Dynamic Stress Changes Research Articles

Optimizing Refracturing Models in Tight Oil Reservoirs: An Integrated Workflow of Geology and Engineering Based on Seepage-Stress Real-Time Cross-Coupling Strategy

Summary Integrated modeling of refracturing is a comprehensive and complex task for engineers involved in field development, especially in previously developed mature fields. The modeling process requires consideration of numerous factors to accurately predict fracture propagation patterns during refracturing and production. During the initial fracturing process, hydraulic fractures generate an altered induced stress field. Later, during the post-fracturing production process, hydrocarbon extraction causes formation pore pressure depletion, leading to further alterations in the stress field. However, due to the complexity of these phenomena, most existing workflows simplify the coupled simulation problems of stresses at different stages in the modeling process. Consequently, this often results in questionable hydraulic fracture geometries during refracturing and suboptimal refracturing designs. The goal of this study is to develop a novel integrated workflow for refracturing, specifically tailored for complex fracture networks in tight oil reservoirs. This model incorporates the hydraulic fracture propagation process during the initial fracturing and dynamic stress changes during the initial production process. It employs artificial intelligence algorithms to calibrate the wellhead treating pressure using a physics-based model, enabling a better understanding of the initial fracturing fracture geometries. The production history match is then conducted based on the initially calibrated hydraulic fracture geometries, preserving the precision of the original fracture geometry. In addition, geomechanics modeling is conducted to obtain dynamic stress changes during the initial production process. For the refracturing design, the fracture propagation model for the refracturing process is later conducted on the depleted stress field. Following a 240-day period after refracturing, the production history is matched using an artificial intelligence–assisted reservoir simulator. Our results indicate that, due to prolonged production, significant changes occur in the stress field during the initial development period, with an average horizontal stress deviation angle of approximately 35° in the near-well zone. With the combined influence of the changing stress field and natural fractures, refracturing results in longer and more complex hydraulic fracture geometries, ultimately increasing individual well production.

Spatial variation in stress orientation in and around Türkiye: rupture propagation across the stress regime transition in the 2023 Mw 7.8 Kahramanmaraş earthquake

SUMMARY On 6 February 2023, the Mw 7.8 Kahramanmaraş, Türkiye, earthquake occurred on the East Anatolian Fault Zone (EAFZ). This study examined the spatial variation of the stress field in and around Türkiye, particularly along the EAFZ, better to understand the rupture process of this event. We first combined focal mechanisms around Türkiye, created a data set consisting of 2984 focal mechanisms, and conducted stress tensor inversion. The results showed that the maximum compressional axis near the EAFZ was oriented north–south and slightly varied along the strike. Moreover, the relative magnitude of north–south compressional stress gradually increased from south to north, and the stress regime changes from a normal fault regime to a strike-slip fault regime. The static stress change caused by the 2023 Mw 7.8 and 7.7 events does not explain this lateral pattern, implying that this stress regime transition existed before the main shock. This suggests that shear stress on the EAFZ was low in this southern segment because it was unfavourably oriented to the regional stress field. Dynamic stress changes due to rupture propagation and dynamic weakening may have triggered the slip at the southern segment under low background shear stress. Previous studies have reported that the Mw 7.8 main shock rupture started at a splay fault, first propagated through the central and northern segments and then backpropagated with a time delay towards the southern segment, where it caused a significant but relatively small slip. The pre-existing along-strike shear stress variation on the fault may have contributed to the smaller and delayed coseismic slip in the southern segment than in the central and northern segments. The main shock rupture possibly caused stress rotation locally near the central segment where the magnitudes of the vertical and north–south compressional stresses were almost equal.

Midcrustal moderate-size earthquake occurrence in paleovolcanic structures off Jeju Island, South Korea

A series of midcrustal moderate-size earthquakes occurred in the Korean Peninsula recently. A midcrustal ML4.9 strike-slip earthquake with a fault-plane strike in N-S occurred on December 14, 2021 at the southwestern offshore region of Jeju Island, South Korea. The fault plane orientation and slip sense (faulting mechanism) hardly conform with the regional stress field. The deep focal depth and N-S directional strike-slip motion require transient changes in the medium properties and stress field. Strong ground motions of the midcrustal earthquake induce preferential dynamic stress changes in NE-SW direction, triggering subsequent aftershocks in NE-SW-directional adjacent faults. Both the static and dynamic stress changes caused by the mainshock contribute to the aftershock sequence. The number and focal depths of aftershocks decrease with distance from the mainshock. The different fault-plane orientations between the mainshock and aftershocks suggest earthquake nucleations in independent fault structures. The mainshock occurred in aseismic midcrustal paleovolcanic structure on the outskirt of a high seismicity region. The ML4.9 earthquake suggests possible nucleation of earthquake in seismically-inactive paleotectonic structures, successively incurring aftershocks conforming to the ambient stress. The mainshock and aftershocks suggest that paleotectonic structures may behave as source structures to spawn earthquakes.

The Response of Taupō Volcano to the M7.8 Kaikōura Earthquake

AbstractSeveral studies suggest that large earthquakes (M > 7.0) can act as external triggers of volcanic unrest, and even eruption. This triggering is attributed to either ground shaking (dynamic stresses) or to permanent ground deformation (associated with static stress changes). However, large earthquakes are rare and testing triggering hypotheses has proven difficult. We use geodetic data to show that the 13 November 2016 Kaikōura earthquake (Mw 7.8) triggered local deformation of up to 11 mm at Taupō volcano, 500 km away, which lasted for approximately twelve days. Using elastic geodetic models, we infer that the observed deformation was caused by either aseismic fault slip or a dike intrusion. We then use strong motion data from the surrounding area to show that the Kaikōura earthquake caused maximum dynamic stress changes in the range of 0.15–0.9 MPa in the vicinity of Taupō volcano and conclude that these dynamic stress changes triggered local faulting or dike activity and the associated deformation at Taupō volcano.

Dynamic Triggering of Earthquakes in Northeast Japan before and after the 2011 M 9.0 Tohoku-Oki Earthquake

ABSTRACT Previous studies documented a relative scarcity of remote dynamic triggering of earthquakes in Japan and suggested that it could be related to Japan’s predominantly compressive tectonic regime or the more frequent occurrence of large earthquakes in Japan. For example, remote triggering in California, characterized by extensional tectonics, occurs at levels of stress change as small as 0.1 kPa, whereas in Japan, transient stresses ≥30 kPa are required. However, the dynamic triggering threshold in Japan, following the 2016 Mw 7.0 Kumamoto earthquake, has been found to be of just a few kilopascals, significantly smaller than reported previously. It was proposed that a decrease in the triggering threshold may have taken place in Japan, in particular at volcanic and geothermal areas, after the 2011 Mw 9.0 Tohoku-Oki earthquake. In this study, we analyze the possible change in dynamic triggering conditions in five areas in northeast Japan, where swarm earthquakes have occurred immediately after the Tohoku-Oki earthquake. The triggering thresholds in these five areas have been estimated based on the analysis of waveform recordings of 49 teleseismic earthquakes that occurred between 2004 and 2020. A decrease of the triggering threshold (or triggering ability) is apparent in all but one region. However, a statistical significance Kolmogorov–Smirnov test could not reject, at a 5% level, the null hypothesis stating that “the distribution of dynamic stress changes for triggering earthquakes that occurred before and after the Tohoku-Oki event is the same.” We interpret the changes in the triggering threshold to be related to the pore pressure increase (and thus a fault strength decrease) in the crust following the Tohoku-Oki earthquake. Our results also indicate that dynamic triggering in Japan is more common than reported previously.

Evolution of shallow volcanic seismicity in the hydrothermal system of La Soufrière de Guadeloupe following the April 2018 Mlv 4.1 earthquake

La Soufrière de Guadeloupe volcano is characterized by seismo-volcanic activity dominantly linked to an active hydrothermal system. This microseismicity is shallow, mainly triggered in swarms and characterized by repeating earthquakes. Four recurrent families of Volcano-Tectonic (VT) repeaters have been identified and the main repeater accounts for 80% of detections. Stacking repeating seismic waveforms produces MASTER events with a high Signal-to-Noise Ratio (SNR) and thanks to the deployment of a temporary seismic nodal array, their absolute locations can be better constrained. Because we observe systematic positive residuals of P-wave arrival times at almost all stations, we investigate potential bias in the velocity model in the shallow part of the dome. By increasing the P-wave velocity and decreasing the VP/VS ratio from 1.8 to 1.69, we reduce these residuals and improve the local velocity model. We use this new velocity model to locate each VT event relatively to is own MASTER hypocenter. This procedure offers a new image of the hydrothermal seismic activity that we locate under the acid lake of the summit Tarissan crater, along a sub-vertical conduit. We also define a linear relationship between the logarithm of the peak amplitude of seismic events and their duration to obtain a pseudo local magnitude. The approach enables to accurately and automatically estimate the magnitude during the event detection. We show that after the occurrence in April 2018 of the largest earthquake (Mlv 4.1) since the last phreatic eruption in 1976–1977, the swarm frequency and amplitude increased significantly, and a new family of VT repeater emerges, accounting for up to 14% of detections. We show a quiescence in the main repeaters activity in the month following the April 2018 earthquake, suggesting a significant stress release within the volcanic system, which is likely related to dynamic stress changes rather than static stress variations caused by the earthquake. Moreover, we detect the emergence of a new family of repeaters 3 months after the event, located ∼100 m above the main repeater family, that could be related to a fluid pressure increase induced by the regional hydrological cycle, superimposed to the internal forcing driven by the hydrothermal system. Finally, using a statistical approach, we highlight seasonal periodicities in the number of events and the released seismic moment at La Soufrière de Guadeloupe, with a dominant peak of seismic activity in October–November and a second, less significant, peak of in April.

Experiments on the Coal Measures Sandstone’s Dynamic Mechanical Parameter Characteristics under the Combined Action of Temperature and Dynamic Load

This paper conducts impact dynamics experiments on coal measures sandstone in a deep mine via the established dynamic load and temperature split Hopkinson pressure bar (SHPB) experimental system. Firstly, the experimental conditions for the impact dynamics of fine sandstone were determined, with temperatures of 18, 40, 60, 80, and 100 °C, an axial static load range of 1–9 MPa, and a preset bullet incidence velocity of 1.0–5.0 m/s. Secondly, based on the analysis of the basic parameters and physical composition, the dynamic stress and strain responses of fine sandstone under different experimental conditions were obtained, and the change mechanism of its dynamic mechanical process was theoretically analyzed. When the temperature rose from 18 °C to 100 °C, the dynamic peak stress of fine sandstone increased from 36.04 MPa to 73.41 MPa, with an increase of 103.7%. At a temperature of 40 °C, when the axial static load increased from 1 MPa to 9 MPa, the dynamic stress peak of fine sandstone increased from 57.25 MPa to 80.01 MPa, and the corresponding peak strain also showed an increasing trend. The experiment analyzed the variation characteristics of dynamic stress in fine sandstone under the combined action of different strain rates or bullet incidence velocities and different temperatures. In the strain rate range of 47.1 s−1 to 140.9 s−1, there was a significant strain rate effect on the dynamic peak stress and peak strain of fine sandstone, which increased with the increase of strain rate. The study found a polynomial relationship between the dynamic mechanical parameters of fine sandstone and the impact of experimental parameters, with a coefficient of determination greater than 0.9. A dynamic stress constitutive model for fine sandstone under one-dimensional stress state under dynamic load and temperature action was established, and the model validation and parameter determination of dynamic stress changes in fine sandstone under different temperature conditions were carried out. The research results provide a new experimental method for the static and dynamic mechanical analysis of coal and rock masses under complex geological conditions and can provide a basic reference for the prevention and control of dynamic disasters in deep mining processes.

Evaluating the Aftershock Duration of Induced Earthquakes

ABSTRACT As the carbon sequestration community prepares to scale up the number and size of commercial operations, the need for tools and methods to assess and mitigate risks associated with these operations becomes increasingly important. One outstanding question is whether aftershocks of induced events decay quickly after injection operations cease or if aftershock activity persists for hundreds of years before returning to background levels more akin to tectonic events (Stein and Liu, 2009). Appropriate estimates of the aftershock duration impact several operational management decisions including mitigation strategies and post-injection monitoring for seismic activity. It is hypothesized that induced earthquake rates may diminish more quickly after injection is stopped, owing to higher stressing rates from injected fluids. Alternatively, it is plausible that only the first event in the sequence is induced by increased fluid overpressures, whereas subsequent events (e.g., aftershocks) respond to the stored tectonic stresses and static and dynamic stress changes due to the mainshock (Keranen et al., 2013). If the aftershock duration can be linked to stressing rates due to injection, then it follows that operational strategies to reduce seismic hazard by reducing injection rates or volumes may be successful. However, if aftershocks of induced events are relieving stored tectonic stresses, then altering injection volumes may not alleviate ongoing seismic activity. Furthermore, knowledge of an aftershock duration could aid in the determination of an appropriate post-injection monitoring period for ongoing seismicity, which is a factor in overall operational costs. In this study, we model induced seismicity sequences in Oklahoma with a coupled Coulomb rate–state earthquake rate model (Dieterich, 1994; Kroll et al., 2017) to estimate aftershocks durations. Results for the current study indicate that elevated rates of aftershock activity following induced mainshocks return to background seismicity rates in less than five years, contrary to the tens to hundreds of years observed for tectonic aftershocks.

Time-lapse imaging of seismic scattering property and velocity in the northeastern Japan

Investigating spatio-temporal changes in crustal structure is a key to understanding the dynamic processes in the earth's crust associated with large earthquakes and volcanic activities. The dense seismic network in the northeast part of Japan (Tohoku region) allows us to apply seismic interferometry to continuous records of ambient noise data for studying crustal changes related to the 2008 Mw 6.9 Iwate-Miyagi Nairiku earthquake and the 2011 Mw 9.0 Tohoku-oki earthquake. We track changes in both seismic scattering properties and velocities over five years and spatially localize them based on the coda wave decorrelation and travel time sensitivity kernels calculated from the solution of the radiative transfer equation. The 2008 Iwate-Miyagi Nairiku earthquake caused significant changes in both seismic scattering properties and seismic velocities in the vicinity of the epicenter subjected to dynamic or/and static stress perturbations. Changes in scattering properties associated with the 2011 Tohoku-Oki earthquake around some active volcanoes in the Tohoku region are likely related to geofluid migrations triggered by strong vibrations during the large earthquake. The decrease in seismic velocity exists mainly along the eastern coastal region and extends to the central Quaternary volcanic chains under the combined dynamic and static stress changes by the earthquake. The waveform correlation is possibly sensitive to the fluid content and might be a useful indicator of fluid migration in the crust. This is the first comprehensive demonstration and comparison of the spatio-temporal changes of seismic scattering properties and velocities at long-term and regional scales under the influence of two large earthquakes.

Flexible gear modeling and stress spectrum compilation of an artillery steering machine

To solve the load data lack problems when researching the fatigue life prediction of artillery steering machines, the flexible body model of an artillery steering gear was established using the recurdyn/fflex, based on the finite element flexible body theory. The complete artillery adjustment process of the steering gear was simulated and calculated, and the dynamic stress change of the gear under variable load conditions was obtained. Eight-level stress spectrum was compiled based on the rain flow counting method, which provides the load data basis for the subsequent reliability design and fatigue life research.

Absolute stress levels in models of low-heat faults: Links to geophysical observables and differences for crack-like ruptures and self-healing pulses

Absolute levels of stress on faults have profound implications for earthquake physics and fault mechanics. A number of observations suggest that well-developed, mature faults such as the San Andreas Fault are generally “weak,” i.e. operate at much lower levels of shear stress compared to the higher expected shear resistance ∼100 MPa at seismogenic depths. In particular, low heat flow measurements suggest shear stress levels of ∼10 MPa or less on highly localized faults. Geodynamic constraints based on topography and similar considerations also support “weak” fault operation, and are comparable with heat-based constraints for some mature faults, but potentially higher for regions with substantial topography. Here, we investigate measures of average fault shear stress and their relationship to geophysically inferable quantities using numerical simulations of earthquake sequences on rate-and-state faults with low heat production, due to chronic fluid overpressure and/or enhanced dynamic weakening from the thermal pressurization of pore fluids. We review the earthquake energy balance, focusing on energy-based definitions of average shear stress and how the average fault prestress (a measure of fault strength plausibly relevant to geodynamic constraints) can be expressed as the sum of the dissipation-based average rupture stress (which can, in principle, be inferred from shear-heating constraints), and seismologically inferable source properties, such as the static stress drop and apparent stress. Our modeling demonstrates that rapid dynamic weakening and healing of shear resistance during ruptures, as exhibited in self-healing pulses, allows faults to maintain higher average interseismic stress levels despite low dynamic resistance and realistic static stress drops, providing a physical explanation for potential differences between topography-based and heat-based constraints on fault shear stress. In our models, the difference is related to stress undershoot and apparent stress, which can be as large as 1-3 times the static stress drop based on our simulations. Yet suitably large values of apparent stress (and hence radiated energy) are rarely inferred for natural earthquakes, either because radiated energy is underestimated, or suggesting that most large earthquakes do not propagate as sharp enough self-healing pulses with sufficiently large undershoot. Our results emphasize the distinction between dynamic versus static stress changes when relating earthquake source observations to absolute levels of fault stress and suggest that reviewing estimates of radiated energy and static stress drop from large earthquakes, with input from finite-fault numerical modeling, may improve constraints on absolute fault stress levels.

Coupled Hydromechanical Modeling and Assessment of Induced Seismicity at FWU: Utilizing Time-Lapse VSP and Microseismic Data

The objective of this work is to utilize integrated geomechanics, field vertical seismic profile (VSP) and microseismic data to characterize the complex subsurface stress conditions at the Farnsworth Unit (FWU). The model is based on a five-spot sector model extracted from a primary geomechanical model. The five-spot well injection pattern is characterized by extensive reservoir characterization data, such well logs, extracted cores and borehole geophone data, to facilitate the detailed examination of stress changes and microseismic event occurrences. The study utilizes field vertical seismic volumes acquired from the injection well 13-10A. The seismic volumes successfully provided snapshots of the behavior of the reservoir at distinct times. The use of VSP and microseismic data provided direct and indirect estimates of the dynamic stress changes occurring in the overburden, reservoir and underburden rock formations. In order to illuminate the stress regions and identify rocks that have undergone inelastic failure, microseismic event occurrences were utilized. Microseismic activity has been detected at the FWU; further study of its locations, timing, and magnitude was needed to deduce the nature of the changing stress state. The results of the study revealed that microseismic events were successfully modeled within the Morrow B formation. Moment magnitudes of seismic events were within the same magnitudes for events in the reservoir, suggesting the suitability of the model. The results of the study showed that the computed moment magnitudes for seismic events were insignificant to warrant safety concerns. The study findings showed the usefulness of coupled hydromechanical models in predicting the subsurface stress changes associated with CO2 injection. The knowledge gained from this study will serve as a guideline for industries planning to undertake underground CO2 storage, and characterize the subsurface stress changes.

3-D Simulations of earthquakes rupture jumps: 1. Homogeneous pre-stress conditions

SUMMARYObservational and modelling studies indicate that earthquake ruptures can jump between fault sections as large as ∼3 and ∼5 km for compressional and extensional offsets, respectively. Here, we compare characteristics of the rupture jump process on parallel but offset fault sections from traditional 3-D dynamic rupture simulations governed by slip weakening friction using the finite element code, FaultMod, to those from quasi-dynamic simulations governed by rate- and state-dependent friction (rate-state friction) using the code RSQSim. These simulations use spatially uniform initial stresses. For a variety of measures the rupture renucleation position on the offset fault, the rate-state friction and slip weakening friction models produce very similar results. The principal difference is the additional occurrence of delayed rupture jumps that arise from the time- and stress-dependent nucleation that is characteristic of rate-state friction. For immediate rupture jumps, models with slip weakening friction span greater offsets than those with rate-state friction. However the jump distances are nearly identical when delayed rupture jumps are included in the comparisons. We propose that delayed rupture jumps are the likely mechanism for adjacent large-earthquake pairs and clusters. Based on the similarity of renucleation positions with both dynamic and quasi-dynamic models, we conclude that the renucleation positions for rupture initiation on the receiver fault (separated by less than ∼3 km from the source fault) are primarily controlled by static stress changes induced by slip on the initiating fault. However, in light of the slightly greater maximum jump distances (>3 km) seen with the dynamic slip weakening friction model, dynamic stress changes from seismic waves play an increasingly important role as offset distances increase.

Analysis of Multistorey RCC Frame subjected to Mainshock and Aftershock Earthquake

Abstract: Structure analysis and design are significantly influenced by earthquake. It is believed that seismic evaluation is required for the viability and serviceability of both present and future building structures. When a structure's base is subjected to a certain type of ground motion, a time history analysis is performed to examine the dynamic reaction of the structure at each time interval. The near-field earthquake ground motion verification may have specific impacts for both forward and backward directivity. The initial's velocity and displacement motions, respectively, exhibit pulse and fling-step characteristics. Therefore, it is crucial to assess how well structures built solely to withstand the primary shock would work through future aftershocks. Using modern seismic protection systems, such as base isolations or / and supplemental dampening devices, that significantly reduce building damages during main shocks and their related aftershocks is one of the appropriate solutions to this issue. Due to changes in both static and dynamic stress that take place during the earthquake process, aftershock events are set off by the primary shock. In order to better understand the ground motion features of a sizable collection of mainshock and subsequent aftershock ground motion data recorded in accelerograph stations around the region, this study reviews pertinent literature in the field. The G+9 RCC building will be used in this study to conduct a time history analysis of the mainshock and aftershock data of the Chamoli earthquake provided by the Centre for Engineering Strong Motion Research Ground Motion Database. For designing purposes, the IS 456-2000 code is taken into consideration. Live loads are measured in accordance with IS 875-part 1, and seismic zone IV is selected for analysis in accordance with IS 1893-2016. Storey drift, base shear, joint reactions, and storey displacement are just a few of the variables that can have an impact on how well a building performs. Since each of these variables has a significant impact on how a structure responds to seismic loads, they are also taken into account when evaluating the results.

Recent Seismic Sequences and Activation of Normal Fault Systems in the Mugello Basin and Surrounding Areas (Northern Apennines, Italy)

The Mugello Basin (North-Eastern Tuscany) is an intermontane basin of the Northern Apennines (Italy) with a well-documented record of seismicity; the two major historical earthquakes occurred in 1542 (Mw = 6.0) and in 1919 (Mw = 6.4). In this study, we integrate different seismic catalogs spanning the 2005–2019 time interval, and complement these data with phase arrival times from a temporary network that specifically operated in the area throughout the 2019–2021 time interval. The subsequent relocation of this data set with a double-difference algorithm allows for accurate analyses of the most relevant seismic sequences which affected the study area in 2008, 2009, 2015–2017, and 2019. These sequences are associated with the activation of adjacent segments of larger NW-striking fault systems, one of which bounds the NE margin of the Mugello Basin (Ronta Fault System). For each seismic sequence, best-fit fault surfaces are derived from orthonormal regression of relocated hypocenters, yielding consistent results with that derived from fault plane solutions. The four sequences mark a significant increase in the seismicity rate with respect to what was recorded in the previous decades. This suggests that, following the 2008 renewal of seismicity, static or dynamic stress changes, or both depending on the case, played a role in advancing the time of failure of the fault segments activated subsequently.

Spatiotemporal Clustering of Seismicity in the Kiskatinaw Seismic Monitoring and Mitigation Area

Induced seismicity has become a concern for industry and nearby residents. Fluid-induced earthquakes are a side effect of industrial operations such as hydraulic fracturing, where high-pressure fluids are pumped into the Earth’s crust to increase hydrocarbon flow to a well from petroleum-bearing rock formations. Previous studies on fluid-induced seismicity showed the existence of spatiotemporal clustering and aftershock activity. Here, we study seismic catalogs from the Kiskatinaw Seismic Monitoring and Mitigation Area (KSMMA), British Columbia, Canada to investigate spatiotemporal clustering and determine the aftershock activity due to secondary triggering mechanisms such as static and dynamic stress changes arising from preceding seismic events. Our analysis is based on modern tools of statistical seismology such as the Bi-test and triggering analysis. We find significant aftershock triggering and spatiotemporal clustering across the KSMMA but with large spatial variations, which might be indicative of pre-existing stresses and fault systems. In particular, aftershock triggering is characterized by rapidly decaying spatial distributions of aftershocks beyond the mainshock rupture area and by a strong dominance of small but frequent triggers, consistent with that observed for hydraulic fracturing and saltwater disposal elsewhere.

Dynamic stress analysis of a metro bogie due to wheel out-of-roundness based on multibody dynamics algorithm

In metro vehicle systems, the dynamic stress and vibration of the bogie frame have a significant impact on the safety and reliability of vehicles. This study presents a fracture failure analysis of the bogie frame for the metro vehicle. The analysis consists of a bogie frame dynamic stress test, a wheel out-of-roundness (OOR) measurement and a vibration transmissibility test of the vehicle which is also used for operational modal analysis of the bogie frame. The field test result shows that the bending of cross beam combined with torsion of side beams(49.5 Hz) which are aroused by the wheel OOR mainly contribute to the fatigue failure of the bogie frame. To investigate the effect of wheel OOR on the fatigue failure of the bogie frame, the multibody system (MBS) model of the metro vehicle was developed. The MBS model accounts for the flexibility of the bogie frame and the nonlinearity of wheel-rail contact. Furthermore, the modal stress recovery method was adopted to study the dynamic responses of the bogie frame. The measured wheel OOR data is used to validate the efficiency of the model and investigate the characteristics of dynamic stress in time histories as well as frequency spectra. Moreover, the dynamic stress of fracture site in the bogie frame with consideration of ideal polygonal wear in radial deviation including different orders and variable amplitudes was investigated. The phase shift of wheel OOR between different wheels proves to be influential in the bogie frame vibration and thus the characteristics of dynamic stress change accordingly. The recommended limit values of wheel OOR amplitude are shown based on numerical results.

Diverse rupture processes of the 2014 Kangding, China, earthquake doublet (MW 6.0 and 5.7) and driving mechanisms of aftershocks

An unusual earthquake doublet ruptured the high-risk middle segment of the Xianshuihe fault in November 2014, exhibiting significant variations in the rupture and aftershock characteristics. By conducting rupture kinematic inversions jointly with local broadband seismic data, strong-motion data, and coseismic InSAR data, we find diverse rupture processes of this earthquake doublet. The first event ruptured upward along the dip and bilaterally along the strike with the duration of 8.0 s, whereas the second event ruptured mainly around the hypocenter with the duration of 3.5 s. The estimated moment magnitudes of the first and second events of this earthquake doublet were MW 6.0 and MW 5.7, respectively. The further analysis of rupture model-based static and dynamic stress change suggests that the first event is likely responsible for triggering the second event. Based on the comprehensive analysis of aftershock migration features, we find complex and diverse driving mechanisms of aftershocks. Aftershocks of the first event of the earthquake doublet were likely driven by afterslip. Early aftershocks of the second event were also likely driven by afterslip; however, later aftershocks were likely driven by fluid diffusion. The revealed diverse rupture characteristics of the 2014 Kanding earthquake doublet sequence suggest the inherent strength and/or stress heterogeneities on the seismogenic faults.

Transient structural load characteristics of reactor coolant pump rotor system in rotor seizure accident

The serviceability of reactor coolant pump (RCP) under extreme conditions is essential assurance to prevent nuclear accidents. Among them, the RCP rotor seizure accident is classified as Type IV nuclear accident condition. This paper mainly studies the transient structural load characteristics of the RCP in rotor seizure accident, and reveals the radial and axial loads of the RCP impeller in the transient process of the rotor seizure accident, as well as the typical area stress distribution, transient dynamic stress and other change laws of the entire rotor. In this paper, the mathematical model of transient flow and speed under rotor seizure conditions is first established based on the rotor seizure test, which is the boundary condition of the calculation of flow-solid coupling, and then the structural module in Workbench is used to analyze the structural characteristics of the rotor in the transient process of rotor seizure condition. The study found that there will be three obvious “impact radial loads” in the transition process of rotor seizure, which are in the initial stage and the mid to late stage of the rotor seizure condition respectively. The transient radial load variation law is similar under different rotor seizure degrees, and the dynamic stress of the monitoring point at the inlet side of the impeller blade has the greatest difference, the closer the monitoring point is to the outlet of the blade, the later the dynamic stress recovery time and the lower the dynamic stress value at the inflection point.

Decadal Timescale Correlations Between Global Earthquake Activity and Volcanic Eruption Rates

AbstractAt the global scale, large tectonic earthquakes and volcanic eruptions are believed to be random and independent events. Here, we compare global time‐series of large earthquakes ( 7) and explosive volcanic eruptions (VEI 2) spanning 1960–2019. Both time‐series exhibit decadal timescale trends, over which annual earthquake and eruption rates vary by a factor of 2. Moreover, global seismic moment release is positively correlated with global eruption rate, with Monte Carlo permutation tests showing that this correlation is significant with a ‐value of 0.05. Although large earthquakes can trigger eruptions at nearby volcanoes, the magnitude of this effect is insufficient to cause the observed global correlation. Other mechanisms, such as triggering of distant eruptions (>1000 km) by earthquake‐induced dynamic stress changes, modulation of global earthquake and eruption rates by variations in Earth's rotational velocity, or natural synchronization of events over repeating cycles, are therefore required to explain the correlation.