Summary Using forward mantle convection models starting at 140 Ma, and assimilating plate reconstructions as surface velocity boundary condition, we predict present-day mantle structure and compare them with tomography models, using geoid as an additional constraint. We explore a wide model parameter space, such as different values of Clapeyron slope and density change across 660 km, density and viscosity of the thermochemical piles at the core-mantle boundary (CMB), internal heat generation rate, and model initiation age. We also investigate the effects of different strengths of a weak layer below 660 km and weaker asthenosphere and slabs. Our results suggest that slab structures at different subduction zones are sensitive to the viscosity of the asthenosphere, strength of slabs, values of Clapeyron slope and the density and viscosity of the thermochemical piles, while different internal heat generation rates do not affect the slab structures. We find that with a moderately weak asthenosphere (1020 Pas) and strong slabs, the predicted slab structures are consistent with the tomography models, and the observed geoid is also matched well. Moreover, our models successfully reproduce the degree-2 structure of the lower mantle beneath Africa and the Pacific, also known as Large Low Shear Velocity provinces (LLSVPs). A moderate Clapeyron slope of -2.5 MPa/K at 660 km aids in slab stagnation while higher values result in massive slab accumulation at that depth, ultimately leading to slab avalanches. We also find that the convective patterns in the thermal and thermochemical cases with slightly denser LLSVPs are similar, although the geoid amplitudes are lower for the latter. However, with more dense LLSVPs, the slabs cannot perturb them and no plumes are generated. Plumes arise as thermal instabilities from the edges of the LLSVPs, when cold and viscous slabs perturb them. While our predicted plume locations are consistent with the observed hotspot locations, matching the plume structures in tomography models is difficult. These plumes are essential in fitting the finer features of the observed geoid. In longer-duration models, more voluminous subducted material reaches the CMB, which tends to erode the LLSVPs significantly, and yields a poor fit to the observed geoid. Our results suggest that with the presence of a thin, moderately weak layer below 660 km, a slightly dense LLSVP, and Clapeyron slope of -2.5 MPa/K, the velocity anomalies in seismic tomography and the long-wavelength geoid can be matched well. One of the limitations of our models is that the assimilated plate motion history may be too short to overcome arbitrary initial conditions effects. Also, assimilated true plate velocities in our models may not represent the true convective vigor of the Earth.
{"title":"Present day mantle structure from global mantle convection models since the Cretaceous","authors":"Debanjan Pal, Attreyee Ghosh","doi":"10.1093/gji/ggae231","DOIUrl":"https://doi.org/10.1093/gji/ggae231","url":null,"abstract":"Summary Using forward mantle convection models starting at 140 Ma, and assimilating plate reconstructions as surface velocity boundary condition, we predict present-day mantle structure and compare them with tomography models, using geoid as an additional constraint. We explore a wide model parameter space, such as different values of Clapeyron slope and density change across 660 km, density and viscosity of the thermochemical piles at the core-mantle boundary (CMB), internal heat generation rate, and model initiation age. We also investigate the effects of different strengths of a weak layer below 660 km and weaker asthenosphere and slabs. Our results suggest that slab structures at different subduction zones are sensitive to the viscosity of the asthenosphere, strength of slabs, values of Clapeyron slope and the density and viscosity of the thermochemical piles, while different internal heat generation rates do not affect the slab structures. We find that with a moderately weak asthenosphere (1020 Pas) and strong slabs, the predicted slab structures are consistent with the tomography models, and the observed geoid is also matched well. Moreover, our models successfully reproduce the degree-2 structure of the lower mantle beneath Africa and the Pacific, also known as Large Low Shear Velocity provinces (LLSVPs). A moderate Clapeyron slope of -2.5 MPa/K at 660 km aids in slab stagnation while higher values result in massive slab accumulation at that depth, ultimately leading to slab avalanches. We also find that the convective patterns in the thermal and thermochemical cases with slightly denser LLSVPs are similar, although the geoid amplitudes are lower for the latter. However, with more dense LLSVPs, the slabs cannot perturb them and no plumes are generated. Plumes arise as thermal instabilities from the edges of the LLSVPs, when cold and viscous slabs perturb them. While our predicted plume locations are consistent with the observed hotspot locations, matching the plume structures in tomography models is difficult. These plumes are essential in fitting the finer features of the observed geoid. In longer-duration models, more voluminous subducted material reaches the CMB, which tends to erode the LLSVPs significantly, and yields a poor fit to the observed geoid. Our results suggest that with the presence of a thin, moderately weak layer below 660 km, a slightly dense LLSVP, and Clapeyron slope of -2.5 MPa/K, the velocity anomalies in seismic tomography and the long-wavelength geoid can be matched well. One of the limitations of our models is that the assimilated plate motion history may be too short to overcome arbitrary initial conditions effects. Also, assimilated true plate velocities in our models may not represent the true convective vigor of the Earth.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-06-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141508208","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}
Yu Hong, Jianghai Xia, Hao Zhang, Jingyin Pang, Bo Guan, Jie Zhou, Yulong Ma
Summary Passive surface-wave methods have found extensive application in near-surface investigation due to their benefits of low costs, noninvasiveness, and high accuracy. Linear arrays are usually adopted in urban environments for their convenience and efficiency. However, the distribution of noise sources in densely populated urban areas varies rapidly in time and space, making it challenging to estimate accurate dispersion spectra using a linear array. To solve this problem, we propose a polarization analysis-based azimuthal correction method. We first obtain the azimuth of each segment by calculating the correlation coefficient of three-component ambient noise data. The normalized correlation coefficient is then applied for quality control to select reliable segments. For selected segments, the overestimated velocity caused by directional sources are corrected to obtain accurate dispersion spectra. A synthetic test is conducted to demonstrate the feasibility of our method. Compared with the dispersion spectra obtained without any correction, the dispersion spectra obtained following the suggested scheme are more consistent with the theoretical dispersion curves. Two real-world examples at crossroads show the superiority of the proposed technique in obtaining higher-resolution dispersion energy and more accurate phase velocities. In addition, our approach can attenuate the artifacts and improve the dispersion measurements.
{"title":"Azimuth correction for passive surface wave dispersion based on polarization analysis","authors":"Yu Hong, Jianghai Xia, Hao Zhang, Jingyin Pang, Bo Guan, Jie Zhou, Yulong Ma","doi":"10.1093/gji/ggae232","DOIUrl":"https://doi.org/10.1093/gji/ggae232","url":null,"abstract":"Summary Passive surface-wave methods have found extensive application in near-surface investigation due to their benefits of low costs, noninvasiveness, and high accuracy. Linear arrays are usually adopted in urban environments for their convenience and efficiency. However, the distribution of noise sources in densely populated urban areas varies rapidly in time and space, making it challenging to estimate accurate dispersion spectra using a linear array. To solve this problem, we propose a polarization analysis-based azimuthal correction method. We first obtain the azimuth of each segment by calculating the correlation coefficient of three-component ambient noise data. The normalized correlation coefficient is then applied for quality control to select reliable segments. For selected segments, the overestimated velocity caused by directional sources are corrected to obtain accurate dispersion spectra. A synthetic test is conducted to demonstrate the feasibility of our method. Compared with the dispersion spectra obtained without any correction, the dispersion spectra obtained following the suggested scheme are more consistent with the theoretical dispersion curves. Two real-world examples at crossroads show the superiority of the proposed technique in obtaining higher-resolution dispersion energy and more accurate phase velocities. In addition, our approach can attenuate the artifacts and improve the dispersion measurements.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-06-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141508207","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 We present a novel strategy for performing joint inversion with guided fuzzy c-means (GFCM) clustering coupling and apply it to electrical resistivity tomography (ERT) and ambient noise surface wave (ANSW) data. To accurately extract a priori clustering information, we use density peak clustering (DPC) rather than fuzzy c-means (FCM). The number and centres of resistivity and shear-wave velocity a priori clusters are extracted by DPC and then used to guide the joint inversion with the GFCM clustering coupling of ERT and ANSW data. Synthetic and field data are used to evaluate the flow and algorithm of DPC-GFCM clustering joint inversion. The results of synthetic examples show that the models recovered by the DPC-GFCM clustering joint inversion are nearly the same as the true models and are more accurate than those inverted using individual inversion and FCM-GFCM clustering joint inversion. In the field case, the depths of the stratigraphic interfaces shown in the resistivity and shear-wave velocity models inverted by DPC-GFCM clustering joint inversion are nearly consistent with those from the drilling data. In contrast, the strata recovered by the individual inversion and FCM-GFCM clustering joint inversion significantly differ from the drilling results. Both the synthetic and field examples verify the effectiveness of the DPC-GFCM clustering coupling method used for the joint inversion of ERT and ANSW data acquired from the near surface with strong heterogeneity. This novel approach can also be applied to other types of geophysical data.
{"title":"Joint inversion of ERT and ambient noise surface wave data with DPC-guided fuzzy c-means clustering for near-surface imaging","authors":"Zhanjie Shi, Chao Wang","doi":"10.1093/gji/ggae227","DOIUrl":"https://doi.org/10.1093/gji/ggae227","url":null,"abstract":"Summary We present a novel strategy for performing joint inversion with guided fuzzy c-means (GFCM) clustering coupling and apply it to electrical resistivity tomography (ERT) and ambient noise surface wave (ANSW) data. To accurately extract a priori clustering information, we use density peak clustering (DPC) rather than fuzzy c-means (FCM). The number and centres of resistivity and shear-wave velocity a priori clusters are extracted by DPC and then used to guide the joint inversion with the GFCM clustering coupling of ERT and ANSW data. Synthetic and field data are used to evaluate the flow and algorithm of DPC-GFCM clustering joint inversion. The results of synthetic examples show that the models recovered by the DPC-GFCM clustering joint inversion are nearly the same as the true models and are more accurate than those inverted using individual inversion and FCM-GFCM clustering joint inversion. In the field case, the depths of the stratigraphic interfaces shown in the resistivity and shear-wave velocity models inverted by DPC-GFCM clustering joint inversion are nearly consistent with those from the drilling data. In contrast, the strata recovered by the individual inversion and FCM-GFCM clustering joint inversion significantly differ from the drilling results. Both the synthetic and field examples verify the effectiveness of the DPC-GFCM clustering coupling method used for the joint inversion of ERT and ANSW data acquired from the near surface with strong heterogeneity. This novel approach can also be applied to other types of geophysical data.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-06-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141514192","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 On 6 February 2023, an Mw 7.8 left-lateral strike-slip fault earthquake occurred on the East Anatolian Fault Zone (EAFZ) in Türkiye. This study examined the spatial variation of the stress field around Türkiye better to understand the generation process of this event. We first combined focal mechanisms around Türkiye, created a dataset 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 increases from south to north, and the stress regime changes from a normal fault stress regime to a strike-slip fault regime. The stress change caused by the Mw 7.8 mainshock does not explain this lateral pattern, implying that this stress regime transition existed before the mainshock. This suggests that shear stress on the EAFZ was low in this southern segment because it was unfavourably oriented to the regional stress field. Previous studies have reported that the Mw 7.8 mainshock rupture started at a splay fault, first propagated through the central and northern segments and then backpropagated with a time delay toward the southern segment, where it caused a significant but relatively small slip. The preexisting 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. Moreover, the mainshock rupture possibly caused stress rotation locally near the central segment where the magnitudes of the vertical and north-south compressional stresses were almost equal.
{"title":"Spatial Variation in Stress Orientation around Türkiye: Rupture Propagation across the Stress Regime Transition in the 2023 Mw 7.8 Türkiye Earthquake","authors":"Keisuke Yoshida","doi":"10.1093/gji/ggae230","DOIUrl":"https://doi.org/10.1093/gji/ggae230","url":null,"abstract":"Summary On 6 February 2023, an Mw 7.8 left-lateral strike-slip fault earthquake occurred on the East Anatolian Fault Zone (EAFZ) in Türkiye. This study examined the spatial variation of the stress field around Türkiye better to understand the generation process of this event. We first combined focal mechanisms around Türkiye, created a dataset 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 increases from south to north, and the stress regime changes from a normal fault stress regime to a strike-slip fault regime. The stress change caused by the Mw 7.8 mainshock does not explain this lateral pattern, implying that this stress regime transition existed before the mainshock. This suggests that shear stress on the EAFZ was low in this southern segment because it was unfavourably oriented to the regional stress field. Previous studies have reported that the Mw 7.8 mainshock rupture started at a splay fault, first propagated through the central and northern segments and then backpropagated with a time delay toward the southern segment, where it caused a significant but relatively small slip. The preexisting 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. Moreover, the mainshock rupture possibly caused stress rotation locally near the central segment where the magnitudes of the vertical and north-south compressional stresses were almost equal.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-06-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141514190","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}
Chunhong Wu, Xinwen Su, Chuang Xu, Guangyu Jian, Jinbo Li
Summary During the inversion of seafloor topography (ST) using the backpropagation neural network (BPNN), the random selection of parameters may decrease the accuracy. To address this issue and achieve a more efficient global search, this paper introduces a genetic algorithm-backpropagation (GA-BP) neural network. Benefiting from the global search and parallel computing capabilities of the GA, this study refines the seafloor topography of the South China Sea using multi-source gravity data. The results indicate that the GA-BP model, with a root mean square (RMS) value of 126.0 m concerning ship-measured water depths. It is noteworthy that when dealing with regions characterized by sparse survey line distributions, the GA-BP neural network stronger robustness compared to BPNN, showing less sensitivity to the distribution of survey data. Furthermore, the paper explores the influence of different data preprocessing methods on the neural network inversion of sea depths. This research introduces an optimization algorithm that reduces instability during BPNN initialization, resulting in a more accurate prediction of seafloor topography.
{"title":"Seafloor topography refinement from multi-source data using genetic algorithm - backpropagation neural network","authors":"Chunhong Wu, Xinwen Su, Chuang Xu, Guangyu Jian, Jinbo Li","doi":"10.1093/gji/ggae229","DOIUrl":"https://doi.org/10.1093/gji/ggae229","url":null,"abstract":"Summary During the inversion of seafloor topography (ST) using the backpropagation neural network (BPNN), the random selection of parameters may decrease the accuracy. To address this issue and achieve a more efficient global search, this paper introduces a genetic algorithm-backpropagation (GA-BP) neural network. Benefiting from the global search and parallel computing capabilities of the GA, this study refines the seafloor topography of the South China Sea using multi-source gravity data. The results indicate that the GA-BP model, with a root mean square (RMS) value of 126.0 m concerning ship-measured water depths. It is noteworthy that when dealing with regions characterized by sparse survey line distributions, the GA-BP neural network stronger robustness compared to BPNN, showing less sensitivity to the distribution of survey data. Furthermore, the paper explores the influence of different data preprocessing methods on the neural network inversion of sea depths. This research introduces an optimization algorithm that reduces instability during BPNN initialization, resulting in a more accurate prediction of seafloor topography.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-06-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141514191","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 Mode conversion of P waves at the boundary between Earth's crust and upper mantle, when analyzed using receiver functions (RFs), allows characterization of Earth structure where seismic station density is high and earthquake sources are favorably distributed. We applied two ensemble decision tree algorithms – Random Forest (RanFor) and eXtreme Gradient Boost (XGBoost) – to synthetic and real RF data to assess these machine learning techniques' potential for crustal imaging when available data are sparse. The synthetic RFs, entailing both sharp increases in seismic velocity across the Moho and gradational Moho structures, calculated with and without added random noise, correspond to idealized crustal structures: a dipping Moho, Moho offset by crustal-scale faults, anti- and synform Moho structures and combinations of these. The RanFor/XGBoost algorithm recovers input structures well regardless of event-station distributions. Useful crustal and upper mantle seismic velocities can also be determined using RanFor and XGBoost, making it possible to image crustal thickness and P and S wave velocities simultaneously from receiver functions alone. We applied the trained RanFor/XGBoost to receiver functions determined from real seismic data recorded in the contiguous U.S., producing a map of the Moho and P and S wave velocities of the lowermost crust and uppermost mantle. Use of XGBoost, which evaluates residuals between input RFs and ground-truth to update the decision tree using the gradient of a penalty function, improves the crustal thickness estimates.
摘要 利用接收函数（RF）分析地壳和上地幔边界处 P 波的模式转换，可以确定地震台站密度高且震源分布合理的地球结构特征。我们将随机森林（RanFor）和极端梯度提升（XGBoost）这两种决策树算法应用于合成和真实射频数据，以评估这些机器学习技术在可用数据稀少的情况下用于地壳成像的潜力。合成射频数据包括莫霍区地震速度的急剧增加和渐变莫霍区结构，计算时添加和不添加随机噪声，这些数据对应于理想化的地壳结构：倾斜莫霍区、被地壳尺度断层抵消的莫霍区、反形和合形莫霍区结构以及这些结构的组合。无论事件站分布如何，RanFor/XGBoost 算法都能很好地恢复输入结构。使用 RanFor 和 XGBoost 算法还能确定有用的地壳和上地幔地震速度，从而有可能仅通过接收函数就能同时对地壳厚度以及 P 波和 S 波速度进行成像。我们将训练有素的 RanFor/XGBoost 应用于从美国毗连地区记录的真实地震数据中确定的接收函数，绘制出了莫霍线图以及最下部地壳和最上部地幔的 P 波和 S 波速度。XGBoost 可以评估输入射频和地面实况之间的残差，利用惩罚函数的梯度更新决策树，从而改进地壳厚度估算。
{"title":"Use of Decision Tree Ensembles for Crustal Structure Imaging from Receiver Functions","authors":"Yitan Wang, R M Russo, Yuanhang Lin","doi":"10.1093/gji/ggae226","DOIUrl":"https://doi.org/10.1093/gji/ggae226","url":null,"abstract":"Summary Mode conversion of P waves at the boundary between Earth's crust and upper mantle, when analyzed using receiver functions (RFs), allows characterization of Earth structure where seismic station density is high and earthquake sources are favorably distributed. We applied two ensemble decision tree algorithms – Random Forest (RanFor) and eXtreme Gradient Boost (XGBoost) – to synthetic and real RF data to assess these machine learning techniques' potential for crustal imaging when available data are sparse. The synthetic RFs, entailing both sharp increases in seismic velocity across the Moho and gradational Moho structures, calculated with and without added random noise, correspond to idealized crustal structures: a dipping Moho, Moho offset by crustal-scale faults, anti- and synform Moho structures and combinations of these. The RanFor/XGBoost algorithm recovers input structures well regardless of event-station distributions. Useful crustal and upper mantle seismic velocities can also be determined using RanFor and XGBoost, making it possible to image crustal thickness and P and S wave velocities simultaneously from receiver functions alone. We applied the trained RanFor/XGBoost to receiver functions determined from real seismic data recorded in the contiguous U.S., producing a map of the Moho and P and S wave velocities of the lowermost crust and uppermost mantle. Use of XGBoost, which evaluates residuals between input RFs and ground-truth to update the decision tree using the gradient of a penalty function, improves the crustal thickness estimates.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-06-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141514189","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}
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}
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