depths below 300 km, northern Izu-Bonin trench WBZ shows a stress regime of pure compression. We infer the dominant forces may be the slab blending in the shallow trench-outer rise regions, slab pull and ridge push at shallow depths, and the slab resistance provided by the mantle to the penetration of the slab at deep depths. Whereas the two-planar stress pattern may be caused by the unbending of the slab, and the unusual stress state along the junction of the trench WBZ at intermediate depths may be influenced by lateral deformation due to increasing slab’s dip.

Fracture‐Mesh Faulting in the Swarm‐Like 2020 Maacama Sequence Revealed by High‐Precision Earthquake Detection, Location, and Focal Mechanisms

AbstractIn August of 2020, an earthquake sequence initiated within the Maacama fault zone in northern California, raising questions about its relationship with the larger‐scale fault. To investigate the faulting geometry and its implications for physical processes driving seismicity, we applied an integrated, multi‐faceted seismic analysis including waveform‐correlation‐based event detection, relative relocation, and high‐precision focal mechanisms. To determine mechanisms for a large population of very small earthquakes (predominantly M < 1), we combined correlation‐derived polarity analysis with a recently developed technique to determine S/P amplitude ratios from single seismic components. Finally, we applied an iterative stress inversion to distinguish between the likely fault and auxiliary planes. This analysis reveals that although the sequence initiated on a right‐lateral fault, plausibly a strand of the Maacama Fault, it also activated numerous en echelon left‐lateral conjugate faults. Together, these interlocking faults form a fracture mesh, consistent with structures associated elsewhere with fluid‐induced faulting and earthquake swarms.

Active Faults Revealed and New Constraints on Their Seismogenic Depth from a High-Resolution Regional Focal Mechanism Catalog in Myanmar (2016–2021)

ABSTRACTWe derive a new earthquake focal mechanism catalog for 86 Mw>4.0 earthquakes that occurred in the Myanmar region from 2016 to 2021. We apply the generalized Cut-and-Paste inversion method to a new set of regional broadband waveform data to obtain the earthquake focal mechanism and centroid depth with uncertainties estimated in a bootstrapping manner. Compared with global earthquake catalogs, our results are better aligned with mapped, active faults and reveal seismic activity along unmapped, blind faults. Our new catalog shows that the Sagaing Fault is more active in its northern segment with deeper seismogenic zone (∼27 km) compared to its southern segment that has a shallower seismogenic zone (∼10 km), sandwiching a seismic gap in its central segment. Earthquakes that occurred on the unmapped, blind faults beneath the Central Myanmar Basin at shallow depths (3–12 km) suggest a dominating northeast–southwest compressional stress field. Shallow earthquakes beneath the Indo-Myanmar Range (IMR) are rare, instead, north–south-oriented strike-slip faults are active within the deep accretionary wedge or lower crust of the Myanmar plate between depths of 20 and 40 km. At the eastern edge of the IMR, earthquakes with high-angle thrust mechanisms occurred between depths of 30 and 48 km, likely along steep faults separating the accretionary wedge from the Myanmar forearc crust. High-resolution intraslab focal mechanisms show that to the north of 22° N, slab deformation is dominated by strike-slip earthquakes with subvertical fault planes down to a depth of ∼25 km beneath the slab, suggesting lateral shear within the slab due to the northward motion of the Indian plate. To the south, more normal-faulting earthquakes suggest a stronger role of plate-bending processes in the slab deformation.

Seismotectonic model of High-Middle Atlas Junction (Morocco) derived from earthquake focal mechanism and stress tensor analysis

Seismotectonic model of High-Middle Atlas Junction (Morocco) derived from earthquake focal mechanism and stress tensor analysis

Modern dextral strain controls active hydrothermal systems in the southeastern Canadian Cordillera

Thermal springs in the southeastern Canadian Cordillera occur in association with several major fault zones, which may permit deep circulation of fluid through fractured reservoirs to depths greater than 2 km. Both the current stress field and the most recent kinematics of these faults likely play a significant role in localizing hydrothermal upwellings but are poorly resolved. In this paper, we present new data from structural mapping along the Columbia River, Slocan Lake, Purcell Trench, Southern Rocky Mountain Trench, and Redwall faults. Fault plane and slickenfiber orientations measured in the field indicate a previously unidentified, post-Eocene phase of dextral strike-slip kinematics on these faults, which have historically been mapped primarily as Eocene extensional structures. The NE−SW maximum principal stress required for these kinematics shares a similar orientation to the present-day stress field derived from crustal earthquake focal mechanisms, which suggests dextral slip may be a recent and ongoing development that continues at the present time. On a regional scale, there is a positive correlation between the locations of springs and regions with elevated seismicity. At the local scale, geothermal upwellings may be controlled by local zones of enhanced permeability including fault intersections and strain-transfer zones. The identification of favorable structural settings may enable discovery of hidden (a.k.a. blind) geothermal energy resources.

Spatially varying crustal stress along the Zagros seismic belt inferred from earthquake focal mechanisms

We determine the stress field of the Zagros fold-and-thrust belt (ZFTB) in the collisional zone between the Arabian and Iranian plates. Using 898 mainshock focal mechanisms, which occurred along the belt between 1956 and 2021, we calculate stress at 32 locations. The results reveal that the crustal stress in the Zagros belt is heterogeneous with variations in the orientation of SHmax as well as in the stress ratio. The interpolation of the Simpson's index Aϕ points to three distinct domains: northern, central and southern, which are characterized by wrench, compressional, and wrench-compressional to compressional tectonic regimes, respectively. Tectonic stress in the study area is characterized by a horizontal to sub-horizontal maximum compression axis in the ∼NE-SW to ∼NNW-SSE direction. The GPS measurements of the strain tensor and the SHmax direction inferred from the stress inversion indicate that both the deformation pattern and the seismicity along the ZFTB zone are primarily affected by convergence of the Arabian and Iranian plates.