Spatiotemporal Variations of Focal Mechanism Solutions and Stress Field of the 2019 Changning Ms 6.0 Earthquake Sequence

The June 17, 2019, MS 6.0 Changning earthquake is the largest recorded event in the Sichuan basin, spatiotemporal variations of stress field may shed light on the seismogenic mechanism of the earthquake. We determined the focal mechanism solutions (FMSs) of 124 earthquakes with MS ≥ 3.0 occurring in the Changning area from April 1, 2007, to February 29, 2020, and analyzed changes of FMSs and stress field before and after Changning earthquake. The Changning aftershocks were predominantly thrust fault earthquakes, followed by strike slip. The P-axis azimuths of the aftershock FMSs were oriented predominantly in the NEE direction, notably differing from the NWW-oriented P-axis azimuths of pre-earthquake FMSs; it shows the rotation of local stress field before and after the Changning earthquake, it is speculated that the change of stress field in Changning area may be caused by long-term water injection and salt mining activities. From the southeast to the northwest of the aftershock zone, the azimuths of principal compressive stress (S1) change from NEE to near-EW in both horizontal and vertical planes. Significant changes occurred in the FMS types and stress field of the aftershock zone following the Changning earthquake, the FMSs became diverse, the S1 azimuth of the Changning area changed from NWW to NEE, and then EW, the plunge and stress tensor variances increased, it reflects that the stress field of the Changning area adjusts continually with time.

Characterizing Stress Orientations in Southern Kansas

ABSTRACT Induced seismicity predominantly occurs along faults that are optimally oriented to the local principal compressive stress direction, and the characterization of these stress orientations is an important component of understanding seismic hazards. The seismicity rate in southern Kansas rapidly increased in 2013 primarily due to the disposal of large volumes of wastewater into the Arbuckle Group. Previously, local stress orientations in this area were poorly constrained, which limited our understanding of the complex faulting and diverse earthquake mechanisms in this region. We use shear-wave splitting and focal mechanism inversion techniques to create multiple, independent estimates of maximum horizontal stress directions (SHmax) and their spatial variation across the study area. We then create an integrated model of stress orientations for southern Kansas and northern Oklahoma using our local results in conjunction with previous, regional stress orientation estimates. We find that SHmax in both southern Kansas and central Oklahoma exhibits an east-northeast (∼N78° E) orientation, and these regions bound a northeast (∼N59° E) rotation within a ∼20 km area in northern Oklahoma near the Nemaha ridge.

Crustal folds alter local stress fields as demonstrated by magma sheet – fold interactions in the Central Andes

<p>For magma chambers to form or volcanic eruptions to occur magma must propagate through the crust as dikes, inclined sheets and sills. The vast majority of models that investigate magma paths assume the crust to be either homogeneous or horizontally layered, often composed of rocks of contrasting mechanical properties. In subduction regions that have experienced orogenesis, like the Andes, the crust has been deformed over several million years, resulting in rock layers that are commonly folded and steeply dipping. The assumption of homogeneous properties or horizontal layering then does not capture all of the potential magma path crustal interactions. Here we tackle this problem by determining the effect of a crust made of steeply inclined layers in which sills and inclined sheets are emplaced. We combine field observations from a sill emplaced in the core of an anticlinal fold at El Juncal in the Chilean Central Andes, such as lithologies, sill and fold limbs attitude, sill length and layers and sill thickness, with a suite of finite element method models to explore the mechanical interactions between inclined layers and magma paths. Our results demonstrate that the properties of the host rock layers as well as the contacts between the layers and the crustal geometry all play an important role on magma propagation and emplacement at shallow levels. Sill propagation and emplacement through heterogeneous and anisotropic crustal segments changes the crustal stress field promoting sill arrest, deflection or propagation. Specifically, sills are more likely to be deflected when encountering shallow dipping layers rather than steeply dipping layers of a fold. Mechanically weak contacts encourage sill deflection due to the related rotation of the maximum principal compressive stress and this effect is attenuated when the fold layers are more steeply dipping. This processes may change the amount and style of surface deformation recorded, with significant implications for monitoring of active volcanoes.</p>

Analysis on Bouguer Gravity Anomaly Characteristics and Boundary Identification in China and Surrounding Regions

China is located in the southeast of the Eurasian Plate and is subject to the effects of subducting, squeezing and collision by the Pacific Plate to the east, Philippine Plate to the southeast and Indian Ocean Plate to the southwest. It has exceptional geotectonic structure. Based on the satellite gravity data with high precision, high resolution and ample geophysical information, combined with geological data, by using satellite gravity potential field and its full tensor gradient, this paper studies the distribution characteristics of gravity anomalies and the identification of tectonic boundaries in China and surrounding regions. Results suggest that the Bouguer gravity anomaly in eastern China reduces gradually from east to west, mostly in the direction of NNE; in the western, it reduces gradually in a wave mode from north to south, mainly in the directions of NW and NWW. In general, the stress field reduces gradually from west to east, and the tectonic of stress field in western China is complex. The maximum principal compressive stress in Xinjiang exists in SN direction and that in Qinghai-Tibet Plateau mostly changes gradually from NNE to SSE; the change in eastern China is relatively simple, and the maximum principal compressive stress direction gradually changes from NE to WE and then to SE. In addition to the above study results, by comprehensively referencing the previous studies by other people and by using the boundary identification methods based on the satellite gravity full-tensor gradient data and its combinations, we update the extension route of Red River fault zone and deduce the tectonic unit boundary between the North China and South China active tectonic block regions. This paper identifies in China and surrounding regions 6 primary active tectonic blocks, 22 secondary active tectonic blocks, 3 tertiary active tectonic blocks and the 20 active tectonic block boundary zones constituted of deformation belts and active tectonic belts with various geometric structures and width variations. The results of this study can improve the understanding of gravity anomalies and boundary structures in China and surrounding regions, and provide certain geophysical supports for geological structure analysis and crustal dynamic process.

Activation of optimally and unfavourably oriented faults in a uniform local stress field during the 2011 Prague, Oklahoma, sequence

SUMMARY The orientations of faults activated relative to the local principal stress directions can provide insights into the role of pore pressure changes in induced earthquake sequences. Here, we examine the 2011 M 5.7 Prague earthquake sequence that was induced by nearby wastewater disposal. We estimate the local principal compressive stress direction near the rupture as inferred from shear wave splitting measurements at spatial resolutions as small as 750 m. We find that the dominant azimuth observed is parallel to previous estimates of the regional compressive stress with some secondary azimuths oriented subparallel to the strike of the major fault structures. From an extended catalogue, we map ten distinct fault segments activated during the sequence that exhibit a wide array of orientations. We assess whether the five near-vertical fault planes are optimally oriented to fail in the determined stress field. We find that only two of the fault planes, including the M 5.7 main shock fault, are optimally oriented. Both the M 4.8 foreshock and M 4.8 aftershock occur on fault planes that deviate 20–29° from the optimal orientation for slip. Our results confirm that induced event sequences can occur on faults not optimally oriented for failure in the local stress field. The results suggest elevated pore fluid pressures likely induced failure along several of the faults activated in the 2011 Prague sequence.

Investigation on shear strain of reinforced concrete membrane panels subjected to pure shear

Hsu recently conducted a shear test on nine reinforced concrete panel elements subjected to applying pure shear using a shear testing device. Modern truss models (i.e. modified compression field theory and a rotating angle softened truss model) are used to perform a complex nonlinear analysis through a trial and error method based on a double loop. This analysis is conducted by employing equilibrium conditions, compatibility conditions, and a ductile stress–strain relationship of a reinforced concrete membrane panel in a biaxial state. In this study, an effective algorithm that uses a revised Mohr compatibility method based on the failure criteria of struts and ties is proposed. This algorithm is used to improve the convergence rate in the analysis of shear history, which was performed in the experiment of Hsu. The result of the analysis indicates that the shear strain energy in a state of extended shear strain is influenced by the relationship between principal compressive stress and strain (crushing failure).

Stress Analysis of Dongjiang Arch dam by Finite Element Method under Different Concrete Elastic Modulus and High-Low Temperature Seasons

Elastic modulus and temperature are the main influences of dam stress. In order to study the Dongjiang dam stress for crack controlling, the critical parts stress of dam under different elastic modulus and high-low temperature seasons is analyzed by the finite element method. The results show that: (1) the arch crown and abutment principal tensile stress at upstream face and principal compressive stress at downstream face under high temperature season are greater than the calculation results under low temperature season for the whole at the normal storage water level; (2) the influence on stress caused by elastic modulus are more significant under high temperature season compared to low temperature season at arch crown and abutment, the maximum stress increase with the increasing of elastic modulus. (3) Dongjiang arch dam should pay more attention to the safety control under high temperature season and surface crack prevention and control work in low temperature season.

Analysis on Stress State of Box-Girder Web under Prestressing

In order to study the stress of box-girder web under prestressing, and confirming the internal stress distribution of the web, analyzing of vertical prestressed box girder, curved beam prestressed sensitivity under the webs. Establishing finite element model of the box-girder webs vertical prestressing effect is analyzed, results show that the principal tensile stress of the web is sensitive to the vertical prestress, applying the vertical prestress can effectively reduce the principal tensile stress of the web; with the decrease of the effective vertical prestress, the neutral axis above the principal compressive stress decreases rapidly, while below the neutral axis decreases relatively slow; Under the same vertical preloading stress level, the roots of cross section of the compressive stress of web reserves than L/4 section of the web.

Inversion of Initial Geo-Stress in High and Steep Slope

At present, due to various reasons, cannot do a large number of field measurements, and the measured results often can only reflect the local stress field distribution. What’s more, the measured results has discreteness, it’s difficult to describe the law of the initial stress field distribution in the entire region. This paper combines topographic and geological conditions and the measured in-situ stress value of Jinping I hydropower station dam area, using FLAC3D4.0, select geological section profile of II1 exploration line for reference, to simulate the incised process of the valley. We get the stress field distribution when rock gravity acting alone, and exert horizontal tectonic stress based on the calculate result of gravity field in the VI terrace model. We inversion the distribution of current Valley stress field, and validate the rationality of the design after compare with the measured data. We obtain the features of stress field of current valley. And we also proved that the regional principal compressive stress plays a decisive role in the formation of the current valley stress field.

The Finite Element Analysis on the Local Compression of the Concrete Filled Steel Tubular Column-Beam Joint

In order to study the mechanical behavior of the joint between concrete filled steel tubular column and beam with discontinuous column tube at the joint zone under axial pressure, the finite element analysis software ANSYS is adopted for parametric analysis and the analysis results are compared with experimental ones. The principal compressive stress is mainly transmitted by the inside area of the joint which is subjected to local compression if it is low, but extends to more outside areas of the joint if it is high. The radial compressive stress, which is the confined stress of the ring beam to the core concrete of the joint, keeps the same as that the width of the ring beam equal to the diameter of the core area of the joint. The vertical strain on the edge of the joint, which would lead to horizontal annular cracks in the side face of the ring beam, changes from tension in the whole height to tension only in the top part and compression in the lower part of the joint, which is consistent with the experimental phenomenon.