Reza Esfahani, Fabrice Cotton, Luis Fabian Bonilla
Summary Strong ground shaking has the potential to generate significant dynamic strains in shallow materials such as soils and sediments, thereby inducing nonlinear site response resulting in changes in near-surface materials. The nonlinear behavior of these materials can be characterized by an increase in wave attenuation and a decrease in the resonant frequency of the soil; these effects are attributed to increased material damping and decreased seismic wave propagation velocity, respectively. This study investigates the “in-situ” seismic velocity changes and the predominant ground motion frequency evolution during the 2016 Kumamoto earthquake sequence. This sequence includes two foreshocks (Mw6, Mw6.2) followed by a mainshock (Mw7.2) that occurred 24 h after the last foreshock. We present the results of the seismic velocity evolution during these earthquakes for seismological records collected by the KiK-net (32 stations) and K-NET (88 stations) networks between 2002 and 2020. We analyze the impulse response and autocorrelation functions to investigate the nonlinear response in near-surface materials. By comparing the results of the impulse response and autocorrelation functions, we observe that a nonlinear response occurs in near-surface materials. We then quantify the velocity reductions that occur before, during, and after the mainshock using both approaches. This allows us to estimate the “in situ” shear modulus reduction for different site classes based on VS30 values (VS30 < 360 m/s, 360 <VS30 < 760 m/s, VS30 > 760 m/s). We also establish the relationships between velocity changes, shear modulus reduction, variations in predominant ground motion frequencies, and site characteristics (VS30). The results of this analysis can be applied to site-specific ground motion modeling, site response analysis, and the incorporation of nonlinear site terms into ground motion models.
{"title":"Temporal variations of the “in-situ” nonlinear behavior of shallow sediments during the 2016 Kumamoto Earthquake sequence","authors":"Reza Esfahani, Fabrice Cotton, Luis Fabian Bonilla","doi":"10.1093/gji/ggae222","DOIUrl":"https://doi.org/10.1093/gji/ggae222","url":null,"abstract":"Summary Strong ground shaking has the potential to generate significant dynamic strains in shallow materials such as soils and sediments, thereby inducing nonlinear site response resulting in changes in near-surface materials. The nonlinear behavior of these materials can be characterized by an increase in wave attenuation and a decrease in the resonant frequency of the soil; these effects are attributed to increased material damping and decreased seismic wave propagation velocity, respectively. This study investigates the “in-situ” seismic velocity changes and the predominant ground motion frequency evolution during the 2016 Kumamoto earthquake sequence. This sequence includes two foreshocks (Mw6, Mw6.2) followed by a mainshock (Mw7.2) that occurred 24 h after the last foreshock. We present the results of the seismic velocity evolution during these earthquakes for seismological records collected by the KiK-net (32 stations) and K-NET (88 stations) networks between 2002 and 2020. We analyze the impulse response and autocorrelation functions to investigate the nonlinear response in near-surface materials. By comparing the results of the impulse response and autocorrelation functions, we observe that a nonlinear response occurs in near-surface materials. We then quantify the velocity reductions that occur before, during, and after the mainshock using both approaches. This allows us to estimate the “in situ” shear modulus reduction for different site classes based on VS30 values (VS30 &lt; 360 m/s, 360 &lt;VS30 &lt; 760 m/s, VS30 &gt; 760 m/s). We also establish the relationships between velocity changes, shear modulus reduction, variations in predominant ground motion frequencies, and site characteristics (VS30). The results of this analysis can be applied to site-specific ground motion modeling, site response analysis, and the incorporation of nonlinear site terms into ground motion models.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-06-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141514193","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Hangtao Yu, Pengbo Qin, Chuang Xu, Hui Zhang, Yi Chai, Ranran Du
Summary Before inverting Moho topography, the traditional Parker-Oldenburg method requires the determination of two important hyperparameters, the average Moho depth and Moho density contrast. The selection of these two hyperparameters will directly affect the inversion results. In this paper, a new method for estimating hyperparameters is proposed which is used to improve the Parker-Oldenburg method. The new method is improved by using simulated annealing to accurately estimate the average Moho depth and Moho density contrast based on the relationship between Moho depths and corresponding gravity anomalies at seismic control points. Synthetic tests show that compared to the improved Bott's method and the trial and error method, our method reduces the error in Moho density contrast and average Moho depth by 0.83% and 1.81% respectively. In addition, compared with the trial and error method, our method greatly improves the computational efficiency. In a practical example, we apply this method to invert the Moho topography in the northern South China Sea. The inversion results show that the Moho topography in the northern South China Sea ranges from 8.2 to 33 km. The root mean squared error between our Moho topography and the seismic validation points is 0.94 km. Compared with the CRUST 1.0 model, our Moho topography is more accurate.
{"title":"Improved Parker-Oldenburg method and its application to Moho topographic inversion in the northern South China Sea","authors":"Hangtao Yu, Pengbo Qin, Chuang Xu, Hui Zhang, Yi Chai, Ranran Du","doi":"10.1093/gji/ggae224","DOIUrl":"https://doi.org/10.1093/gji/ggae224","url":null,"abstract":"Summary Before inverting Moho topography, the traditional Parker-Oldenburg method requires the determination of two important hyperparameters, the average Moho depth and Moho density contrast. The selection of these two hyperparameters will directly affect the inversion results. In this paper, a new method for estimating hyperparameters is proposed which is used to improve the Parker-Oldenburg method. The new method is improved by using simulated annealing to accurately estimate the average Moho depth and Moho density contrast based on the relationship between Moho depths and corresponding gravity anomalies at seismic control points. Synthetic tests show that compared to the improved Bott's method and the trial and error method, our method reduces the error in Moho density contrast and average Moho depth by 0.83% and 1.81% respectively. In addition, compared with the trial and error method, our method greatly improves the computational efficiency. In a practical example, we apply this method to invert the Moho topography in the northern South China Sea. The inversion results show that the Moho topography in the northern South China Sea ranges from 8.2 to 33 km. The root mean squared error between our Moho topography and the seismic validation points is 0.94 km. Compared with the CRUST 1.0 model, our Moho topography is more accurate.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-06-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141508209","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Raymond Ng, Xiaowei Chen, Nori Nakata, Jacob I Walter
SUMMARY Microseismic monitoring is an important technique to obtain detailed knowledge of in-situ fracture size and orientation during stimulation to maximize fluid flow throughout the rock volume and optimize production. Furthermore, considering that the frequency of earthquake magnitudes empirically follows a power law (i.e. Gutenberg–Richter), the accuracy of microseismic event magnitude distributions is potentially crucial for seismic risk management. In this study, we analyse microseismicity observed during four hydraulic fracture treatments of the legacy Cotton Valley experiment in 1997 at the Carthage gas field of East Texas, where fractures were activated at the base of the sand-shale Upper Cotton Valley formation. We perform waveform cross-correlation to detect similar event clusters, measure relative amplitude from aligned waveform pairs with a principal component analysis, then measure precise relative magnitudes. The new magnitudes significantly reduce the deviations between magnitude differences and relative amplitudes of event pairs. This subsequently reduces the magnitude differences between clusters located at different depths. Reduction in magnitude differences between clusters suggests that some attenuation-related biases could be effectively mitigated with relative magnitude measurements. The maximum likelihood method is applied to understand the magnitude frequency distributions and quantify the seismogenic index of the clusters. Statistical analyses with new magnitudes suggest that fractures that are more favourably oriented for shear failure have lower b-value and higher seismogenic index, suggesting higher potential for relatively larger earthquakes, rather than fractures subparallel to maximum horizontal principal stress orientation.
摘要 微震监测是一项重要的技术，可用于在激发过程中详细了解原位裂缝的大小和走向，从而最大限度地提高流体在整个岩体中的流动性并优化生产。此外，考虑到地震震级频率根据经验遵循幂律（即古登堡-里克特），微震事件震级分布的准确性对于地震风险管理至关重要。在本研究中，我们分析了 1997 年在德克萨斯州东部迦太基气田进行的传统棉花谷实验的四次水力压裂处理过程中观察到的微震。我们通过波形交叉相关来检测类似的事件集群，利用主成分分析从对齐的波形对中测量相对振幅，然后测量精确的相对振幅。新的振幅大大减少了事件对的振幅差和相对振幅之间的偏差。随后，位于不同深度的事件群之间的振幅差异也随之减小。群集之间振幅差异的减少表明，一些与衰减相关的偏差可以通过相对振幅测量得到有效缓解。应用最大似然法了解震级频率分布并量化震群的成震指数。利用新震级进行的统计分析表明，更有利于剪切破坏的断裂具有更低的 b 值和更高的成震指数，这表明发生相对较大地震的可能性比与最大水平主应力方向不平行的断裂更大。
{"title":"Precise relative magnitude measurement improves fracture characterization during hydraulic fracturing","authors":"Raymond Ng, Xiaowei Chen, Nori Nakata, Jacob I Walter","doi":"10.1093/gji/ggae204","DOIUrl":"https://doi.org/10.1093/gji/ggae204","url":null,"abstract":"SUMMARY Microseismic monitoring is an important technique to obtain detailed knowledge of in-situ fracture size and orientation during stimulation to maximize fluid flow throughout the rock volume and optimize production. Furthermore, considering that the frequency of earthquake magnitudes empirically follows a power law (i.e. Gutenberg–Richter), the accuracy of microseismic event magnitude distributions is potentially crucial for seismic risk management. In this study, we analyse microseismicity observed during four hydraulic fracture treatments of the legacy Cotton Valley experiment in 1997 at the Carthage gas field of East Texas, where fractures were activated at the base of the sand-shale Upper Cotton Valley formation. We perform waveform cross-correlation to detect similar event clusters, measure relative amplitude from aligned waveform pairs with a principal component analysis, then measure precise relative magnitudes. The new magnitudes significantly reduce the deviations between magnitude differences and relative amplitudes of event pairs. This subsequently reduces the magnitude differences between clusters located at different depths. Reduction in magnitude differences between clusters suggests that some attenuation-related biases could be effectively mitigated with relative magnitude measurements. The maximum likelihood method is applied to understand the magnitude frequency distributions and quantify the seismogenic index of the clusters. Statistical analyses with new magnitudes suggest that fractures that are more favourably oriented for shear failure have lower b-value and higher seismogenic index, suggesting higher potential for relatively larger earthquakes, rather than fractures subparallel to maximum horizontal principal stress orientation.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-06-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141514194","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
J M Guerrero, T T Ojo, C W Fairservice, P Javaheri, J P Lowman
Summary Evidence that the Earth’s surface is divided into a tessellation of piece-wise rigidly translating plates is the primary observation supporting the solid-state creep-enabled convection paradigm, utilised to investigate evolution of the Earth’s mantle. Accordingly, identifying the system properties that allow for obtaining dynamically generated plates remains a primary objective in numerical global mantle convection simulations. The first challenge for analysing fluid dynamic model output for the generation of rigid plates is to identify candidate plate boundaries. Here, we utilise a previously introduced numerical tool for plate boundary detection which employs a user specified threshold (tolerance) to automatically detect candidate plate boundaries. The numerical tool is applied with different sensitivities, to investigate the nature of the surface velocity fields generated in three calculations described in earlier work. The cases examined differ by the values that they specify for the model yield stress, a parameter that can allow the formation of tightly focussed bands of surface deformation. The three calculations we examine include zones comprising possible plate boundaries that are characterised by convergence, divergence, and strike-slip behaviour. Importance of the potential plate boundaries is assessed by examining the rigidity of the inferred model generated plates. The rigidity is measured by comparing the model velocities to the rigid rotation velocities implied by the statistically determined Euler poles for each candidate plate. We quantify a lack in rigidity by calculating a deformity field based on disagreement of actual surface velocity with rotation about the Euler pole. For intermediate yield stress and boundary detection threshold value, we find that the majority of the model surface can translate almost rigidly about distinct plate Euler poles. Regions that conform poorly to large-scale region rigid translation are also obtained but we find that generally these regions can be decomposed into subsets of smaller plates with a lower tolerance value. Alternatively, these regions may represent diffuse boundary zones. To clarify the degree to which the mantle convection model behaviour shows analogues with Earth’s current-day surface motion, we apply the plate boundary detection and Euler pole calculation methods to previously published terrestrial strain-rate data. Strong parallels are found in the response of the terrestrial data and mantle convection calculations to the threshold value, such that appropriate choice of that parameter results in very good agreement between observations and convection model character. We conclude that plates generated by fluid dynamic convection models can exhibit motion that is markedly rigid, and define statistics (plateness) and fields (deformity) by which the generation of self-consistently determined plate rigidity can be quantified, as well as describing how plate recognition might b
{"title":"Utilising Euler poles for the evaluation of plate rigidity in numerical mantle convection models","authors":"J M Guerrero, T T Ojo, C W Fairservice, P Javaheri, J P Lowman","doi":"10.1093/gji/ggae219","DOIUrl":"https://doi.org/10.1093/gji/ggae219","url":null,"abstract":"Summary Evidence that the Earth’s surface is divided into a tessellation of piece-wise rigidly translating plates is the primary observation supporting the solid-state creep-enabled convection paradigm, utilised to investigate evolution of the Earth’s mantle. Accordingly, identifying the system properties that allow for obtaining dynamically generated plates remains a primary objective in numerical global mantle convection simulations. The first challenge for analysing fluid dynamic model output for the generation of rigid plates is to identify candidate plate boundaries. Here, we utilise a previously introduced numerical tool for plate boundary detection which employs a user specified threshold (tolerance) to automatically detect candidate plate boundaries. The numerical tool is applied with different sensitivities, to investigate the nature of the surface velocity fields generated in three calculations described in earlier work. The cases examined differ by the values that they specify for the model yield stress, a parameter that can allow the formation of tightly focussed bands of surface deformation. The three calculations we examine include zones comprising possible plate boundaries that are characterised by convergence, divergence, and strike-slip behaviour. Importance of the potential plate boundaries is assessed by examining the rigidity of the inferred model generated plates. The rigidity is measured by comparing the model velocities to the rigid rotation velocities implied by the statistically determined Euler poles for each candidate plate. We quantify a lack in rigidity by calculating a deformity field based on disagreement of actual surface velocity with rotation about the Euler pole. For intermediate yield stress and boundary detection threshold value, we find that the majority of the model surface can translate almost rigidly about distinct plate Euler poles. Regions that conform poorly to large-scale region rigid translation are also obtained but we find that generally these regions can be decomposed into subsets of smaller plates with a lower tolerance value. Alternatively, these regions may represent diffuse boundary zones. To clarify the degree to which the mantle convection model behaviour shows analogues with Earth’s current-day surface motion, we apply the plate boundary detection and Euler pole calculation methods to previously published terrestrial strain-rate data. Strong parallels are found in the response of the terrestrial data and mantle convection calculations to the threshold value, such that appropriate choice of that parameter results in very good agreement between observations and convection model character. We conclude that plates generated by fluid dynamic convection models can exhibit motion that is markedly rigid, and define statistics (plateness) and fields (deformity) by which the generation of self-consistently determined plate rigidity can be quantified, as well as describing how plate recognition might b","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-06-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141514081","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Summary Geothermal heat flow beneath the Greenland and Antarctic ice sheets is an important boundary condition for ice sheet dynamics, but is rarely measured directly and therefore is inferred indirectly from proxies (e.g. seismic structure, magnetic Curie depth, surface topography). We seek to improve the understanding of the relationship between heat flow and one such proxy—seismic structure—and determine how well heat flow data can be predicted from the structure (the characterization problem). We also seek to quantify the extent to which this relationship can be extrapolated from one continent to another (the transportability problem). To address these problems, we use direct heat flow observations and new seismic structural information in the contiguous US and Europe, and construct three Machine Learning models of the relationship with different levels of complexity (Linear Regression, Decision Tree, Random Forest). We compare these models in terms of their interpretability, the predicted heat flow accuracy within a continent, and the accuracy of the extrapolation between Europe and the US. The Random Forest and Decision Tree models are the most accurate within a continent, while the Linear Regression and Decision Tree models are the most accurate upon extrapolation between continents. The Decision Tree model uniquely illuminates the regional variations of the relationship between heat flow and seismic structure. From the Decision Tree model, uppermost mantle shear wavespeed, crustal shear wavespeed and Moho depth together explain more than half of the observed heat flow variations in both the US (r2 ≈ 0.6 (coefficient of determination), RMSE ≈ 8mW/m2 (Root Mean Squared Error)) and Europe (r2 ≈ 0.5, RMSE ≈ 13mW/m2), such that uppermost mantle shear wavespeed is the most important. Extrapolating the US-trained models to Europe reasonably predicts the geographical distribution of heat flow (ρ = 0.48 (correlation coefficient)), but not the absolute amplitude of the variations (r2 = 0.17), similarly from Europe to the US (ρ = 0.66, r2 = 0.24). The deterioration of accuracy upon extrapolation is caused by differences between the continents in how seismic structure is imaged, the heat flow data, and intrinsic crustal radiogenic heat production. Our methods have the potential to improve the reliability and resolution of heat flow inferences across Antarctica and the validation and cross-validation procedures we present can be applied to heat flow proxies other than seismic structure, which may help resolve inconsistencies between existing subglacial heat flow values inferred using different proxies.
{"title":"Applying Machine Learning to Characterize and Extrapolate the Relationship Between Seismic Structure and Surface Heat Flow","authors":"Shane Zhang, Michael H Ritzwoller","doi":"10.1093/gji/ggae218","DOIUrl":"https://doi.org/10.1093/gji/ggae218","url":null,"abstract":"Summary Geothermal heat flow beneath the Greenland and Antarctic ice sheets is an important boundary condition for ice sheet dynamics, but is rarely measured directly and therefore is inferred indirectly from proxies (e.g. seismic structure, magnetic Curie depth, surface topography). We seek to improve the understanding of the relationship between heat flow and one such proxy—seismic structure—and determine how well heat flow data can be predicted from the structure (the characterization problem). We also seek to quantify the extent to which this relationship can be extrapolated from one continent to another (the transportability problem). To address these problems, we use direct heat flow observations and new seismic structural information in the contiguous US and Europe, and construct three Machine Learning models of the relationship with different levels of complexity (Linear Regression, Decision Tree, Random Forest). We compare these models in terms of their interpretability, the predicted heat flow accuracy within a continent, and the accuracy of the extrapolation between Europe and the US. The Random Forest and Decision Tree models are the most accurate within a continent, while the Linear Regression and Decision Tree models are the most accurate upon extrapolation between continents. The Decision Tree model uniquely illuminates the regional variations of the relationship between heat flow and seismic structure. From the Decision Tree model, uppermost mantle shear wavespeed, crustal shear wavespeed and Moho depth together explain more than half of the observed heat flow variations in both the US (r2 ≈ 0.6 (coefficient of determination), RMSE ≈ 8mW/m2 (Root Mean Squared Error)) and Europe (r2 ≈ 0.5, RMSE ≈ 13mW/m2), such that uppermost mantle shear wavespeed is the most important. Extrapolating the US-trained models to Europe reasonably predicts the geographical distribution of heat flow (ρ = 0.48 (correlation coefficient)), but not the absolute amplitude of the variations (r2 = 0.17), similarly from Europe to the US (ρ = 0.66, r2 = 0.24). The deterioration of accuracy upon extrapolation is caused by differences between the continents in how seismic structure is imaged, the heat flow data, and intrinsic crustal radiogenic heat production. Our methods have the potential to improve the reliability and resolution of heat flow inferences across Antarctica and the validation and cross-validation procedures we present can be applied to heat flow proxies other than seismic structure, which may help resolve inconsistencies between existing subglacial heat flow values inferred using different proxies.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-06-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141508210","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Summary The mechanism responsible for the lateral expansion and uplift of the eastern Tibetan Plateau remains a topic of ongoing debate, partly due to discrepancies in the results of seismic velocity and anisotropy. In local earthquake tomography, hypocentral uncertainties can cause significant errors in the tomographic model. However, this issue has received limited attention in previous studies. In this work, we employ the weighted least-squares (WLS) method to solve the tomographic inversion problem. A power exponent coefficient, which is called weighting level, is introduced into the weighting matrix to control the relative contribution of the data with different hypocentral errors to the final tomographic result. Our data set contains high-quality Pg, Pn and Sg arrival times of local earthquakes recorded by the dense Chinese seismic network in eastern Tibet during 2008 to 2022. We comprehensively analyze the inversion results derived from the WLS inversions with different weighting levels to evaluate the robustness of isotropic velocity anomalies and azimuthal anisotropy. The most robust feature of our results is a striking low-velocity (low-Vp) zone surrounded by high-velocity (high-Vp) anomalies and fault parallel fast-velocity directions (FVDs) of azimuthal anisotropy in the lower crust beneath the western side of the Longmenshan fault zone. Taking into account many previous results of the region, we deem that the low-Vp zone reflects hot and wet upwelling flow from the deep asthenosphere, which ascends to the lower crust along the fault zone. At the NE margin of the Tibetan Plateau, significant low-Vp anomalies exist in the lower crust and the FVDs are consistent with the motion direction of the Tibetan block revealed by GPS observations. We think that lower crustal flow exists beneath NE Tibet, which controls the plateau expansion toward the northeast. A low-Vp anomaly appears at 30 km depth beneath the Sichuan Basin. However, as the weighting level increases, the amplitude of this low-Vp anomaly decreases by more than 6%, suggesting that this low-Vp anomaly has a larger uncertainty than the other features.
{"title":"Anisotropic tomography of eastern Tibet and its uncertainty from hypocentral errors","authors":"Ruo Jia, Dapeng Zhao, Rizheng He","doi":"10.1093/gji/ggae221","DOIUrl":"https://doi.org/10.1093/gji/ggae221","url":null,"abstract":"Summary The mechanism responsible for the lateral expansion and uplift of the eastern Tibetan Plateau remains a topic of ongoing debate, partly due to discrepancies in the results of seismic velocity and anisotropy. In local earthquake tomography, hypocentral uncertainties can cause significant errors in the tomographic model. However, this issue has received limited attention in previous studies. In this work, we employ the weighted least-squares (WLS) method to solve the tomographic inversion problem. A power exponent coefficient, which is called weighting level, is introduced into the weighting matrix to control the relative contribution of the data with different hypocentral errors to the final tomographic result. Our data set contains high-quality Pg, Pn and Sg arrival times of local earthquakes recorded by the dense Chinese seismic network in eastern Tibet during 2008 to 2022. We comprehensively analyze the inversion results derived from the WLS inversions with different weighting levels to evaluate the robustness of isotropic velocity anomalies and azimuthal anisotropy. The most robust feature of our results is a striking low-velocity (low-Vp) zone surrounded by high-velocity (high-Vp) anomalies and fault parallel fast-velocity directions (FVDs) of azimuthal anisotropy in the lower crust beneath the western side of the Longmenshan fault zone. Taking into account many previous results of the region, we deem that the low-Vp zone reflects hot and wet upwelling flow from the deep asthenosphere, which ascends to the lower crust along the fault zone. At the NE margin of the Tibetan Plateau, significant low-Vp anomalies exist in the lower crust and the FVDs are consistent with the motion direction of the Tibetan block revealed by GPS observations. We think that lower crustal flow exists beneath NE Tibet, which controls the plateau expansion toward the northeast. A low-Vp anomaly appears at 30 km depth beneath the Sichuan Basin. However, as the weighting level increases, the amplitude of this low-Vp anomaly decreases by more than 6%, suggesting that this low-Vp anomaly has a larger uncertainty than the other features.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-06-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141514080","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Summary The prominent Pamir plateau holds considerable significance in comprehending the processes of Asian continental collisional orogeny. However, due to harsh natural conditions and low seismic activity within the Pamir hinterland, our understanding of this region remains deficient. Recent major events and the accumulation of geodetic observations present a rare opportunity for us to get insights into the tectonic activities and orogenic processes occurring in this region. Firstly, employing Sentinel-1 and ALOS-2 SAR images, we acquire coseismic displacements associated with the most recent earthquakes in 2015 and 2023. Subsequently, we conduct the souce models inversion with the constraints of surface displacements based on a finite-fault model. Our results reveal displacements ranging from -0.8 m to 0.8 m for the 2015 Mw 7.2 Tajik earthquake and -0.25 m to 0.25 m for the 2023 Mw 6.9 Murghob event, respectively. The optimal three-segment model for the 2015 event ruptured a fault length of 89 km with a surface rupture extending 59 km along the Sarez-Karakul fault (SKF), characterized predominantly by left-lateral strike-slip motion, with a maximum slip of 3.5 m. Meanwhile, our preferred uniform slip model suggests that the 2023 event ruptured an unmapped fault in the southern Pamir region with a strike angle of 31° and a dip angle of 76.8°. The distributed slip model indicates that the 2023 event ruptured a fault length of 32 km, resulting in an 8 km surface rupture. This event is characterized by left-lateral strike slip, with a peak slip of 2.2 m. Secondly, the Coulomb stress calculations demonstrate that the 2023 event was impeded by the 2015 event. Finally, interseismic GPS data reveals a relative motion of 3.4–5.7 mm/yr in the N-S component and 3.2–3.8 mm/yr in the E-W component along the SKF in the Pamir hinterland, respectively. These N-S direction strike-slip activities and slip behaviors support an ongoing strong shear and extension in the Pamir regime, which is a response to the oblique convergence between the Indian and Eurasian plates.
{"title":"The N-S direction strike-slip activities in the Pamir hinterland under oblique convergence: the 2015 and 2023 earthquakes","authors":"Ping He, Yangmao Wen, Xiaohang Wang, Jianfeng Cai","doi":"10.1093/gji/ggae214","DOIUrl":"https://doi.org/10.1093/gji/ggae214","url":null,"abstract":"Summary The prominent Pamir plateau holds considerable significance in comprehending the processes of Asian continental collisional orogeny. However, due to harsh natural conditions and low seismic activity within the Pamir hinterland, our understanding of this region remains deficient. Recent major events and the accumulation of geodetic observations present a rare opportunity for us to get insights into the tectonic activities and orogenic processes occurring in this region. Firstly, employing Sentinel-1 and ALOS-2 SAR images, we acquire coseismic displacements associated with the most recent earthquakes in 2015 and 2023. Subsequently, we conduct the souce models inversion with the constraints of surface displacements based on a finite-fault model. Our results reveal displacements ranging from -0.8 m to 0.8 m for the 2015 Mw 7.2 Tajik earthquake and -0.25 m to 0.25 m for the 2023 Mw 6.9 Murghob event, respectively. The optimal three-segment model for the 2015 event ruptured a fault length of 89 km with a surface rupture extending 59 km along the Sarez-Karakul fault (SKF), characterized predominantly by left-lateral strike-slip motion, with a maximum slip of 3.5 m. Meanwhile, our preferred uniform slip model suggests that the 2023 event ruptured an unmapped fault in the southern Pamir region with a strike angle of 31° and a dip angle of 76.8°. The distributed slip model indicates that the 2023 event ruptured a fault length of 32 km, resulting in an 8 km surface rupture. This event is characterized by left-lateral strike slip, with a peak slip of 2.2 m. Secondly, the Coulomb stress calculations demonstrate that the 2023 event was impeded by the 2015 event. Finally, interseismic GPS data reveals a relative motion of 3.4–5.7 mm/yr in the N-S component and 3.2–3.8 mm/yr in the E-W component along the SKF in the Pamir hinterland, respectively. These N-S direction strike-slip activities and slip behaviors support an ongoing strong shear and extension in the Pamir regime, which is a response to the oblique convergence between the Indian and Eurasian plates.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-06-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141514082","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Jin Cao, Yunhe Liu, Changchun Yin, Haoman Wang, Yang Su, Luyuan Wang, Xinpeng Ma, Bo Zhang
Summary We propose a novel method for three-dimensional (3D) magnetotelluric (MT) forward modeling based on hybrid meshless and finite-element (FE) methods. This method divides the earth model into a central computational region and an expansion one. For the central region, we adopt scatter points to discretize the model, which can flexibly and accurately characterize the complex structures without generating unstructured mesh. The meshless method using compact support radial basis function is applied to simulate this area's electromagnetic (EM) field. While in the expansion region, to avoid the heavy time consumption and numerical error of the meshless method caused by non-uniform nodes, we adopt a node-based finite-element method with regular hexahedral mesh for stability. Finally, the two discretized systems are coupled at the interface nodes according to the continuity conditions of vector and scalar potentials. Considering that the normal electric field is discontinuous at the interface with resistivity discontinuity, while the shape functions for the meshless method are continuous, we further adopt the visibility criterion in constructing the support region. Numerical experiments on typical models show that using the same degree of freedom (DOF), the hybrid meshless-FEM (HMF) algorithm has higher accuracy than the node-based finite element method (FEM) and meshless method. In addition, the electric field discontinuity at interfaces is well preserved, which proves the effectiveness of the visibility criterion method. In general, compared to the conventional grid-based method, this new approach doesn't need the complex mesh generation for complex structures and can achieve high accuracy, thus it has the potential to become a powerful 3D MT forward modeling technique.
{"title":"Hybrid meshless-FEM method for 3D magnetotelluric modeling using non-conformal discretization","authors":"Jin Cao, Yunhe Liu, Changchun Yin, Haoman Wang, Yang Su, Luyuan Wang, Xinpeng Ma, Bo Zhang","doi":"10.1093/gji/ggae215","DOIUrl":"https://doi.org/10.1093/gji/ggae215","url":null,"abstract":"Summary We propose a novel method for three-dimensional (3D) magnetotelluric (MT) forward modeling based on hybrid meshless and finite-element (FE) methods. This method divides the earth model into a central computational region and an expansion one. For the central region, we adopt scatter points to discretize the model, which can flexibly and accurately characterize the complex structures without generating unstructured mesh. The meshless method using compact support radial basis function is applied to simulate this area's electromagnetic (EM) field. While in the expansion region, to avoid the heavy time consumption and numerical error of the meshless method caused by non-uniform nodes, we adopt a node-based finite-element method with regular hexahedral mesh for stability. Finally, the two discretized systems are coupled at the interface nodes according to the continuity conditions of vector and scalar potentials. Considering that the normal electric field is discontinuous at the interface with resistivity discontinuity, while the shape functions for the meshless method are continuous, we further adopt the visibility criterion in constructing the support region. Numerical experiments on typical models show that using the same degree of freedom (DOF), the hybrid meshless-FEM (HMF) algorithm has higher accuracy than the node-based finite element method (FEM) and meshless method. In addition, the electric field discontinuity at interfaces is well preserved, which proves the effectiveness of the visibility criterion method. In general, compared to the conventional grid-based method, this new approach doesn't need the complex mesh generation for complex structures and can achieve high accuracy, thus it has the potential to become a powerful 3D MT forward modeling technique.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-06-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141514083","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
V Ruiz González, E M Renda, H Vizán, F Martín-Hernández, A Palencia-Ortas, M L Osete
Summary In this study, we present the results of palaeomagnetic research conducted on Jurassic units of the Cañadón Asfalto Basin (CAB) in Patagonia, formed during Gondwana breakup. This basin is a key locality for understanding intra-plate deformation within Patagonia during the Jurassic. The nature of this basin has been a subject of debate, based on the dynamics of the blocks that constitute its depocentres. In this context, the palaeomagnetic study of the Jurassic units of this basin provides a unique methodology to characterise the tectonic motions of its crustal blocks during its formation and development. To achieve this, we collected 350 samples from 53 sites in the sedimentary units of Las Leoneras (ca. 189 Ma) and Cañadón Calcáreo Formations (ca. 160 Ma – 157 Ma), as well as the volcanic Lonco Trapial Group (ca. 185 Ma – 172 Ma). The palaeomagnetic results from the sedimentary units show a regional remagnetisation due to hydrothermal activity that obliterated the original remanence and overprinted a new one, simultaneously imprinting a secondary remanence in the volcanic units of the Lonco Trapial Group. When comparing the direction of the palaeomagnetic pole obtained from the remagnetised units with respect to average poles of equivalent ages, it is observed that the remagnetisation must have occurred during the Late Jurassic (ca. 145 Ma). The age range in which this process occurred (Oxfordian to Aptian) and the direction of the calculated pole dispute a monster polar shift postulated for Late Jurassic to Early Cretaceous times. In addition, the primary magnetisation recorded in the units of the Lonco Trapial Group indicates a counterclockwise rotation of the studied crustal blocks between 21º and 11º, which, in line with previous studies, refutes large-scale dextral motion along the Gastre Fault System since the Jurassic. Similar counterclockwise rotations of equivalent magnitudes are found along the units overlying the Palaeozoic Central Patagonian Igneous-Metamorphic Belt, which represents the opposite shear sense compared to the Jurassic units beyond this belt. This is interpreted as a reactivation of the Palaeozoic belt structures in the opposite sense, from transpressive during the Palaeozoic to transtensive during the Mesozoic.
{"title":"Intra-plate deformation during Gondwana breakup: a study of the Jurassic units of the Cañadón Asfalto Basin (extra-Andean Patagonia, Argentina)","authors":"V Ruiz González, E M Renda, H Vizán, F Martín-Hernández, A Palencia-Ortas, M L Osete","doi":"10.1093/gji/ggae217","DOIUrl":"https://doi.org/10.1093/gji/ggae217","url":null,"abstract":"Summary In this study, we present the results of palaeomagnetic research conducted on Jurassic units of the Cañadón Asfalto Basin (CAB) in Patagonia, formed during Gondwana breakup. This basin is a key locality for understanding intra-plate deformation within Patagonia during the Jurassic. The nature of this basin has been a subject of debate, based on the dynamics of the blocks that constitute its depocentres. In this context, the palaeomagnetic study of the Jurassic units of this basin provides a unique methodology to characterise the tectonic motions of its crustal blocks during its formation and development. To achieve this, we collected 350 samples from 53 sites in the sedimentary units of Las Leoneras (ca. 189 Ma) and Cañadón Calcáreo Formations (ca. 160 Ma – 157 Ma), as well as the volcanic Lonco Trapial Group (ca. 185 Ma – 172 Ma). The palaeomagnetic results from the sedimentary units show a regional remagnetisation due to hydrothermal activity that obliterated the original remanence and overprinted a new one, simultaneously imprinting a secondary remanence in the volcanic units of the Lonco Trapial Group. When comparing the direction of the palaeomagnetic pole obtained from the remagnetised units with respect to average poles of equivalent ages, it is observed that the remagnetisation must have occurred during the Late Jurassic (ca. 145 Ma). The age range in which this process occurred (Oxfordian to Aptian) and the direction of the calculated pole dispute a monster polar shift postulated for Late Jurassic to Early Cretaceous times. In addition, the primary magnetisation recorded in the units of the Lonco Trapial Group indicates a counterclockwise rotation of the studied crustal blocks between 21º and 11º, which, in line with previous studies, refutes large-scale dextral motion along the Gastre Fault System since the Jurassic. Similar counterclockwise rotations of equivalent magnitudes are found along the units overlying the Palaeozoic Central Patagonian Igneous-Metamorphic Belt, which represents the opposite shear sense compared to the Jurassic units beyond this belt. This is interpreted as a reactivation of the Palaeozoic belt structures in the opposite sense, from transpressive during the Palaeozoic to transtensive during the Mesozoic.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-06-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141514084","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Benjamin Fernando, Pierrick Mialle, Göran Ekström, Constantinos Charalambous, Steven Desch, Alan Jackson, Eleanor K Sansom
Summary We conduct a thorough analysis of seismic and acoustic data purported to be from the so-called ‘Interstellar Meteor’ which entered the Earth’s atmosphere off the coast of Papua New Guinea on 2014-01-08. Previous work had suggested that this meteor may have been caused by an alien spacecraft burning up in the atmosphere. We conclude that both previously-reported seismic signals are spurious - one has characteristics suggesting a local vehicular-traffic based origin; whilst the other is statistically indistinguishable from the background noise. As such, previously-reported localisations based on this data are unreliable. Analysis of acoustic data provides a best fit location estimate which is very far (∼170 km) from the reported fireball location. Accordingly, we conclude that material recovered from the seafloor and purported to be from this event is almost certainly unrelated to it, and is likely of more mundane (non-interstellar) origin.
{"title":"Seismic and acoustic signals from the 2014 ‘Interstellar Meteor’","authors":"Benjamin Fernando, Pierrick Mialle, Göran Ekström, Constantinos Charalambous, Steven Desch, Alan Jackson, Eleanor K Sansom","doi":"10.1093/gji/ggae202","DOIUrl":"https://doi.org/10.1093/gji/ggae202","url":null,"abstract":"Summary We conduct a thorough analysis of seismic and acoustic data purported to be from the so-called ‘Interstellar Meteor’ which entered the Earth’s atmosphere off the coast of Papua New Guinea on 2014-01-08. Previous work had suggested that this meteor may have been caused by an alien spacecraft burning up in the atmosphere. We conclude that both previously-reported seismic signals are spurious - one has characteristics suggesting a local vehicular-traffic based origin; whilst the other is statistically indistinguishable from the background noise. As such, previously-reported localisations based on this data are unreliable. Analysis of acoustic data provides a best fit location estimate which is very far (∼170 km) from the reported fireball location. Accordingly, we conclude that material recovered from the seafloor and purported to be from this event is almost certainly unrelated to it, and is likely of more mundane (non-interstellar) origin.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-06-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141514085","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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