Fault Geometry and Mechanism of the Mw 5.7 Nakchu Earthquake in Tibet Inferred from InSAR Observations and Stress Measurements

Different types of focal mechanism solutions for the 19 March 2021 Mw 5.7 Nakchu earthquake, Tibet, limit our understanding of this earthquake’s seismogenic mechanism and geodynamic process. In this study, the coseismic deformation field was determined and the geometric parameters of the seismogenic fault were inverted via Interferometric Synthetic Aperture Radar (InSAR) processing of Sentinel-1 data. The inversion results show that the focal mechanism solutions of the Nakchu earthquake are 237°/69°/−70° (strike/dip/rake), indicating that the seismogenic fault is a NEE-trending, NW-dipping fault dominated by the normal faulting with minor sinistral strike-slip components. The regional tectonic stress field derived from the in-situ stress measurements shows that the orientation of maximum principal compressive stress around the epicenter of the Nakchu earthquake is NNE, subparallel to the fault strike, which controlled the dominant normal faulting. The occurrence of seven M ≥ 7.0 historical earthquakes since the M 7.0 Shenza earthquake in 1934 caused a stress increase of 1.16 × 105 Pa at the hypocenter, which significantly advanced the occurrence of the Nakchu earthquake. Based on a comprehensive analysis of stress fields and focal mechanisms of the Nakchu earthquake, we propose that the dominated normal faulting occurs to accommodate the NE-trending compression of the Indian Plate to the Eurasian Plate and the strong historical earthquakes hastened the process. These results provide a theoretical basis for understanding the geometry and mechanics of the seismogenic fault that produced the Nakchu earthquake.

Stratigraphically Controlled Stress Variations at the Hydraulic Fracture Test Site-1 in the Midland Basin, TX

We investigated the relationship between stratigraphy, stress, and microseismicity at the Hydraulic Fracture Test Site-1. The site comprises two sets of horizontal wells in the Wolfcamp shale and a deviated well drilled after hydraulic fracturing. Regional stresses indicate normal/strike-slip faulting with E-W compression. Stress measurements in vertical and horizontal wells show that the minimum principal stress varies with depth. Strata with high clay and organic content show high values of the least compressive stress, consistent with the theory of viscous stress relaxation. By integrating data from core, logs, and the hydraulic fracturing stages, we constructed a stress profile for the Wolfcamp sequence, which predicts how much pressure is required for hydraulic fracture growth. We applied the results to fracture orientation data from image logs to determine the population of pre-existing faults that are expected to slip during stimulation. We also determined microseismic focal plane mechanisms and found slip on steeply dipping planes striking NW, consistent with the orientations of potentially active faults predicted by the stress model. This case study represents a general approach for integrating stress measurements and rock properties to predict hydraulic fracture growth and the characteristics of injection-induced microseismicity.

Movement of gas slug in annular channels with different diameter ratios

The motion of gas slugs in annular channels was studied experimentally. The outer tube diameter was 32 mm. The inner tube diameter varied from 4 to 25 mm. The gas slugs were produced by injecting air through a capillary tube. The shapes of gas slugs were studied by high-speed videos. The paper presents data on the rise velocity of gas slugs in the channels, and wall shear stress measurements, performed by electrodiffusional technique. The probes were mounted on both walls of the channel. The evolution of wall shear stress during slug passage was recorded.

Technology development for in-situ measurement of residual stress in arc welded joints of MDN 250 by portable Cosα X-ray diffraction method

Residual stresses are inherent stresses that exist in engineering components even though no external load is applied. They are caused by the non-uniform volumetric shift of the metallic component during manufacturing processes. Welding is a key manufacturing technique that has a substantial impact on the economy since it is required for the production of a diverse variety of products used in the engineering sector. The residual stress primarily affects the stability, durability and performance of the welded joints. Hence its determination is of utmost importance. X-ray diffraction (XRD) is the most commonly used method for residual stress analysis. There are mainly two approaches for measuring residual stress using XRD; one is the sin2ψ method and the other is the cosα method. The residual stress measurements using the cosα method are handy, quick and convenient compared to the sin2ψ method. This method is well suited for welded joints, as it provides flexibility for testing immediately after the welding operation. Apart from residual stress measurements, the cosα method also gives valuable insights in the form of Debye-Scherrer (DS) rings and full width at half maximum. The present study focuses on the development of a novel technique that not only enables residual stress measurement but also provides a quantitative estimation of hardness and qualitative estimation of grain size without performing metallurgical or mechanical characterization. The material used for the present study is an arc-welded joint of MDN 250 grade maraging steel. The residual stress results show a compressive profile throughout the weldment, with a maximum value of compressive residual stress of 428 MPa at the fusion zone.

High Resolution Residual Strain/Stress Measurements on Three Axis Neutron Diffractometer

Focusing 3-axis diffractometer set-up equipped with bent perfect crystal (BPC) monochromator and analyzer offers the sensitivity in determination of strains Dd/d < 10-4 in polycrystalline materials which is about one order of magnitude higher with respect to that of conventional 2-axis neutron scanners. It also offers possibility of line profile analysis for reasonable sample volumes and counting times. In this paper, the feasibility of using the 3-axis set-up even for measurements of rather large bulk polycrystalline samples with an acceptable resolution is presented. As the 3-axis set-up exploits focusing in real and momentum space, by a proper adjustment of the curvature of the analyzer, a high-resolution determination of the lattice changes can also be achieved even on large irradiated gauge volumes, though with a slightly relaxed resolution. It can be successfully exploited namely, in the strain/stress measurements on samples exposed to an external load, e.g. in tension/compression rig, in aging machine etc. In addition to the original performance where the analysis is carried out by rocking the BPC analyzer and the neutron signal registered by a point detector, a new alternative is offered which uses a fixed rocking angle position of the analyzer and the detector signal is registered by a one-dimensional position sensitive detector (PSD). This trick makes possible in some cases the elastic strain/stress measurements considerably faster and thus reduces the drawback of the time consuming step-by-step analysis.