Pure Strike-slip Research Articles

Quantifying damage zones width in strike-slip faults: Insights from a two-dimensional finite-element modeling approach

We use a two-dimensional finite-element modeling approach to investigate the structural evolution of strike-slip faults under five different confining pressures and determine how their damage zone widths vary depending on the kinematic. Three representative fault segments are modeled, accounting for movement obliquity forming in pure strike-slip, oblique convergence and oblique divergence scenarios. The elastoplastic constitutive model (Drucker-Prager) was employed to couple an associated flow rule into the initial stress state of a geostatic stage. We applied two empirical methods for determining the width of the damage zone from progressive model response. The first method involves using the plastic strain distribution curve's inflection points, to mathematically fixate the width. The second method uses the standard deviation obtained from a trend Gaussian/Normal probability distribution, with over 98% fit to the plastic strain curve data, simplifying the inference of the strain propagation along the model. The simulation results indicate inverse proportional relationships between confining pressure and both porosity and plastic strain intensity, while showing a direct proportional relationship with the damage zone width. For fault zone width quantification, results show convergence between the methods, revealing widths of 2.46–2.65 m for oblique divergence (displacement of 0.20 m), 2.65–3.55 m for pure strike-slip (displacement of 0.20 m), and 4.36–4.46 m for oblique convergence zones (displacement of 0.70 m). The pure strike-slip and the oblique convergence damage zone width results align well with outcrop observations of faults with similar kinematics from the literature, underscoring the significance of numerical modeling as a valuable tool for measure and study the mechanics of characterizing the nucleation of fault zones and quantifying the width of the damage zone.

Late Quaternary deformation in the western extension of the North Anatolian Fault (North Evia, Greece): Insights from very high-resolution seismic data (WATER surveys)

Since the Pliocene, rift basins have developed in the North Evia region at the south-western extension of the North Anatolian Fault. North Evia, therefore, is a key area for investigating Plio-Quaternary deformation within the diffuse Anatolian-Aegean–Eurasian plate boundary. In this study, we present a detailed map of offshore faults in the North Evia region based on three seismic reflection surveys: an airgun (2004) and two very high-resolution Sparker (2017 and 2021) single-channel surveys. Careful and systematic interpretation of these datasets revealed i) the identification of a new basin (Skiathos Basin) located at the western extension of the North Anatolian Fault and ii) two main fault sets striking oblique to the active N–S-directed extension. The seismicity analysis of the study area reveals pure strike-slip displacements along faults previously identified as normal. The NW–SE-striking faults bordering the North Evia Gulf rift are characterised by left-lateral displacements, and the NE–SW faults present dextral focal mechanisms, following the kinematic character of the North Anatolian Fault. We show that dextral strike-slip deformation recently occurred (in the late Quaternary) in the south-western extension of the North Anatolian Fault, suggesting the significant role of this fault system in the complex structural development of the North Evia region.

Channel Profiles Reveal Fault Activity along the Longmen Shan, Eastern Tibetan Plateau

Assessing fault activity in regions lacking Quaternary sedimentary constraints remains a global challenge. In this study, we used channel slope distribution to examine variations in rock uplift along faults. By comparing channel steepness with published low-temperature thermochronology and paleo-seismic data, we identified deformation changes both perpendicular to and along the Longmen Shan at various time scales. Our data revealed distinct fault segments displaying distinct thrust activities along the Longmen Shan’s strike. In the southern segment, the Dachuan fault exhibited the highest activity, and its movement had persisted for millions of years. In the central segment, the Wenchuan fault was active during theearly Quaternary but has become dormant since the late Pleistocene. Within the past millions of years, the Yingxiu and Pengguan faults displayed significant vertical displacement. Fault activity in the northern Longmen Shan was relatively weak, with the Qingchuan fault transitioning from thrust movement during the Neogene to pure strike-slip activity since the Pleistocene. Overall, the Dachuan and Huya faults exhibited deformation patterns similar to the Yingxiu fault during the Quaternary. Similar to the Yingxiu fault, which triggered the Wenchuan earthquake, the Dachuan and Huya faults possess the capacity to produce significant earthquakes in the future. The variations in deformation perpendicular to and along the Longmen Shan fault system underscore the importance of upper crustal shortening in shaping the rock uplift patterns and topography of the eastern Tibetan Plateau margin.

Architecture of intraplate strike-slip fault zones in the Yanchang Formation, Southern Ordos Basin, China: Characterization and implications for their control on hydrocarbon enrichment

Commercial discoveries in the Ordos, Tarim, and Sichuan Basins of China have aroused widespread interest in strike-slip fault-controlled reservoirs. The productivity of different wells, however, can vary within one strike-slip fault zone, suggesting that variability in fault zone architecture controls hydrocarbon enrichment. To date, very few studies have finely and quantitatively explored fault zone architecture in the southern Ordos Basin, inhibiting hydrocarbon development. With the goal of understanding the architecture variability and scale of strike-slip fault zones in the subsurface, we carried out a refined and quantitative study of faults in the Jinghe Oilfield in the southern Ordos Basin by integrating outcrops, wellbore cores, well logs, and 3D seismic data. We quantitatively constrained the subsurface damage zone boundaries according to the slope changes in the cumulative CFI (comprehensive fracture index log) curves at logging scale and the cumulative FSI (fault shape index seismic attribute) curves at seismic scale. We proposed a model highlighting the effects of fault segmentation, architectural configuration, and damage zone asymmetry on the variability of fault zone architecture. Results showed that strike-slip faults can be divided into transtensional, pure strike-slip, and transpressional segments along the fault strike, with transtensional and pure strike-slip segments dominant in the Jinghe Oilfield. These segments exhibited six types of geometric structures, forming complex architectures with multi-fault cores and multi-damage zones surrounding the main and secondary faults. Each segment was further complicated by three configurations of gouge, breccia, and damage zones along the fault dip. The damage zone asymmetry further increased the architecture variability, with the fracture density and damage zone width of the hanging wall being greater than those of the footwall. Quantitative analysis showed that the fault zone width was the greatest for transtensional segments, intermediate for transpressional segments, and the lowest for pure strike-slip segments. And there was a positive linear correlation between the fault core width and the fault zone width (including the fault core and damage zone). Fault segmentation and damage zone asymmetry affected the hydrocarbon enrichment along the fault strike and dip within one strike-slip fault zone. We conclude that damage zones in transtensional segments, particularly in hanging walls, are primary targets for hydrocarbon development in the southern Ordos Basin.

Accuracy evaluation on five non-linear methods for fault parameter inversion of different-type fault earthquakes

Accurate inversion of seismic fault parameters has been a challenge in the studies of geophysical non-linear inversion problems. Many non-linear methods such as Simulated Annealing (SA), Genetic Algorithm (GA), Particle Swarm Optimization (PSO), and Multipeaks Particle Swarm Optimization (MPSO), have usually been applied to inverse the fault parameters from geodetic observation data. However, their accuracy and availability can vary from different-type fault earthquakes (pure strike-slip, pure dip-slip fault, oblique-slip fault earthquakes). In order to evaluate the accuracy and availability of these non-linear methods on inversion for fault parameters of different-type fault earthquakes, we applied the SA, GA, PSO, MPSO methods and a new non-linear method—Black Hole Particle Swarm Optimization (BHPSO), to inverse fault parameters of different-type earthquakes from synthetic and observed GPS and InSAR data. We found that the MPSO and BHPSO performed better than SA, GA, and PSO for inversion from both the synthetic and observed data. The synthetic data simulation results showed that the Root-Mean-Square Errors (RMSEs) of MPSO and BHPSO methods were 0.01–0.06 m, smaller than those of SA, GA and PSO. We then applied these five methods to inverse fault parameters of two real earthquakes—the 2020 Nevada Mw 6.4 earthquake and 2021 Maduo Mw 7.4 earthquake, from observed GPS and InSAR data. We found that the RMSEs of MPSO and BHPSO were 0.005–0.195 m, also smaller than those of SA, GA, and PSO, and the MPSO and BHPSO performed better than SA, GA, and PSO. The results in this study demonstrated that the MPSO and BHPSO, can hold high accuracy and availability for inversion of fault parameters of different-type fault earthquakes.

Variations of the seismic b-value along the Dead Sea transform

The frequency-magnitude distribution follows the Gutenberg-Richter empirical law, in which the scaling between small and large earthquakes is represented by the b-value. Laboratory experiments have shown that the b-value is related to fault mechanics with an inverse dependency to the differential stress, as was also inferred from observational datasets through relations with earthquake depth and style of faulting. In this study, we aim to obtain a better understanding of the geological structure and tectonics along the Dead Sea transform (DST), by examining relations of the b-value to three source parameters: the earthquake depth, the seismic moment release, and the predominant style of faulting. We analyse a regional earthquake catalogue of ∼20,300 earthquakes that were recorded between 1983 and 2020 in a regional rectangle between latitudes 27.5°N−35.5°N and longitudes 32°E−38°E. We convert the duration magnitudes, Md, to moment magnitudes, Mw, applying a new regional empirical relation, by that achieving a consistent magnitude type for the entire catalogue. Exploring the variations in the b-value for several regions along and near the DST, we find that the b-value increases from 0.93 to 1.19 as the dominant style of faulting changes from almost pure strike-slip, along the DST, to normal faulting at the Galilee, northern Israel. Focusing on the DST, our temporal analysis shows an inverse correlation between the b-value and the seismic moment release, whereas the spatial variations are more complex, showing combined dependencies on seismogenic depth and seismic moment release. We also identify seismic gaps that might be related to locking or creeping of sections along the DST and should be considered for hazard assessment. Furthermore, we observe a northward decreasing trend of the b-value along the DST, which we associate to an increase of the differential stress due to structural variations, from more extensional deformation in the south to more compressional deformation in the north.

Using a grid-search approach to validate the Graves–Pitarka broadband simulation method

This work assesses the ability of the Graves–Pitarka simulation approach to reproduce observed ground motions for 12 California and Baja California earthquakes. A total of 240 realizations are computed for each earthquake and compared with recorded strong motions from near-fault sites. In addition to spatial variability in slip, each realization samples from discrete combinations of average rupture speed, rise time, and down-dip fault width. Ground motions for each realization of a given earthquake are compared to the observations using a pseudo-spectral acceleration goodness-of-fit (GoF) metric. Parameters for the lowest misfit cases are then tabulated to develop relations for estimating median values and ranges for future applications. The results are generally consistent with published scaling relations for estimating rupture dimensions as a function of magnitude. Additionally, I find the average rise time scales as {2.3times alpha }_{T}times {10}^{-9}{times M}_{o}^{1/3} (Mo is seismic moment in dyne-cm) and average rupture speed as (0.765 pm 0.075)times {alpha }_{T}^{-1}times {V}_{s} (Vs is local shear wave velocity), where aT is a mechanism adjustment ranging from a value of 1.0 for pure strike-slip to 0.9 for pure reverse-slip cases. There are cases where slightly different combinations of parameters produce equally good fits to the observations, demonstrating the non-uniqueness of using a single GoF metric in this approach. Nonetheless, the results reinforce the importance of adequately sampling ranges of rupture parameters when performing validations, as well as when simulating ground motions for future events.Graphical

Genetic Types of the tp12cx Strike-Slip Fault Segments and Their Role in Controlling Reservoirs in the Tarim Basin

The change of motion mode of multistage active strike-slip faults controls the segmentary types of strike-slip faults, which is seldom studied.Based on high-precision 3D seismic data and the principle of structural analysis, this paper defines the structural evolution characteristics of the tp12cx strike-slip fault in the key structural period and identifies the fault segmentation types. Combined with the statistical results of drilling production data and fault width, and fault width, it is demonstrated that different fault segments display various reservoir architecture and hydrocarbon potential. The tp12cx strike-slip fault experienced two phases of tectonic activity controlling reservoir development: the middle Caledonian and the late Caledonian to early Hercynian. During the middle Caledonian period, a left-lateral and left-step strike-slip fault was formed. The overlapping segments of the left steps were transtension zones, and the rest were pure strike-slip segments. From the late Caledonian to the early Hercynian, the movement mode changed from left-lateral to right-lateral, and the arrangement of left steps remained unchanged, forming right-lateral and left-step strike-slip faults. That is, as a weak zone, the transtension zones of all the preexisting overlapping segments took the lead in moving into many pure strike-slip segments and maintained the transtensional property. During the right-lateral slipping process of all the original pure strike-slip segments along the fault, they were blocked and squeezed by the surrounding rocks on both sides, forming a series of “positive” flower-shaped fault anticlines, which became overlapping segments, and the fault property became transpressional. Under the continuous action of the right-lateral slipping, a regional right-lateral and right-step strike slip fault formed. The interiors of the right-step-arranged faults were composed of the left-step arranged faults. Among them, the right-step overlapping segments were weakly step overlapping segments were weakly transtensional, and the larger the fault width of the internal left step pure strike slip and overlapping segments, the stronger the dissolution. The deformation of the right-step pure strike-slip segments was weak and basically maintained the characteristics of the previous stage. According to the evolution and superposition of pure strike-slipped and overlapped segments and the changes in fault properties, four types of strike-slip fault segments and corresponding reservoir models are divided. Type I: left-step pure strike-slip segment + left-step transpressional segment + right-step transtensional segment; Type II: left-step transtensional segment + left-step pure strike-slip segment + right-step transtensional segment; Type III: left-step pure strike-slip segment + left-step transpressional segment; and Type IV: left-step transtensional segment + left-step pure strike-slip segment. The fault width and oil production of type II and type IV with transtensional properties are much larger than those of type I and type III with transpressional properties.

The 14 December 2021 Mw 4.9 Offshore Jeju Island, Korea, Earthquake: Seismological Observation of an Intraplate Earthquake Provides Insight into Regional Seismotectonics

AbstractThe Mw 4.9 Offshore Jeju Island, Korea, earthquake of 14 December 2021 is the most significant event that occurred close to the young intraplate volcano on the continental shelf south of the Korean Peninsula. About 185 small earthquakes occurred during nine days following the mainshock. We accurately located 39 events despite limited station azimuthal coverage. We found that the mainshock ruptured along a ∼1.4 km long fault striking east–west (275°) and triggered two distinct clusters in the north–northeast direction. The mainshock is pure strike-slip faulting on the vertical plane. Its horizontal P axis strikes 50°, which differs substantially from the average trend of ∼70° for earthquakes in the southern Korean Peninsula. However, it is consistent with the P-axes orientation of shallow earthquakes in the offshore western Kyushu and along Okinawa trough. The 2021 Offshore Jeju Island earthquake may be the first significant earthquake in southern Korea–East China Sea continental shelf to date, whose causative fault is well constrained by accurate aftershock locations and detailed space–time analysis of the earthquake sequence. This earthquake may represent intraplate deformation in response to the tectonic evolution of the Philippine Sea plate, which is manifested through volcanic activities.

Strike-slip salt tectonics in the Shuntuoguole Low Uplift, Tarim Basin, and the significance to petroleum exploration

The connection between source rock and reservoir separated by salt strata is a global problem. Taking Tarim Basin as a typical case, this paper studies how the No. 5 strike-slip fault passes through the salt layers. The No. 5 strike-slip fault system in the Shuntuoguole Low Uplift in the Tarim Basin, China, interacts with Cambrian salt layers at depth. In the present study, a part of this fault system was investigated to assess structural characteristics of the Yijianfang (Ordovician carbonate layer) and the Wusonggeer (Cambrian salt layer) formations. Seismic data reveal three structural styles in the salt layer including transpression, transtension, and pure strike-slip. In transpressional segments, the salt layer is thinned accross the fault plane; transtensional segments involve minor reactive salt diapirs, and pure strike-slip segments display no noticeable variation in the salt thickness. The impact of the deep strike-slip salt tectonics on petroleum exploration in the area is discussed using data from seven wells. Wells in transpressional segments, where the salt is thin, are characterized by good production, whereas those in transtensional segments, where the salt is thick, exhibit poor production. The salt prevents the migration of oil and gas along the strike-slip fault. Strike-slip salt structures examined in the present study improve the understanding of the associated tectonics in the central Tarim Basin and provide insights on hydrocarbon migration and accumulation in the basin.

A new analysis of the intraplate stress regime and stress ratio in numerically modeled mantle convection

I propose a new analysis method for determining the intraplate stress in geodynamic models using a series of numerical simulations of mantle convection in 3D spherical-shell geometry. In the present study, the intraplate stress was evaluated from numerically obtained velocity and stress fields of mantle, and quantitatively classified into nine types by analyzing the principal deviatoric stress axes and the “stress ratio,” which is a continuous parameter accounting for the stress regimes. The sensitivity of model parameters and physical conditions associated with the basic characteristics of mantle convection, such as internal heating ratio, viscosity stratification, and temperature-dependent viscosity of the mantle as well as viscoplastic rheology that causes plate-like surface motion, on the intraplate stress regimes were studied. The results demonstrated that the radial viscosity structure of the mantle interior strongly affected intraplate stress regimes, and the combination of increased viscosity in the lower mantle and the low-viscosity asthenosphere enhanced the pure strike-slip faulting regime in the stable part of plate interiors. The temporally averaged toroidal-poloidal ratio (T/P ratio) at the top surface of mantle convection with surface plate-like motion and the mantle's viscosity stratification generally ranged ~20–40%, which is comparable to the observed T/P ratio of present-day and past Earth. Under such Earth-like surface conditions, normal faulting regime with strike-slip component or strike-slip regime with normal faulting component, as well as pure strike-slip faulting regime, were broadly found in the stable parts of the plate interiors. From the definition of the stress regime in the present study, strike-slip faults on the real Earth are likely to occur where the strike-slip faulting component is dominant in the present models. The analysis method proposed herein is effective for evaluating the intraplate stress in research target regions, for which observation data is insufficient to determine the intraplate stress.

Composite pull-apart basin evolution: A discontinuum based numerical study

Traditional models of pull-apart basins usually develop along two strike-slip faults. However, there is an important variant that does not fit with the traditional pull-apart basins. This study presents this type of pull-apart basin called composite pull-apart basin formed in a strike-slip system consisting of multiple master fault segments. A set of two-dimensional discontinuum based, scaled, elastic models in upper crust are built to provide insights on composite pull-apart basin evolution. Three representative fault segment interaction geometries are modeled, showing underlapping, neutral, and overlapping releasing steps. They produce similar composite pull-apart basins consisting of two or three en echelon rhomboidal subbasins as offset increases. However, a change of initial fault geometry from underlapping to neutral and overlapping causes the fracture pattern to be more diffuse. The underlapping model produces a composite pull-apart basin with a throughgoing fault evolving from cross-basin faults in the subbasins whereas composite pull-apart basins produced in neutral and overlapping systems have no cross-basin faults and throughgoing faults. Modeling results show that local extension in underlapping systems with both pure strike-slip and 5° transtension is accommodated through development of R-shears, small subbasins, cross-basin faults obliquely cutting the subbasins, and throughgoing fault. The morphology and structural evolution of composite pull-apart basins are determined by factors including pre-existing master strike-slip fault geometries (underlapping, neutral, or overlapping), fault kinematics (pure strike-slip, transtension, or transpression), and strike-slip displacements corresponding to evolution stages. The composite pull-apart basin geometries and fracture patterns of our numerical models fit well with natural examples such as the Gulf of Aqaba and the northern part of the Marmara Sea.

The Storfjorden, Svalbard, Earthquake Sequence 2008–2020: Transtensional Tectonics in an Arctic Intraplate Region

AbstractAn earthquake sequence in the Storfjorden offshore area southwest of Spitsbergen in the Svalbard archipelago initiated with a 21 February 2008 magnitude Mw 6.1 event. This area had previously not produced any significant earthquakes, but between 2008 and 2020, a total of ∼2800 earthquakes were detected, with ∼16 of them being of moderate size (ML≥4.0). Applying double-difference relocation to improve relative locations reveals that the activity is linked to several subparallel faults striking southwest–northeast that extend across the entire crust. The southwest–northeast trend is also found as a possible fault plane from regional moment tensor inversion. The solutions range from oblique normal in the center of the cluster to pure strike slip farther away and are consistent with the compressional σ1 axis roughly in the east–west direction and plunging 57°, and the extensional σ3 axis subhorizontal trending north–south. The mainshock fault is steeply dipping to the southeast, but several other faults appear to be near vertical. The existence of oblique, right-lateral strike-slip motion on southwest–northeast-trending faults with a normal component and pure normal faulting events in between suggests transtensional tectonics that in and around Storfjorden result in activation of a complex fault system.

Recent Seismic Activity in the Bejaia–Babors Region (Northeastern Algeria): The Case of the 2012–2013 Bejaia Earthquake Sequences

From November 2012 to May 2013, the coastal region of Bejaia (northeastern Algeria), experienced three distinct seismic sequences in about 6 months. The first one was on 28 November 2012 started with a mainshock of magnitude Mw = 5.1, and the second one occurred on 22 February 2013, a few kilometers southeast from the first one and culminated with a magnitude Mw = 4.3 earthquake. The last sequence comprised two shocks of moderate magnitude: a Mw 5.2 event on 19 May 2013, followed by a Mw 5.0 event on 26 May. Earthquakes were relocated using the double-difference method with differential times derived from phase-picked data and waveform cross-correlation delays. We compiled a dataset of 252 accurately relocated events that are associated with focal mechanism solutions consistent with a near-vertical, right-lateral strike-slip fault striking NW–SE. The data analysis is coherent with a new fault extending over 35 km from the Gulf of Bejaia to the northeast of Kherrata. This newly discovered active fault, we name it the Babors Transverse Fault, appears to be segmented in four segments (S1–S4) evidenced by separate earthquake clusters. Static Coulomb stress changes and the spatiotemporal evolution of seismicity suggest static stress triggering, with the rupture first activated in the S1 segment in the northwest, migrating to S2, and terminating in S3 and S4 in the southeast. Stress tensor inversion from focal mechanisms implies that the dominant local stress field is a pure strike-slip regime, with N–S maximum horizontal compression in good agreement with the velocity field displaying reorientation compared to the oblique convergence of Africa–Eurasia. In this study, the newly identified Babors Transverse and Tizi N’Berber–Darguinah faults, in association with the NNW–SSE offshore active fault of the Greater Kabylian block (in the north) and the NNW–SSE right-lateral strike-slip Lâalam fault (in the south), form an en echelon fault system that acts as a transfer zone between offshore thrust faulting and the Ghardimaou–North Constantine strike-slip fault.

The September 21, 2020, M w = 5.5, Bystraya Earthquake in the Southern Baikal Region: Preliminary Results of Instrumental and Macroseismic Observations

We consider the September 21, 2020, Mw = 5.5 seismic event, which occurred in the eastern part of the Tunka rift basin system and was followed by a relatively strong mb = 4.6 aftershock. The earthquake epicenter is localized within the Bystraya basin in the zone of prolonged seismic quiescence and is possibly confined to the Main Sayan fault zone. The hypocentral depth, according to the solution of the Baikal Branch, United Geophysical Survey, Russian Academy of Sciences (UGS RAS), is estimated as h = 18 km. The focal mechanisms of the main shock and strong aftershock are characterized by almost pure strike-slip movements, which agrees well with modern ideas about the tectonics of the area. The maximum shaking intensity observed in the populated areas closest to the epicenter is estimated at VI–VII on the MSK-64 scale. A shaking intensity of V and IV was observed in populated areas at distances over 180 and 500 km, respectively. This earthquake made it possible to obtain new data on the dynamic ground motion parameters. The maximum peak ground acceleration (PGA) of 187 cm/s2 was recorded at the Talaya seismic station, which is the highest value for the territory of the Southern Baikal Region during the digital period of earthquake detection.

Assessing the brittle crust thickness from strike-slip fault segments on Earth, Mars and Icy moons

Segment lengths along major strike-slip faults exhibit a size dependency related to the brittle crust thickness. These segments result in the formation of the localized “P-shear” deformation crossing and connecting the initial Riedels structures (i.e. en-echelon fault structures) which formed during the genesis stage of the fault zone. Mechanical models show that at all scales, the geometrical characteristics of the Riedels exhibit dependency on the thickness of the brittle layer. Combining the results of our mechanical discrete element model with several analogue experiments using sand, clay and gypsum, we have formulated a relationship between the orientation and spacing of Riedels and the thickness of the brittle layer. From this relationship, we derive that for a pure strike-slip mode, the maximum spacing between the Riedels is close to three times the thickness of the layer. For a transtensional mode, as the extensive component becomes predominant, the spacing distance at the surface becomes much smaller than the thickness. Applying this relationship to several well-characterized strike-slip faults on Earth, we show that the predicted brittle thickness is consistent with the seismogenic depth. Supposing the ubiquity of this phenomenon, we extented this relationship to characterize en-echelon structures observed on Mars, in the Memnonia region located West of Tharsis. Assuming that the outer ice shells of Ganymede, Enceladus and Europa, exhibit a brittle behavior, we suggest values of the corresponding apparent brittle thicknesses.

Extreme Quaternary plate boundary exhumation and strike slip localized along the southern Fairweather fault, Alaska, USA

AbstractThe Fairweather fault (southeastern Alaska, USA) is Earth’s fastest-slipping intracontinental strike-slip fault, but its long-term role in localizing Yakutat–(Pacific–)North America plate motion is poorly constrained. This plate boundary fault transitions northward from pure strike slip to transpression where it comes onshore and undergoes a <25°, 30-km-long restraining double bend. To the east, apatite (U-Th)/He (AHe) ages indicate that North America exhumation rates increase stepwise from ∼0.7 to 1.7 km/m.y. across the bend. In contrast, to the west, AHe age-depth data indicate that extremely rapid 5–10 km/m.y. Yakutat exhumation rates are localized within the bend. Further northwest, Yakutat AHe and zircon (U-Th)/He (ZHe) ages gradually increase from 0.3 to 2.6 Ma over 150 km and depict an interval of extremely rapid >6–8 km/m.y. exhumation rates that increases in age away from the bend. We interpret this migration of rapid, transient exhumation to reflect prolonged advection of the Cenozoic–Cretaceous sedimentary cover of the eastern Yakutat microplate through a stationary restraining bend along the edge of the North America plate. Yakutat cooling ages imply a long-term strike-slip rate (54 ± 6 km/m.y.) that mimics the millennial (53 ± 5 m/k.y.) and decadal (46 mm/yr) rates. Fairweather fault slip can account for all Pacific–North America relative plate motion throughout Quaternary time and indicates stability of highly localized plate boundary strike slip on a single fault where extreme rock uplift rates are persistently localized within a restraining bend.

SPECTRAL and SOURCE PARAMETERS of CRIMEAN-BLACK SEA EARTHQUAKES

The spectral and dynamic source parameters (М0, r0, , , ησ, , ū, Eu and Mw) of 13 Crimean earthquakes with КП = 7.2–11.2 (Mw=2.6–4.1), restored by 123 amplitude spectra of longitudinal and transverse seismic waves recorded by digital regional seismic stations is shown. Approximation of the spectra and the source dynamic parameters calculation based on their parameters is performed in the framework of the Brune dislocation model. Four to seven station definitions participated in the averaging of focal parameters, which ensured a small standard deviation, the scattering degree index of the individual estimates. The best convergence of the station definitions is obtained for the radius of a circular dislocation. The highest values of dynamic para-meters have been obtained for the perceptible earthquake on March 2 with КП=11.2, and the lowest values have been obtained for its weak aftershock on April 6 with КП = 7.2. Within the total range of energies the value of the stress drop does not exceed = 106Pa(10 bar), and the apparent stress drop does not exceed ησ<3∙105Pa (3 bar). The average values of seismic moments and circular dislocation radius within the errors of their deter-mination match the average long-term correlations of the parameters on the earthquake energy level obtained by the analog recording.For the strongest earthquakes on March 2 with Mwreg=4.1 and October 18 with Mwreg=3.8 solutions of the focal mechanisms were obtained. The March 2 earthquake occurred in the central part of the region near the South Coast of Crimea under the action of horizontal tensile forces oriented near the latitude. The type of movement in the focus is a obligue slip with predominance of a normal component over a strike-slip. The earthquake on October 18 occurred in the central part of the Black Sea basin in condition of sublatitudinal compression and submeridional extension. Type of movement in the source is a pure strike-slip. For the main shock on March 2 and its six aftershocks, energy spectra according to the data of Alushta station and their main parameters are given. The analysis of obtained results is given.

Dynamic Fault Interaction during a Fluid-Injection-Induced Earthquake: The 2017 Mw 5.5 Pohang Event

ABSTRACT The 15 November 2017 Mw 5.5 Pohang, South Korea, earthquake has been linked to hydraulic stimulation and fluid injections, making it the largest induced seismic event associated with an enhanced geothermal system. To understand its source dynamics and fault interactions, we conduct the first 3D high-resolution spontaneous dynamic rupture simulations of an induced earthquake. We account for topography, off-fault plastic deformation under depth-dependent bulk cohesion, rapid velocity weakening friction, and 1D subsurface structure. A guided fault reconstruction approach that clusters spatiotemporal aftershock locations (including their uncertainties) is used to identify a main and a secondary fault plane that intersect under a shallow angle of 15°. Based on simple Mohr–Coulomb failure analysis and 180 dynamic rupture experiments in which we vary local stress loading conditions, fluid pressure, and relative fault strength, we identify a preferred two-fault-plane scenario that well reproduces observations. We find that the regional far-field tectonic stress regime promotes pure strike-slip faulting, whereas local stress conditions constrained by borehole logging generate the observed thrust-faulting component. Our preferred model is characterized by overpressurized pore fluids, nonoptimally oriented but dynamically weak faults and a close-to-critical local stress state. In our model, earthquake rupture “jumps” to the secondary fault by dynamic triggering, generating a measurable non-double-couple component. Our simulations suggest that complex dynamic fault interaction may occur during fluid-injection-induced earthquakes and that local stress perturbations dominate over regional stress conditions. Therefore, our findings have important implications for seismic hazard in active georeservoir.

Earthquake source dynamics and kinematics of the Eastern Indian Shield and adjoining regions

The Eastern Indian Shield (EIS) consists of two cratonic nuclei, namely Singhbhum craton and Chhotanagpur Granitic Gneissic terrain. This area contains several crisscross faults, lineaments, shear zones, numerous hot springs and three major rivers (e.g., Ganga, Brahmaputra and Damodar). The area is regionally covered by 7 seismic stations and jointly recorded 16 events from the study area, and less noisy waveforms of 4 events were used for focal mechanism analysis using the Cut and Paste method. The focal parameters of these 4 events were compiled with results of 8 events computed by different workers for the study area. To understand the detailed tectonics, focal mechanisms of 21 events for the Himalayan segment were taken from CMT Harvard catalog of duration 1976–2017. Spatial variations of operative stress fields for major tectonic domains were analyzed in the present study based on stress inversion of focal mechanism parameters. We observed strike-slip-dominated movements in the EIS, which changes partially into extension in the northeast part between the Ganga and Brahmaputra Rivers. Thrust-dominated movements evidenced by focal mechanisms and the pure compression in the western segment of the Himalaya in the north might be promoting shear movements in the EIS and adjoining regions. The pure strike-slip in the eastern segment of the Himalaya and its deeper level account for lateral shearing and eastward movements of different tectonic blocks. The normal faulting earthquakes in the northeast part might be indicating stretching in the basement because of convergence of Indian lithosphere beneath the Myanmar plate.