Regional Stress Direction Research Articles

Cenozoic structure of the Zhu 3 Depression in the Pearl River Mouth Basin and its response to the dynamic background of the South China Sea

The Zhu 3 Depression is located at the transition between the South China Sea and the South China Block. The Cenozoic structure reflects the dynamic background of the South China Sea. We used 3D seismic and log data to study the structural evolution of the Zhu 3 Depression. We defined the characteristics of Cenozoic rift deformation and the migration of the depocentre based on fault activity and the distribution of sediment thickness. The spatial and temporal differences in the Cenozoic structure of the Zhu 3 Depression are due to the influence of pre-existing faults and regional stresses. Our results show that there are three populations of faults in the Zhu 3 Depression, striking NE, east–west and NW. The NE-striking faults are mainly large-scale boundary faults. The east–west-striking faults are small and were activated in the later stages of deformation. The NW-striking faults are continuous at depth, but are en echelon and parallel to each other at shallow depths. Based on these results, we suggest that the NE- and NW-striking pre-existing faults divide the Zhu 3 Depression into different structural zones and controlled the differences in deformation in the basin. The regional stress direction, which is controlled by regional plate interactions and the Red River Fault Zone, changed clockwise from NW-trending during the Paleocene to NE-trending during the Mid-Miocene. These findings will contribute to a better understanding of the evolution of the entire South China Sea. Thematic collection: This article is part of the Emerging knowledge on the tectonics of the South China Sea collection available at: https://www.lyellcollection.org/topic/collections/south-china-sea

Low-permeability reservoirs flooding using horizontal wells

For the first time in the practice of horizontal well flooding the study investigates the influence of horizontal well length, number of stages, and proppant loading on productivity during hydraulic development of low-permeability reservoirs using horizontal wells. To increase the sweep efficiency in systems with horizontal wells, a new development systems configuration is proposed, relying on longitudinal horizontal production wells and transverse horizontal injection wells with respect to the regional stress direction. The use of this configuration will not only increase the efficiency of reservoir pressure maintenance systems, but also reduce capital costs for well construction by reducing the injection wells to production wells ratio per development unit while maintaining system stiffness. Keywords: horizontal injection well; Auto-HF fracture; low-permeability reservoir; reservoir pressure maintenance system; oil recovery ratio increasing

Preliminary analysis of crustal anisotropy in the southernmost part of Simeulue Island

The study location is in the southernmost region of Simeulue Island near the Indo-Australian subduction margin. The deformation and geodynamic evolution around the island have been studied through some geophysical and geological studies. However, information regarding crustal seismic anisotropy around this region is less investigated, which may be very useful in delineating the deformation process in this area. In this work, we conducted an initial crustal anisotropy study using shear wave splitting analysis for two seismic stations from two datasets. One seismic station is from temporary deployment data recording data between December 2005 and March 2006, and the other one is from IA permanent network data. The objective of using these two different datasets is to assess the splitting analysis’s consistency and constrain the dominant cause of crustal anisotropy around the study area. The anisotropy orientations at all seismic stations are parallel to the regional stress direction. This situation may imply that anisotropy around the study area is mainly due to stress-induced anisotropy. Furthermore, the delay time distribution with depth suggests that the anisotropic source is concentrated around 15-20 km depth of the crust. This indicates a strong influence from the interaction between the lower crust and the upper part of the subducting slab.

The role of the Ebro Block on the deformation experienced within the Pyrenean realm: Insights from deformable plate tectonic models

Plate kinematic models of the Pyrenees have been extensively debated due to discrepancies between plate kinematic constraints for the Iberian plate and Atlantic/Tethyan related plate motions. Recently, the morphology of the Iberian plate and its partitioning into several continental blocks has been proposed as a solution towards reconciling discrepancies between previously published reconstructions that treat Iberia as a single, rigid, tectonic plate. Herein, the first deformable plate tectonic modeling study of the Pyrenean realm is presented using previously published and newly presented reconstructions of Iberia. Special emphasis is given to the kinematics of the Ebro Block, a continental block situated between the Pyrenees and Iberian Ranges, whose kinematics are considered to play a key role in the extensional deformation experienced within the Pyrenean realm. Temporal variations in strain rate and crustal thickness calculated by deformable plate models provide insights regarding the pre-orogenic template of the Pyrenees and the variability in regional stress directions along the Iberia-Eurasia plate boundary from the Triassic to Cenomanian. Models that propose transtensional rift phases within the Pyrenean realm induced by the Landes High and Ebro Block kinematics since the Triassic are successful in deriving crustal thicknesses indicative of a pre-orogenic hyperextended rifted margin within the Pyrenean realm. The results of this study demonstrate the importance of continental block kinematics during rift-related deformation and their impact on the evolution and partitioning of rift domains. Furthermore, this study also highlights potential avenues to consider for improving future plate kinematic models of Iberia, and regions elsewhere.

Flexural subsidence modelling of post-rift paleobathymetry and sedimentary infill in the northern South China Sea margin

The South China Sea (SCS) is the largest marginal sea on the SE Asian continental margin, located at the junction of the Pacific, Indo-Australia and Eurasian plates, and has undergone the joint effect of the Tethys and Pacific tectonic domains. Currently, the SCS remains debatable for its evolution and origin. However, its northern margin preserves the Cenozoic tectonic and subsidence history, which allow us to restore the past continental margin (0–33.7 Ma), to estimate subsidence, thickness and sediment accumulation correlated with regional tectonics. This paper has employed a 2D flexural backstripping technique and spatio-temporal variations by enhancing the seismic profiles with flexurally interpolated modelled sections. According to our findings, the transition was mainly controlled by three structural stages. (1) Early-Late Oligocene: records the highest subsidence due to the initiation of seafloor spreading, isostatic response of continental breakup, associated uplifts therein, and retreat of the Pacific Plate. (2) Early-Middle Miocene: mature spreading attributed to a slow phase of subsidence during the Early Miocene and a rapid increase of subsidence associated with the closure of the SCS in the Middle Miocene. (3) Thermal subsidence: sediments shifted since the Late Miocene towards mechanically weak and stretched regions (central-southern and eastern) which were triggered by a gravitational pull force. A fault system and differential stretched crust relocated sediment to the regional stress directions of the Pacific Plate.

Influence of pre-existing faults on Cenozoic structures in the Chengbei sag and the Wuhaozhuang area, Bohai Bay Basin, East China

Pre-existing fault reactivation, interaction, and linkage with new fault segments are critical for understanding the evolution of fault network development in petroliferous basins. This study analyses the influence of different types of pre-existing faults on the tectonic evolution characteristics of two adjacent areas in the Bohai Bay Basin. We use high-resolution 2D and 3D seismic and well data to discuss the influence of different types of pre-existing faults on the evolution of fault networks and basins. The pre-existing faults began to move in the Mesozoic times. In the Cenozoic, the Chengbei fault reactivated, but the Wuhaozhuang fault did not. The Chengbei fault developed “zigzag” plane-view geometries and controlled the sedimentary characteristics of the Chengbei sag. In this area, the near-EW-striking faults developed perpendicular to the Cenozoic regional stress direction. The Wuhaozhuang fault did not reactivate and was cut by NNE-striking faults, which controlled the Cenozoic sedimentation. Based on the above mentioned observations, we discuss the factors that influence the reactivation of pre-existing faults, including their geometric characteristics (fault strike and dip), the influence of regional stress, and the heterogeneity of basement rocks. These observations demonstrate the influence of pre-existing faults and regional stress on the later normal faults development and basin evolution.

Lineament mapping for a part of the Central Sulaiman Fold\u2013Thrust Belt (SFTB), Pakistan

This study aims to analyze the lineaments using the field data and a remote sensing approach, to describe their relationship with the folds, faults, and regional tectonic stress of the central Sulaiman Fold-Thrust Belt. Joint data, from nine anticlines, has been collected using the scanline method and classified into three sets (JS1, JS2, and JS3) based on their geometrical relationships. Lineaments extracted from the 30 m digital elevation model have been classified subsequently into three lineament sets (LS1, LS2, and LS3) based on the azimuths from the corresponding joint sets. A very high correlation coefficient (rs = 0.97), between the azimuths of the field joints and the remotely sensed lineaments, has been observed which validates that the lineaments are the regional representation of the local field joints. The geometrical relationship of the lineament sets with the fold hinges indicates that the older LS1 and LS2 are strongly related to the regional folding episode, while the younger LS3 is a result of local shears. The chronological interpretation of the deformational events responsible for the lineament sets is constrained by the presence of the Kingri Fault, which induces a strike-slip component within the study area. Furthermore, the controls on the joint and lineament sets, established using multivariate statistics to decipher the effects of lithological and structural contrasts on the lineament density, reveal that an increase in the lineament density can be attributed to the competence and thickness of the rock units as well as the variable local stresses across the different folds. Based on the orientations of these lineament sets, the cumulative direction of the compressive event in the NW-SE direction (310–320) coincides with the regional stress direction of the SFTB.

Investigating the Plate Kinematics of the Bay of Biscay Using Deformable Plate Tectonic Models

AbstractThe plate kinematics of the Iberian plate and their implications on the plate tectonic evolution of the Bay of Biscay during rifting and subsequent opening of the southern North Atlantic continue to be topics of scientific discussion and debate. Constrained by previous plate reconstruction, geophysical, and geological studies, deformable plate tectonic reconstructions of the Bay of Biscay‐Parentis rift system are created in this study using the GPlates software. These deformable plate models are used to investigate the kinematics of the Landes High, Le Danois High, and Ebro Block previously recognized as independent continental blocks within deformable regions. A comparison between results calculated via deformable plate models with previously published and newly presented gravity inversion crustal thickness estimates provided a good metric for delineating the most probable plate kinematic scenario for the Bay of Biscay‐Parentis system and investigating the effect of the plate kinematic scenarios on the present‐day structure. The preferred plate kinematic model implies a transtensional Bay of Biscay‐Parentis rift system from the Jurassic to Early Cretaceous, with the onset of significant crustal thinning initiating during the Late Jurassic (∼150 Ma) induced by the motion of the Landes High and its interplay with the Ebro Block. Aside from demonstrating the role of the Landes High and its suggested interplay with structural inheritance on the deformation experienced within the Bay of Biscay, the timing and orientation of regional stress directions, and the velocities induced by the preferred deformable model provide good comparisons with previously calculated subsidence rates.

Transfer of stress from the 2004 Mw9.2 Sumatra subduction earthquake promoted widespread seismicity and large strike‐slip events in the Wharton Basin

AbstractThe 2004 Mw9.2 Sumatra and 2012 Mw8.6 Wharton Basin (WB) earthquakes provide the unprecedented opportunity to investigate stress transfer from a megathrust earthquake to the subducting plate. Comprehensive analyses of this study revealed that the 2004 earthquake excited widespread seismicity in the WB, especially in regions of calculated stress increase greater than 0.3 bars. The 2004 earthquake stressed all three rupture planes of the 2012 Mw8.6 strike‐slip mainshock and the largest Mw8.2 aftershock with mean values of Coulomb stress between 0.3 and 2.1 bars. For the 77 Mw ≥ 4 regional events since 2012, at least one nodal plane for 95% of the events, and both nodal planes for 72% of the events experienced stress increase due to the 2004 earthquake. Results of the analyses also revealed that the regional stress directions in the WB may have controlled the sub‐fault orientations of the 2012 Mw8.6 strike‐slip earthquake.

High-Resolution Imaging of Hydraulic-Fracturing-Induced Earthquake Clusters in the Dawson-Septimus Area, Northeast British Columbia, Canada

Abstract The number of earthquakes in the western Canada sedimentary basin (WCSB) has increased drastically in the last decade related to unconventional energy production. The majority of reported earthquakes are correlated spatially and temporally with hydraulic fracturing (HF) well stimulation. In this study, we use waveform data from a new deployment of 15 broadband seismic stations in a spatial area of roughly 60×70km2, covering parts of the Montney Formation, to study the relationship between earthquakes and HF operations in the Dawson-Septimus area, British Columbia, Canada, where the two largest HF-related earthquakes in WCSB to date, an Mw 4.6 on 17 August 2015 and an ML 4.5 on 30 November 2018, have occurred. We use an automated short-term average/long-term average algorithm and the SeisComP3-software to detect and locate 5757 local earthquakes between 1 July 2017 and 30 April 2019. Using two clustering techniques and double-difference relocations of the initial catalog, we define event families that are spatially associated with specific wells, and exhibit temporal migration along a horizontal well bore and/or multiple fractures close to wells. Relocated clusters align in two dominant orientations: one roughly perpendicular to the maximum horizontal regional stress direction (SH) and several conjugate structures at low angles to SH. Comparing the two predominant seismicity lineations to regional earthquake focal mechanisms suggests that deformation occurs via thrust faulting with fault strike oriented perpendicular to SH and via strike-slip faulting with strike azimuth at low angles to SH. Local scale seismicity patterns exhibit clustering around individual HF wells, whereas regional scale patterns form lineations consistent with deformation on faults optimally oriented in the regional stress field.

Predicting Structural Permeability in the Deep Coal Play, Tirrawarra-Gooranie fields, Cooper Basin

A 2D numerical stress modelling study was conducted for the Tirrawarra-Gooranie oil and gas field in the Cooper Basin, which produces from a mixture of conventional and non-conventional (deep coal) reservoirs. The aim of this study is to understand the current state of stress, both magnitudes and orientation, within the zones of interest. A high confidence 3D structural framework was used as a key input to the model, which sought to demonstrate how the in-situ stress is distributed or perturbed throughout the field. The understanding of the current stress state at borehole scale can assist with the prediction of natural fractures or structural permeability, fracture stimulation design and placement, and production performance. Extensive sensitivity studies were conducted, with the most important inputs found to be boundary stress magnitudes and stress orientation with respect to the faults. The models were interrogated at both the primary coal target and the underlying sandstone horizons. They were found to have significantly different stress states. The output models were compared against production data, 1D mechanical earth models, fracture stimulation data, image logs and core data. The model predictions are in reasonable agreement with the mechanical earth models and the stress orientations determined from the image logs. Predicted stress rotations greater than 4° from the regional stress direction appear to be associated with poorer frac placement, higher bottom hole treating pressures and poor gas rate outcomes. There is a clear association of mineralised natural fractures with areas of low differential stress. The models provide an additional element of subsurface information which will inform future appraisal activities and may improve the understanding of variability in the deep coal fracture stimulation outcomes.

Tectonic Study of Lineaments for Part of Zagros Belt, Northeast Iraq

The study area situated in the western part of Zagros fold-thrust belt, north east (NE) Iraq. Google earth image for study area was used to build up lineaments map. Studying and processing of these lineaments revealed that there are two dominant directions of lineaments northeast-southwest (NE-SW) and northwest- southeast (NW-SE), compatible with the main regional stress direction and Zagros belt direction respectively. It was found that there is no significant change of lineaments density in different tectonic zones and there is no obvious effect of lithology on lineament density in the study area. Length rose diagrams showed an obvious increase in the value of NW-SE lineaments direction, starting from low folded zone toward NE, despite frequency rose diagrams of different tectonic zones showed no significant change of the main prevailing direction. It is found that some lineaments shifted or bended some folds or ridges in the study area. Field observations demonstrated that some lineaments of study area had field criteria of faults while they did not described in tectonic map.

High‐resolution relocation of aftershocks of the Mw 7.1 Darfield, New Zealand, earthquake and implications for fault activity

AbstractLow‐slip‐rate regions often represent under‐recognized hazards, and understanding the progression of seismicity when faults in such areas rupture will help us to better understand earthquake rupture patterns. The 3 September 2010 (UTC) Mw 7.1 Darfield earthquake revealed a formerly unrecognized set of faults in the Canterbury region of New Zealand, an area that had previously been mapped as one of the lower‐hazard areas in the country. In this study, we analyze the first four months of its aftershock sequence to identify active faults and temporal changes in seismicity along them. We jointly invert for three‐dimensional P wave and S wave velocities and hypocentral locations, using data for 2840 aftershocks recorded at 36 temporary and permanent seismic stations within 70 km of the main shock epicenter. These relocations delineate eight individual faults active prior to the 22 February 2011 Mw 6.3 Christchurch earthquake, the largest aftershock of the Darfield earthquake. Two of these faults are in the Christchurch region, one of which corresponds to geodetically determined rupture planes of the Christchurch earthquake. Using focal mechanisms calculated from first‐motion polarities, we find mainly strike‐slip faulting events, with some reverse and normal faulting events as well. We compare the orientations of these faults to the prevailing regional stress directions to identify which faults may have been active prior to the Darfield earthquake and which may be newly developed.

Fault Delineation and Regional Stress Direction from the Analysis of Background Microseismicity in the southern Apennines, Italy

Abstract The present active faults and stress field acting in the southern Apennines (Italy), a structurally complex area characterized by high seismic potential, are studied by analyzing the background microseismicity ( M ≤3). We used a microearthquake data set consisting of 1312 events that occurred from August 2005 to April 2011 by integrating the data recorded at 42 seismic stations of various networks. The refined seismicity locations and focal mechanisms delineate a system of northwest–southeast striking normal faults along the Apenninic chain and an approximately east–west oriented strike‐slip fault transversely cutting the belt. The seismicity along the chain does not occur on a single fault but in a volume, delimited by the faults activated during the 1980 Irpinia M 6.9 earthquake on subparallel predominantly normal faults. Results show that the recent low magnitude earthquakes belong to the background seismicity, and they are likely generated along the major fault segments activated during the most recent earthquakes, suggesting that they are still active today, 30 years after the mainshock occurrences. The stress inversion from the whole data set suggests that a unique anti‐Apenninic extensional regional stress field could explain the two different faulting styles characterizing the earthquakes that occur along the chain and the east–west fault dissecting the belt. On the other hand, the results obtained by separately inverting the Irpinia and the Potenza clusters indicate a more complex model that would predict a change from a normal‐faulting regime, acting in the inner sector of the chain, to a strike‐slip regime moving eastward and down‐depth in the Potenza area. Online Material: Seismicity maps and focal mechanisms.

Mechanics, slip behavior, and seismic potential of corrugated dip‐slip faults

To better understand the mechanics and seismic potential of nonplanar fault surfaces, we present results from a suite of numerical models of faults with sinusoidal corrugations in the downdip direction. Systematic variations in corrugation wavelength, amplitude, and loading angle are introduced to determine the effects on slip behavior and seismic moment release. We find that corrugated faults, in general, slip less than planar faults. Changes in slip behavior are nearly scale‐independent and are dominantly controlled by the amplitude/wavelength of corrugations. Model results suggest that obliquely loaded corrugated faults accumulate less strike slip than a planar fault with the same tip line dimensions and average orientation. This result implies that slip direction is not a reliable indicator of regional stress direction and may at least partially explain repeated, nearly pure dip‐slip coseismic events at oblique plate boundaries. Though the scalar seismic moment release is always less for corrugated fault surfaces due to a greater reduction in slip compared to increased surface area, for geologically reasonable corrugation geometries, changes in total scalar moment release are not significantly different than planar faults. Techniques that utilize highly simplified fault geometries may therefore accurately reproduce scalar moment release but will nonetheless incorrectly predict coseismic slip magnitudes and distributions, as well as regional stress orientations.

Anisotropic PP and PSv prestack depth migration of 4C seismic data; Pamberi, Trinidad

SummaryIn November 2004, EOG Resources contracted WesternGeco to acquire an ocean-bottom cable (OBC) 4C swath survey across the Pamberi-1 well in the Lower Reverse L block of the Columbus basin, eastern offshore Trinidad. The purpose of the 4C survey was to evaluate the potential of long-offset multicomponent technology for resolving lithology and stratigraphic detail in an area perturbed by shallow gas, overpressure and illumination shadows from normal regional faults and major anticlinal ridge trends acting as pressure seals. A key motivation for EOG was the realization that a conventional 3D towed-streamer survey acquired the previous year failed to adequately image the target reflectors comprising the reservoir under the main fault.Details of the P- and PSv-wave processing of this dataset through anisotropic prestack time migration were previously described (Johns et al., 2006) in which it was demonstrated that there existed a qualitative correlation between derived parameters and attributes from P and Sv anisotropic migration velocities and known regional geology. This observation was quite remarkable considering only a limited effort to constrain or validate parameters (in this case, velocities to the Pamberi-1 well checkshots) was performed. Under the “Future work” section of the previous publication, it was suggested that further data quality enhancement in preparation for more quantitative rock property classification could only be achieved after prestack depth imaging.In this paper, we present precisely that next phase in the 4C processing, advancing the P- and PSv-wave data through anisotropic prestack depth migration, using cellbased tomography with a top-down approach. The Pamberi-1 well was used to constrain the anisotropy in the shallow section, with the deeper, spatial trend away from the proximity of the well determined from the anisotropy derived previously in the time processing.Prior to proceeding with the anisotropic depth imaging, the magnitude of shear splitting from the presence of azimuthal anisotropy (HTI) was first examined to assess its potential impact on the radial rotated P-S signal. The shear wave splitting analysis revealed a principal angle of polarization that was closely aligned with the regional stress direction delineated by the normal major faults blocks acting as pressure seals.

Interpretation of the Gravity and Magnetic Anomalies of the Cappadocia Region, Central Turkey

The Cappadocia region, located in Central Turkey, is characterized by widespread lava flows and volcanoclastic deposits dating from Miocene to Quaternary. Gravity and aeromagnetic anomalies of the region appear to present similar high and low amplitude regions, although the aeromagnetic anomalies exhibit a rather complex pattern which is thought to be caused by remanent magnetization. The low-pass filtered aeromagnetic map shows a deep-seated magnetic anomaly which may be linked to the widespread volcanic activity at the surface. The pseudogravity transformation of the upward continued anomaly has been constructed. The pseudogravity anomaly demonstrates some form of clockwise rotation. This anomaly was modelled by means of a three-dimensional method. The top and bottom of the body are at 6.3km and 11km (including the flight height) from the ground surface, respectively. This deep body is ellipsoidal and extends along an E-W direction, which is in line with the regional stress direction deduced from GPS measurements. A new mobilistic dynamo-tectonic system appears to explain the body’s E-W elongation. The modelled body may be the source for the inferred geothermal energy of the region. Magnetic measurements were carried out on oriented rock samples collected from outcrops of ignimbrites and basalts, providing directions and intensities of remanent magnetization, susceptibilities and Koeningsberger (Q) ratios. Standard deviations of remanent directions of the Natural Remanent Magnetization (NRM) display a wide scatter implying unreliability of the surface data. Reduction to pole (RTP) transformation of magnetic anomalies was successful with the induced magnetization angle despite the complex pattern of magnetic anomalies.

Rupture progression along discontinuous oblique fault sets: implications for the Karadere rupture segment of the 1999 Izmit earthquake, and future rupture in the Sea of Marmara

Large earthquakes in strike-slip regimes commonly rupture fault segments that are oblique to each other in both strike and dip. This was the case during the 1999 Izmit earthquake, which mainly ruptured E–W-striking right-lateral faults but also ruptured the N60°E-striking Karadere fault at the eastern end of the main rupture. It will also likely be so for any future large fault rupture in the adjacent Sea of Marmara. Our aim here is to characterize the effects of regional stress direction, stress triggering due to rupture, and mechanical slip interaction on the composite rupture process. We examine the failure tendency and slip mechanism on secondary faults that are oblique in strike and dip to a vertical strike-slip fault or “master” fault. For a regional stress field well-oriented for slip on a vertical right-lateral strike-slip fault, we determine that oblique normal faulting is most favored on dipping faults with two different strikes, both of which are oriented clockwise from the strike-slip fault. The orientation closer in strike to the master fault is predicted to slip with right-lateral oblique normal slip, the other one with left-lateral oblique normal slip. The most favored secondary fault orientations depend on the effective coefficient of friction on the faults and the ratio of the vertical stress to the maximum horizontal stress. If the regional stress instead causes left-lateral slip on the vertical master fault, the most favored secondary faults would be oriented counterclockwise from the master fault. For secondary faults striking ±30° oblique to the master fault, right-lateral slip on the master fault brings both these secondary fault orientations closer to the Coulomb condition for shear failure with oblique right-lateral slip. For a secondary fault striking 30° counterclockwise, the predicted stress change and the component of reverse slip both increase for shallower-angle dips of the secondary fault. For a secondary fault striking 30° clockwise, the predicted stress change decreases but the predicted component of normal slip increases for shallower-angle dips of the secondary fault. When both the vertical master fault and the dipping secondary fault are allowed to slip, mechanical interaction produces sharp gradients or discontinuities in slip across their intersection lines. This can effectively constrain rupture to limited portions of larger faults, depending on the locations of fault intersections. Across the fault intersection line, predicted rakes can vary by >40° and the sense of lateral slip can reverse. Application of these results provides a potential explanation for why only a limited portion of the Karadere fault ruptured during the Izmit earthquake. Our results also suggest that the geometries of fault intersection within the Sea of Marmara favor composite rupture of multiple oblique fault segments.

How well constrained are well‐constrained T, B, and P axes in moment tensor catalogs?

The T, B and P axes of earthquake moment tensors (MT) are often used to evaluate regional stress directions and other tectonic parameters; we here undertake three comparisons to assess the uncertainty in the orientations of these axes. These are (1) a direct comparison of common MT in the Harvard, U.S. Geological Survey, or Earthquake Research Institute (ERI) catalogs; (2) a comparison of MT slip vectors and plate motion vectors in several tectonically straightforward regions; and (3) an analysis of the axial variability in the Harvard and ERI catalogs implied by the reported uncertainties in individual MT components. All three comparisons indicate that there is considerable variability within the catalog concerning the axial orientation of MT, but all suggest that axis orientations of the majority of Harvard MT have uncertainties of 15° or less. For compensated linear vector dipole (CLVD) components among the three catalogs, the correlation is very low. For the Harvard catalog, three statistics are especially useful for selecting better constrained MT; these are (1) the relative error Erel, which is the ratio of the scalar moments of the reported error tensor and of the MT itself; (2)ƒCLVD, a measure of the strength of the CLVD component; and (3) nfree, the number of MT elements not fixed at zero in the inversion. For selecting better constrained MT, the appropriate statistical cutoffs chosen depend on the problem of interest, the data available, arid personal preference. However, for analysis of shallow earthquakes we have used Erel ≤ 0.15, ƒCLVD ≤ 0.20, and nfree = 6. While this eliminates 53% of the catalog, our calculations suggest that nearly all the remaining events have T, B, and P axes with azimuth and inclination angle uncertainties of 5°–10° or less.

Regional stress alignments in the Kutai Basin, East Kalimantan, Indonesia: a contribution from a borehole breakout study

Borehole breakout data from 134 wells located within the Kutai Basin region, East Kalimantan were analyzed to determine the present day regional horizontal stress alignments. The data were extracted from various types of dipmeter logs. The study reveals that the majority of the data give a coherent picture of breakout orientation. The mean azimuths for the entire-unweighted, ellipticity-weighted and magnitude-weighted data sets are preferentially aligned in the regional-mean direction of 48.9° N or 128.9° N. Most of the data have a low dispersion value ( So) and the ranking of reliability in Zoback’s classification is ‘A’. There is no significant azimuth variation with depth. These relatively consistent alignments of breakout azimuths indicate that the maximum regional stress direction in the study area is NW–SE. This regional-mean of breakout azimuths deviates from the axis of the anticlinorium trends and from the strike of the thrust–fault patterns in the region. It is believed that these structural patterns are influenced by reactivation of weak zones related to sediment loading (structural inversion).