Ambient Stress Field Research Articles

Rupture Dynamics of Cascading Earthquakes in a Multiscale Fracture Network

AbstractFault‐damage zones comprise multiscale fracture networks that may slip dynamically and interact with the main fault during earthquake rupture. Using 3D dynamic rupture simulations and scale‐dependent fracture energy, we examine dynamic interactions of more than 800 intersecting multiscale fractures surrounding a listric fault, emulating a major listric fault and its damage zone. We investigate 10 distinct orientations of maximum horizontal stress, probing the conditions necessary for sustained slip within the fracture network or activating the main fault. Additionally, we assess the feasibility of nucleating dynamic rupture earthquake cascades from a distant fracture and investigate the sensitivity of fracture network cascading rupture to the effective normal stress level. We model either pure cascades or main fault rupture with limited off‐fault slip. We find that cascading ruptures within the fracture network are dynamically feasible under certain conditions, including: (a) the fracture energy scales with fracture and fault size, (b) favorable relative pre‐stress of fractures within the ambient stress field, and (c) close proximity of fractures. We find that cascading rupture within the fracture network discourages rupture on the main fault. Our simulations suggest that fractures with favorable relative pre‐stress, embedded within a fault damage zone, may lead to cascading earthquake rupture that shadows main fault slip. We find that such off‐fault events may reach moment magnitudes up to Mw ≈ 5.5, comparable to magnitudes that can be otherwise hosted by the main fault. Our findings offer insights into physical processes governing cascading earthquake dynamic rupture within multiscale fracture networks.

Potential Seismic Hazard in Seoul, South Korea: A Comprehensive Analysis of Geology, Seismic, and Geophysical Field Observations, Historical Earthquakes, and Strong Ground Motions

ABSTRACT A series of moderate-size (Mw 4.0–6.0) earthquakes occurred in South Korea after the 2011 Mw 9.0 Tohoku–Oki megathrust earthquake, incurring public concern about possible occurrence of devastating earthquakes in Seoul—the capital city of South Korea, where historical seismic damage was reported. The seismicity is distributed in Seoul, being dominated by strike-slip earthquakes. The fault planes are oriented in north-northeast–south-southwest, which is a favorable direction to respond to the ambient stress field. Higher rates of seismicity are observed in the northwestern Seoul at depths of <10 km. Micro-to-small earthquakes occur episodically in the central Seoul along the Chugaryeong fault system that traverses Seoul in north–south. Seismic, geophysical, and geological properties illuminate the fault structures. Stochastic modeling of ground motions reproduces the seismic damages of historical earthquakes reasonably, supporting the occurrence of devastating historical earthquakes in Seoul. The seismicity distribution, focal mechanism solutions, geological features, and seismic and geophysical properties suggest the possible presence of earthquake-spawning blind faults in Seoul. The peak ground motions are assessed for moderate-size scenario earthquakes (Mw 5.4 with focal depth of 7 km) at six representative subregions in Seoul. The upper bounds of peak ground accelerations reach ∼11 m/s2. The seismic damage potentials for moderate-size earthquakes are high in most areas of Seoul, particularly around river sides covered by alluvium.

Unravelling a midcrustal seismogenic fault structure from a seismic sequence and geophysical data: application to the 28 October 2022ML 4.1 Goesan earthquake in the central Korean Peninsula

SUMMARYEarthquakes in the Korean Peninsula often occur in subsurface hidden faults that are hardly identified before the seismic activity. We investigate a midcrustal subsurface hidden fault in a seismically quiescent region of the central Korean Peninsula that produced the 28 October 2022 ML 4.1 earthquake. A combined analysis of seismicity, geophysical properties and geological features constrain the subsurface fault geometry. The midcrustal fault may extend to the lineament on the surface that presents low gravity anomalies. The focal mechanism solutions and seismicity distribution suggest that the lineament may correspond to the surface trace of the fault. Further, the responsible fault is a left-lateral strike-slip fault with dip of 88° and bilateral rupture at depths of ∼12–14 km. The lateral extent of the fault may reach >25 km. A series of foreshocks formed the source zone to induce the main shock. The main shock nucleated at a location of lateral contrasts of heat fluxes and magnetic anomalies in the fault. The strong ground motions by the main shock are amplified by the sedimentary layers in the event site, inducing aftershocks to spread along the fault with time. The fault orientation is laid to respond to the ambient stress field. The Coulomb stress changes induced by the main shock load stress the fault, increasing the possibility of another earthquake occurrence. The study suggests that a combined analysis of seismicity, geophysical properties and geological features may provide constraints on seismogenic subsurface hidden faults.

Emplacement of a lamprophyric sheet swarm under ductile transpression in continental crust. Insights from a Variscan magmatic complex in the West Armorican belt, Western France

We present structural data from a Variscan magmatic complex of dominantly lamprophyric (kersantite) and associated felsic (microgranodiorite) sheets - the West Armorican swarm (WAS) - injected into Paleozoic metasedimentary series at the western extremity of a transpressional system in the Armorican belt, Western France. The comparative structural analysis of nearly 100 individual intrusive sheets and their host-rocks demonstrates that the WAS complex is a syntectonic magmatic structure which intruded during the onset of regional shortening via successive pulses alternating with three strain episodes (D1-D2-D3). The moderate/high dipping attitude of a great majority of intrusions, along with their prominent orientation parallel to the regional cleavage (S1), i.e. orthogonal to the direction of shortening, challenge theoretical predictions about magma-filled fractures in contractional settings. These apparently atypical features are reconciled in a kinematic model which emphasizes the influence of both host-rock planar anisotropies (S1 cleavage and D3 shear patterns) and structural inheritance on magma pathways, and instead hypothesizes the negligible role of the ambient stress field. Comparisons are attempted with syncollisional lamprophyre systems present in the European Variscides, and more especially in the SE Bohemian massif.

Role of Backbone Fault System on Earthquake Spawning and Geohazards in the Seoul Metropolitan Area

AbstractMajor earthquakes in continental regions may cause significant damage. Preexisting fault system across megacity receives high attention for possible seismic damages. Earthquake occurrence mechanism is important to assess the geohazard potentials. Continental‐scale Quaternary fault system is developed across the Seoul metropolitan area where the population is the largest in the Korean Peninsula. Historical seismic‐damage records suggest potential seismic hazards in the Seoul metropolitan area. We investigate the fault motions and spatial distribution of earthquakes in the Seoul metropolitan area using a matched‐filter technique that is based on stacked waveform crosscorrelation functions among densely‐deployed seismic stations. The analysis detects 1103 earthquakes that include 360 events with magnitudes (ML) of −0.6 to 2.0 around the Chugaryeong fault and 34 events with magnitudes of −0.5 to 2.7 around Wangsukcheon, Pocheon, and Yeseonggang faults. The seismicity suggests a set of near‐vertical subparallel (or orthogonal) faults that develop from the major faults. A major fault system behaves as a backbone structure that makes branch faults develop, producing seismicity including major earthquakes. The backbone structure may control the fault development that conforms to the ambient stress field. The backbone faults may play a role to increase geohazard potentials.

Fault slip potential induced by fluid injection in the Matouying enhanced geothermal system (EGS) field, Tangshan seismic region, North China

Abstract. The Tangshan region is one of the most seismically active areas in the North China, and the 1976 M 7.8 earthquake occurred on 28 July near the Tangshan fault zone. The Matouying enhanced geothermal system (EGS) field is located ∼90 km away from the city of Tangshan. Since late 2020, preliminary hydraulic stimulation tests have been conducted at depths of ∼3965–4000 m. Fluid injection into geothermal reservoir facilitates a heat exchanger system. However, fluid injection may also induce earthquakes. In anticipation of the EGS operation at the Matouying uplift, it is essential to assess how the fault slip potential of the nearby active and quiescent faults will change in the presence of fluid injection. In this study, we first characterize the ambient stress field in the Tangshan region by performing stress tensor inversions using 98 focal-mechanism data (ML≥2.5). Then, we estimate the principal stress magnitudes near the Matouying EGS field by analyzing in situ stress measurements at shallow depths (∼600–1000 m). According to these data, we perform a quantitative risk assessment using the Mohr–Coulomb framework in order to evaluate how the main active faults might respond to hypothetical injected-related pore pressure increases due to the upcoming EGS production. Our results mainly show that most earthquakes in the Tangshan seismic region have occurred on the faults that have relatively high fault slip potential in the present ambient stress field. At well distances of less than 15 km, the probabilistic fault slip potential on most of the boundary faults increases with continuing fluid injection over time, especially on the faults with well distances of ∼6–10 km. The probabilistic fault slip potential (fsp) increases linearly with the fluid injection rate. However, the fsp values decrease exponentially with increased unit permeability. The case study of the Matouying EGS field has important implications for deep geothermal exploitation in China, especially for Gonghe EGS (in Qinghai Province) and Xiong'an New Area (in Hebei Province) geothermal reservoirs that are close to the Quaternary active faults. Ongoing injection operations in the regions should be conducted with these understandings in mind.

Spatio‐Temporal Evolution of Earthquake Static Stress Drop Values in the 2016–2017 Central Italy Seismic Sequence

AbstractThe static stress drop of an earthquake is an indicator of the stress state of a specific fault before rupture initiation. The stress state is primarily controlled by the ambient stress field, fault strength, fault complexity, and the presence of fluids. This study aims to investigate the spatio‐temporal distribution of static stress drop values of the 2016–2017 multi‐fault rupture seismic sequence in central Italy, which includes three earthquakes with Mw ≥ 5.9 (Amatrice, Visso, and Norcia earthquakes), and over 95,000 aftershocks (M 0.5–6.5). We estimate stress drop values using a circular crack model with corner frequency and seismic moment estimates from single‐spectra fitting, a cluster‐event method, and spectral‐ratio fitting. The temporal distribution of stress drop values shows an apparent increase of stress drop following a large earthquake (Mw ≥ 5.9). The spatial distribution shows comparably high stress drop values for early aftershocks surrounding the mainshock rupture area. High stress drop events correlate with fault complexity, such as fault intersections at depth and reactivated thrust fronts. We observe a constant stress drop for Mw ≥ ∼3, in contrast to previous studies. Instrument response and signal‐to‐noise bandwidth limitations likely govern the observed decrease in stress drop with decreasing magnitude for events with Mw ≤ 3. The spatio‐temporal distribution of stress drop values in a complex seismic sequence could support a more complete understanding of the earthquake rupture process and the evolution of seismic sequences. It could also highlight areas where stress loading is focused, which would have implications for short and intermediate term seismic hazard estimates.

Back‐Arc Extension of the Central Bransfield Basin Induced by Ridge–Trench Collision: Implications From Ambient Noise Tomography and Stress Field Inversion

AbstractThe Bransfield Basin is a young (∼4 Ma) back‐arc basin related to the remnant subduction of the Phoenix Plate that once existed along the entire Pacific margin of the Antarctic Peninsula. Based on a recently deployed amphibious seismic network, we use ambient noise tomography to obtain the S‐wave velocity structure in the Central Bransfield Basin (CBB). Combining with the stress field inverted from focal mechanisms, our images reveal that the CBB suffers a significant extension in the northwest‐southeast direction. The extension is strongest in the northeastern CBB with associated mantle exhumation and weakens to the southwest with decoupled deformations between the upper crust and lithospheric mantle. Such an along‐strike variation of extension can be explained by slab window formation and forearc rotation, which are associated with the Phoenix Plate detachment during the ridge–trench collisions at the southwest of the Hero Fracture Zone.

Stress‐Driven Failure of Cylindrical Volcanic Conduits

AbstractMany explosive volcanic eruption sequences are characterized by exceedingly unsteady behavior, often punctuated by multiple explosive phases, separated by hours to days. What causes this unsteady behavior is not fully understood. Here, we explore one possible mechanism: stress‐driven failure of the volcanic conduit wall rock. We present a multiphase conduit flow model that predicts the shear and normal tractions exerted on the conduit wall, and couple (one way) these tractions with a numerical model of host rock failure. While analytical estimations of host rock stress serve as useful guides, analytical approximations which ignore shear tractions are insufficient to adequately predict conduit instability. Failure generally initiates near the fragmentation depth, where shear tractions are highest and magma pressure is low relative to the lithostatic stress. Shear tractions on the conduit wall, caused by viscous drag, promote strain localization and failure. The in situ stress state not only determines the range of conduit pressures which promotes failure, but also influences the style of failure. The mechanism that is most likely to lead to the disruption of flow, where wall rock moves radially into the conduit along normal faults, occurs in extensional stress regimes when large shear tractions at the fragmentation depth enhance normal fault formation. The results of this study indicate that conduit stability strongly depends on the ambient stress field and the nature of conduit flow at the fragmentation depth.

An investigation of how intracratonic rifting is “seeded”: Case study of the Late Devonian Dniepr-Donets Basin rift within the East European Craton

This paper analyses the role of pre-existing Precambrian structures for the localisation of the intracratonic rifts from a case study of the Dniepr-Donets Basin (DDB) in Ukraine. The DDB was formed as a result of Late Palaeozoic rifting in the Archaean-Paleoproterozoic Sarmatian segment of the East European craton (EEC). It separates the Ukrainian Shield (UkS) to its southwest from the Voronezh Massif (VM) to its northeast. The Donbas Foldbelt (DF) constitutes the tectonically inverted part of the DDB in its southeastern extent and has been imaged by coincident wide-angle reflection and refraction seismic as well as deep near-vertical reflection seismic profiles (project “DOBRE”). It is almost completely filled with a highly indurated succession of upper Palaeozoic sediments and metasediments, up to some 20 km in thickness, now exposed at the surface. Here, a crustal and upper mantle structural-compositional model that is tightly constrained by the seismic data and gravity anomalies along the profile is used to search for Precambrian pre-rift crustal features that could have played a role in localising Late Palaeozoic rifting. The results suggest that there may be a different tectonic history for Sarmatian crystalline crust on either side of the DF. Density isolines in the AM crust are shallower than corresponding ones of equal value in the VM crust and, accordingly, the mean density of the crust of the AM is higher. This effect, calibrated on the DOBRE profile, is expressed along the margins of the entire DDB with a higher background level of the gravity field seen generally for the UkS than for the VM. The associated gravity gradient coinciding with the location of Palaeozoic rifting means that there is a perturbation to the horizontal deviatoric stress in this position that likely predates rifting. Further, a well-constrained upper crustal low-density granitic body beneath the northeastern flank of the DF produces a significant negative gravity anomaly superimposed upon the background gravity gradient. This adds a significant additional extensional component to the ambient deviatoric stress field. It cannot be concluded with certainty that either of these crustal features was necessary or sufficient for “seeding” Late Palaeozoic rifting but modern passive seismology surveys across the DDB as well as new bedrock geological studies would help test such a hypothesis.

One-off deep crustal earthquake swarm in a stable intracontinental region of the southwestern Korean Peninsula

A deep crustal strike-slip earthquake swarm that occurred in the intracontinental region of the southwestern Korean Peninsula is investigated. The earthquakes are detected using matched filter analysis. The hypocenters are then refined, showing more than 500 micro to small strike-slip earthquakes concentrated on a fault plane with dimensions of 500 m by 300 m at a depth of ~21 km. The low temperature in the source depth facilitates the seismicity. The earthquake swarm was transiently active for 15 days from 25 April 2020 to 12 May 2020, presenting a high seismicity rate with a Gutenberg-Richter b value of 2.01 (±0.05). The earthquake swarm began in the central area and expanded outward with time. The spatiotemporal seismicity distribution reflects bilateral rupture. The focal mechanism solutions suggest a left-lateral strike-slip fault with a strike of 103.6∘ ± 3.2∘ and a dip of 73.3∘ ± 11.7∘. The spatial distribution of seismicity generally agrees with the focal mechanism solutions. The orientation of the inverted stress field agrees with the ambient stress field, suggesting that the earthquake swarm occurred as a consequence of ambient lithostatic stress loading. Coulomb stress changes suggest that the stress increased in the directions of WNW-ESE and NNE-SSW from the fault. The transiently active earthquake swarm began to occur after the 2011 MW9.0 Tohoku-Oki megathrust earthquake, suggesting a rapid release of accumulated stress in response to changes in the properties of the medium caused by the megathrust earthquake.

Mesozoic‐Cenozoic Regional Stress Field Evolution in Svalbard

AbstractCooling fracture orientations in diabase sills associated with the Cretaceous High Arctic Large Igneous Province and syn‐sedimentary Triassic faults help constrain a model for Svalbard's (NE Barents Shelf) Mesozoic stress field evolution. Fracture data from Edgeøya and adjacent islands in SE Svalbard, from S Spitsbergen, and from literature were used to model preferred orientations and temporal relationships. Orthogonal, roughly E‐W and N‐S, joints and veins in sills from SE Svalbard are interpreted as cooling fractures influenced by the ambient stress field. Aligned preferred orientations within the Triassic host strata are associated with a regional Cretaceous jointing episode driven by sill emplacement and/or erosional unloading. The regional maximum horizontal stress (likely σ1) is inferred to have been parallel to a dominant ≈E‐W set. Spitsbergen's more complex joint patterns are associated with proximity to the Cenozoic West Spitsbergen Fold‐and‐Thrust Belt, but ≈E‐W and ≈N‐S orientations occur and are typically the earlier set. Syn‐sedimentary, ≈NW‐SE striking, Triassic normal faults in SE Svalbard aligned with the maximum horizontal stress indicate a Triassic to Cretaceous counterclockwise stress field shift, with additional counterclockwise shifting during Cenozoic dextral transpression between Svalbard and Greenland. Localized joint preferred orientations consistent with both decoupled and coupled transpression occur. Changes in the regional maximum horizontal stress and deformation regime may reflect timing of which plate margin was crucial in influencing Svalbard's plate interior stress field, starting with Triassic Uralian activity to the E, then Cretaceous Amerasian Basin development to the NW, culminating with Cenozoic dextral transpression and transtension to the SW.

Structural geological methods in the geotechnical practice – rock mass rating. Advantages and problems of the rating methods

This paper presents a review of the most common geotechnical rock rating methods. The directional properties of the rock’s anisotropies are exemplified, by a case of resolution of stresses in folded and foliated metamorphic rocks. In such rocks, depending on the geometry of the folds, the ambient stress field can generate varying shear potential along the foliation planes in different parts of the excavation. The commonly used rating schemes, with accent on the geological feasibility of the classifications, are discussed – Protodiakonov’s rock scale, Terzaghi’s grading for construction of tunnels, then the rock quality designation of Deere, the Bieniawski’s rock mass rating (RMR), the Hoek, and Brown’s GSI and the Barton’s Q-system. It is emphasized that in spite of its broad use, the RQD is distorting the statistical weight of the joint groups as some steeply dipping joints may be completely neglected. It is recognized that the RMR is the simplest to use but underestimates the directional properties of the rock anisotropies, which require better definition and has no provision for lithologically varying rock packages, although it has the advantage of using the uniaxial compressive strength, which other systems do not employ. The Hoek and Brown’s criterion went too far with complex empirical relationships, which rely on extensive laboratory testing, so it is no more field criterion. Besides, its geological descriptive powers are rather poor and now, new form of GSI classification is offered for nearly every lithological type. Barton’s Q-system, being best suited to case studies of actual underground constructions, suffers from the fact that is centered nearly exclusively on joints, which may be justified in Norway, where mainly magmatic and high grade metamorphic rocks are present but should be applied cautiously in areas, where sedimentary, volcanic and strongly foliated rocks are exposed. In general, for all the discussed geomechanical classification systems (RMR, GSI, Q) the rule is valid, that they work better in an isotropic, strong but jointed rock masses and do not work well in week layered and foliated rocks.

Fault failure modes, deformation mechanisms, dilation tendency, slip tendency, and conduits v. seals

Abstract Faults have complicated shapes. Non-planarity of faults can be caused by variations in failure modes, which in turn are dictated by mechanical stratigraphy interacting with the ambient stress field, as well as by linkage of fault segments. Different portions of a fault or fault zone may experience volume gain, volume conservation and volume loss simultaneously depending on the position along a fault's surface, the stresses resolved on the fault and the associated deformation mechanisms. This variation in deformation style and associated volume change has a profound effect on the ability of a fault to transmit (or impede) fluid both along and across the fault. In this paper we explore interrelated concepts of failure mode and resolved stress analysis, and provide examples of fault geometry in normal faulting and reverse faulting stress regimes that illustrate the effects of fault geometry on failure behaviour and related importance to fluid transmission. In particular, we emphasize the utility of using relative dilation tendency v. slip tendency on fault patches as a predictor of deformation behaviour, and suggest this parameter space as a new tool for evaluating conduit v. seal behaviour of faults.

2. 45 Ga break-up of the Archaean continent in Northern Fennoscandia: Rifting dynamics and the role of inherited structures within the Archaean basement

Constraints from restoration of minor rift-related faults preserved from subsequent Palaeoproterozoic overprint along the southern margin of the Peräpohja Belt, northern Fennoscandia indicate local NE-SW extension directions prevailed at the time of 2.44 Ga rifting of the Archaean continent. This palaeostress field is attributed to pull-apart setting of the Peräpohja Belt at a left overstepping zone of major sinistral N-S trending deformation zones. Consequently, the variably overprinted NW-SE trending structures of the Peräpohja Belt, including the NW-SE central graben were originally generated as normal faults. The proposed setting is compatible with a conducted reconnaissance sandbox analogue model where the orientation of the major normal faults was controlled by the ambient stress field within the step-over zone between strike-slip faults. By contrast, the faults bounding E-W to ENE-WSW trending basement horsts are the result of reactivation of older basement structures underlying the pull-apart basin. Based on the results, we infer that the final break-up of the Archaean continent in northern Fennoscandia utilized the 2.44 Ga rift-related NW-SE trending crustal anisotropy (parallel with dyke swarms), with pre-existing Archaean N-S trending structures reactivated as second-order shear zones in-between. The voluminous syn-rift 2.5–2.4 Ga and later pulses of mafic magmatism (2.32–1.98 Ga) are considered indicative of active-type rifting which onset was controlled by a mantle plume centred within a supercraton formed by the Superior, Hearne and Karelian (-Kola) cratons. The presented model supports the continuity of the Archaean craton beyond the Caledonian Orogen towards north-west and explains its segmentation, suggesting a model for the localization of the Palaeoproterozoic supracrustal belts of Northern Fennoscandia. Moreover, this paper provides an approach to understand the basement-cover relationships and the structural signatures within these relatively shallow supracrustal belts which are highly prospective for mineral deposits.

Azimuthal Seismic Anisotropy in the Upper Crust of the Japanese Islands Induced by the 2011 Tohoku‐Oki Megathrust Earthquake

AbstractWe investigated VP/VS changes in the upper crust of the Japanese islands after the 2011 MW9.0 Tohoku‐Oki megathrust earthquake. Abrupt VP/VS changes with azimuth‐dependent variations were observed after the megathrust earthquake. The VP/VS changes ranged between −0.0458 (±0.0012) and 0.0422 (±0.0033). Large localized VP/VS changes over regional distances suggested medium‐dependent deformation. Peak VP/VS changes were observed along paths subparallel or subperpendicular to the directions toward the megathrust earthquake. The VP/VS changes displayed characteristic 2θ variations as a function of the azimuth difference, suggesting azimuthal seismic anisotropy. The VP/VS ratio recovered gradually over time. Some regions presented permanent VP/VS changes. The azimuthal seismic anisotropy may have developed from preferential crack orientation as a consequence of the combined effects of lithospheric displacements and a depth‐dependent ambient stress field. It appears that recovery of the medium properties may take decades.

Holocene tectonic activity along Kachchh Mainland Fault: Impact on late mature Harappan civilization, Kachchh, western India

Paleoseismic and archaeological records of the Kachchh Rift Basin (KRB) of western India indicates that the region become tectonically active during the period of Middle to Late Holocene. To understand the relationship between strain accumulation, earthquake genesis, and landform development, we analyzed uplifted Late Quaternary terraces and archaeological evidence along the Kachchh Mainland Fault (KMF) zone. Dividing the elevation of the bedrock strath at each site by their ages, yields vertical uplift rates of 0.29–1.17 mm/y for the KMF zones. Archaeological data from the excavation of the Kotada Bhadli site suggest that the site was occupied during the Late mature Harappan (2300-2500 BCE) period and abandoned around 1900 BCE. The ruins of Harappan civilization at Kotada Bhadli location are resting over the Middle Holocene fluvial sediments. The oldest fluvial sediment is dated to 4 ka, whereas the youngest sediment yielded an age of 3 ka. The chronological constraints in geomorphic and archaeological records suggest that the area was hit by an earthquake around 2200 BC (∼3.5ka). The geomorphic and chronologically constrained uplift rates reveal that the KMF zone of the intraplate region of the Kachchh is geodynamically controlled by several fault segments, which are consistent with the ambient tectonic stress field owing to the northward movement of the Indian Plate concerning the Eurasian Plate.

Evidence for Basement Reactivation during the Opening of the Labrador Sea from the Makkovik Province, Labrador, Canada: Insights from Field Data and Numerical Models

The onshore exposures adjacent to modern, offshore passive continental margins may preserve evidence of deformation from the pre-, syn-, and post-rift phases of continental breakup that allow us to investigate the processes associated with and controlling rifting and breakup. Here, we characterize onshore brittle deformation and pre-rift basement metamorphic mineral fabric from onshore Labrador in Eastern Canada in the Palaeoproterozoic Aillik Domain of the Makkovik Province. Stress inversion (1) was applied to these data and then compared to (2) numerical models of hybrid slip and dilation tendency, (3) independent calculations of the regional geopotential stress field, and (4) analyses of palaeo-stress in proximal regions from previous work. The stress inversion shows well-constrained extensional deformation perpendicular to the passive margin, likely related to pre-breakup rifting in the proto-Labrador Sea. Hybrid slip and dilatation analysis indicates that inherited basement structures were likely oriented in a favorable orientation to be reactivated during rifting. Reconstructed geopotential stresses illuminate changes of the ambient stress field over time and confirm the present paleo-stress estimates. The new results and numerical models provide a consistent picture of the late Mesozoic-Cenozoic lithospheric stress field evolution in the Labrador Sea region. The proto-Labrador Sea region was characterized by a persistent E–W (coast-perpendicular) extensional stress regime, which we interpret as the pre-breakup continental rifting that finally led to continental breakup. Later, the ridge push of the Labrador Sea spreading ridge maintained this general direction of extension. We see indications for anti-clockwise rotation of the direction of extension along some of the passive margins. However, extreme persistent N–S-oriented extension as indicated by studies further north in West Greenland cannot be confirmed.

Consistency of stress state, locations and source mechanisms of events induced by hydraulic fracturing: downhole monitoring

ABSTRACTWe present results of processed microseismic events induced by hydraulic fracturing and detected using dual downhole monitoring arrays. The results provide valuable insight into hydraulic fracturing. For our study, we detected and located microseismic events and determined their magnitudes, source mechanisms and inverted stress field orientation. Event locations formed a distinct linear trend above the stimulated intervals. Source mechanisms were only computed for high‐quality events detected on a sufficient number of receivers. All the detected source mechanisms were dip‐slip mechanisms with steep and nearly horizontal nodal planes. The source mechanisms represented shear events and the non‐double‐couple components were very small. Such small, non‐double‐couple components are consistent with a noise level in the data and velocity model uncertainties. Strikes of inverted mechanisms corresponding to the nearly vertical fault plane are (within the error of measurements) identical with the strike of the location trend. Ambient principal stress directions were inverted from the source mechanisms. The least principal stress, σ3, was determined perpendicular to the strike of the trend of the locations, indicating that the hydraulic fracture propagated in the direction of maximum horizontal stress. Our analysis indicated that the source mechanisms observed using downhole instruments are consistent with the source mechanisms observed in microseismic monitoring arrays in other locations. Furthermore, the orientation of the inverted principal components of the ambient stress field is in agreement with the orientation of the known regional stress, implying that microseismic events induced by hydraulic fracturing are controlled by the regional stress field.

Earthquake Focal Mechanism Studies in Koyna-Warna Region in the Last Five Decades — Current Understanding on Tectonics and Seismogenesis

ABSTRACT On 10th December 1967, the world’s largest reservoir triggered seismic (RTS) event of magnitude 6.3 shook the Koyna region, the prime site of RTS globally. Ever since, several studies have attempted to infer the seismotectonics and to comprehend the actual causative mechanism of triggered seismicity in this region. Initial studies, including those of the 1967 Koyna main shock and its aftershocks, were based on the conventional P wave polarity or the first motion approach. These studies provided the first ever understanding of a predominantly strike-slip environment in the Koyna region, concurrent with the direction of ambient stress field due to the Indian plate motion. Subsequent studies pointed to a normal faulting environment in the Warna region further south, subsequent to impoundment later in 1985. A few studies did report solutions based on composite focal mechanisms, which however, only represent the average picture of the region. More recent studies based on modelling of seismic broadband waveform data provided more accurate focal mechanisms with unprecedented location accuracies including focal depths. A catalog of 50 focal mechanism solutions is now available for the earthquakes of magnitude ∼4 and larger that occurred during the last 50 years, which has paved way for a clear understanding of the stress field and the causative model of seismogenesis in this active intra-plate seismic RTS zone in western India. Based on stress inversion using this catalog, a new tectonic model depicting a periodically varying stress field and hence faulting mechanism has been inferred.