Maximum Horizontal Compressive Stress Research Articles

Source characteristics and b-values of the Tuz Gölü Fault Zone in Central Anatolia, Turkey

We investigate significant earthquake sequences in the observation period between 2000 and 2018 along the Tuz Gölü Fault Zone in Central Anatolia. These sequences include a series of strong events with moment magnitudes (Mw) >5.0, e.g. 30 July 2005 Bala-Ankara Mw 5.2, 20 December 2007 Bala-Ankara Mw 5.7 and 26 December 2007 Mw 5.6 Afşar-Ankara earthquakes. The mainshocks are shallow focus strike-slip events with minor normal component at depths of 12–15 km. Focal depths of aftershocks range from 2 to 30 km. The focal mechanisms of the aftershocks are mainly normal faulting with variable strike-slip components. The geometry of focal mechanisms reveals a normal faulting regime with E-W and also NE-SW trending directions of T-axis in the entire activated region. A stress tensor inversion of focal mechanism data is performed to acquire a more accurate picture of the Tuz Gölü Fault Zone stress field. The stress tensor inversion results display a predominant normal stress regime with a NNE–SSW oriented maximum horizontal compressive stress (SH). According to high-resolution hypocenter relocations of the Tuz Gölü Fault Zone seismic sequences, eight clusters are revealed. The aftershock activities in the observation period between 5 March 2000 and 2 June 2018 extend from NW to SE direction. Seismic cross-sections show that a complex pattern of the hypocenter distribution with the activation of Tuz Gölü Fault Zone segments. The western clusters are associated with a fault plane trending mainly N-S and dipping towards almost vertical, while the eastern clusters are related to a fault plane trending NW and dipping towards SE direction. The best constrained focal depths are mainly confined in the crust (depth < 30 km) and range from 2 to 30 km. In this study, we compare the results of an analysis of b-values from seismicity and stress tensors of focal mechanism solutions to understand their coupling in terms of faulting and earthquake hazard implications. In particular, this comparison allows to investigate the spatiotemporal correlation between b-values and stress distributions and it is therefore able to locate fault segments that have a high potential of generating large earthquakes. According to recent seismicity, b-values range from 0.5 to 1.5 along the Tuz Gölü Fault Zone in Central Anatolia. The maximum horizontal compressive stress (SH) indicates N-S and NE-SW directions while the minimum horizontal compressive stress (Sh) displays E-W. These results represent that low stress variances are correlated with high b-values. Low b-values are predominant in areas with high stress rates, consistent with a high heterogeneity of focal mechanisms.

Stress field modelling of the Late Oligocene tectonic inversion in the Liaodong Bay Subbasin, Bohai Bay Basin (northern China): Implications for geodynamics and petroleum accumulation

The Liaodong Bay Subbasin (LDBS), Bohai Bay Basin, is one of the most petroliferous basins in China. The basin experienced a strong tectonic inversion in the Late Oligocene, which has been proven to have had a major influence on petroleum accumulation. However, the dynamic origin of this tectonic event is poorly understood. In this paper, we present a finite element-based dynamic modelling study of the Late Oligocene tectonic inversion in the basin based on geological, geophysical and geomorphological data with different boundary conditions. We compared the modelling results with actual geological data and concluded that the NE-SW-directed compression model (labelled with BC-1) is the preferred model, suggesting that the effects of the India-Eurasia collision were the main cause of the Late Oligocene tectonic inversion. The results suggested that the maximum horizontal principal compressive stress direction (σHmax) generally trends EN E-W SW in the whole basin. The majority of the basin features a strike-slip faulting stress regime; the normal fault regimes are limited to the major faults, and the thrust faulting regimes are present only in the northern area of the Liaodong Uplift and Liaozhong Depression. The predicted fault slip rates occurring along the LZ1 fault in the Liaozhong area are significantly higher than those in the Liaoxi and Liaodong areas. Two high shear strain rates belts are present along the major faults in the Liaoxi and Liaozhong areas, indicating that the major faults accommodated the majority of the strain rates. This tectonic inversion play a significant positive role in the accumulation of petroleum in the basin due to the favourable relationship among the high strain rates, the formation of inversion-related traps and the timing of hydrocarbon charging. The high strain rate belts are suggested as the preferred targets for oil and gas exploration.

Temporal changes in the internal stresses and pore pressures in a large-scale submarine mass transport deposit

We examined the temporal changes in the internal stresses and pore fluid pressures of a submarine mass transport deposit (MTD) in the Akkeshi Formation of the Upper Cretaceous–Paleocene Nemuro Group, eastern Hokkaido Island, Japan. We first analyzed previous paleostress field results from meso-scale faults in the MTD blocks, which indicated two phases during the evolution of the debris flow: phase I, radial spreading of the flow body during downslope movement; phase II, the flow body underwent compression during deposition on the basin plain. We also estimated the pore fluid pressure ratio from the fault orientation distribution. There was a large increase in the pore fluid pressure ratio during the transition from phase I to phase II that continued to rise during the initial stage of phase II and then decreased in its latter stages, whereas the maximum horizontal compressive stress increased throughout phase II. This variation in pore fluid pressure relates to the dynamics and evolution of the debris flow, where the clasts in the central part of the flow were supported by the excess pore pressure due to the compression of the debris flow as the flow head decelerated. Although pore fluid pressure plays a critical role in the dynamics of debris flows, there was no previous methodology to quantify both the stress fields and pore fluid pressures in large debris flows and their resultant MTDs. Our results implemented for outcrop studies imply that meso-scale faults in MTDs can provide clues to better understand these paleoflow mechanisms.

Recovery of tectonic traces and its influence on coalbed methane reservoirs: A case study in the Linxing area, eastern Ordos Basin, China

The physical properties of coal reservoirs, i.e., porosity, permeability, fractures, and coal structures, are generally evaluated based on its structural morphology, however, its tectonic evolution history plays a much more important role. The key problems associated include the time when the dominant variations on properties of coal occurs and how to quantitatively characterize the influences of different tectonic movements. Based on field joint observations and well logging (micro-resistivity imaging logging (MIL), interval transit time (DT24), density (DEN), and natural gamma-ray (GR)) interpretations, the paleotectonic stress fields and tectonic traces were recovered and the present coal structures were identified. Subsequently, the quantitative correlation between the tectonic deformation and coal reservoir property were established with fluid intrusion tested porosity and permeability and scanning electron microscope (SEM) results.The results showed that the maximum horizontal compressive stress was oriented at the NW-SE direction in Yanshanian and at the NEE-SWW direction in the Himalayan period as obtained from hundreds of joint measurement and fracture interpretation results. Six combinations (by superposition of anticline, syncline, and the flanks) from the Yanshanian and Himalayan movements were found, which indicate different deformation degrees. The maximum tectonic curvature (r'), which occurred in either the Yanshanian or Himalayan period, shows differences at different structural areas and reflects the real deformation degree of coal seam in geological history. When r'< 12 × 10−6 m−1, primary coals are developed; when 12 × 10−6 m−1 <r' < 35 × 10−6 m−1, cataclastic coals are generally developed; when r'> 35 × 10−6 m−1, granulated coals are dominant. Mylonitized coals are barely developed in the study area. With the increase of deformation degrees of coal seams, the number of microfractures increases and fracture stretches tend to be more complicated. When r' < 25 × 10−6 m−1 (if r' > 25 × 10−6 m−1, the coal cores are much too broken for drilling core samples), with the increase of r', the displacement pressure (Pd) achieved by mercury intrusion method decreases in an linear form, and the permeability tested by helium presents a linear growth. However, the porosity shows a weak correlation with r', mainly caused by the great burial depth and long-term compaction resulting in low porosity ranges (2.8%–8.1%) with little difference shown. This presents an applicable method for evaluating the influence of tectonic movement on the physical properties of coal during different times and would benefit the evaluation of coalbed methane reservoirs in the study area as well as other coal basins in the world.

New constraints on the neotectonic stress pattern of the Flinders and Mount Lofty Ranges, South Australia

The majority of published in-situ stress information in the Australian continent is confined to petroleum provinces where industry technologies facilitate the capture of contemporary crustal stress information. In contrast, the stress pattern of non-petroleum regions such as the Flinders and Mount Lofty Ranges in South Australia, where intraplate deformation is localised, has not been investigated comprehensively so far. The ongoing activities of the mining industry in South Australia has enabled us to access recently drilled boreholes for image logging techniques that have typically been under-utilised by the mineral sector. Herein, we conduct stress analysis by analysing borehole image logs from 16 boreholes in the basement rocks of South Australia as well as two geothermal wells and one coal seam gas well in South Australia’s northern Flinders Ranges, the Gawler Craton and the Eyre Peninsula. The resulting dataset of stress orientations is further accomplished by including recent seismological observations, which provide crustal stress information derived from focal mechanism solutions. The results of this study suggest a regional E–W orientation of the maximum horizontal compressive stress that is consistent with numerous observed neotectonic structures in this region. The focal mechanism solutions in this study suggest that the majority of events occur in a thrust faulting stress regime, which is consistent with the observed Quaternary fault scarps. However, our data compilation also indicates the presence of strike-slip and normal faulting stress regimes in the region, which has not been suggested extensively before this study.

Cretaceous stress field evolution and origin of the Jiaolai Basin, Eastern North China

The Jiaolai Basin, to the east of the Tan-Lu Fault Zone (TLFZ), is a Cretaceous NE-trending basin in eastern North China and has a multi-episodic evolution. Different from other contemporary NW-trending en-echelon graben systems west to the TLFZ, the origin of the Jiaolai Basin (three episodes: K1l, 135–120 Ma; K2q, 120–105 Ma; K2w, 88–65 Ma) cannot be simply explained by the sinistral displacement of the TLFZ at the beginning of the Early Cretaceous (around ca. 135 Ma). The purpose of this paper is to investigate the critical factors (e.g., bounding faults, tectonic setting, stress field) driving the development of the Jiaolai Basin and constraint its evolution. Three episodes of two-dimensional finite element models were generated in order to determine the roles of the boundary conditions and to reveal the tectonic development of the basin. When compared to the calculated maximum horizontal compressive stress (σHmax) trajectories and the observed data, along with a comparison between the Von Mises Stress (uVon) and the depocenters of the basin, the calculated results can be well proved by the geological evidence both in stress field evolution and in the basin patterns. Our analysis show that the three episodes of the maximum principle stress trajectories changed from NE-SW (earlier Early Cretaceous, K1l) NNE-SSW (later Early Cretaceous, K1q) then E-W (Late Cretaceous, K2w). Moreover, the results show a good consistency between the uVonand the depocenters in the basin through geological cross-sections. Via models we suggest that the origin and evolution of the Jiaolai Basin were controlled by three episodes of tectonic events, (i) the reactivated bounding faults and the post-orogenic extension of the Sulu Orogen during the earlier Early Cretaceous, (ii) the lithospheric delamination of the thickened lithosphere during the later Early Cretaceous, and (iii) the likely far-field effects produced by the collision between the Kohistan-Drags arc complex and Asia during the Late Cretaceous. The Jiaolai Basin evolved from extension to pull-apart experiencing volcanic activity during the three episodes of the Cretaceous.

State of stress in the Permian Basin, Texas and New Mexico: Implications for induced seismicity

Since the 1960s, the Permian Basin of west Texas and southeast New Mexico has experienced earthquakes that were possibly triggered by oil and gas activities. In recent years, seismicity has been concentrated near Pecos, Texas; around the Dagger Draw Field, New Mexico; and near the Cogdell Field, Snyder, Texas. We have collected hundreds of measurements of stress orientation and relative magnitude to identify potentially active normal, normal/strike-slip, or strike-slip faults that might be susceptible to earthquake triggering in this region. In the Midland Basin and Central Basin Platform, the faulting regime is consistently normal/strike slip, and the direction of the maximum horizontal compressive stress (SHmax) is approximately east–west, although modest rotations of the SHmaxdirection are seen in some areas. Within the Delaware Basin, however, a large-magnitude clockwise rotation (∼150°) of SHmaxoccurs progressively from being nearly north–south in the north to east-southeast–west-northwest in the south, including the western Val Verde Basin. A normal faulting stress field is observed throughout the Delaware Basin. We use these stress data to estimate the potential for slip on mapped faults across the Permian Basin in response to injection-related pressure changes at depth that might be associated with future oil and gas development activities in the region.

The Role of Texture, Cracks, and Fractures in Highly Anisotropic Shales

AbstractOrganic shales generally have low permeability unless fractures are present. However, how gas, oil, and water flows into these fractures remains enigmatic. The alignment of clay minerals and the alignment of fractures and cracks are effective means to produce seismic anisotropy. Thus, the detection and characterization of this anisotropy can be used to infer details about lithology, rock fabric, and fracture and crack properties within the subsurface. We present a study characterizing anisotropy using S wave splitting from microseismic sources in a highly anisotropic shale. We observe very strong anisotropy (up to 30%) with predominantly VTI (vertical transverse isotropy) symmetry, but with evidence of an HTI (horizontal transverse isotropy) overprint due to a NE striking vertical fracture set parallel to the maximum horizontal compressive stress. We observe clear evidence of a shear wave triplication due to anisotropy, which to our knowledge is one of only a very few observations of such triplications in field‐scale data. We use modal proportions of minerals derived from X‐ray fluorescence data combined with realistic textures to estimate the contribution of intrinsic anisotropy as well as possible contributions of horizontally aligned cracks. We find that aligned clays can explain much of the observed anisotropy and that any cracks contributing to the vertical transverse isotropy (VTI) must have a low ratio of normal to tangential compliance (ZN/ZT), typical of isolated cracks with low hydraulic connectivity. Subhorizontal cracks have also been observed in the reservoir, and we propose that their reactivation during hydraulic fracturing may be an important mechanism to facilitate gas flow.

Reactivation of normal faults as high-angle reverse faults due to low frictional strength: Experimental data from the Moonlight Fault Zone, New Zealand

Large normal faults are frequently reactivated as high-angle reverse faults during basin inversion. Elevated fluid pressure is commonly invoked to explain high-angle reverse slip. Analogue and numerical modeling have demonstrated that frictional weakening may also promote high-angle reverse slip, but there are currently no frictional strength measurements available for fault rocks collected from large high-angle reverse faults. To test the hypothesis that frictional weakening could facilitate high-angle reverse slip, we performed single- and double-direct friction experiments on fault rocks collected from the Moonlight Fault Zone in New Zealand, a basin-bounding normal fault zone that was reactivated as a high-angle reverse fault (present-day dip angle 60°–75°). The fault core is exposed in quartzofeldspathic schists exhumed from c. 4–8 km depth and contains a <20 m thick sequence of breccias, cataclasites and foliated cataclasites that are enriched in chlorite and muscovite. Friction experiments on water-saturated, intact samples of foliated cataclasite at room temperature and normal stresses up to 75 MPa yielded friction coefficients of 0.19<μ < 0.25. On the assumption of horizontal maximum compressive stress, reactivation analysis indicates that a friction coefficient of <0.25 will permit slip on high-angle reverse faults at hydrostatic (or even sub-hydrostatic) fluid pressures. Since foliated and phyllosilicate-rich fault rocks are common in large reactivated fault zones at basement depths, long-term frictional weakening is likely to act in concert with episodic build-ups of fluid pressure to promote high-angle reverse slip during basin inversion.

A Decade of Induced Slip on the Causative Fault of the 2015Mw4.0 Venus Earthquake, Northeast Johnson County, Texas

AbstractOn 7 May 2015, aMw4.0 earthquake occurred near Venus, northeast Johnson County, Texas, in an area of the Bend Arch‐Fort Worth Basin that reports long‐term, high‐volume wastewater disposal and that has hosted felt earthquakes since 2009. In the weeks following theMw4.0 earthquake, we deployed a local seismic network and purchased nearby active‐source seismic reflection data to capture additional events, characterize the causative fault, and explore potential links between ongoing industry activity and seismicity. Hypocenter relocations of the resulting local earthquake catalog span ~4–6 km depth and indicate a fault striking ~230°, dipping to the west, consistent with a nodal plane of theMw4.0 regional moment tensor. Fault plane solutions indicate normal faulting, withBaxes striking parallel to maximum horizontal compressive stress. Seismic reflection data image the reactivated basement fault penetrating the Ordovician disposal layer and Mississippian production layer, but not displacing post‐Lower Pennsylvanian units. Template matching at regional seismic stations indicates that low‐magnitude earthquakes with similar waveforms began in April 2008, with increasing magnitude over time. Pressure data from five saltwater disposal wells within 5 km of the active fault indicate a disposal formation that is 0.9–4.8 MPa above hydrostatic. We suggest that the injection of 28,000,000 m3of wastewater between 2006 and 2015 at these wells led to an increase in subsurface pore fluid pressure that contributed to inducing this long‐lived earthquake sequence. The 2015Mw4.0 event represents the largest event in the continuing evolution of slip on the causative fault.

Crustal stress pattern in China and its adjacent areas

During the update of the World Stress Map (WSM) database, we integrated the China stress database by strictly using the internationally developed quality ranking scheme for each individual stress data record. This effort resulted in a comprehensive and reliable dataset for the crustal stress of China and its adjacent areas with almost double the amount of data records from the WSM database release 2008, i.e., a total of 8228 data records with reliable A-C qualities in the region of 45–155° East and 0–60° North. We use this dataset for an analysis of the stress pattern for the orientation of maximum compressive horizontal stress (SHmax). In contrast to earlier findings that suggested that the mean SHmax orientation would be aligned with the direction of plate motion, we clearly see from our results that the plate boundary forces, as well as topography and faulting, are important control factors for the overall stress pattern. Furthermore, the smoothing results indicate that the SHmax orientation in China rotates clockwise from the west to the east, which results in a fan-shaped crustal stress pattern for the continental scale. The plate boundary forces around China, which are the Indian-Eurasian plate collision in the west and the Pacific plate subduction and the push from the Philippine plate in the east, can still be seen as the key driving processes and the first-order controls for the crustal stress pattern. The South-North seismic zone can be seen as the separation zone for the western and eastern plate boundary forces. Topographic variation and faulting activity, however, provide second-order changes, and lead to local variations and different inhomogeneity scales for the stress pattern. Due to differences in these factors, Northeast China and the central part of the Tibetan plateau have notably homogeneous stress patterns, while the South-North seismic zone, the Hindu Kush-Pamir region, and the Taiwan region have extremely inhomogeneous stress patterns. Furthermore, the different behaviors of stress orientations around continental and oceanic plate boundaries could imply that complicated mechanisms exist and warrant further and more specific studies.

Source characteristics and geological implications of the January 2016 induced earthquake swarm near Crooked Lake, Alberta

On 2016 January 12, an intraplate earthquake with an initial reported local magnitude (ML) of 4.8 shook the town of Fox Creek, Alberta. While there were no reported damages, this earthquake was widely felt by the local residents and suspected to be induced by the nearby hydraulic-fracturing (HF) operations. In this study, we determine the earthquake source parameters using moment tensor inversions, and then detect and locate the associated swarm using a waveform cross-correlation based method. The broad-band seismic recordings from regional arrays suggest a moment magnitude (M) 4.1 for this event, which is the largest in Alberta in the past decade. Similar to other recent M ∼ 3 earthquakes near Fox Creek, the 2016 January 12 earthquake exhibits a dominant strike-slip (strike = 184°) mechanism with limited non-double-couple components (∼22 per cent). This resolved focal mechanism, which is also supported by forward modelling and P-wave first motion analysis, indicates an NE–SW oriented compressional axis consistent with the maximum compressive horizontal stress orientations delineated from borehole breakouts. Further detection analysis on industry-contributed recordings unveils 1108 smaller events within 3 km radius of the epicentre of the main event, showing a close spatial-temporal relation to a nearby HF well. The majority of the detected events are located above the basement, comparable to the injection depth (3.5 km) on the Duvernay shale Formation. The spatial distribution of this earthquake cluster further suggests that (1) the source of the sequence is an N–S-striking fault system and (2) these earthquakes were induced by an HF well close to but different from the well that triggered a previous (January 2015) earthquake swarm. Reactivation of pre-existing, N−S oriented faults analogous to the Pine Creek fault zone, which was reported by earlier studies of active source seismic and aeromagnetic data, are likely responsible for the occurrence of the January 2016 earthquake swarm and other recent events in the Crooked Lake area.

An example of triggered earthquakes in western Turkey: 2000–2015 Afyon-Akşehir Graben earthquake sequences

We investigate significant earthquake sequences in the observation period between 2000 and 2015 along the Afyon-Akşehir Graben system in western Anatolia. These sequences include a series of strong events with moment magnitudes (Mw) larger than 5.5, e.g. Eber (Mw 5.8), Sultandağı (Mw 6.5) and Çay (Mw 5.8) earthquakes. The mainshocks are shallow focus normal events at depths of 8–15km. Focal depths of aftershocks range from 2 to 30km. The focal mechanisms of the aftershocks are mainly normal faulting with variable strike-slip components. The geometry of focal mechanisms reveals a normal faulting regime with NNE-SSW trending direction of T-axis in the entire activated region. A stress tensor inversion of focal mechanism data is performed to acquire a more accurate picture of the Afyon-Akşehir Graben stress field. The stress tensor inversion results display a predominant normal stress regime with a NW–SE oriented maximum horizontal compressive stress (SH). According to high-resolution hypocenter relocations of the Afyon-Akşehir Graben seismic sequences, six clusters are revealed. The aftershock activities in the observation period between 17 January 2000 and 26 October 2015 extend from east to west direction. Seismic cross-sections show that a complex pattern of the hypocenter distribution with the activation of Sultandağı Fault segments. The western clusters are associated with a fault plane trending mainly E-W and dipping towards SE, while the eastern clusters are related to a fault plane trending NW-SE and dipping towards vertical. The best constrained focal depths are mainly confined in the crust (depth<30km) and run in the approximate depth range from 2 to 30km. Moreover, Coulomb stress analysis is also performed to estimate the stress loading for the activated region. We obtain more than 3bars for positive lobes. This result defines that the values are large enough to increase the Coulomb stress failure towards NE-SW direction.

Spatial analysis of an intra-plate basaltic volcanic field in a compressional tectonic setting: South-eastern Australia

The Newer Volcanics Province (NVP) is a Pliocene to Recent intra-plate basaltic volcanic field (BVF) that has formed in a compressive tectonic setting (σv<σhmin<σHmax) and is not readily attributed to a single geodynamic process. A comprehensive spatial analysis of both monogenetic eruption centres and coeval vents of the NVP constrain factors that control the distribution and emplacement of volcanoes. A point-set of 434 eruption centres totalling 726 vents are divided into three geographical sub-provinces for analysis. Kernel density estimation and Poisson nearest neighbour analysis are used to scrutinize the distribution of eruption centres. A simple and novel fitted regression line method is used to determine the orientation of coeval vents, and Hough transform and two-point azimuth methods are used to identify alignments and alignment trends between eruption centres. The distribution of eruption centres and their relative spatial density corresponds with the extent of thinner lithosphere. Eruption centres of the NVP have a clustered distribution whilst smaller sub-sets of eruption centres are distributed more uniformly. The main alignment trends between coeval vents related to individual dikes and between eruption centres related to successive temporally discrete dikes are primarily oriented nearly parallel with pre-existing crustal fault trends. The frequency of volcanic alignments shows faults oriented nearly parallel to the orientation of the regional maximum horizontal compressive stress (σ1) are favourably utilised by intrusions over other fault trends. The depth from which pre-existing faults facilitate dike propagation is not constrained. We interpret they are likely important in preventing dikes from stalling and forming sills under a compressive stress field in the case of the NVP. It is also observed that coeval vent alignments are more strongly aligned in areas overlying consolidated basement relative to areas of poorly consolidated basin sediments. This could be explained by poorly aligned groups of vents being fed by shallow sills, preferentially forming in layered basin sediments.

Upper crustal stress and seismotectonics of the Garhwal Himalaya using small-to-moderate earthquakes: Implications to the local structures and free fluids

The work presents new focal-mechanism data of small-to-moderate (3.0⩾ML⩽5.0) upper crustal earthquakes for the Garhwal Himalaya from a local seismic network installed in July 2007. Majority of the epicenters of these earthquakes are located close to the Main Central Thrust (MCT) zone. We retrieved Moment Tensor (MT) solutions of 26 earthquakes by waveform inversion. The MT results and 11 small-to-moderate earthquakes from the published records are used for stress inversions. The MT solutions reveal dominatingly thrust mechanisms with few strike slip earthquakes near Chamoli. The seismic cross sections illustrate that, these earthquakes are located around the Mid-Crustal-Ramp (MCR) in the detachment. The optimally oriented faults from stress inversions suggest that, the seismogenic fault in this region is similar to a fault plane having dip angle between 12 and 25 degrees, which is compatible with the dip angle of the MCR (∼16°) in this region. P-axes and the maximum horizontal compressive stress are NE-SW oriented; the direction of the relative motion of Indian plate with respect to the Eurasian plate. The Friction Coefficient estimated from stress inversions show that the Chamoli region having low friction in comparison to the overall values. The free fluids trapped beneath the detachment are penetrating into the local faults, hence, decreasing the frictional strength and altering the prevailing stress conditions of the surroundings. The present study reveals that the MCR structure is seismogenically active and producing the small-moderate earthquakes in the region, while the MCT is probably dormant at present.

Source Mechanisms and Stress Fields of the 15–16 June 2013 Crete Earthquake Sequence Along Hellenic Subduction Zone

15 June 2013 M w 6.1 off-shore southern Crete earthquake and its aftershock sequence along Hellenic Subduction Zone are examined. Centroid moment tensors (CMTs) for 40 earthquakes with moment magnitudes (M w) between 3.5 and 6.1 are determined by applying a waveform inversion method. The mainshock is shallow focus thrust event with a minor strike-slip component at a depth of 20 km. The seismic moment (M o) of the mainshock is estimated as 2.07 × 1018 Nm, and rupture duration of the mainshock is 4 s. The focal mechanisms of aftershocks are mainly thrust faulting with a strike-slip component. The geometry of the moment tensors (M w ≥ 3.5) reveals a thrust faulting regime with N–S trending direction of P axis in the entire activated region. According to high-resolution CMT solutions of the off-shore southern Crete earthquake sequence, one main cluster consisting of 40 events is revealed. The aftershock activity in the observation period between 15 June and 15 July 2013 extends from N to S and NW to SE directions. Seismic cross sections indicate a complex pattern of the hypocenter distribution with the activation of two segments. The subduction interface is clearly revealed with high-resolution hypocenter source relocation and moment tensor solution. The best-constrained focal depths indicate that the aftershock sequence is mainly confined in the upper plate (depth < 30 km) and is ranging from about 5–28 km depth. A stress tensor inversion of focal mechanism data is performed to obtain a more precise picture of the off-shore southern Crete stress field. The stress tensor inversion results indicate a predominant thrust stress regime with a NE–SW-oriented maximum horizontal compressive stress (S H). According to variance of the stress tensor inversion, to first order, the southern Crete region is characterized by a homogeneous interplate stress field. We also investigate the Coulomb stress change associated with the mainshock to evaluate any significant enhancement of stresses along southern Crete and surrounding regions. Positive lobes with stress more than 0.3 bars are obtained for the mainshock, indicating that these values are large enough to increase the Coulomb stress failure toward WNW–ESE direction.

Orientation of the Eruption Fissures Controlled by a Shallow Magma Chamber in Miyakejima

Orientation of the eruption fissures and composition of the lavas of the Miyakejima volcano indicate tectonic influence of a shallow magma chamber on the distribution of eruption fissures. We examined the distributions and magmatic compositions of 23 fissures that formed within the last 2800 years, based on a field survey and a new dataset of 14C ages. The dominant orientation of the eruption fissures in the central portion of the volcano was found to be NE-SW, which is perpendicular to the direction of regional maximum horizontal compressive stress (σHmax). Magmas that show evidences of magma mixing between basaltic and andesitic magmas erupted mainly from the eruption fissures with a higher offset angle from the regional σHmax direction. The presence of a shallow dike-shaped magma chamber controls the distribution of the eruption fissures. The injection of basaltic magma into the shallow andesitic magma chamber caused the temporal rise of internal magmatic pressure in the shallow magma chamber. Dikes extending from the andesitic magma chamber intrude along the local compressive stress field which is generated by the internal excess pressure of the andesitic magma chamber. As the result, the eruption fissures trend parallel to the elongation direction of the shallow magma chamber. Injection of basaltic magma into the shallow andesitic magma chamber caused the magma mixing. Some basaltic dikes from the deep-seated magma chamber reach the ground surface without intersection with the andesitic magma chamber. The patterns of the eruption fissures can be modified in the future as was observed in the case of the destruction of the shallow magma chamber during the 2000 AD eruption.

Horizontal compressive stress regime on the northern Cascadia margin inferred from borehole breakouts

During Integrated Ocean Drilling Program Expedition 311 five boreholes were drilled across the accretionary prism of the northern Cascadia subduction zone. Logging-while-drilling borehole images are utilized to determine breakout orientations to define maximum horizontal compressive stress orientations. Additionally, wireline logging data at two of these sites and from Site 889 of Ocean Drilling Program Leg 146 are used to define breakouts from differences in the aperture of caliper arms. At most sites, the maximum horizontal compressive stress SHmax is margin-normal, consistent with plate convergence. Deviations from this trend reflect local structural perturbations. Our results do not constrain stress magnitudes. If the margin-normal compressional stress is greater than the vertical stress, the margin-normal SHmax direction we observe may reflect current locking of a velocity-weakening shallow megathrust and thus potential for trench-breaching, tsunamigenic rupture in a future megathrust earthquake.

The 16 April 2015 M w 6.0 offshore eastern Crete earthquake and its aftershock sequence: implications for local/regional seismotectonics

We examine the 16 April 2015 M w 6.0 offshore eastern Crete earthquake and its aftershock sequence in southern Aegean Sea. Centroid moment tensors for 45 earthquakes with moment magnitudes (M w) between 3.3 and 6.0 are determined by applying a waveform inversion method. The mainshock is shallow focus thrust event with a strike-slip component at a depth of 30 km. The seismic moment (M o) of the mainshock is estimated as 1.33 × 1018 Nm, and rupture duration of the mainshock is 3.5 s. The focal mechanisms of aftershocks are mainly thrust faulting with a strike-slip component. The geometry of the moment tensors (M w ≥ 3.3) reveals a thrust-faulting regime with NE–SW-trending direction of T axis in the entire activated region. According to high-resolution hypocenter relocation of the eastern Crete earthquake sequence, one main cluster consisting of 352 events is revealed. The aftershock activity in the observation period between 5 January 2015 and 7 July 2015 extends from N to S direction. Seismic cross sections indicate a complex pattern of the hypocenter distribution with the activation of three segments. The subduction interface is clearly revealed with high-resolution hypocenter relocation and moment tensor solution. The best constrained focal depths indicate that the aftershock sequence is mainly confined in the upper plate (depth <40 km) and are ranging from about 4.5 to 39 km depth. A stress tensor inversion of focal mechanism data is performed to obtain a more precise picture of the offshore eastern Crete stress field. The stress tensor inversion results indicate a predominant thrust stress regime with a NW–SE-oriented maximum horizontal compressive stress (S H). According to variance of the stress tensor inversion, to first order, the Crete region is characterized by a homogeneous interplate stress field. We also investigate the Coulomb stress change associated with the mainshock to evaluate any significant enhancement of stresses along Crete and surrounding regions. Positive lobes with stress more than 3 bars are obtained for the mainshock, indicating that these values are large enough to increase the Coulomb stress failure toward NE–SW and NW–SE directions, respectively.

Frictional strength of North Anatolian fault in eastern Marmara region

Frequency distribution of azimuth and plunges of P- and T-axes of focal mechanisms is compared with the orientation of maximum compressive stress axis for investigating the frictional strength of three fault segments of North Anatolian fault (NAF) in eastern Marmara Sea, namely Princes’ Islands, Yalova–Cinarcik and Yalova–Hersek fault segments. In this frame, we retrieved 25 CMT solutions of events in Cinarcik basin and derived a local stress tensor incorporating 30 focal mechanisms determined by other researches. As for the Yalova–Cinarcik and Yalova–Hersek fault segments, we constructed the frequency distribution of P- and T-axes utilizing 111 and 68 events, respectively, to correlate the geometry of the principle stress axes and fault orientations. The analysis yields low frictional strength for the Princes’ Island fault segments and high frictional strength for Yalova–Cinarcik, Yalova–Hersek segments. The local stress tensor derived from the inversion of P- and T-axes of the fault plane solutions of Cinarcik basin events portrays nearly horizontal maximum compressive stress axis oriented N154E which is almost parallel to the peak of the frequency distribution of the azimuth of the P-axes. The fitting of the observed and calculated frequency distributions is attained for a low frictional coefficient which is about μ ≈ 0.1. Evidences on the weakness of NAF segments in eastern Marmara Sea region are revealed by other geophysical observations. Our results also show that the local stress field in Cinarcik basin is rotated ≈30° clockwise compared to the regional stress tensor in Marmara region derived from the large earthquakes, whereas the local stress tensor in Yalova–Cinarcik area is found to be rotated ≈30° counterclockwise. The rotation of the two local stress fields is derived in the area where NAF bifurcates into two branches overlaying large electrical conductor.