Crustal Stress Field Research Articles

Insights into seismicity from the perspective of the crustal stress field: a comment

Insights into seismicity from the perspective of the crustal stress field: a comment

Contemporary background stress field in the eastern Tibetan Plateau: Insights from 3D geomechanical modeling

Knowledge of the crustal stress state is important for understanding tectonic processes and seismic hazard assessment as well as for various practical applications. In this study, we present a model describing the contemporary 3D crustal background stress field of the eastern Tibetan Plateau. The model incorporates a complex fault system and an appropriate initial stress state that matches both model-independent stress observations and kinematic data well. Our results show that the modeled maximum horizontal stress SH is generally oriented in NW-SE direction. The Bayan Har Block and the Chuan-Dian Block are dominated by a strike-slip faulting stress regime. However, in some local areas, the crust is governed by a normal faulting regime. We infer that the local normal faulting stress regime in the Bamei-Kangding area is caused by the clockwise rotation of the strike of the Xianshuihe fault at Kangding. This normal faulting stress regime would promote the ascent of intrusions and provide the space necessary for the emplacement of the Zheduoshan granite. The stress state on the Longmen Shan fault zone is rather inhomogeneous. For the Yingxiu-Beichuan fault, the northeastern segment has a lower normal stress but a higher shear stress than the southwestern segment. This inhomogeneous stress state on the Yingxiu-Beichuan fault could provide an explanation for the northeastward propagation of the ultra-long unilateral rupture in the Wenchuan Ms8.0 earthquake. According to the stress state on the Xianshuihe fault, we conclude that the Kangding-Shimian segment has a high potential to produce a strong earthquake due to its high normal stress and a fast slip rate and should be carefully monitored. On the Bamei-Kangding segment, which is governed by a normal faulting stress regime and may be lubricated by fluids, slip seems to be facilitated favoring more frequent small to moderate magnitude earthquakes to occur on this segment.

Origin of Three‐Dimensional Crustal Stress Over the Conterminous United States

AbstractThe crustal stress field determines continental deformation, including intraplate seismicity and topographic undulations. However, the sources of observed crustal stress patterns remain debated, with proposed mechanisms including lateral variations in gravitational potential energy and mantle flow, the latter of which comprises plate boundary interactions and basal tractions. Here, we present a series of geodynamic models that simultaneously consider lithospheric and mantle dynamics in the same physical framework, based on which we investigate the sources of crustal stress over the conterminous U.S. The data‐oriented nature of these models allows us to systematically explore the relative contributions of different dynamic sources to the three‐dimensional crustal stress field. These models reveal that forces from the plate boundaries play a dominant role in generating the directional pattern of long‐wavelength horizontal crustal stress across the conterminous U.S. In the central U.S., especially regions of high‐topography, lithospheric density heterogeneities locally modify the crustal stress field. Similarly, mantle flow beneath the North American plate modulates crustal stress orientation in the eastern U.S., particularly in regions with thin lithosphere. Furthermore, we find that a denser‐than‐ambient lithospheric mantle beneath the central and eastern U.S. is required to match the observed continental‐scale E‐W topographic contrast.

Characteristics of crustal stress field in Tienshan area

Based on 1143 focal mechanisms in Tienshan area from 1900 to 2019, according to the world stress map classification criterion, the main types of focal dislocations are thrust and strike slip; The results of parameter statistics show that the nodal plane of focal mechanism solution is basically consistent with the main tectonic strike in Tienshan area, and the orientation of P axis is perpendicular to the main tectonic strike in Tienshan area, which shows the horizontal nearly NS thrust. The direction of the maximum principal compressive stress is 175.1°, the minimum principal stress direction is 67.8°, the plunge angle of the maximum principal compressive stress axis is 4.3°, the plunge angle of the minimum principal compressive stress axis is 75.8°, the relative stress ratio R is 0.80, and the stress state is compressional, which indicates that the area is in the background of compressional orogeny.

3D crustal stress state of Germany according to a data-calibrated geomechanical model

Abstract. The contemporary stress state in the upper crust is of great interest for geotechnical applications and basic research alike. However, our knowledge of the crustal stress field from the data perspective is limited. For Germany basically two datasets are available: orientations of the maximum horizontal stress (SHmax) and the stress regime as part of the World Stress Map (WSM) database as well as a complementary compilation of stress magnitude data of Germany and adjacent regions. However, these datasets only provide pointwise, incomplete and heterogeneous information of the 3D stress tensor. Here, we present a geomechanical–numerical model that provides a continuous description of the contemporary 3D crustal stress state on a regional scale for Germany. The model covers an area of about 1000×1250 km2 and extends to a depth of 100 km containing seven units, with specific material properties (density and elastic rock properties) and laterally varying thicknesses: a sedimentary unit, four different units of the upper crust, the lower crust and the lithospheric mantle. The model is calibrated by the two datasets to achieve a best-fit regarding the SHmax orientations and the minimum horizontal stress magnitudes (Shmin). The modeled orientations of SHmax are almost entirely within the uncertainties of the WSM data used and the Shmin magnitudes fit to various datasets well. Only the SHmax magnitudes show locally significant deviations, primarily indicating values that are too low in the lower part of the model. The model is open for further refinements regarding model geometry, e.g., additional layers with laterally varying material properties, and incorporation of future stress measurements. In addition, it can provide the initial stress state for local geomechanical models with a higher resolution.

An integrated critical approach to off-fault strike-slip motion triggered by the 2011 Van mainshock (Mw 7.1), Eastern Anatolia (Turkey): New stress field constraints on subcrustal deformation

In this study, we retrieved the finite source characteristics of the October 23, 2011 Van earthquake (Mw 7.1) using the teleseismic waveforms to focus on the source location. The outstanding off-fault aftershock sequence of the Van mainshock was readily explained by calculating the Coulomb stress changes imparted to the surrounding crust. This may be accomplished through finite source modelling to examine the stress interaction between the fault, ruptured by the Van mainshock, and the surrounding fault(s) triggered by the same mainshock. In addition, to provide further support for the Coulomb failure stress changes in the off-fault area, centroid moment tensor (CMT) inversion of the off-fault aftershocks was performed and stress tensors were derived from their focal solutions. This identified the dominant fault slip, the constraints of the crustal stress fields and illuminated the crustal nature of the stress interaction. The off-fault aftershocks showed a strike-slip stress regime in rotational (to NW) and non-rotational (to N) stress fields of the upper and lower crusts, respectively. However, this was inconsistent with a horizontal compressional stress direction striking to the north. This suggests that a local source and/or rotation of lateral variation in stress magnitudes in crustal and sub-crustal structures strongly perturbed the regional stress field. It was also evident that these strike-slip aftershocks increased the intensity of stress in an off-fault area, NE of the source rupture. This revealed a uniquely triggered strike-slip motion, activated and rooted in the weak lower crust. We conclude that the Van mainshock rupture source area, associated with the stress changes imparted to the surrounding crust, had undergone anomalous modifications to generate distinctive off-fault aftershock responses in the entire crust, and also triggered and loaded the weak lower crust. We hypothesize that the strike-slip motion, the so called “transfer fault”, as a distinctly triggered slip event, was generated or selectively activated by subcrustal ductile processes in the absence of mantle lid beneath the study area. However, local slab fragmentation, tearing and cold mantle beneath the study area lead to paradigm changes in interpreting the strike-slip motion and subcrustal deformation. The presence of a small piece of oceanic lithosphere, consistent with fragmented, torn slab and cold mantle, may be an alternative hypothesis that remains to be tested. The Van earthquake, combined with careful examination of associated off-fault aftershocks, revealed new information about stress field constraints on subcrustal deformation. This investigation also provided insights into an important role of stress interaction, with a newly discovered transfer fault within the off-fault area, which extends through the entire crust beneath Lakes Van and Erçek areas.

Crustal folds alter local stress fields as demonstrated by magma sheet — Fold interactions in the Central Andes

For magma chambers to form or volcanic eruptions to occur magma must propagate through the crust as dikes, inclined sheets and sills. Most models that investigate magma paths assume the crust to be either homogeneous or horizontally layered, often composed of rocks of contrasting mechanical properties. In regions that have experienced orogenesis, like the Andes, the crust has been deformed over several million years, resulting in rock layers that are commonly folded and steeply dipping. The assumption of homogeneous properties or horizontal layering then does not capture all of the potential magma path-crustal interactions. Here we tackle this problem by determining the effect of a crust made of steeply inclined layers in which sills and inclined sheets are emplaced. We combine field observations from a sill emplaced in the core of an anticlinal fold at El Juncal in the Chilean Central Andes, including lithologies, sill and fold limbs attitude, length and thickness with a suite of finite element method models to explore the mechanical interactions between inclined layers and magma paths. Our results demonstrate that the properties of the host rock layers as well as the contacts between the layers and the geometry of crustal structures all play an important role in magma propagation and emplacement at shallow levels. Sill propagation and emplacement in heterogeneous and anisotropic crustal segments change the crustal stress field promoting sill arrest, deflection or further propagation. Specifically, sills are more likely to be deflected when encountering shallow dipping layers rather than steeply dipping layers of a fold. Mechanically weak contacts encourage sill deflection due to the related rotation of the stress field and this effect is attenuated when the folded layers are steeper. These processes may change the amount and style of recorded surface deformation, with implications for monitoring of active volcanoes.

Study on the crustal stress field of the Tengchong volcanic area using composite focal mechanism method

Study on the crustal stress field of the Tengchong volcanic area using composite focal mechanism method

The Crustal Stress Field Inferred From Focal Mechanisms in Northern Chile

AbstractWe ​study the spatial variability of the crustal stress in northern Chile. A margin‐parallel compressive crustal stress regime is inferred along the coastal region between 19° and 23.5°S, similar to stress observations in Cascadia and Japan. The Andean Precordillera shows a distinct stress field associated with a strike‐slip faulting regime around 21°S. These results are constrained by over a decade of observations, for which earthquake catalogs report thousands of events in the continental crust. We present focal mechanisms for 817 of these crustal earthquakes, including mechanism qualities. The best mechanisms were grouped and inverted to infer the stress‐field variability. We interpret the margin‐parallel compression to be caused by the concave shape of the margin and the locking of the plate interface. The inferred strike‐slip regime in the Andes agrees with previous studies and has been proposed to be mostly caused by local stresses imposed by a thicker crust.

Crustal Strain and Stress Fields in Egypt from Geodetic and Seismological Data

The comparison between crustal stress and surface strain azimuthal patterns has provided new insights into several complex tectonic settings worldwide. Here, we performed such a comparison for Egypt taking into account updated datasets of seismological and geodetic observations. In north-eastern Egypt, the stress field shows a fan-shaped azimuthal pattern with a WNW–ESE orientation on the Cairo region, which progressively rotated to NW–SE along the Gulf of Aqaba. The stress field shows a prevailing normal faulting regime, however, along the Sinai/Arabia plate boundary it coexists with a strike–slip faulting one (σ1 ≅ σ2 > σ3), while on the Gulf of Suez, it is characterized by crustal extension occurring on near-orthogonal directions (σ1 > σ2 ≅ σ3). On the Nile Delta, the maximum horizontal stress (SHmax) pattern shows scattered orientations, while on the Aswan region, it has a WNW–ESE strike with pure strike–slip features. The strain-rate field shows the largest values along the Red Sea and the Sinai/Arabia plate boundary. Crustal stretching (up to 40 nanostrain/yr) occurs on these areas with WSW–ENE and NE–SW orientations, while crustal contraction occurs on northern Nile Delta (10 nanostrain/yr) and offshore (~35 nanostrain/yr) with E–W and N–S orientations, respectively. The comparison between stress and strain orientations over the investigated area reveals that both patterns are near-parallel and driven by the same large-scale tectonic processes.

A Systematic Approach to Mapping Regimes of Earthquake‐Induced Static Stress Changes Acting on Magmatic Pathways

AbstractLarge earthquakes alter the crustal stress field across great distances (hundreds to thousands of kilometers) over geologically short timescales (seconds to years). These stress changes can affect magmatic systems, triggering (or suppressing) volcanic unrest and eruption, along with other deeper processes. We use simple kinematic source models in an isotropic elastic half‐space to assess earthquake‐induced static stress changes (>1 kPa) over the entire thickness of the lithosphere and consider the implications for magma ascent and storage. Modeling subduction zone earthquakes, we calculate static normal stress changes with depth on three mutually perpendicular end‐member magma pathways: vertical arc‐parallel, vertical arc‐perpendicular, and horizontal. From this, we define seven stress change regimes within the adjacent volcanic arc. Three of these regimes may strongly encourage magma ascent in dykes by inducing unclamping (decreased compressive normal stress) of vertical pathways which increases in magnitude toward the surface and clamping of horizontal pathways. Two of the regimes may encourage stalling and storage of magma in sills near the base of the crust by inducing unclamping of horizontal pathways at depth. The spatial distribution of the regimes is largely dependent on earthquake magnitude, but also varies with slip distribution and interface dip. We show how the responses of magmatic systems to earthquakes also depends on the stress change magnitude and the state of the magmatic system, with a greater impact expected for larger stress changes acting on weaker, more thermally mature systems.

Pattern of Global Crustal Stresses of the Earth

The results of tectonophysical inversion of the global tectonic stress field are considered from the position of explaining the active forces that cause the lithospheric plate movements. The analysis of regularities of the stress state of the main structural elements of Earth’s lithosphere—ocean spreading, transformations, active continental margins, and intracontinental orogeny—is performed. It is shown that the stresses in most of the ocean lithosphere, including subduction zones, correspond to active traction forces from the submerged and weighted part of the lithosphere. A low level of deviatoric stresses in subduction zones and the presence of dynamo pair with sharply different mode of stress state were found. Horizontal compression is observed in the crust of the continental slope, and horizontal tension is observed in the ocean lithosphere, so the boundaries of lithospheric plates should not be considered as a source of increased horizontal compressive stress in the crust of continents. The formation of a horizontal compression regime in the crust of intracontinental orogens, as well as in the crust of active continental margins orogens, is associated with residual stresses of rocks that have been exhumed from the depths of the crust. A generalizing scheme for the formation of the global crustal stress field of the Earth is proposed, which corresponds to modern data on the movements of lithospheric plates and the deep structure of the upper mantle.

Database of Italian present-day stress indicators, IPSI 1.4

The Italian Present-day Stress Indicators (IPSI) database is a freely available Italian georeferenced repository of information regarding the crustal stress field. It consists of horizontal stress orientations that have been analysed, compiled in a standardised format and quality-ranked for reliability and comparability on a global scale. The database contains a collection of information regarding contemporary stress within the shallow crust from the following main stress-indicator categories: borehole breakouts; earthquake focal mechanisms; seismic sequences and active fault-slip data. The present database (IPSI 1.4) released in January 2020 is accessible through a web interface which facilitates findability, accessibility, interoperability and reusability of the dataset. Moreover, it contains 928 records updated up until December 2019 with an increase of 10% with respect to the first one, and improved metadata information. The uniform spread of stress data over a given territory is relevant for earth crustal modelling or as starting point in many applied studies. It is therefore necessary to continue collecting new data and update present-day stress maps to obtain more reliable evaluations.

Focal mechanisms of small earthquakes beneath the Japanese islands based on first-motion polarities picked using deep learning

SUMMARY Knowledge of crustal stress fields is essential for understanding tectonics and earthquake generation. One approach for estimating the crustal stress field is based on the focal mechanisms of earthquakes. This study investigated the focal mechanisms of approximately 110 000 microearthquakes in the area of the Japanese islands that occurred at a depth shallower than 20 km, based on the first-motion polarities picked by a simple neural network model. The model was first trained using a data set of mainly moderate to large earthquakes throughout Japan. Following on, the model was re-trained using a data set of microearthquakes in two regions of Japan. The threshold of the confidence score from the neural network model was chosen to maximize the overall quality of the focal mechanism solutions. The P- and T-axes of the numerous focal mechanism solutions provided more detailed distributions of the crustal stress field. For example, in the Chugoku region, small differences were observed in the trend of P-axes azimuths between the northern and southern areas, spatially corresponding to geodetic observations. The results of this study are useful for revealing the crustal stress field, and, as such, for assessing past and current tectonic activities and potential future earthquake generation.

Field observations and numerical models of a Pleistocene-Holocene feeder dyke swarm associated with a fissure complex to the east of the Tatara-San Pedro-Pellado complex, Southern Volcanic Zone, Chile

Magma is transported through the lithosphere as dykes which, during periods of unrest, may feed eruptions at the surface. The propagation path of dykes is influenced by the crustal stress field and can be disturbed by crustal heterogeneities such as contrasting rock units or faults. Moreover, as dykes propagate, they themselves influence the surrounding stress field through processes of stress transfer, crustal deformation and seismic failure. The result is the formation of arrested dykes, as well as contrasting strike and dip angles and dyke segmentation. Here, we study the mechanisms of dyke injection and the role played in modifying the stress field and potential propagation paths of later dyke injections. To do this we combine field data from an eroded and well-exposed shallow feeder dyke swarm with a suite of two-dimensional FEM numerical models. We mapped 35 dyke segments over a ~1 km long dyke swarm exposed ~5 km to the East of Pellado Volcano, in the Tatara-San Pedro-Pellado (TSPP) volcanic complex, Southern Volcanic Zone of the Andes. Detailed mapping of the swarm elucidates two preferential strike orientations, one ~N80°E and the other ~N60°E. Our numerical models simulate both the TSPP volcanic complex and the studied dyke swarm as zones of either magmatic excess pressure or as a rigid inclusion. The crustal segment hosting the volcanic complex and dykes is modelled using an elastic domain subjected to regional compression in select model cases. Model outputs provide the stress and strain fields resulting from the different geometries and applied boundary loads. The model results indicate that individual dyke injections can locally rotate the principal stresses such as to influence the range of orientations over which later dykes will form. The orientation of σ1 at the dyke tip ranges over 60° (±30° either side of the dyke tip) indicating that the strike orientation of later dykes will fall within this range. The effect of adding a bulk regional compression is to locally increase the magnitude of favorably oriented tensile stresses in the bedrock but to reduce the range of σ1 orientations to 40° (±20°). This implies that under a far-field transpressive stress regime, as is common in Andean settings, regional dyke swarms will tend to maintain their strike orientation parallel to the regional bulk stress. These results should be accounted for when studying periods of volcanic unrest in order to discern the location and orientation of potential fissure eruptions in active volcanic areas such as the Southern Volcanic Zone of the Andes.

New insights on the Neogene tectonic evolution of the Aksu Basin (SE Turkey) from the Anisotropy of Magnetic Susceptibility (AMS) and paleostress data

The junction between the Aegean and Cyprus arc in the southern Turkey forms a triangular-shaped morphological structure so-called Isparta Angle (IA). In Neogene time, the inner part of the Isparta Angle became overlain by sedimentary basins including Manavgat, Köprüçay and Aksu, characterized by marine clastics and carbonates.Aksu Basin, one of these marine basins, provides geological records about the Neogene crustal deformation and stress field in the Eastern Mediterranean region. Therefore, we kinematically assess if and when tectonic stress fields affected the Aksu basin in early Miocene to recent times. For this purpose, Anisotropy of Magnetic Susceptibility (AMS) data (AMS) of cored samples and fault-slip data from the brittle mesoscopic faults were collected. The ~490 oriented samples for Anisotropy of Magnetic Susceptibility measurements were analyzed and two different maximum magnetic lineation's directions are recognized; 1) N–S magnetic lineation in Pliocene and 2) from N–S to NW-SE magnetic lineation in the Miocene. The results inferred from more than 1000 fault-slip data collected from 83 different sites combined with AMS results demonstrate that the Aksu Basin developed under four different tectonic phases; 1) ~E-W extensional phase (related to basin formation), 2) ~N–S compressional (Lycian) phase, 3) ~(N)E-(S)W compressional (Aksu) phase and 4) N–S extensional (Recent) phase.

Discriminating Possible Causes of Microwave Brightness Temperature Positive Anomalies Related With May 2008 Wenchuan Earthquake Sequence

Based on the spatiotemporally weighted two-step method (STW-TSM), the spatiotemporal characteristics of the residual microwave brightness temperature (MBT) with the Mw7.9 Wenchuan earthquake on May 12, 2008 are revealed by satellite data from the Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E) sensor. Two significant MBT positive anomalies are found to be exactly in spatial accordance with two geological Quaternary zones, and the detailed geometric information of the MBT positive anomaly is found to be correlated with the microwave frequencies. After eliminating other possible influential factors, including surface temperature, vegetation index, land-surface roughness, and surface soil moisture under the conditions of space, time, and magnitude, and according to the microwave radiative transfer model, the dielectric variation in the ground surface is suggested to be the primary contributor of the MBT positive anomaly. The positive-hole (P-hole) theory is applied to interpret the geological preference of the MBT positive anomalies through a chain process: crustal stress enhancing—P-hole producing and flowing down stress gradients—surface P-hole accumulating—dielectric constant decreasing—and microwave radiation increasing. The stress-resulting effect of the dielectric decrease on MBT the increase provides a novel mechanism for microwave remote-sensing monitoring of crustal stress field alteration, earthquake preparation, and upcoming shocks. This research has a particular significance for searching potential georelations between the tectonic earthquake preparation and the abnormal satellite MBT.

Dynamic earthquake rupture modeling considering regional crustal stress conditions in southeastern Korea

The Korean Peninsula is located in a low-seismicity zone at the eastern margin of the Eurasian plate. However, the 2016 Gyeongju and 2017 Pohang earthquakes, which occurred in southeastern Korea, highlight the importance of advanced seismic hazard assessment and mitigation in this region. We successfully performed spontaneous dynamic rupture modeling by considering the crustal stress field in southeastern Korea, constrained by earthquake focal mechanism data, to better understand the seismic hazard in the region. We found that the geologically recognized Yangsan fault trace near the location of the Gyeongju earthquake is not aligned well with the favorable shear rupture directions, given the current crustal stress field in the region. The fault needs to be very weak (i.e., have a low static friction coefficient) to initiate a large earthquake rupture. However, our dynamic model results show that the fault plane constrained by the focal mechanism solution of the 2016 Gyeongju earthquake is more favorably aligned for shear rupture under the current stress condition and that it may be able to produce a relatively large earthquake if the extension of the fault is quite long (~60 km). This type of dynamic rupture modeling study may help to more accurately elucidate the seismic hazard in a low-seismicity zone such as southeastern Korea.

Experimental and theoretical analysis of hydraulic fracturing and gas fracturing of rock under true triaxial compressions

A test system was developed for true triaxial hydraulic fracturing (HF) and gas fracturing (GF) combined with acoustic emission (AE) monitoring and 3D computerized tomography (CT) scanning reconstruction to carry out a series of HF and GF tests under different true triaxial loading conditions. The critical breakdown pressure characteristics, the AE activities, and the fracture propagation and spatial morphologies were obtained during the fracturing process. The test results showed that the GF breakdown pressure was approximately 29.0–31.3% less than the HF breakdown pressure. Moreover, for the GF specimen, the gas pore pressure induced a large number of new microfractures before critical fracture. However, both HF and GF under true axial compression can only produce simple main longitudinal fractures (LFs, along the borehole) and transverse fractures (TFs, across the borehole) that are strictly dependent on the initial true triaxial stress field. Based on the experimental observations, a hypothesis for the initiation of the HF and GF under true triaxial compression was proposed by using the maximum tangential stress on the borehole wall and the maximum axial stress along the borehole. Accordingly, analytical solutions of the critical breakdown pressure for the HF (impermeable) and GF (permeable) specimens were derived. The theoretical predictions agree well with the observations from the 3D numerical simulations and tests. Lastly, a novel pulsed GF method (dynamic loading fracturing) was proposed to overcome the dependence of the in-situ crustal stress field associated with traditional HF and GF methods (static loading fracturing). Laboratory test results showed that the dynamic pulsed GF at moderate pressure can generate a complex and random fracture network independent of the in-situ stress field in rock.

Multiscale variations of the crustal stress field throughout North America

The Earth’s crustal stress field controls active deformation and reflects the processes driving plate tectonics. Here we present the first quantitative synthesis of relative principal stress magnitudes throughout North America together with hundreds of new horizontal stress orientations, revealing coherent stress fields at various scales. A continent-scale transition from compression (strike-slip and/or reverse faulting) in eastern North America to strike-slip faulting in the mid-continent to predominantly extension in western intraplate North America is likely due (at least in part) to drag at the base of the lithosphere. Published geodynamic models, incorporating gravitational potential energy and tractions from plate motions or relative mantle flow, successfully predict most large-wavelength stress rotations but not the shorter-wavelength (<~200 km) rotations observed in the western USA. The stresses resulting from glacial isostatic adjustment appear to be much smaller than the magnitude of ambient tectonic stresses in the crust at depth.