Maximum Principal Stress Axis Research Articles

Post-Deccan Trap stress reorientation under transpression: Evidence from fault slip analyses from SW Saurashtra, Western India

We analyze paleostress from a region in SW Saurashtra (India) using three free software, T-Tecto, WinTensor and FaultKin based on attitudes of ∼ E-W trending, sub-vertical, brittle fault planes and associated horizontal, inclined and subvertical slickensides, developed over them. We present a detailed fault slip analysis of a previously unmapped E trending fault (herein referred as the Katar Fault) that cuts across and offset the southeastward dipping Deccan lava flows with a dense NE-SW trending dyke swarm belonging to Deccan Trap near Rajula (Gujarat, India). Measured fault strikes do not match exactly with ∼ NW/NNW trending basement structures, deduced by previous geophysical studies, nor with faults reported from the offshore Saurashtra region. However, a few geomorphic features around Katar indicate a similar strike slip offset. A stress ratio of up to 0.2, and a SE maximum principal stress axis were deduced using T-Tecto, while WinTensor and FaultKin gave plunging stress axes. Therefore, stress analyses by T-Tecto seem more plausible. The SHmax direction usually remains close to one of the stress axes, and shows up to 36° rotation when compared with the offshore region. We attribute the formation of Katar Fault to post-Deccan Trap oblique slip reverse faulting with sinistral slip due to intraplate transpressional stresses induced by the anticlockwise rotation and collision of the Indian plate with the Eurasian plate at ∼ 55 Ma. This transpressional faulting post-dates the eastward tilting of lava flows and anticlockwise rotation in the dyke swarm patterns caused by the emplacement of large Girnar intrusive body. Detailed structural geological field-work in Saurashtra would be much required to understand its onshore tectonics.

Development of branching brittle and ductile shear zones: A numerical study

AbstractContinental collision zones are usually associated with large‐scale strike‐slip shear zones. In most cases, these shear zones are complex and consist of multiple strands, varying in width, length, and total displacement. Here we present 2‐D numerical models to simulate the formation of such shear zones at different depth levels within the crust, under either brittle (frictional/plastic) or ductile conditions. Localization of shear zones is initiated by a material contrast (heterogeneity) of the material parameters. We systematically test the rate of strain weakening in brittle and in ductile regimes to understand its influence on the development of shear zone networks. Our simulations suggest that the development of antithetic faults in a brittle shear zone system is closely linked to a decrease in the angle of friction during deformation. In general, variation of the strain weakening also has a significant influence on ductile shear zones. Numerical results show that the geometry and thickness of the localized high strain zone are especially affected by weakening mechanisms during deformation. Furthermore, the interconnection and interaction of the shear strands lead to a more complex kinematic pattern, which lead to a local change in the maximum principal stress axis. These interaction of shear strands may explain the occurrence of shear‐related structures (e.g., folds) or differing characteristics of shear zones, such as the thickness of shear zones or the orientation of the faults to the stress field, which are consistent with field observations.

Stress accumulation process in and around the Atotsugawa fault, central Japan, estimated from focal mechanism analysis

We estimated 275 focal mechanisms from P-wave first-motion polarities of small earthquakes obtained in an extensive seismic survey during 2004–2008 in and around the Atotsugawa fault, central Japan, where ongoing dextral shear strain concentration has been observed. Along the fault trace, the azimuth direction of P-axes is oriented WNW–ESE, which agrees well with previous studies. The regional stress disturbance is detected by stress inversion analysis. The azimuth of the maximum principal stress axis systematically rotates counterclockwise as the distance from the fault trace decreases. The regional stress disturbance is explained by a cumulative slip deficit in the shallower portion of the Atotsugawa fault relative to the surrounding fault surface (i.e., the eastern, western, and deeper extensions of the fault plane).

Crustal stress field in Yunnan: implication for crust-mantle coupling

We applied the gCAP algorithm to determine 239 focal mechanism solutions $$\left( {3.0 \le M_{\text{W}} \le 6.0} \right)$$ with records of dense ChinArray stations deployed in Yunnan, and then inverted 686 focal mechanisms (including 447 previous results) for the regional crustal stress field with a damped linear inversion. The results indicate dominantly strike-slip environment in Yunnan as both the maximum (σ 1) and minimum (σ 3) principal stress axes are sub-horizontal. We further calculated the horizontal stress orientations (i.e., maximum and minimum horizontal compressive stress axes: S H and S h, respectively) accordingly and found an abrupt change near ~26°N. To the north, S H aligns NW-SE to nearly E-W while S h aligns nearly N-S. In contrast, to the south, both S H and S h rotate laterally and show dominantly fan-shaped patterns. The minimum horizontal stress (i.e., maximum strain axis) S h rotates from NW-SE to the west of Tengchong volcano gradually to nearly E-W in west Yunnan, and further to NE-SW in the South China block in the east. The crustal strain field is consistent with the upper mantle strain field indicated by shear-wave splitting observations in Yunnan but not in other regions. Therefore, the crust and upper mantle in Yunnan are coupled and suffering vertically coherent pure-shear deformation in the lithosphere.

Compaction-driven melt segregation in migmatites

A common and puzzling feature of migmatite terranes is the presence of synkinematic leucosomes oriented perpendicular to the maximum principal compressive stress axis, σ1, particularly leucosomes oriented parallel to the axial plane of folds. These are parallel to planes of compaction, but experiments predict that leucosomes should form in dilational bands oriented instead at small angles to σ1. This discrepancy suggests that our understanding of melt extraction is incomplete. We propose that leucosomes at high angles to σ1 are results of ductile compaction instabilities that arise in compressive environments as a result of compaction of porous media with nonlinear rheology and interstitial fluids. These instabilities form at high angles to σ1 and their spacing can be regular, controlled by the compaction length.

Changes in the stress field after the 2008 M7.2 Iwate‐Miyagi Nairiku earthquake in northeastern Japan

AbstractIn order to investigate changes in the stress field following the 2008 M7.2 Iwate‐Miyagi Nairiku earthquake in NE Japan, we determined many focal mechanisms within the focal area, using data from both the dense aftershock observation network deployed just after the main shock and other temporary and routinely operated stations in the surrounding area. Applying stress tensor inversions to these focal mechanism data, we estimated the detailed spatial distribution of the principal stress orientations in this event's source area. Estimated orientations of the maximum principal stress (σ1) axes differ significantly before and after the main shock. Before the main shock, the σ1 axis is oriented WNW‐ESE across the entire studied area, which is parallel to the plate convergence direction. After the main shock, however, the orientation of the σ1 axis changed significantly and showed remarkable spatial variation. In the northern and southern parts of the aftershock area, located relatively far from the main shock large slip area, the σ1 axes are oriented WNW‐ESE, similar to those before the main shock. In contrast, to the east and west of the central part of the aftershock area, the σ1 axes are oriented N‐S and NE‐SW, respectively. This characteristic spatial pattern of σ1 axis orientations is approximately consistent with that of static stress change associated with the main shock rupture, which strongly suggests that the spatially heterogeneous stress after the main shock is caused by such static stress change. The differential stress magnitude before the main shock is estimated to be less than 10–30 MPa, suggesting that the main shock faults are weak.

An assessment of the microseismic activity and focal mechanisms of the Izmir (Smyrna) area from a new local network (IzmirNET)

Izmir metropolitan area is one of the biggest cities of Turkey in terms of population and industrial density. Amount of recordings and quality of located events increased since the new local seismic array, IzmirNET, was installed in 2008. Present study addresses to elucidate seismotectonics of Izmir vicinity by a new data set and reveal first results of seismological analysis obtained for 4.5 years.930 small earthquakes are well located within less than 1.0km horizontal error (RMS≤0.15s) over a total of 1421 recorded events. Data were processed by using SEISAN Earthquake Analysis Software implemented at SeismoLab of Dokuz Eylul University. The Orhanli–Tuzla Fault Zone and Izmir faults are capable of producing a destructive earthquake. Events show clear clusters around the Outer Bay of Izmir Gulf and Sigacik Bay. We also detected some dense clusters Sigacik Bay to the South, and in the outer bay of Izmir Gulf along NW–SE direction.We investigated kinematics of active faults around the Izmir and Foca areas by using 41 focal mechanism solutions from different algorithms. Foca is located 60km in the NW direction of Izmir. Mechanisms generally resulted in pure normal or dominant normal faulting with minor strike-slip components in agreement with the kinematics of the other fault zones in Western Turkey. A stress tensor study using selected fault plane solution indicates that the study area is under extension regime in an N–S direction with 71° plunge for maximum compressive principal stress axis (S1), and closer to horizontals for intermediate (S2) and minimum compressive principal stress axis (S3).

Dyke intrusion into a pre-existing joint network: The Aiguablava lamprophyre dyke swarm (Catalan Coastal Ranges)

A structural analysis has been performed in the Upper Permian lamprophyric dyke swarm of Aiguablava (Costa Brava, NE Iberia). Emplacement of sub-vertical dykes is related to the presence of a widespread joint network, mostly developed during the cooling and decompression of the late Variscan leucogranitic host rocks. The joint pattern consists on multiple sub-vertical joint sets among which two orthogonal sets at ≈N23° and ≈N113° are predominant. A sub-horizontal set is also prominent. The sub-vertical dykes have a mean N100°–N125° trend, which corresponds to the trend of one of the main joint sets. However, dyke segmentation is noticeable at the Dm- to cm-scale, and this is inferred to be related to dyke propagation and emplacement along the variably oriented pre-existing joints. A mean robust sub-horizontal NNE–SSW net dilation direction was measured from matching dyke jogs, markers in the host rock and analysis of maximum dyke thicknesses, and this is in line with the minimum principal stress axis (3) derived from a three-dimensional paleostress analysis from dyke orientations. The inferred maximum principal stress axis (1) is sub-vertical, indicating that dykes intruded under conditions of tectonic extension. Furthermore, a Mohr construction allowed calculation of the stress ratio φ=0.29 (close to a prolate stress ellipsoid) and a driving pressure ratio R′=0.27, which corresponds to a magmatic pressure almost equal to the intermediate principal stress axis, σ2. It is inferred that many of the pre-existing joint sets were exploited by the magma, the WNW–ESE joint set (normal to σ3) being the most favorable for dyke emplacement. The present study highlights the structural control of a pre-existing fracture network on emplacement of the Aiguablava lamprophyres in the upper crust during late Permian NNE–SSW brittle extension.

Creep strength of Mod.9Cr1Mo steel under biaxial stress states

This paper describes the development of tension, torsion, and combined tension-torsion creep testing machine with hydro static thrust bearing, criterion on creep rupture strength under biaxial stress states was examined. Tension, torsion, and combined tension-torsion creep tests were carried out at 873 K in air employing the developed creep testing machine and hollow cylindrical specimen of Mod.9Cr1Mo steel. Ratio of rupture time tR/tR* increased with the decreasing principal stress ratio λ at constant Mises' equivalent stress, where tR, and tR* denote rupture times for biaxial and tension creep tests, and λ expresses ratio of minimum principal stress to maximum principal stress. Hayhurst's stress successfully correlated with tension, torsion, and combined tension-torsion loading creep lives but Mises' type equivalent stress and maximum principal stress were inadequate to predict creep lives of Mod.9Cr1Mo steel. Oxide film and micro cracks on specimen surfaces were observed using SEM. Micro cracks and oxide film cracks initiated to the perpendicular direction to maximum principal stress axis for -1≦λ≦0. Main crack grew in direction perpendicular to specimen axis independent on the maximum stress ratio λ. The residual lives under biaxial stress states were estimated using Omega method. The Omega method was able to estimate the residual lives within a factor of 4 under biaxial stress states.

Spatial variation of crustal strain in the Kachchh region, India: Implication on the Bhuj earthquake of 2001

The Kachchh province of Western India is a major seismic domain in an intraplate set-up. This seismic zone is located in a rift basin, which was developed during the early Jurassic break-up of the Gondwanaland. The crustal strain determined from the GPS velocity data of post-seismic time period following the 2001 Bhuj earthquake indicates a maximum strain rate of ∼266×10−9 per year along N013°. Focal mechanism solutions of the main event of 26 January 2001 and the aftershocks show that the maximum principal stress axis is close to this high strain direction. Maximum shear strain rate determined from the GPS data of the area has similar orientation. The unusually high strain rate is comparable in magnitude to the continental rift systems. The partitioning of the regional NE–SW horizontal stress (SHmax) by the pre-existing EW-striking boundary fault developed the strike–slip components parallel to the regional faults, the normal components perpendicular to the faults, NE-striking conjugate Riedel shear fractures and tension fractures. The partitioned normal component of the stress is considered to be the major cause for compression across the regional EW faults and development of the second-order conjugate shear fractures striking NE–SW and NW–SE. The NE-striking transverse faults parallel to the anti-Riedel shear planes have become critical under these conditions. These anti-Riedel planes are interpreted to be critical for the seismicity of the Kachchh region. The high strain rate in this area of low to moderate surface heat flow is responsible for deeper position of the brittle–ductile transition and development of deep seated seismic events in this intraplate region.

Possible stress states adjacent to the rupture zone of the 1999 Chi-Chi, Taiwan, earthquake

We explore regional stress states in the vicinity of the rupture area of the 1999 Chi-Chi, Taiwan, earthquake by reconciling data from the Taiwan Chelungpu-Fault Drilling Project (TCDP) in-situ measurements and earthquake focal mechanisms. Given the background deviatoric stress in the range of 10–50 MPa and the horizontal NW–SE directed maximum principal stress axis, the predicted fault types show strike–slip and normal faulting near the coseismic surface rupture and thrust and strike–slip faulting in central Taiwan. Such predictions are able to fit TCDP in-situ observations in a local scale and aftershock earthquake focal mechanisms in a regional scale. Additionally, the proposed stress state explains remarkable rotations of the maximum stress axes observed near the northern segment of the Chelungpu Fault. This result provides key information for forecasting of consequent earthquakes and evaluation of focal mechanisms after the occurrence of a large earthquake.

Seismotectonics in the Tanzawa Mountains area in the Izu-Honshu collision zone of central Japan, as revealed by precisely determined hypocenters and focal mechanisms

We investigate the detailed distribution of hypocenters and focal mechanisms beneath the Tanzawa Mountains, central Japan, where the Izu-Bonin arc has collided into the central part of the Honshu arc. Remarkable differences are found to exist between the hypocenter distributions in the western and eastern parts. The hypocenters of earthquakes in the eastern part tend to be distributed in a horizontal zone, whereas those in the western part are distributed in a volume. The focal mechanisms in the eastern part are right-lateral reverse faulting mechanisms, and one of the nodal planes is consistent with the geometry of the Philippine Sea (PHS) plate in the region. These results suggest that most earthquakes in the eastern part occur along the upper surface of the subducting PHS plate. In contrast, the focal mechanisms in the western part, especially deep in the western part, exhibit a different feature. The stress states in these two regions are found to be significantly different. The maximum and minimum principal stress axes in the eastern part are slightly inclined, whereas those in the western part are oriented in approximately the vertical and horizontal directions, respectively. The stress field in the eastern part may be caused by a slab pull force induced from the deeper part of the subducted plate.

Present-day stress state of the Shanxi tectonic belt

The Shanxi tectonic belt is a historically earthquake­abundant area. For the majority of strong earthquakes in this area, the distribution of earthquake foci was controlled by the N-S oriented local structures on the tectonic belt. Studies of the present stress state of the Shanxi tectonic belt can contribute to the understanding of the relationship between strong earth­ quakes' occurrence and their structural distribution and also facilitate assessments of regional seismic danger and determina­ tion of the regions wherein strong earthquakes may occur in future. Using the Cataclastic Analysis Method (CAM), we per­ formed stress inversion based on the focal mechanism data of earthquakes which took place in the Shanxi tectonic belt from 1967 to 2010. Our results show that orientations of the maximum principal compressive stress axis of the Shanxi tectonic belt might have been variable before and after the 2001 Kunlun MS=8.1 strong earthquake, with two different superior trends of the NW-SE and NE-SW orientation in different periods. When the maximum principal compressive stress axis is oriented in the NE-SW direction, the pattern of the space distribution of the seismic events in the Shanxi tectonic belt shows a trend of their concentration in the N-S oriented tectonic segments. At the same time, the stress state is registered as horizontal shea­ ring and horizontal extension in the N-S and NE-SW oriented local segments in turn. When the maximum principal com­ pressive stress axis is NW-SE oriented, the stress state of the N-S and NE-SW oriented tectonic segments is primarily regis­ tered as horizontal shearing. Estimations of plunges of stress axes show that seismicity in the Shanxi belt corresponds prima­ rily to the activity of low­angle faults, and high­angle stress sites are located in the NE-SW oriented extensional tectonic seg­ ments of the Shanxi belt. This indicates that the stress change of the Shanxi belt is caused by adjustment of the regional stress field, rather than by the diverse seismic activities.

End Late Paleozoic tectonic stress field in the southern edge of Junggar Basin

This paper presents the end Late Paleozoic tectonic stress field in the southern edge of Junggar Basin by interpreting stress-response structures (dykes, folds, faults with slickenside and conjugate joints). The direction of the maximum principal stress axes is interpreted to be NW–SE (about 325°), and the accommodated motion among plates is assigned as the driving force of this tectonic stress field. The average value of the stress index R′ is about 2.09, which indicates a variation from strike-slip to compressive tectonic stress regime in the study area during the end Late Paleozoic period. The reconstruction of the tectonic field in the southern edge of Junggar Basin provides insights into the tectonic deformation processes around the southern Junggar Basin and contributes to the further understanding of basin evolution and tectonic settings during the culmination of the Paleozoic.

Crustal stress and strain patterns in the Indian plate interior: implications for the deformation behaviour of a stable continent and its seismicity

Terra Nova, 23, 407–415, 2011AbstractThe present analyses indicate that the maximum principal stress axis of the Indian plate interior does not coincide with the plate motion vector. The strain rate curves for the Indian plate indicate interference of two different orders of deformation. The magnitudes of the first‐order maximum strain rates are lower than local extreme strain rates of the second‐order deformation. It has been shown herein that the oblique motion of the Indian plate with respect to a state of dominant strike‐slip deformation in the Indian plate interior is partitioned into a principal deformation zone, synthetic shear and antithetic shear. The strain rates along these regional shear zones show complex association of second‐order high shortening rate coinciding with high extension rate. The earthquakes of the Indian plate interior are interpreted to be results of periodic accumulation of stress/strain due to shortening and release of strain along the same directions during the extension.

Anomalous depth dependency of the stress field in the 2007 Noto Hanto, Japan, earthquake: Potential involvement of a deep fluid reservoir

[1] We have elucidated depth variations in the stress field associated with the 2007 Noto Hanto, Japan, earthquake by stress tensor inversion using high-quality aftershock data obtained by a dense seismic network. Aftershocks that occurred above 4 km in depth indicated a strike-slip stress regime. By contrast, aftershocks in deeper parts indicated a thrust faulting stress regime. This depth variation in the stress regime correlates well with that in the slip direction derived from a finite source model using geodetic data. Furthermore, the maximum principal stress (σ1) axis was stably oriented approximately W20°N down to the depth of the mainshock hypocenter, largely in agreement with the regional stress field, but, below that depth, the σ1 axis had no definite orientation, indicating horizontally isotropic stress. One likely cause of these drastic changes in the stress regime with depth is the buoyant force of a fluid reservoir localized beneath the seismogenic zone.

The 8 June 2008 Mw 6.4 Earthquake in Northwest Peloponnese, Western Greece: A Case of Fault Reactivation in an Overpressured Lower Crust?

Abstract On 8 June 2008 a strong ( M w 6.4) earthquake occurred in the area of northwest Peloponnese, western Greece. The event originated in the lower crust and was caused by the rupture of a previously unknown fault probably inherited from past tectonic phases. In this study we perform a stress inversion of all available focal mechanisms in this area in order to obtain an estimate of the regional stress field. It is shown that the maximum principal stress axis has an azimuth of N273°E and forms an angle of 63° with the fault’s strike, which is well-constrained by seismological observations. This implies that the fault was severely misoriented with respect to the prevailing stress field assuming friction coefficients in the range 0.65–0.85. Calculation of pore-fluid factors for a variety of input parameters seems to confirm the presence of elevated fluid pressure near the hypocenter because they reach superhydrostatic to lithostatic values (0.80–1.0). The source of these fluids is probably of deep origin and may have to do with upper mantle degassing. This suggestion is supported by the presence of mantle helium in spring waters close to the epicenter and by low Pn velocities consistent with partially molten mantle beneath northwest Peloponnese.

Contemporary tectonic stress field in China

The contemporary tectonic stress field in China is obtained on the basis of Chinese stress field database and Harvard CMT catalogue. Result of the inverted tectonic stresses shows that the maximum principal stress axis strikes nearly north-south direction in the west part of Tibet plateau, ENE direction in North China. In Central China, its strikes show a radiated pattern, i.e., NNE in north part and NNW in south part. The detailed stress field parameters of nearly whole China are given and can be used in geodynamic stress field simulation and earthquake prediction.

Spatial variation of the stress field along the fault rupture zone of the 1999 Izmit earthquake

We have investigated the spatial variation of the stress field along the fault rupture zone of the 1999 Izmit earthquake (Mw 7.4) using first-motion polarity data at seven distinct aftershock clusters. In our approach, the first-motion polarities of all the aftershocks in a cluster are simultaneously inverted to determine the stress tensor parameters and fault plane solutions of individual events, in accordance with the method of Horiuchi et al. (1995). Where post-seismic slip was significant (e.g., Sapanca, Sakarya-Akyazı, and Karadere segments), we obtained stress tensors with the fault parallel or fault normal maximum (σ1) and minimum (σ3) principal compressive stress axes, which may imply either low frictional coefficients or fault weakness. A stress tensor with similar features was derived from the Çınarcık cluster, where the aftershocks lie in a low-velocity zone beneath the geothermal area. The maximum principal stress axis tends to remain parallel to the trend of the pre-mainshock σ1 around the Yalova segment; this segment experienced little to no co-seismic displacements. The stress tensor around the Gölcük segment, where the largest surface displacement of 5.5 m was observed, was determined to be 20–25° counterclockwise rotated, but the aftershock alignment remained fault parallel. We interpret these results in terms of the strong crust. On the other hand, both the aftershock alignment and the stress tensor were found to be rotated in the Izmit earthquake epicentral region despite the lower co-seismic displacements. We attribute this feature to the weak crust.

Local stress concentration in the seismic belt along the Japan Sea coast inferred from precise focal mechanisms: Implications for the stress accumulation process on intraplate earthquake faults

Spatial changes in the azimuth of the maximum principal stress (σ1) axis are found in the intraplate region of southwest Japan. It is inferred from stress inversions of numerous precise focal mechanisms that the azimuths of the σ1 axis in the seismic belt along the Japan Sea coast are oriented in N110°E–N130°E directions, while in the surrounding region they are aligned in almost the EW direction, N90°E–N100°E. Maximum horizontal compressional stresses in an EW direction are widely observed in the shallow crust in the inland plate along the Nankai Trough. However, the maximum horizontal stress in a WNW–ESE direction is seen only in the seismic belt. This spatial change in the stress field in and around the seismic belt is explained by the deformation of an aseismic fault or a ductile fault zone in the lower crust beneath the seismic belt.