Local Tectonic Stress Field Research Articles

Transtensional coseismic fault slip of the 2021 Mw 6.7 Turt Earthquake and heterogeneous tectonic stress surrounding the Hovsgol Basin, Northwest Mongolia

Knowledge of the regional crustal deformation and stress field is fundamental to understanding and constrain the ongoing evolution of Hovsgol basin, northwest Mongolia. The 2021 Mw 6.7 Turt earthquake provides an unprecedented opportunity to probe the local tectonic stress field and upper crust deformation. We investigate the coseismic surface displacements and invert fault slip models using Interferometric Synthetic Aperture Radar observations and teleseismic data. The mainshock occurred as a result of normal faulting with a right-lateral strike-slip component on an NW striking plane, which is consistent with the transtensive local stress field inverted from regional focal mechanisms. It is surrounded by transpressive and strike-slip stress fields proposed by previous studies indicating regional stress heterogeneity probably controlled by the distinct structure geometries in and around Hovsgol basin. Our results also suggest that the current deformation of the Hovsgol basin is dominated by half-graben forming. Seven historical strong earthquakes (M 7) may advance the 2021 Turt earthquake by ~12% recurrence interval, based on the assumption of equal stress release from recurrence earthquakes, meanwhile, the 2021 Turt earthquake may increase the potential seismic hazard on the neighbor Mondy and South Hovsgol faults, which deserves more attention.

Seismogenic Faults of the Changning Earthquake Sequence Constrained by High-Resolution Seismic Profiles in the Southwestern Sichuan Basin, China

Abstract The seismicity rate in the southwestern Sichuan basin, China, dramatically increased after 2014. The associated moderate earthquakes may have been induced by salt mining or shale gas exploration. The location of the seismogenic faults causing these moderate earthquakes has not been confirmed, resulting in a lack of understanding of the earthquake mechanisms in the study area. The detailed structural characteristics of pre-existing faults, which are typically responsible for induced seismicity, are unclear. In this study, we used high-resolution seismic reflection profiles in conjunction with geological, seismologic, and geodetic data to reveal the 3D distributions of the seismogenic faults. Basement thrust faults in the Changning anticline were identified using seismic interpretations and are associated with the 2019 Changning earthquake sequence. The geometry and location of these pre-existing faults are consistent with previous studies of the seismology and structural geology in the area. The well-developed pre-existing fault system in the sedimentary cover and basement makes the Changning area vulnerable to induced earthquakes. Present-day reactivation of the basement fault system reveals the unstable state of the local tectonic stress field. It is possible that the potential seismic risk in this region could be increased by industrial activity in the southwestern Sichuan basin.

Estimation of near-surface attenuation in the tectonically complex contact area of the northwestern External Dinarides and the Adriatic foreland

Abstract. Seismically induced ground motion at a site is generally influenced by the seismic source, the propagation path and the local site conditions. Over the last several decades, researchers have consistently asserted that for near-site attenuation, the spectral parameter κ is subject primarily to the site conditions. In this research, we estimated the parameter κ based on the acceleration amplitude spectrum of shear waves from local earthquakes recorded by seismological stations situated in the western part of Croatia from the slope of the high-frequency part. The spatial distribution of κ values is comparable with seismological, geophysical and geological features, with the published coda-Q values for each station as well as with the isoseismal maps for selected stronger earthquakes in the study area. The complex pattern of longitudinal and transversal major late-orogenic fault zones dissecting early-orogenic thin-skinned tectonic cover in the Kvarner area and the shallow depth to the Moho (Mohorovičić discontinuity) in the Adriatic foreland (southern Istria) are probably responsible for a significant part of wave attenuation and for the anisotropy of attenuation. Regional near-surface attenuation distribution and modelled macroseismic fields point to the conclusion that attenuation properties of rocks in the northwestern External Dinarides are far from isotropic, and the most likely anisotropy sources are the preferential orientations of cracks and fractures under the local tectonic stress field, trapping of waves along major faults (waveguides), and/or attenuation within the fault zones. These results are important for gaining further insight into the attenuation of near-surface crust layers in the northwestern External Dinarides and the associated Adriatic foreland as well as in similar geotectonic settings.

Tectonic control on the genesis of magmas in the New Hebrides arc (Vanuatu)

We present here new bathymetric, petrological and geochemical whole rock, glass and mineral data from the submarine Epi volcano in the New Hebrides (Vanuatu) island arc. The structure has previously been interpreted to be part of a larger caldera structure but new bathymetric data reveal that the volcanic cones are aligned along shear zones controlled by the local tectonic stress field parallel to the recent direction of subduction. We aim to test if there is an interaction between local tectonics and magmatism and to what extent the compositions of island arc volcanoes may be influenced by their tectonic setting. Primitive submarine Epi lavas and those from the neighbouring Lopevi and Ambrym islands originate from a depleted mantle wedge modified by addition of subduction zone components. Incompatible element ratios sensitive to fluid input (e.g., Th/Nb, Ce/Yb) in the lavas are positively correlated with those more sensitive to mantle wedge depletion (e.g., Nb/Yb, Zr/Nb) amongst the arc volcanoes suggesting that fluids or melts from the subducting sediments have a stronger impact on the more depleted compositions of the mantle wedge. The whole rock, glass and mineral major and trace element compositions and the occurrence of exclusively normally zoned clinopyroxene and plagioclase crystals combined with the absence of inversely zoned crystals and water-bearing phases in both mafic and evolved lavas suggest that the erupted melt was relatively dry compared to other subduction zone melts and has experienced little disequilibrium modification by melt mixing or assimilation. Our data also imply that differentiation of amphibole is not required to explain the incompatible element patterns but may rather result from extensive clinopyroxene fractionation in agreement with petrographic observations. Thermobarometric calculations indicate that the melts fractionated continuously during ascent, contrasting with fractionation during stagnation in an established crustal magma reservoir. We interpret the occurrence of this fractional crystallisation end-member in a relatively thick island arc crust (~30 km thickness) to result from isolated and relatively rapid ascent of melts, most likely through a complex system of dykes and sills that developed due to the tectonic positioning of Epi in a complex tectonic zone between a compressional environment in the north and an extensional setting in the south. We can show that the alignment of the cones largely depends on the local tectonic stress field at Epi that is especially influenced by a large dextral strike-slip zone, indicating that structural features have a significant impact on the location and composition of volcanic edifices.

Clustering and Stress Inversion in the Song Tranh 2 Reservoir, Vietnam

The seismicity in the vicinity of the Song Tranh 2 reservoir that commenced in late 2010 is an example of reservoir‐induced seismicity. Moment tensor (MT) solutions are mostly normal‐faulting mechanisms with shearing as the main component of full MT, which suggest reservoir exploitation stress field changes as a triggering origin of this seismicity. However, the local tectonic stress field plays a role in the seismogenic process, which is accelerated by the reservoir exploitation. Two main orientations of discontinuities reactivated by reservoir exploitation are identified, with clustering of events located in two main areas: northern and southern. In addition to the northern cluster (NC) and southern cluster (SC), further smaller clusters are distinguishable, assuming both clustering of epicenters and similarity of focal mechanisms as the clustering criteria. Maximum seismic activities in clusters appear in different periods, with high seismicity in the SC preceding that in the NC. The results indicate significant differences between the NC and SC. It is shown that the SC is an example of rapid triggering, whereas the NC is a delayed triggering type. Also, magnitude distribution differs between the NC ( b≈1.1b≈1.1 ) and SC ( b≈1.3b≈1.3 ).

Seismic activity and stress tensor inversion at Las Tres Vírgenes Volcanic and Geothermal Field (México)

We analyze local earthquakes occurring between 2003 and 2012 at the Las Tres Vírgenes Volcanic and Geothermal Field (TVVGF) to establish their temporal and spatial distribution, and relationships with local and regional fault systems, water injection, acid stimulation and steam production tests. We obtained focal mechanisms and inverted data for the stress tensor to understand the local and regional stress fields. We analyzed 423 local earthquakes with magnitudes between 0.1 and 2.9Mc and hypocentral depths from 0.2 to 7.4km b.s.l. The cutoff depth at ~7.4km possibly delineates the brittle–ductile transition zone. We identified seven swarms (from 1 to 7). Swarms 1 (December 2009), 2 (May 2010), 3 (June–July 2010) and 7 (December 2012) are strongly correlated with injection processes; whereas swarms 5 (April 2012) and 6 (September 2012) are correlated with local tectonic faults. Stress inversion showed NW–SE, E–W and NE–SW extensional orientations (Shmin), in agreement with the local tectonic stress field; while NE–SW compressional orientations (SHmax) are correlated with the regional tectonic stress field.

A workflow for building and calibrating 3-D geomechanical models &ndash a case study for a gas reservoir in the North German Basin

Abstract. The optimal use of conventional and unconventional hydrocarbon reservoirs depends, amongst other things, on the local tectonic stress field. For example, wellbore stability, orientation of hydraulically induced fractures and – especially in fractured reservoirs – permeability anisotropies are controlled by the present-day in situ stresses. Faults and lithological changes can lead to stress perturbations and produce local stresses that can significantly deviate from the regional stress field. Geomechanical reservoir models aim for a robust, ideally "pre-drilling" prediction of the local variations in stress magnitude and orientation. This requires a numerical modelling approach that is capable to incorporate the specific geometry and mechanical properties of the subsurface reservoir. The workflow presented in this paper can be used to build 3-D geomechanical models based on the finite element (FE) method and ranging from field-scale models to smaller, detailed submodels of individual fault blocks. The approach is successfully applied to an intensively faulted gas reservoir in the North German Basin. The in situ stresses predicted by the geomechanical FE model were calibrated against stress data actually observed, e.g. borehole breakouts and extended leak-off tests. Such a validated model can provide insights into the stress perturbations in the inter-well space and undrilled parts of the reservoir. In addition, the tendency of the existing fault network to slip or dilate in the present-day stress regime can be addressed.

Hierarchical properties of the tectonic stress field in the source region of the 2003 Chuya earthquake

Local tectonic stress field have been calculated based on aftershock focal mechanisms using materials of epicentral studies with temporary networks of stations in the epicentral area of the 2003 Chuya earthquake. The originality of this work is that the stress state is studied invoking earthquakes with large differences in energy and using highly accurate data on the location and depth in the analysis of events. The main result is the selection of two hierarchical levels of the stress field. It is shown that the stress state determined from data on large earthquakes is homogeneous (horizontal shear) throughout the aftershock zone, and the stress state determined from data on small events varies according to the block structure of the aftershock region.

Intraplate earthquake swarms in West Bohemia/Vogtland (Central Europe)

West Bohemia (Czech Republic) and Vogtland (Germany) are among the most active intraplate earthquake-swarm areas in Europe with the largest events mostly of magnitudes $M_L$ \< 4.0. The principal char-acteristics of the West Bohemia/Vogtland earthquake swarms are derived on the basis of local observations from the network WEBNET during the period between 1991 and 2009. Swarm microearthquakes clustered in number of small focal areas; however, about 90\% of the total seismic moment was released in the Nový Kostel (NK) focal zone, which was formed by an NNW striking and steeply dipping fault plane. The focal depths ranged between 5 and 22 km in the whole region and between 4.5 and 11 km in the NK zone, while most of the swarm events clustered at depths from 8.5 to 9.5 km. All larger earthquake swarms took place in the NK zone; though they were located close, they differed significantly in their evolution. Two swarms, the 2000 ($M_L$ ≤ 3.3) and 2008 ($M_L$ ≤ 3.8) swarms were located on the same portion of the NK focal zone which implies reactivation of the same fault segment. Attention was paid to source mechanisms of the 1997 and 2000 swarms, particularly to the non-shear components in the 1997-swarm earthquakes. All the 1997 sources were of dipole character: the slightly compressive dipoles dominating in the first swarm phase were replaced by tensile dipoles in the second phase. On the contrary, the 2000 swarm events possessed pure double-couple sources which resulted in pure shears along the NK fault. We infer that the pure-shear rupturing in the 2000 swarm was on account of the favourably oriented NK fault plane with respect to the local tectonic stress field whereas the additional tensile forces were needed for rupturing due to the unfavourably oriented 1997-swarm fault segments. Further, we analyzed statistic characteristics and we show that the magnitude-frequency distribution with the b-value ≈ 1.0 is typical for the West Bohemia/Vogtland earthquake swarms and that scalar moments $M_0$ and the WEBNET magnitudes $M_L$ are related according to power-law $M_0$ ∝ 10$^{M_L}$, which is inconsistent with the definition of the moment magnitude $M_0$ ∝ 10$^{1.5 M_w}$ given by Kanamori (1977). We also show that this inconsistency results in a discrepancy in b-value of the magnitude-frequency distribution. Taking into account total seismic moment of the 2000 and 2008 swarms we infer that $M_{L_{MAX}}$ ∼ 5.0 to 5.3 is the maximum expected magnitude in the main NK focal zone and thus in the whole region. There is still an open question concerning the internal and external triggering of the West Bohemia/Vogtland earthquake swarms. We refer to the results of our recent study and infer that the earthquake swarm probably represents gradually propagating rupture along the fault and that both static and dynamic Coulomb stress changes along the fault plane due to co-seismic slip contribute significantly to triggering of the swarm events. Repeatedly observed almost simultaneous occurrence of seismic activity in different focal zones suggests that common triggering forces act in a broader area of West Bohemia/Vogtland.

Azimuthal anisotropy of the Pg-wave velocity in hypocentral volumes of NW Croatia

Based on a large set of arrival times of the Pg phase reported by local and regional stations, we estimate azimuthal anisotropy of the Pg-wave velocity in focal volumes of the upper crust in NW Croatia. The method is based on analyses of the azimuthal dependence of ratios of cumulative differences of arrival times and travel paths between foci of earthquake pairs, computed for rays propagating within narrow azimuthal windows. The results clearly indicate the presence of anisotropy of 3.3% with the direction of fast velocity (approximately NNE-SSW) coinciding with the direction of the maximum tectonic pressure as revealed by 23 available focal mechanisms and previous geological investigations. Although a large part of observed anisotropy can be explained assuming that focal volumes are pervaded by a system of vertical extensive-dilatancy anisotropy (EDA) cracks aligned under the influence of local tectonic stress field, there is indication that — to a smaller extent — some role was also played by alignment of structural features in the region.

Spatio-temporal variation and focal mechanism of the Wenchuan MS8.0 earthquake sequence

Based on abundant aftershock sequence data of the Wenchuan MS8.0 earthquake on May 12, 2008, we studied the spatio-temporal variation process and segmentation rupture characteristic. Dense aftershocks distribute along Longmenshan central fault zone of NE direction and form a narrow strip with the length of 325 km and the depth between several and 40 km. The depth profile (section of NW direction) vertical to the strike of aftershock zone (NE direction) shows anisomerous wedgy distribution characteristic of aftershock concentrated regions; it is related to the force form of the Longmenshan nappe tectonic belt. The stronger aftershocks could be divided into northern segment and southern segment apparently and the focal depths of strong aftershocks in the 50 km area between northern segment and southern segment are shallower. It seems like ‘to be going to rupture’ segment. We also study focal mechanisms and segmentation of strong aftershocks. The principal compressive stress azimuth of aftershock area is WNW direction and the faulting types of aftershocks at southern and northern segment have the same proportion. Because aftershocks distribute on different secondary faults, their focal mechanisms present complex local tectonic stress field. The faulting of seven strong earthquakes on the Longmenshan central fault is mainly characterized by thrust with the component of right-lateral strike-slip. Meantime six strong aftershocks on the Longmenshan back-range fault and Qingchuan fault present strike-slip faulting. At last we discuss the complex segmentation rupture mechanism of the Wenchuan earthquake.

Study on Temporal Variations of Shear‐Wave Splitting in the Chiayi Area, Aftershock Zone of 1999 Chichi Earthquake, Taiwan

AbstractBased on the data from three seismic stations CHY, CHN2 and CHN8 of Taiwan Central Weather Bureau Seismic Network (CWBSN), the systematic analysis method (SAM) of shear‐wave splitting has been used to do the near‐source stress‐forecast research of temporal preseismic variations of polarization directions and time delays for nearly 22 months before 1999 ML 6.4 and ML 6.0 in the Chiayi area‐aftershock zone of 1999 Chichi earthquake with MW7.6 on September 20, Taiwan. The result shows that the normal polarization directions of station CHY, CHN2 and CHN8 are approximately EW, in good agreement with the direction of maximum compressional stress, suggesting that the anisotropy in this area is controlled by local tectonic stress field. The results strongly indicate that in the stress critical stage, the polarization 90°‐flips occurred frequently, and that there were abrupt decreases in time delays shortly before the Chichi mainshock and two Chiayi earthquakes. These can be considered as two precursors of stress‐forecast. We think that the temporal variations of shear‐wave splitting are of great importance, since they provide information about source stress processes and critical warning information before large earthquakes.

On correlation between seismic activities and focal mechanisms of subsequent strong earthquakes

On the basis of the results from spatial-temporal scanning of moderate seismic activities prior to 63 strong earthquakes occurred since 1966, we investigate in this paper the characteristics of anomalous evolution of moderate seismic activities prior to strong earthquakes with different focal mechanisms in different tectonic blocks, as well as the correlation between principal seismic activities and nodal planes of focal mechanisms. The research results indicate that the moderate seismic activities prior to strong earthquakes display two significant stages of stress concentration and stress weakening, which are consistent to the evolvement of local tectonic stress field. The first stage, defined as medium-term stage, is characterized by seismic belts, seismic gaps and centralized activities; The second stage, defined as short-term stage, is predominated by continuous quiescence. For the earthquakes occurred on the strike-slip and normal faults, the two stages are relatively evident; while for the earthquakes occurred on the thrust faults, there are not characteristic images and seismic quiescence in both stages. The spatial distribution of moderate shock belts before strong earthquakes show a good coherence with that of the two nodal planes of focal mechanisms, while the spatial distribution of seismic gaps enclosed by moderate shocks display a poor coherence with the nodal planes.

Hydrologic properties of coal beds in the Powder River Basin, Montana I. Geophysical log analysis

As part of a multidisciplinary investigation designed to assess the implications of coal-bed methane development on water resources for the Powder River Basin of southeastern Montana, six wells were drilled through Paleocene-age coal beds along a 31-km east–west transect within the Tongue River drainage basin. Analysis of geophysical logs obtained in these wells provides insight into the hydrostratigraphic characteristics of the coal and interbedded siliciclastic rocks and their possible interaction with the local stress field. Natural gamma and electrical resistivity logs were effective in distinguishing individual coal beds. Full-waveform sonic logs were used to determine elastic properties of the coal and an attendant estimate of aquifer storage is in reasonable agreement with that computed from a pumping test. Inspection of magnetically oriented images of the borehole walls generated from both acoustic and optical televiewers and comparison with coal cores infer a face cleat orientation of approximately N33°E, in close agreement with regional lineament patterns and the northeast trend of the nearby Tongue River. The local tectonic stress field in this physiographic province as inferred from a nearby 1984 earthquake denotes an oblique strike-slip faulting regime with dominant east–west compression and north–south extension. These stress directions are coincident with those of the primary fracture sets identified from the televiewer logs and also with the principle axes of the drawdown ellipse produced from a complementary aquifer test, but oblique to apparent cleat orientation. Consequently, examination of these geophysical logs within the context of local hydrologic characteristics indicates that transverse transmissivity anisotropy in these coals is predominantly controlled by bedding configuration and perhaps a mechanical response to the contemporary stress field rather than solely by cleat structure.

Azimuthal anisotropy of the P-wave velocity in the hypocentral volume of the Krn Mt. (Slovenia) earthquake sequence

Azimuthal anisotropy of P-wave velocity in the hypocentral volume of the Krn Mt. (Slovenia) earthquake sequence is measured using the differences of travel times and of travel paths of the Pg-phase towards the recording stations of the local and regional networks. The observed velocity varies between 6.0 km/s in the ENE–WSW direction and 6.4 km/s for waves propagating NNW–SSE. These directions closely match those of the mean regional principal stress components obtained from individual fault-plane solutions for events from the Krn Mt. sequence. A large part of observed anisotropy may be explained if hypocentral volume is assumed to be pervaded by a system of vertical/subvertical extensive-dilatancy anisotropy (EDA) cracks aligned under the influence of local tectonic stress field.

Spatial distribution of atypical aftershocks of the 1995 Hyogo-ken Nanbu earthquake

The 1995 Hyogo-ken Nanbu earthquake (MJMA 7.3) occurred on January 17, 1995. To reveal the detailed stress field after the mainshock, we relocate hypocenters of aftershocks (M ≥ 2.3) and determine their focal mechanisms using seismic records obtained by GROUPS-95, a temporary dense seismic network in and around the aftershock region. Along the mainshock rupture zone, some aftershocks are nearly N-S compression or normal fault type events, which is inconsistent with the regional stress field of approximately horizontal E-W compression. We call these aftershocks atypical, defined as events which have focal mechanisms with P-axis directions more than 45° from the regional stress field. The atypical aftershocks amount to about 17% of the total. No temporal variations in aftershock mechanism are found in the analysis period. Their spatial distribution is compared with the slip and stress distribution of the mainshock, as well as the P- and S-wave velocity structure. Most of them are located at the upper boundary between the slipped and unslipped zones of the mainshock. We suggest that the atypical aftershocks are caused by the disturbance of the local tectonic stress field due to the heterogeneous coseismic slip.

Pulsed uplift estimated from terrace elevations in the coast of Rome: evidence for a new phase of volcanic activity?

The most recent phase of activity from the Roman volcanoes, since 45 ka, has been interpreted to be the waning stage of the older explosive activity that peaked between 550 and 250 ka. Yet substantial regional uplift from approximately 250 ka suggests that magma chambers beneath the area are growing rather than shrinking. We have estimated the uplift history of the region using radioisotopically and paleomagnetically constrained coastal terrace elevation data that allow us to correlate these terraces with marine oxygen isotopic stages 1–22. Elevations have been corrected for glacio-eustatic sea-level change. Our study shows that over the last million years, regional uplift of coastal terraces occurred in pulses that preceded volcanic activity at roughly 800 ka (in the Monti Sabatini) and 600 ka (in the Alban Hills). The recent pulse of uplift since 250 ka occurred primarily during a period of relative volcanic dormancy that lasted from 250 to 45 ka. We hypothesize that the most recent volcanic activity since 45 ka may reflect the completion of magma chamber recharge, suggesting the start of a new volcanic epoch. It is also possible that the local tectonic stress field changed recently from one dominated by transpression to one dominated by extension, allowing magma to more easily reach the surface via normal faults.