Seismic Source Mechanisms Research Articles

Multiscale monitoring and analysis of complex rupture and source mechanisms of mining-related seismicity on fault networks

Mining-related seismicity poses significant challenges in underground coal mining due to its complex rupture mechanisms and associated hazards. To bridge gaps in understanding these intricate processes, this study employed a multi-local seismic monitoring network, integrating both in-mine and local instruments at overlapping length scales. We specifically focused on a damaging local magnitude (ML) 2.6 event and its aftershocks that occurred on 10 September 2022 in the vicinity of the 3308 working face of the Yangcheng coal mine in Shandong Province, China. Moment tensor (MT) inversion revealed a complex cascading rupture mechanism: an initial moment magnitude (Mw) 2.2 normal fault slip along the DF60 fault in an ESE–WNW direction, transitioning to a Mw 3.0 event as the FD24 and DF60 faults unclamped. The scale-independent self-similarity and stress heterogeneity of mining-related seismicity were investigated through source parameter calculations, providing valuable insights into the driving mechanism of these seismic sequences. The in-mine network, constrained by its low dynamic changes, captured only the nucleation phase of the DF60 fault. Furthermore, standard decomposition of the MT solution from the seismic network proved inadequate for accurately identifying the complex nature of the rupture. To enhance safety and risk management in mining environments, we examined the implications of source reactivation within the cluster area post-stress-adjustment. This comprehensive multiscale analysis offers crucial insights into the complex rupture mechanisms and hazards associated with mining-related seismicity. The results underscore the importance of continuous multi-local network monitoring and advanced analytical techniques for improved disaster assessment and risk mitigation in mining operations.

Dynamic changes of gravity field before the Luding MS6.8 earthquake and its crustal material migration characteristics

On September 5, 2022, an earthquake of magnitude MS6.8 occurred in Luding County, Sichuan Province. This earthquake occurred at the key part of the southeast-clockwise extrusion of material on the eastern margin of the Qinghai–Xizang Plateau, the Y-shaped confluence of the Xianshuihe, Longmenshan and Anninghe fault zones. In this study, the three-dimensional dynamic crustal density changes in the earthquake area are obtained by the typical gravity change data from 2019 to 2022 before the earthquake and gravity inversion by growing bodies. The results indicate that gravity changes presented an obvious four-quadrant and gradient belt distribution in the Luding area before the earthquake. The three-dimensional density horizontal slices show that small density changes occurred at the epicenter in the mid-to-upper crust between 2019.9 - 2020.9 and 2019.9–2021.9. At the same time, the surrounding areas exhibited a positive and negative quadrant distribution. These observations indicate that the source region was likely in a stable locked state, with locking-in shear forces oriented in the NW and NE directions. From 2021.9 to 2022.8, the epicentral region showed negative density changes, indicating that the source region was in the expansion stage, approaching a near-seismic state. The three-dimensional density vertical slices reveal a southeastward migration of positive and negative densities near the epicenter and on the western of the Xianshuihe Fault Zone, indicating that the material is flowing out to the southeast. The observed local negative density changes at the epicenter along the Longmenshan Fault Zone are likely associated with the NE-oriented extensional stress shown by the seismic source mechanism. The above results can provide a basis for interpreting pre-earthquake gravity and density changes, thereby contributing to the advancement of earthquake precursor theory.

Improved numerical solution of anisotropic poroelastic wave equation in microseismicity: Graphic process unit acceleration and moment tensor implementation

AbstractThe accuracy and computational efficiency of full waveform forward modelling in poroelastic media are crucial for microseismic monitoring. It enables intuitive, precise and efficient simulation of subsurface responses, thereby improving the reliability of moment tensor inversion and seismic source mechanism interpretation. Additionally, it reflects the role of fluid effects in waveform evolution. In this paper, based on the Biot mechanism, we derived the first‐order velocity–stress equation of poroelastic media and discretized the model using a rotated staggered grid. The rotated staggered grid can well adapt to anisotropic media with high contrast parameters. We provide the finite difference formula based on graphic process unit–acceleration and moment tensor and also provide the graphic process unit workflow for forward modelling of anisotropic poroelastic media. First, two models with different grid sizes were run based on single graphic process unit, 1‐thread Central Processing Unit (CPU) and 16‐thread CPU. The results show that the speedup factors are approximately 14.3 and 3.7, respectively, compared with the running time of 1‐thread CPU and 16‐thread CPU. Then, we compare and evaluate the response of three basic source mechanisms (isotropic expansion, double couple and compensated linear vector dipole) in the model. The comparison of analytical and numerical results verifies the effectiveness of the method. Furthermore, wavefield snapshots of two typical anisotropic media (vertical transversely isotropic and horizontal transversely isotropic) are analysed to correspond to different moment tensor sources. The results showed that the source mechanism does not change the isotropic and anisotropic plane and the wave travel time, but it does change the polarization amplitude of the velocity component. The attenuation of slow qP‐wave increases along with the increase of the value of viscosity. The effect of permeability on wavefield appears with the opposite effect of viscosity. Finally, the seismic waveform differences between multi‐layer elastic media and poroelastic media are compared and analysed. The results showed that the seismic wavefield and waveform of poroelastic media are more complex, the propagation speed of seismic waves is faster, but the attenuation of seismic waves is stronger.

Extract and analysis of surface deformation caused by Mengyuan earthquake in Qinghai using ascending and descending tracks D-InSAR technology

Precision in uncovering the seismic source mechanism and conducting a thorough monitoring of deformation characteristics resulting from surface rupture is of paramount importance for geological comprehension, disaster management, and emergency response. This study employs both ascending and descending orbit Sentinel-1 data to capture the horizontal and vertical deformation traits of the Ms6.9 earthquake in Menyuan, Qinghai, on January 8, 2022, consequently exposing its seismogenic structure. The research outcomes suggest: the Menyuan earthquake generated an elliptical deformation zone measuring 30 × 20 km, with the maximum line-of-sight seismic displacement reaching 6.8 cm. And then, the vertical deformation field exhibited a range between -0.28 m and 0.42 m, while the horizontal deformation field ranged from -0.89 m to 0.94 m. This indicates that the earthquake's deformation is primarily oriented in the east-west direction. The left plate exhibited an upward trend with a NWW orientation, while the right plate displayed a downward trend with a SEE orientation, suggesting that the Menyuan earthquake can be classified as a "NWW-SEE" type. Furthermore, the seismic epicenter of this earthquake was predominantly concentrated in the western segment of the Lenglongling Fault. Two powerful earthquakes sequentially struck along this fault zone, intensifying the imperative for seismic geological research in the region. Additionally, this instance can serve as a benchmark for monitoring deformations and elucidating the seismic source mechanism in earthquakes with comparable seismogenic structures.

Rupture Process of the 2017 Mw 5.5 Pohang, South Korea, Earthquake via an Empirical Green’s Function Method

ABSTRACTThe Mw 5.5 earthquake occurred in Pohang, South Korea on 15 November 2017, which is known as a “runaway earthquake,” and was triggered from a critical-state fault as a result of fluid injection. As such earthquakes rarely occur, spatiotemporal slip distributions were investigated via the finite-fault inversion based on the empirical Green’s function in this study. The rupture process can be divided into three steps: first, slip initiated and propagated only to the southwest from the hypocenter during the initial 0.6 s; in the second step from 0.6 to 2.4 s, the slip occurred to the southwest and northeast parts, in which the maximum seismic moment was released; in the third step from 2.4 to 6.0 s, slip occurred around the edge of the fault plane farther from the hypocenter, particularly in the deep part in the northeast direction. In each step, the seismic moment was released as approximately 6%, 59%, and 35%, respectively. The first step can be interpreted not as a part of the rupture process of the mainshock but as the immediate and distinct foreshock. Overall, most of the slip distributed southwest is consistent with the results of the directivity analysis using apparent source time functions. Although the average stress drop (~1 MPa) of the Pohang earthquake is considerably lower than that (~20 MPa) of the Mw 5.5 Gyeongju earthquake that naturally occurred in the vicinity of the Pohang, it is difficult to attribute it only to the fluid injection effect. Through this study, we improve our comprehension of the seismic source physics and mechanisms of the Pohang earthquake by analyzing the spatiotemporal slip history, the directivity of rupture process, and the spatial distribution of the stress drop on the fault plane.

Possible Seismic Source Mechanism of the Catastrophic Tsunamigenic Earthquake on May 9, 1877 in Northwestern Chile

Possible Seismic Source Mechanism of the Catastrophic Tsunamigenic Earthquake on May 9, 1877 in Northwestern Chile

Persistent shallow micro-seismicity at Llaima volcano, Chile, with implications for long-term monitoring

Identifying the source mechanisms of low-frequency earthquakes at ice-covered volcanoes can be challenging due to overlapping characteristics of glacially and magmatically derived seismicity. Here we present an analysis of two months of seismic data from Llaima volcano, Chile, recorded by the permanent monitoring network in 2019. We find over 2000 repeating low-frequency events split across 82 families, the largest of which contains over 200 events. Estimated locations for the largest families indicate shallow sources directly beneath or near the edge of glaciers around the summit vent. These low-frequency earthquakes are part of an annual cycle in activity at the volcano that is strongly correlated with variations in atmospheric temperature, leading us to conclude that meltwater from ice and snow strongly affects the seismic source mechanisms related to glacier dynamics and shallow volcanic processes. The results presented here should inform future assessments of eruptive potential at Llaima volcano, as well as other ice-covered volcanoes in Chile and worldwide.

Quantitative Acoustic Emissions Source Mechanisms Analysis of Soft and Competent Rocks through Micromechanics-Seismicity Coupled Modeling

This research studied seismic source mechanisms of acoustic emission (AE) events generated during the failure of intact and jointed rock samples using a microscale mechanical-seismicity coupled microscale model. During rock failure, forces and displacements around microcracks were measured in the model to determine seismic moment tensor. Interpretation of AE data was carried out by decomposing the moment tensor into a double couple (DC), a compensated linear vector dipole (CLVD), and isotropic component (ISO). In this study, microscale numerical models of Lac du Bonnet (LDB) granite (intact and jointed) and a Berea sandstone sample were built to represent the mechanical behavior of hard and soft rocks. Subsequently, several numerical triaxial tests under different confining pressures were conducted to analyze source mechanisms of AE events during rock failure. Numerical analysis shows the significant contribution of positive non-DC component during the LDB granite rock failure and significant DC component during the Berea rock failure. The source mechanisms of AE move to the negative non-DC component at higher confining for both granite and sandstone samples. Joint failure causes non-DC component toward crack opening at low confining pressure and a non-DC component toward crack closure events under higher confining pressure. Simulation results, presented in this work, show how micromechanical properties of rocks and joints as well as stress conditions give rise to different types of AE source mechanisms during rock failure.

Evaluating the Seismic Hazards by Chang-Hua Fault at Nantou County, Taiwan and Disaster Resistant Strategy

Because of geology and complex seismic source mechanisms, Taiwan suffers attacks of earthquake calamity. In this research, we investigate the seismic hazards caused by Changhua fault and focus on the hazards at Nantou County, Taiwan. The PGA and spectral intensities, SIs, including acceleration, velocity, and displacement (SIa, SIv, and SId), are employed to estimate potential damages. The maximum seismic intensity will occur at Ming-Chien town and the buildings at Caotun Township and Nantou city will be destroyed seriously. The part of freeway 3 which connects Caotun Township and Nantou city has to be paid more attentions to prevent the destructive fractures. Based on the predicted seismic hazards, the best way to reduce the losses of lives and properties of people is preparing a well-rounded mitigation plan in advance. When an earthquake occurs abruptly, a community-based approach to establish an excellent self-aid and mutual-aid system is an efficient method for reducing the casualties. It requires a community-wide effort over a long period of time for making the community comfortable and safe. A detailed process of a community-based mitigation plan for the seismic hazards is discussed in the paper.

Rayleigh-wave multicomponent crosscorrelation-based source strength distribution inversions. Part 2: a workflow for field seismic data

SUMMARYEstimation of ambient seismic source distributions (e.g. location and strength) can aid studies of seismic source mechanisms and subsurface structure investigations. One can invert for the ambient seismic (noise) source distribution by applying full-waveform inversion (FWI) theory to seismic (noise) crosscorrelations. This estimation method is especially applicable for seismic recordings without obvious body-wave arrivals. Data pre-processing procedures are needed before the inversion, but some pre-processing procedures commonly used in ambient noise tomography can bias the ambient (noise) source distribution estimation and should not be used in FWI. Taking this into account, we propose a complete workflow from the raw seismic noise recording through pre-processing procedures to the inversion. We present the workflow with a field data example in Hartoušov, Czech Republic, where the seismic sources are CO2 degassing areas at Earth’s surface (i.e. a fumarole or mofette). We discuss factors in the processing and inversion that can bias the estimations, such as inaccurate velocity model, anelasticity and array sensitivity. The proposed workflow can work for multicomponent data across different scales of field data.

Do stopes contribute to the seismic source?

Parameters such as source location, seismic moment, energy, source size, and stress drop are routinely calculated from mining-induced seismic data. Seismic moment tensors are inverted less routinely because their calculation is more complex and their accuracy depends on the network geometry, among a number of other factors. The models utilized in the source parameter calculations, the most well-known of which is the Brune model, were developed for the global seismicity problem and assume a solid, homogeneous Earth model. However, the tabular orebodies in South African gold and platinum mines are mined extensively and the excavations (stopes) can extend for many kilometres. The seismic source mechanisms on deep-level gold mines are generally compatible with shear failure (Hoffmann et al., 2013), whereas the source mechanisms of events at intermediate-level bord and pillar mines in the platinum district are more compatible with pillar failure and accompanying stope closure (Spottiswoode, Scheepers, and Ledwaba, 2006; Malovichko, van Aswegen, and Clark, 2012). In this paper we investigate the influence of the stope on seismic inversions for the scalar moment, corner frequency/source radius, stress drop through numerical modelling using WAVE3D. The main objective is to determine whether the source parameters calculated from the recorded waveforms are due to a combination of the stope and shearing sources, rather than being related only to a shear source in the host rock. The modelled source is shear rupture in the footwall of a stope. The results show that the stope appears to have an appreciable effect on the seismic inversions. The seismic moment and source radius of the shear source in the stope are larger for the model with a stope compared to the model with no stope. The stress drop for the case with a stope is less than the applied stress drop, which could be an effect of the apparently larger source. This work provides a possible explanation of the second corner frequency often observed in the spectra of seismograms recorded in South Africa platinum mines. This has implications for the accurate determination of source parameters and the assessment of the intensity of shaking in stopes.

A comparison of seismic response to conventional and face destress blasting during deep tunnel development

Abstract A novel design of development face destress blasting was implemented during the construction of an experimental tunnel at great depth. A second tunnel was developed nearby using conventional blasting as a control. The tunnels were developed parallel to one another and perpendicular to a high sub-horizontal stress. High resolution seismic monitoring was used to record and compare the seismic response generated by each excavation. Analysis of the seismic data from the conventionally blasted tunnel indicated that the seismogenic zone of stress-driven instability extended up to 3.6 m ahead of the face. Destress blasting within the corresponding zone of the adjacent tunnel had the effect of reducing the rock mass stiffness, primarily due to weakening of the pre-existing natural discontinuities. The reduction in rock mass stiffness was inferred from the spatial broadening of the seismogenic zone and associated reduction in the measured spatial density of events, radiated energy and seismic potency ahead of the face. High strain gradients around the unsupported portion of the conventionally blasted excavation were implied by the rate at which the spatial density of seismicity changed with respect to the tunnel face position. In contrast, the change in the spatial density of seismicity around the destressed development face was much more gradual. This was indicative of lower strain gradients in the rock there. A reduction in rock mass stiffness following destress blasting was also indicated by the much wider variety of seismic source mechanisms recorded adjacent to the destressed tunnel. Seismic source mechanisms associated with destress blasting were also more clearly characteristic of compressive overstressing with fracture closure. The source mechanism data also indicated that destress blasting induced instability on all natural joint sets. When compared to conventional development blasting, destress blasting typically reduced violent strain energy release from the rock mass and the associated seismicity, but not always.

Delineation of fault segments in mines using seismic source mechanisms and location uncertainty

The identification and quantification of faults or other geological discontinuities is an important task in managing seismic hazard in mines. Unstable slip along these faults may lead to seismic events with fatal or major economic consequences. Current approaches for delineating faults that are employed in mines rely on the interpretation of geological mapping. This mapping, however, may be sparse and can miss structures of potential concern. Clustering techniques are often used to associate seismic events to a common source process, but as previously used make no connection to the underlying physical processes.The Expectation Maximisation Algorithm is used in this study to identify probabilistic kernels representing active segments of faults. This soft assignment clustering method describes the kernels according to the seismic source mechanism, location and location uncertainty of the microseismic events. The method is tested on synthetic data and real data from an underground mine with the aim to delineate a previously unknown structure. Results using synthetic data illustrate how the incorporation of the seismic source mechanism improves the association of events to kernels in the case of scattered events with large location uncertainty. Application of the method to the real data indicates that the results can be interpreted at different levels, a smaller number of kernels provides good and robust description of the overall seismic behaviour, while using more kernels can provide insight into local variations in the location and orientation of faults.The resultant kernels have physical meaning in terms of the location and orientation of the structures. This provides geotechnical engineers at mines with improved tools to identify potentially hazardous areas in the mine and therefore manage these risks.

Source Analysis of Post-Blasting Events Recorded in Deep Copper Mine, Poland

Seismic and rock burst hazard is a very important factor that has to be considered in seismically active mining areas. Several different methods are used to decrease seismic activity in mines. Among others, an active prevention method known as destress blasting, is considered the most effective technique. The aim of this work is to find possible influences of such active prevention on observed seismic activity recorded in the established waiting time after blasts. Using technological knowledge and seismic data recorded by an underground seismic network, we estimated several parameters that characterize sources provoked by blasting works and compared them with the parameters obtained for spontaneous, non-provoked mining tremors. According to our studies the source mechanisms of post-blasting seismicity are characterized by the similar non-DC part of the full moment tensor whereas the source mechanisms of spontaneous events are not characterized by any specific features. On the other hand non-provoked seismicity presents lowest values of apparent stress. Also ES to EP ratio suggest some differences between events occurred immediately after blasting (up to 30 s) and the rest dataset. We believe that these parameters could be considered valuable tools supporting established safety time limitations after active prevention.

Velocity structure building and ground motion simulation of the 2014 Ludian Ms 6.5 Earthquake

This study constructs a 3D velocity structure model of the Ludian region in the Yunnan province, southwestern China, and simulates ground motion propagation of the 2014 Ludian Ms 6.5 earthquake. It aims to construct the local velocity structure of the Ludian region in three dimensions and with high precision. The simulation, using the spectral element method, is validated by field data from the Ludian earthquake records. Thus, it demonstrates that the adopted key parameters, such as the seismic source mechanism, propagation medium and geographical features of the engineering site, are appropriated for the simulation. Meanwhile, the simulation generates the ground motion distribution of the study region with an earthquakeinduced landslide in Ludian earthquake.

Magnitude, moment, and measurement: The seismic mechanism controversy and its resolution

This paper examines the history of two related problems concerning earthquakes, and the way in which a theoretical advance was involved in their resolution. The first problem is the development of a physical, as opposed to empirical, scale for measuring the size of earthquakes. The second problem is that of understanding what happens at the source of an earthquake. There was a controversy about what the proper model for the seismic source mechanism is, which was finally resolved through advances in the theory of elastic dislocations. These two problems are linked, because the development of a physically-based magnitude scale requires an understanding of what goes on at the seismic source. I will show how the theoretical advances allowed seismologists to re-frame the questions they were trying to answer, so that the data they gathered could be brought to bear on the problem of seismic sources in new ways.

Near‐Surface Velocity Structure of Pacaya Volcano, Guatemala, Derived from Small‐Aperture Array Analysis of Seismic Tremor

Knowledge of the typically complicated near‐surface structure on volcanoes is critical for determining accurate seismic‐event locations and seismic source mechanisms associated with precursory and eruptive activity. To generate a near‐surface velocity model for the active cone of Pacaya volcano, Guatemala, we recorded seismic tremor and ambient noise with a small‐aperture array of 11 short‐period seismometers in January 2011. The diffuse wavefield was investigated using the spatial autocorrelation (SPAC) method, and we computed Rayleigh and Love dispersion curves at the array location. The phase velocities for Rayleigh waves range from 1000 m/s at 2 Hz to 230 m/s at 10 Hz; those for Love waves range from 600 to 250 m/s over the same frequency band. Such low velocities are consistent with the presence of unconsolidated tephra on the surface of the volcano interspersed with lava flows. Assuming they represent the fundamental modes of Rayleigh and Love waves, we inverted the dispersion curves to produce a shear‐wave velocity model of the upper 500 m beneath the array. Online Material: S ‐wave velocity models obtained from inversion, and Love‐ and Rayleigh‐wave dispersion curves.

Fully probabilistic seismic source inversion – Part 1: Efficient parameterisation

Abstract. Seismic source inversion is a non-linear problem in seismology where not just the earthquake parameters themselves but also estimates of their uncertainties are of great practical importance. Probabilistic source inversion (Bayesian inference) is very adapted to this challenge, provided that the parameter space can be chosen small enough to make Bayesian sampling computationally feasible. We propose a framework for PRobabilistic Inference of Seismic source Mechanisms (PRISM) that parameterises and samples earthquake depth, moment tensor, and source time function efficiently by using information from previous non-Bayesian inversions. The source time function is expressed as a weighted sum of a small number of empirical orthogonal functions, which were derived from a catalogue of >1000 source time functions (STFs) by a principal component analysis. We use a likelihood model based on the cross-correlation misfit between observed and predicted waveforms. The resulting ensemble of solutions provides full uncertainty and covariance information for the source parameters, and permits propagating these source uncertainties into travel time estimates used for seismic tomography. The computational effort is such that routine, global estimation of earthquake mechanisms and source time functions from teleseismic broadband waveforms is feasible.

Microscale modeling of fluid flow‐geomechanics‐seismicity: Relationship between permeability and seismic source response in deformed rock joints

AbstractStudying rock joint deformation including both slippage and opening mechanisms provides an opportunity to investigate the connection between the permeability and seismic source mechanisms. A microscale fluid flow‐geomechanics‐seismicity model was built to evaluate the transport response and failure mechanism of microcracks developed along a joint in Berea sandstone samples during deformation. The modeling method considers comprehensive grain‐cement interactions. Fluid flow behavior is obtained through a realistic network model of the pore space in the compacted assembly. The geometric description of the complex pore structure is characterized to predict permeability of the rock sample as a function of rock deformation by using a dynamic pore network model. As a result of microcracks development, forces and displacements in grains involved in bond breakage are measured to determine seismic moment tensor. Shear and nonshear displacements are applied to the joint samples to investigate their effects on permeability evolution and failure mechanism of microcracks during joint deformation. In addition, the effect of joint roughness is analyzed by performing numerical compression tests. We also investigate how confining pressure affects volumetric deformation leading to opening or closure of developed microcracks and permeability changes of samples with joints.

Estimation of rockburst wall-rock velocity invoked by slab flexure sources in deep tunnels

For rock support in burst-prone ground, the wall-rock velocity adjacent to the surface of underground openings is a vital support design parameter, and depends on the seismic source mechanism inducing rockburst damage. In this study, to estimate the wall-rock velocity evoked only by rock slab buckling (an important rockburst source mechanism), a comprehensive velocity assessment method is proposed, using an excellent slab column buckling model with a small eccentricity, which relies on a novel compressive or tensile buckling failure criterion of rock slab. The true-triaxial loading–unloading tests and rockburst case analyses reveal that rock mass slabbing induced by high rock stress has major impacts on the evolution and formation of buckling rockburst in deep tunnels. Using a method based on the energy balance principle, the slabbing thickness of intact rock mass is also calculated by an analytical method, which indicates that the slabbing thickness parameter has a nonlinear relation to the following six parameters: uniaxial tensile strength (UTS), uniaxial compressive strength (UCS), normal stress (σn), length of joint (L), friction angle ([Formula: see text]), and joint roughness coefficient (JRC). These proposed models and methods have been quite successfully applied to rockburst and slabbing cases occurring in deep tunnels. These applications show that slab flexure is an important source mechanism invoking high wall-rock velocities and leading to severe rockburst damages in the area surrounding deep tunnels.