Summary The 1934 Mw 8.2 Bihar-Nepal earthquake was one of the devastating earthquakes, which made seismologists realize the importance of proper seismic hazard analysis and design aspects in India. The event occurred way before proper seismic networks were implemented and hence there are no recorded ground motions available for this event. The present study, thus aims to generate possible ground motions for the 1934 Mw 8.2 Bihar-Nepal event. The complex geographical features, ambiguous source information, and lack of ground motion data make the simulation and validation of ground motions very difficult. In this regard, the broadband (BB) ground motions are simulated and validated for the most recent well-documented Himalayan event, i.e., the 2015 Mw 7.9 Nepal earthquake in order to calibrate the model and simulation methodology. For this purpose, the computational model presented by Sreejaya et al. (2023) is extended up to a region of 1000 km × 670 km (longitude 80-89 °E and latitude 23-30 °N) in the Indo-Gangetic Basin to simulate the low-frequency (LF) ground motions using spectral element method (Komatitsch and Tromp 1999). These deterministically simulated LF ground motions are combined with stochastically simulated high-frequency (HF) ground motions based on an improved seismological model following Otarola and Ruiz (2016). The seismic moment and dimensions of the rupture plane presented by Pettanati et al. (2017) are used to generate ten samples for the finite fault source model having different slip distribution along the rupture plane as a random field (Mai and Beroza 2000; 2002). The BB ground motions (0.01–25 Hz) are obtained by merging LF and HF ground motions in the time domain by matching them at a frequency of ∼0.3 Hz. Such BB results are simulated at a grid of stations and at locations where Modified Mercalli Intensity (MMI) intensity values are available. The estimated MMI values and the observed MMI values are compared to emphasize the efficacy of the model. The maximum PGA estimated from the simulated ground motions in horizontal and vertical directions are observed to be 0.48 g and 0.4 g. Further, 5% damped response spectra and spectral amplification are analyzed concerning the sediment depth of the Indo-Gangetic Basin. The results from the study can serve as inputs for dynamic analysis and the design of earthquake-resistant structures across different locations in the Indo-Gangetic Basin.
{"title":"Revisiting the 1934 Mw 8.2 Bihar Nepal earthquake – Simulation of Broadband ground motions","authors":"Jahnabi Basu, Sreejaya KP, S T G Raghukanth","doi":"10.1093/gji/ggae336","DOIUrl":"https://doi.org/10.1093/gji/ggae336","url":null,"abstract":"Summary The 1934 Mw 8.2 Bihar-Nepal earthquake was one of the devastating earthquakes, which made seismologists realize the importance of proper seismic hazard analysis and design aspects in India. The event occurred way before proper seismic networks were implemented and hence there are no recorded ground motions available for this event. The present study, thus aims to generate possible ground motions for the 1934 Mw 8.2 Bihar-Nepal event. The complex geographical features, ambiguous source information, and lack of ground motion data make the simulation and validation of ground motions very difficult. In this regard, the broadband (BB) ground motions are simulated and validated for the most recent well-documented Himalayan event, i.e., the 2015 Mw 7.9 Nepal earthquake in order to calibrate the model and simulation methodology. For this purpose, the computational model presented by Sreejaya et al. (2023) is extended up to a region of 1000 km × 670 km (longitude 80-89 °E and latitude 23-30 °N) in the Indo-Gangetic Basin to simulate the low-frequency (LF) ground motions using spectral element method (Komatitsch and Tromp 1999). These deterministically simulated LF ground motions are combined with stochastically simulated high-frequency (HF) ground motions based on an improved seismological model following Otarola and Ruiz (2016). The seismic moment and dimensions of the rupture plane presented by Pettanati et al. (2017) are used to generate ten samples for the finite fault source model having different slip distribution along the rupture plane as a random field (Mai and Beroza 2000; 2002). The BB ground motions (0.01–25 Hz) are obtained by merging LF and HF ground motions in the time domain by matching them at a frequency of ∼0.3 Hz. Such BB results are simulated at a grid of stations and at locations where Modified Mercalli Intensity (MMI) intensity values are available. The estimated MMI values and the observed MMI values are compared to emphasize the efficacy of the model. The maximum PGA estimated from the simulated ground motions in horizontal and vertical directions are observed to be 0.48 g and 0.4 g. Further, 5% damped response spectra and spectral amplification are analyzed concerning the sediment depth of the Indo-Gangetic Basin. The results from the study can serve as inputs for dynamic analysis and the design of earthquake-resistant structures across different locations in the Indo-Gangetic Basin.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"27 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-09-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142255439","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}
Jan Phillip Kruse, Georg Rümpker, Frederik Link, Thibault Duretz, Harro Schmeling
Summary The analysis of the splitting signature of XKS phases is crucial for constraining seismic anisotropy patterns, especially in complex subduction settings such as outward-dipping double subduction. A natural example of this is found in the Central Mediterranean, where the Apennine and the Dinaride slabs subduct in opposite directions, with the Adriatic plate separating them. To assess the capability of XKS-splitting analysis in revealing anisotropic seismic properties, such as fast polarization directions and shear wave anisotropy (in per cent), we use three-dimensional numerical geodynamic models combined with texture evolution simulations. In these models, two identical outward-dipping oceanic plates are separated by a continental plate. Using the full elastic tensors—directly derived from the texture evolution simulations—we compute anisotropic seismic properties and synthetic teleseismic waveforms. From these waveforms synthetic observables are determined, including apparent splitting parameters (fast polarization directions and delay times) and splitting intensities. Based on these observables, we (1) derive models for a single anisotropic layer (one-layer model), (2) identify regions with significant depth-dependent anisotropic seismic properties, and (3) perform inversions at selected locations in terms of two anisotropic layers (two-layer model). We consider two geodynamic models: one with a strong (M1) and one with a weak (M2) continental plate. Model M1 exhibits significant retreat of the subducting plates with no horizontal stretching of the continental plate, whereas Model M2 shows less retreat, substantial horizontal stretching, and detachment of the subducting plates. These different subduction styles result in distinct flow and deformation patterns in the upper mantle, which are reflected in the anisotropic seismic properties. In Model M1, the fast polarization directions below the continental plate are predominantly trench-parallel, whereas in Model M2, they are mostly trench-normal. In most regions of both models, the one-layer models are sufficient to resolve the anisotropic seismic properties, as these properties are nearly constant with depth. However, for both models, we identify some isolated regions—primarily near the tips of the subducting plates and beneath the continental plate—where fast polarization directions exhibit significant variations with depth. Inverting the apparent splitting parameters in these regions yields multiple two-layer models at each location that excellently fit the observables. However, their anisotropic seismic properties can vary significantly, and not all these two-layer models adequately approximate the true depth variations. This ambiguity can be partially reduced by selecting two-layer models in which the summed shear wave anisotropy closely matches that of one of the one-layer models, as these models better capture the true variations.
{"title":"Anisotropy and XKS-splitting from geodynamic models of double subduction: testing the limits of interpretation","authors":"Jan Phillip Kruse, Georg Rümpker, Frederik Link, Thibault Duretz, Harro Schmeling","doi":"10.1093/gji/ggae328","DOIUrl":"https://doi.org/10.1093/gji/ggae328","url":null,"abstract":"Summary The analysis of the splitting signature of XKS phases is crucial for constraining seismic anisotropy patterns, especially in complex subduction settings such as outward-dipping double subduction. A natural example of this is found in the Central Mediterranean, where the Apennine and the Dinaride slabs subduct in opposite directions, with the Adriatic plate separating them. To assess the capability of XKS-splitting analysis in revealing anisotropic seismic properties, such as fast polarization directions and shear wave anisotropy (in per cent), we use three-dimensional numerical geodynamic models combined with texture evolution simulations. In these models, two identical outward-dipping oceanic plates are separated by a continental plate. Using the full elastic tensors—directly derived from the texture evolution simulations—we compute anisotropic seismic properties and synthetic teleseismic waveforms. From these waveforms synthetic observables are determined, including apparent splitting parameters (fast polarization directions and delay times) and splitting intensities. Based on these observables, we (1) derive models for a single anisotropic layer (one-layer model), (2) identify regions with significant depth-dependent anisotropic seismic properties, and (3) perform inversions at selected locations in terms of two anisotropic layers (two-layer model). We consider two geodynamic models: one with a strong (M1) and one with a weak (M2) continental plate. Model M1 exhibits significant retreat of the subducting plates with no horizontal stretching of the continental plate, whereas Model M2 shows less retreat, substantial horizontal stretching, and detachment of the subducting plates. These different subduction styles result in distinct flow and deformation patterns in the upper mantle, which are reflected in the anisotropic seismic properties. In Model M1, the fast polarization directions below the continental plate are predominantly trench-parallel, whereas in Model M2, they are mostly trench-normal. In most regions of both models, the one-layer models are sufficient to resolve the anisotropic seismic properties, as these properties are nearly constant with depth. However, for both models, we identify some isolated regions—primarily near the tips of the subducting plates and beneath the continental plate—where fast polarization directions exhibit significant variations with depth. Inverting the apparent splitting parameters in these regions yields multiple two-layer models at each location that excellently fit the observables. However, their anisotropic seismic properties can vary significantly, and not all these two-layer models adequately approximate the true depth variations. This ambiguity can be partially reduced by selecting two-layer models in which the summed shear wave anisotropy closely matches that of one of the one-layer models, as these models better capture the true variations.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"7 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-09-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142268883","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 Rocks near a fault plane are commonly damaged by multiple earthquake ruptures, forming damage zones. The damage zone is important structures controlling various properties of a fault, yet its fine scale (tens to hundreds of meters) structure is difficult to resolve with surface seismic observations. We propose to use earthquakes that occur at depth within a fault zone as virtual seismometers (VSs) and use surface observations to extract Green's function (GFs) between VS pairs (VSGFs) . This method resembles that of ambient noise tomography and the retrieved VSGFs are related to the structures between event pairs. In this study, we develop the theory about how to extract VSGFs using surface stations deployed across a fault zone. Firstly, we use a half-space model and Fresnel zone analysis to determine the upper and lower limits of the GF frequency band, which is controlled by the station spacing and aperture of a given seismic array. Then, for VS in a fault zone, we demonstrate that the VSGF can be retrieved by linear seismic arrays deployed across the fault, and that the VSGF is equivalent to waves emitted simultaneously from an array of mirror sources of one event and received by the other. Secondly, the half-space result is directly adopted to determine the corresponding frequency band in the damage zone situation. Thirdly, we analyze different combinations of VS pair geometry and conclude that a relatively larger VS distance (much larger than the damage zone width) is more effective to recover damage zone structures for the available frequency bands. In this situation, VSGFs are trapped waves, that is represented by the interference of mirror sources. In such a case, the trapped waves are equivalent to surface waves, which have dispersion features to extract damage zone structures. Finally, we adopt the VSGF method to the Ridgecrest earthquake aftershock monitoring array and use a profile of aftershocks to extract 6 pairs of VSGFs. The spatial variation of VSGFs may reflect the depth-dependent variation of damaged zone. Our analysis shows a promising direction to use VSGFs to extract spatial variations of fault damaged zones.
摘要 断层面附近的岩石通常会受到多次地震破裂的破坏，形成破坏带。破坏带是控制断层各种性质的重要结构，但其精细尺度（几十米到几百米）结构却很难通过地表地震观测来解析。我们建议将断层带深处发生的地震作为虚拟地震仪（VS），利用地表观测数据提取 VS 对之间的格林函数（GF）（VSGF）。这种方法类似于环境噪声层析成像法，提取的 VSGF 与事件对之间的结构相关。在本研究中，我们建立了如何利用部署在断层带的地面站提取 VSGF 的理论。首先，我们使用半空间模型和菲涅尔区分析来确定 GF 频带的上限和下限，这由特定地震阵列的台站间距和孔径控制。然后，对于断层带中的 VS，我们证明了 VSGF 可以通过跨断层部署的线性地震阵列检索到，并且 VSGF 等同于从一个事件的镜像源阵列同时发射并被另一个事件接收的波。其次，直接采用半空间结果确定破坏带情况下的相应频带。第三，我们分析了 VS 对几何形状的不同组合，并得出结论：相对较大的 VS 距离（远大于损伤区宽度）对恢复现有频段的损伤区结构更为有效。在这种情况下，VSGF 是陷波，表现为镜像源的干涉。在这种情况下，滞留波相当于表面波，具有色散特征，可以提取损伤区结构。最后，我们将 VSGF 方法应用于 Ridgecrest 地震余震监测阵列，并利用余震剖面提取了 6 对 VSGF。VSGFs 的空间变化可能反映了受损区随深度的变化。我们的分析为利用 VSGFs 提取断层破坏带的空间变化指明了方向。
{"title":"Extracting fault-zone structures using the virtual seismometer method: from theoretical to synthetic test","authors":"Wei Liu, Han Yue, Nan Hu","doi":"10.1093/gji/ggae335","DOIUrl":"https://doi.org/10.1093/gji/ggae335","url":null,"abstract":"Summary Rocks near a fault plane are commonly damaged by multiple earthquake ruptures, forming damage zones. The damage zone is important structures controlling various properties of a fault, yet its fine scale (tens to hundreds of meters) structure is difficult to resolve with surface seismic observations. We propose to use earthquakes that occur at depth within a fault zone as virtual seismometers (VSs) and use surface observations to extract Green's function (GFs) between VS pairs (VSGFs) . This method resembles that of ambient noise tomography and the retrieved VSGFs are related to the structures between event pairs. In this study, we develop the theory about how to extract VSGFs using surface stations deployed across a fault zone. Firstly, we use a half-space model and Fresnel zone analysis to determine the upper and lower limits of the GF frequency band, which is controlled by the station spacing and aperture of a given seismic array. Then, for VS in a fault zone, we demonstrate that the VSGF can be retrieved by linear seismic arrays deployed across the fault, and that the VSGF is equivalent to waves emitted simultaneously from an array of mirror sources of one event and received by the other. Secondly, the half-space result is directly adopted to determine the corresponding frequency band in the damage zone situation. Thirdly, we analyze different combinations of VS pair geometry and conclude that a relatively larger VS distance (much larger than the damage zone width) is more effective to recover damage zone structures for the available frequency bands. In this situation, VSGFs are trapped waves, that is represented by the interference of mirror sources. In such a case, the trapped waves are equivalent to surface waves, which have dispersion features to extract damage zone structures. Finally, we adopt the VSGF method to the Ridgecrest earthquake aftershock monitoring array and use a profile of aftershocks to extract 6 pairs of VSGFs. The spatial variation of VSGFs may reflect the depth-dependent variation of damaged zone. Our analysis shows a promising direction to use VSGFs to extract spatial variations of fault damaged zones.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"18 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-09-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142255442","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}
In this paper, we present a new seismic travel-time tomography approach that combines ensemble Kalman inversion (EKI) with Neural Networks (NNs) to facilitate the inference of complex underground velocity fields. Our methodology tackles the challenges of high-dimensional velocity models through an efficient neural network parameterization, enabling efficient training on coarse grids and accurate output on finer grids. This unique strategy, combined with a reduced-resolution forward solver, significantly enhances computational efficiency. Leveraging the robust capabilities of EKI, our method not only achieves rapid computations but also delivers informative uncertainty quantification for the estimated results. Through extensive numerical experiments, we demonstrate the exceptional accuracy and uncertainty quantification capabilities of our EKI-NNs approach. Even in the face of challenging geological scenarios, our method consistently generates valuable initial models for full wave inversion (FWI).
{"title":"Seismic travel-time tomography based on Ensemble Kalman Inversion","authors":"Yunduo Li, Yijie Zhang, Xueyu Zhu, Jinghuai Gao","doi":"10.1093/gji/ggae329","DOIUrl":"https://doi.org/10.1093/gji/ggae329","url":null,"abstract":"In this paper, we present a new seismic travel-time tomography approach that combines ensemble Kalman inversion (EKI) with Neural Networks (NNs) to facilitate the inference of complex underground velocity fields. Our methodology tackles the challenges of high-dimensional velocity models through an efficient neural network parameterization, enabling efficient training on coarse grids and accurate output on finer grids. This unique strategy, combined with a reduced-resolution forward solver, significantly enhances computational efficiency. Leveraging the robust capabilities of EKI, our method not only achieves rapid computations but also delivers informative uncertainty quantification for the estimated results. Through extensive numerical experiments, we demonstrate the exceptional accuracy and uncertainty quantification capabilities of our EKI-NNs approach. Even in the face of challenging geological scenarios, our method consistently generates valuable initial models for full wave inversion (FWI).","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"34 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-09-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142177359","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 Intermediate-depth earthquakes, accommodating intraslab deformation, typically occur within subduction zone settings at depths between 60–300 km. These events are in a unique position to inform us about the geodynamics of the subducting slab, specifically the geometry of the slab and the stress state of the host material. Improvements in the density and quality of recorded seismic data enhance our ability to determine precise locations of intermediate-depth earthquakes, in order to establish connections between event nucleation and the tectonic setting. Depth phases (near-source surface reflections, e.g. pP and sP) are crucial for the accurate determination of earthquake source depth using global seismic data. However, they suffer from poor signal-to-noise ratios in the P wave coda. This reduces the ability to systematically measure differential traveltimes to the direct P arrival, particularly for the frequent lower magnitude seismicity which highlights considerable seismogenic regions of the subducted slabs. To address this limitation, we have developed an automated approach to group globally distributed stations at teleseismic distances into ad-hoc arrays with apertures of 2.5$^circ$, before optimizing and applying phase-weighted beamforming techniques to each array. Resultant vespagrams allow automated picking algorithms to determine differential arrival times between the depth phases and their corresponding direct P arrival. Using these differential times we can then determine the depths of earthquakes, which in turn can be used to create a catalogue of relocated events. This will allow new comparisons and insights into the governing controls on the distribution of earthquakes in subducted slabs. We demonstrate this method by relocating intermediate-depth events associated with northern Chile and the Peruvian flat slab regions of the subducting Nazca plate. The relocated Chilean catalogue contains comparable event depths to an established catalogue, calculated using a semi-automated global methodology, which serves to validate our fully automatic methodology. The new Peruvian catalogue we generate indicates three broad zones of seismicity approximately between latitudes 1–7$^circ$S, 7–13$^circ$S and 13–19$^circ$S. These align with flat to steep slab dip transitions and the previously identified Pucallpa Nest. We also find a regionally deeper slab top than indicated by recent slab models, with intraslab events concentrated at points where the slab bends, suggesting a link between slab flexure and intermediate-depth earthquake nucleation.
摘要 中深度地震通常发生在俯冲带内 60-300 千米的深度范围内，并伴有板块内部变形。这些地震对我们了解俯冲板块的地球动力学，特别是板块的几何形状和主材料的应力状态具有独特的作用。记录地震数据的密度和质量的提高增强了我们确定中深层地震精确位置的能力，从而建立地震成核与构造环境之间的联系。深度相位（近震源表面反射，如 pP 和 sP）对于利用全球地震数据准确确定震源深度至关重要。然而，它们在 P 波尾音中的信噪比较低。这就降低了系统测量直接 P 波到达的差分旅行时间的能力，特别是对于频繁发生的低震级地震，因为低震级地震突出了俯冲板块中相当大的成震区。为了解决这一限制，我们开发了一种自动方法，将远震距离上的全球分布台站组合成孔径为 2.5$^circ$ 的特设阵列，然后对每个阵列进行优化并应用相位加权波束成形技术。由此产生的 Vespagrams 允许自动选取算法确定深度相位与其对应的直接 P 波到达之间的差分到达时间。利用这些差分时间，我们就可以确定地震的深度，进而建立一个重新定位的地震事件目录。这样就可以对俯冲板块中地震分布的控制因素进行新的比较和深入了解。我们通过重新定位与智利北部和纳斯卡俯冲板块的秘鲁平板区域相关的中深度地震事件来演示这种方法。重新定位的智利地震目录包含的事件深度与使用半自动全球方法计算的已有目录相当，这也验证了我们的全自动方法。我们生成的新秘鲁地震目录显示，大约在南纬 1-7$^circ$、南纬 7-13$^circ$ 和南纬 13-19$^circ$ 之间有三个广泛的地震带。这些区域与从平坦到陡峭的板块倾角转换以及之前确定的普卡尔帕巢相吻合。我们还发现该区域的板顶比最近的板块模型所显示的要深，板内事件集中在板块弯曲的地方，这表明板块弯曲与中深层地震成核之间存在联系。
{"title":"Automatic relocation of intermediate-depth earthquakes using adaptive teleseismic arrays","authors":"Alice Blackwell, Timothy Craig, Sebastian Rost","doi":"10.1093/gji/ggae289","DOIUrl":"https://doi.org/10.1093/gji/ggae289","url":null,"abstract":"SUMMARY Intermediate-depth earthquakes, accommodating intraslab deformation, typically occur within subduction zone settings at depths between 60–300 km. These events are in a unique position to inform us about the geodynamics of the subducting slab, specifically the geometry of the slab and the stress state of the host material. Improvements in the density and quality of recorded seismic data enhance our ability to determine precise locations of intermediate-depth earthquakes, in order to establish connections between event nucleation and the tectonic setting. Depth phases (near-source surface reflections, e.g. pP and sP) are crucial for the accurate determination of earthquake source depth using global seismic data. However, they suffer from poor signal-to-noise ratios in the P wave coda. This reduces the ability to systematically measure differential traveltimes to the direct P arrival, particularly for the frequent lower magnitude seismicity which highlights considerable seismogenic regions of the subducted slabs. To address this limitation, we have developed an automated approach to group globally distributed stations at teleseismic distances into ad-hoc arrays with apertures of 2.5$^circ$, before optimizing and applying phase-weighted beamforming techniques to each array. Resultant vespagrams allow automated picking algorithms to determine differential arrival times between the depth phases and their corresponding direct P arrival. Using these differential times we can then determine the depths of earthquakes, which in turn can be used to create a catalogue of relocated events. This will allow new comparisons and insights into the governing controls on the distribution of earthquakes in subducted slabs. We demonstrate this method by relocating intermediate-depth events associated with northern Chile and the Peruvian flat slab regions of the subducting Nazca plate. The relocated Chilean catalogue contains comparable event depths to an established catalogue, calculated using a semi-automated global methodology, which serves to validate our fully automatic methodology. The new Peruvian catalogue we generate indicates three broad zones of seismicity approximately between latitudes 1–7$^circ$S, 7–13$^circ$S and 13–19$^circ$S. These align with flat to steep slab dip transitions and the previously identified Pucallpa Nest. We also find a regionally deeper slab top than indicated by recent slab models, with intraslab events concentrated at points where the slab bends, suggesting a link between slab flexure and intermediate-depth earthquake nucleation.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"1146 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-09-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142177360","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}
A Revil, A Ghorbani, X Zhao, A Mouyeaux, L Barrère, J Richard, L Peyras, P Vaudelet
SUMMARY The dam of Lampy (Black Mountain, Aude, France) is considered as one of the oldest dams in France. A geophysical survey is performed to better understand the pattern of groundwater flow downstream of this dam in the granitic substratum. Induced polarization is first used to image both electrical conductivity and normalized chargeability. Eight core samples of granite from this site are measured and analysed in the laboratory. Their electrical conductivity and normalized chargeability are expressed as a function of the porosity and cation exchange capacity (CEC). The field data and the petrophysical results are used to image the water content, the CEC and the permeability distribution of the substratum. Then, self-potential is used as a complementary passive geophysical technique, which, in absence of metallic bodies, is directly sensitive to groundwater flow through the so-called streaming potential effect. Indeed, the excess of electrical charges in the vicinity of the solid grains, in the so-called double layer, is dragged by the ground water flow generating in turn an electrical (streaming) current and therefore an electrical field. A map of the resulting self-potential signals is done over the area covered by the induced polarization profiles. This map shows a large positive anomaly with an amplitude of ∼80 mV possibly associated with upwelling groundwater in an area where the soil is water-saturated. A groundwater flow simulation is performed to model this anomaly. This is done in two steps. A preliminary groundwater flow model is built using the permeability and water content distributions obtained from the induced polarization data. Then, this groundwater flow model is updated using the information contained in the self-potential data including the electrical conductivity distribution obtained through resistivity tomography. The algorithm for the inversion of the self-potential data is validated through a 2-D numerical test. This analysis yields a groundwater flow model with the flow being focused through a high permeability zone. This study shows how three geoelectrical methods (self-potential, induced polarization and electrical resistivity) can be efficiently combined to image groundwater flow in the vicinity of a dam.
{"title":"Groundwater flow paths using combined self-potential, electrical resistivity, and induced polarization signals","authors":"A Revil, A Ghorbani, X Zhao, A Mouyeaux, L Barrère, J Richard, L Peyras, P Vaudelet","doi":"10.1093/gji/ggae291","DOIUrl":"https://doi.org/10.1093/gji/ggae291","url":null,"abstract":"SUMMARY The dam of Lampy (Black Mountain, Aude, France) is considered as one of the oldest dams in France. A geophysical survey is performed to better understand the pattern of groundwater flow downstream of this dam in the granitic substratum. Induced polarization is first used to image both electrical conductivity and normalized chargeability. Eight core samples of granite from this site are measured and analysed in the laboratory. Their electrical conductivity and normalized chargeability are expressed as a function of the porosity and cation exchange capacity (CEC). The field data and the petrophysical results are used to image the water content, the CEC and the permeability distribution of the substratum. Then, self-potential is used as a complementary passive geophysical technique, which, in absence of metallic bodies, is directly sensitive to groundwater flow through the so-called streaming potential effect. Indeed, the excess of electrical charges in the vicinity of the solid grains, in the so-called double layer, is dragged by the ground water flow generating in turn an electrical (streaming) current and therefore an electrical field. A map of the resulting self-potential signals is done over the area covered by the induced polarization profiles. This map shows a large positive anomaly with an amplitude of ∼80 mV possibly associated with upwelling groundwater in an area where the soil is water-saturated. A groundwater flow simulation is performed to model this anomaly. This is done in two steps. A preliminary groundwater flow model is built using the permeability and water content distributions obtained from the induced polarization data. Then, this groundwater flow model is updated using the information contained in the self-potential data including the electrical conductivity distribution obtained through resistivity tomography. The algorithm for the inversion of the self-potential data is validated through a 2-D numerical test. This analysis yields a groundwater flow model with the flow being focused through a high permeability zone. This study shows how three geoelectrical methods (self-potential, induced polarization and electrical resistivity) can be efficiently combined to image groundwater flow in the vicinity of a dam.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"26 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-09-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142177361","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}
Romain Sylvain, Louise Watremez, Isabelle Thinon, Frank Chanier, Fabien Caroir, Virginie Gaullier
Summary When interpreting marine Very High-Resolution (VHR) single-channel seismic reflection data, the signal in the water-column is generally considered as noise and is often eliminated by a water-mute application to focus on geological information under the seafloor. Alternatively, the signal in the water-column can be used to study ocean currents or gas/fluid emissions. To provide images of the sedimentary formations and tectonic structures beneath the seafloor in shallow water regions, such as continental shelves and lakes, marine seismic reflection profiles are often acquired using a single-channel streamer and sparker-type source, providing VHR data, with limited penetration-depth. To exploit the full potential of these single-channel data, we propose a simple algorithm, called REWARE (Recovery of Water-column Acoustic Reflectors). This algorithm allows to extract further geological information from the water-column data using open-source codes (Seismic Un*x), adding the coherent signal from the previous shots, recorded in the water-column, to the previous traces. The record length becomes longer while maintaining a very high trace-to-trace consistency. To demonstrate its efficiency, we present two examples of the REWARE processing in two different geological contexts: the East Sardinia shelf (Italy) and the North Evia Gulf (Greece). This method provides deeper images than with original data for seismic data acquired across steep slopes, such as canyons or continental shelf breaks. Thus, depending on the seafloor geometry and sub-seafloor structures, it is possible to image or map sediment layers and tectonic structures at depth, keeping a very high structural resolution.
{"title":"REWARE: a seismic processing algorithm to retrieve geological information from the water-column","authors":"Romain Sylvain, Louise Watremez, Isabelle Thinon, Frank Chanier, Fabien Caroir, Virginie Gaullier","doi":"10.1093/gji/ggae319","DOIUrl":"https://doi.org/10.1093/gji/ggae319","url":null,"abstract":"Summary When interpreting marine Very High-Resolution (VHR) single-channel seismic reflection data, the signal in the water-column is generally considered as noise and is often eliminated by a water-mute application to focus on geological information under the seafloor. Alternatively, the signal in the water-column can be used to study ocean currents or gas/fluid emissions. To provide images of the sedimentary formations and tectonic structures beneath the seafloor in shallow water regions, such as continental shelves and lakes, marine seismic reflection profiles are often acquired using a single-channel streamer and sparker-type source, providing VHR data, with limited penetration-depth. To exploit the full potential of these single-channel data, we propose a simple algorithm, called REWARE (Recovery of Water-column Acoustic Reflectors). This algorithm allows to extract further geological information from the water-column data using open-source codes (Seismic Un*x), adding the coherent signal from the previous shots, recorded in the water-column, to the previous traces. The record length becomes longer while maintaining a very high trace-to-trace consistency. To demonstrate its efficiency, we present two examples of the REWARE processing in two different geological contexts: the East Sardinia shelf (Italy) and the North Evia Gulf (Greece). This method provides deeper images than with original data for seismic data acquired across steep slopes, such as canyons or continental shelf breaks. Thus, depending on the seafloor geometry and sub-seafloor structures, it is possible to image or map sediment layers and tectonic structures at depth, keeping a very high structural resolution.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"9 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-09-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142177388","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 This paper offers a comprehensive re-analysis of the Beni-Ilmane 2010 seismic sequence, using a dataset that is 100 per cent larger than previous studies. This unprecedented sequence in Algeria features three mainshocks with magnitudes Mw 5.4, 5.1, and 5.1. Our approach involves high-precision relocation, which includes the development of a new 1D minimum velocity model, followed by a double-difference (DD) procedure and hierarchical clustering. We determined the focal mechanisms (FMs) for 128 key events and identified 21 multiplet groups using an average cross-correlation threshold of 0.8. Our analysis offers new insights into fault geometry and addresses ongoing debates, by proposing a seismotectonic model that reveals the activation of fourteen (14) fault segments during the sequence, in contrast to previous oversimplified models that suggested two or three faults. The computed stress field from the inversion of 128 FMs aligns with a tectonic loading force due to the convergence of the African and Eurasian plates. These findings highlight the complexity of the fault network in the study area and shed light on the role of strike-slip faults in shaping the thrust belt. We found a strong link between multiplet groups and fluid movement along the fault network. Analysis of the temporal history of these multiplet groups provides new insights into fluid dynamics timescales, with an estimated hydraulic diffusivity (D) of 0.36 m2/s suggesting a fluid pressure diffusion process. The observed expansion of the aftershock area with the logarithm of time and the existence of repeating earthquakes indicates, for the first time, an aseismic slip mechanism that adds an additional layer to the driven processes. In conclusion, our results suggest that the underlying mechanisms governing the BI-2010 seismic sequence involve a complex interplay of tectonic loading, coseismic stress transfer, fluid dynamics, and aseismic slip transients. We attempt to correlate our findings with various studies linking the structure, mechanics, and fluid flow properties of fault zones and fault systems. The activation of smaller fault segments potentially averted a larger quake, resulting in three moderate mainshocks and numerous aftershocks. This work not only enrich our understanding of seismic phenomena but also provides useful insights for seismic hazard assessment and risk mitigation strategies.
{"title":"Unveiling complex fault geometry and driving mechanisms: insights from a refined data processing and multiplet analysis of the 2010 Beni-Ilmane seismic sequence (NE Algeria)","authors":"El-Mahdi Tikhamarine, Issam Abacha, Oualid Boulahia, Hichem Bendjama, Khaled Roubeche, Sofiane Taki-Eddine Rahmani","doi":"10.1093/gji/ggae327","DOIUrl":"https://doi.org/10.1093/gji/ggae327","url":null,"abstract":"Summary This paper offers a comprehensive re-analysis of the Beni-Ilmane 2010 seismic sequence, using a dataset that is 100 per cent larger than previous studies. This unprecedented sequence in Algeria features three mainshocks with magnitudes Mw 5.4, 5.1, and 5.1. Our approach involves high-precision relocation, which includes the development of a new 1D minimum velocity model, followed by a double-difference (DD) procedure and hierarchical clustering. We determined the focal mechanisms (FMs) for 128 key events and identified 21 multiplet groups using an average cross-correlation threshold of 0.8. Our analysis offers new insights into fault geometry and addresses ongoing debates, by proposing a seismotectonic model that reveals the activation of fourteen (14) fault segments during the sequence, in contrast to previous oversimplified models that suggested two or three faults. The computed stress field from the inversion of 128 FMs aligns with a tectonic loading force due to the convergence of the African and Eurasian plates. These findings highlight the complexity of the fault network in the study area and shed light on the role of strike-slip faults in shaping the thrust belt. We found a strong link between multiplet groups and fluid movement along the fault network. Analysis of the temporal history of these multiplet groups provides new insights into fluid dynamics timescales, with an estimated hydraulic diffusivity (D) of 0.36 m2/s suggesting a fluid pressure diffusion process. The observed expansion of the aftershock area with the logarithm of time and the existence of repeating earthquakes indicates, for the first time, an aseismic slip mechanism that adds an additional layer to the driven processes. In conclusion, our results suggest that the underlying mechanisms governing the BI-2010 seismic sequence involve a complex interplay of tectonic loading, coseismic stress transfer, fluid dynamics, and aseismic slip transients. We attempt to correlate our findings with various studies linking the structure, mechanics, and fluid flow properties of fault zones and fault systems. The activation of smaller fault segments potentially averted a larger quake, resulting in three moderate mainshocks and numerous aftershocks. This work not only enrich our understanding of seismic phenomena but also provides useful insights for seismic hazard assessment and risk mitigation strategies.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"9 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-09-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142177362","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}
Adrian White, James Boyd, Paul Wilkinson, Holly E Unwin, James Wookey, John Michael Kendall, Andrew Binley, Jonathan Chambers
Summary Electrical resistivity tomography (ERT), a geophysical imaging method, is commonly used on flood embankments (dykes or levees) to characterise their internal structure and look for defects. These surveys often use a single line of electrodes to enable 2D imaging through the embankment crest, an approach that enables rapid and efficient surveying compared to 3D surveys. However, offline variations in topography can introduce artefacts into these 2D images, by affecting the measured resistivity data. Such topographic effects have only been explored on a site-specific basis. If the topographic effects can be assessed for a distribution of embankment geometries (e.g. slope angle and crest width) and resistivity variations, it would allow for targeted correction procedures and improved survey design. To investigate topographic effects on ERT measurements, we forward-modelled embankments with different trapezoidal cross-sections sat atop a flat foundation layer with contrasting resistivity values. Each was compared to a corresponding flat model with the same vertical resistivity distribution. The modelling workflow was designed to minimise the effect of forward modelling errors on the calculation of topographic effect. We ran 1872 unique embankment forward models, representing 144 geometries, each with 13 different resistivity contrasts. Modelling results show that offline topography affects the tested array types (Wenner-Schlumberger, Dipole-Dipole, and Multiple-Gradient) in slightly different ways, but the magnitudes are similar, so all are equally suitable for embankment surveys. Three separate mechanisms are found to cause topographic effects. The dominant mechanism is caused by the offline topography confining the electrical current flow, increasing the measured transfer resistance from the embankment model. The two other mechanisms, previously unidentified, decrease the measured transfer resistances from the embankment model compared to a layered half-space but only affect embankments with specific geometries and resistivity distributions. Overall, we found that for typical embankment geometries and resistivity distributions, the resistivity distribution has a greater control on the magnitude of the topographic effect than the exact embankment geometry: the subsurface resistivity distribution cannot be neglected. 2D inversions are suitable when both the embankment is more resistive than the foundations and when the embankment's cross-sectional area is greater than 4 m2/m2 (area scaled to an embankment with a height of 1 m). Topographic corrections, 3D data acquisition or 3D forward models are required when these conditions are not met. These are demonstrated using field data from an embankment at Hexham, Northumberland, UK. Improving the accuracy of the resistivity values in ERT models will enable more accurate ground models, better integration of resistivity data with geotechnical datasets, and will improve the translation of resistivity
{"title":"Assessing the effect of offline topography on electrical resistivity measurements: insights from flood embankments","authors":"Adrian White, James Boyd, Paul Wilkinson, Holly E Unwin, James Wookey, John Michael Kendall, Andrew Binley, Jonathan Chambers","doi":"10.1093/gji/ggae313","DOIUrl":"https://doi.org/10.1093/gji/ggae313","url":null,"abstract":"Summary Electrical resistivity tomography (ERT), a geophysical imaging method, is commonly used on flood embankments (dykes or levees) to characterise their internal structure and look for defects. These surveys often use a single line of electrodes to enable 2D imaging through the embankment crest, an approach that enables rapid and efficient surveying compared to 3D surveys. However, offline variations in topography can introduce artefacts into these 2D images, by affecting the measured resistivity data. Such topographic effects have only been explored on a site-specific basis. If the topographic effects can be assessed for a distribution of embankment geometries (e.g. slope angle and crest width) and resistivity variations, it would allow for targeted correction procedures and improved survey design. To investigate topographic effects on ERT measurements, we forward-modelled embankments with different trapezoidal cross-sections sat atop a flat foundation layer with contrasting resistivity values. Each was compared to a corresponding flat model with the same vertical resistivity distribution. The modelling workflow was designed to minimise the effect of forward modelling errors on the calculation of topographic effect. We ran 1872 unique embankment forward models, representing 144 geometries, each with 13 different resistivity contrasts. Modelling results show that offline topography affects the tested array types (Wenner-Schlumberger, Dipole-Dipole, and Multiple-Gradient) in slightly different ways, but the magnitudes are similar, so all are equally suitable for embankment surveys. Three separate mechanisms are found to cause topographic effects. The dominant mechanism is caused by the offline topography confining the electrical current flow, increasing the measured transfer resistance from the embankment model. The two other mechanisms, previously unidentified, decrease the measured transfer resistances from the embankment model compared to a layered half-space but only affect embankments with specific geometries and resistivity distributions. Overall, we found that for typical embankment geometries and resistivity distributions, the resistivity distribution has a greater control on the magnitude of the topographic effect than the exact embankment geometry: the subsurface resistivity distribution cannot be neglected. 2D inversions are suitable when both the embankment is more resistive than the foundations and when the embankment's cross-sectional area is greater than 4 m2/m2 (area scaled to an embankment with a height of 1 m). Topographic corrections, 3D data acquisition or 3D forward models are required when these conditions are not met. These are demonstrated using field data from an embankment at Hexham, Northumberland, UK. Improving the accuracy of the resistivity values in ERT models will enable more accurate ground models, better integration of resistivity data with geotechnical datasets, and will improve the translation of resistivity","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"20 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-09-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142177387","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 Terrestrial heat flow plays a vital role in determining the present thermal regimes of sedimentary basins, offering a robust foundation for understanding hydrocarbon maturation processes and the geothermal resource potential. The Junggar basin is one of the largest and most petroliferous superimposed petroleum basins in China. However, research on heat flow is scarce. In this study, 94 new high-quality heat flow values are derived from through borehole temperature analysis and thermal conductivity measurements of rocks. The results indicate that (1) the geothermal gradient in the basin varies from 11.4 to 28.3° C/km, with a mean value of 20.9 ± 3.4° C/km, and the heat flow varies from 23.4 to 64.5 mW/m2, with a mean value of 45.1 ± 8.4 mW/m2. The overall low geothermal gradient and heat flow are attributed to the continuous cooling during the Meso-Cenozoic. (2) At basin scale, the high heat flow values are primarily concentrated in areas characterized by basement uplift, whereas the low heat flow values are mainly located in the depressions. This suggests that thermal refraction is the primary factor influencing the heat flow variations. (3) Although large-scale development and utilization of geothermal resources face challenges, certain local areas in the basin show promise for geothermal resource utilization.
{"title":"An updated terrestrial heat flow dataset for the Junggar basin, northwest China: implications for geothermal resources","authors":"Chao Zhang, Fei Wang, Yidan Zhang, Hui Lu, Haozhu Zhang, Ronghua Huang, Zepeng Liu, Junji Chen","doi":"10.1093/gji/ggae325","DOIUrl":"https://doi.org/10.1093/gji/ggae325","url":null,"abstract":"Summary Terrestrial heat flow plays a vital role in determining the present thermal regimes of sedimentary basins, offering a robust foundation for understanding hydrocarbon maturation processes and the geothermal resource potential. The Junggar basin is one of the largest and most petroliferous superimposed petroleum basins in China. However, research on heat flow is scarce. In this study, 94 new high-quality heat flow values are derived from through borehole temperature analysis and thermal conductivity measurements of rocks. The results indicate that (1) the geothermal gradient in the basin varies from 11.4 to 28.3° C/km, with a mean value of 20.9 ± 3.4° C/km, and the heat flow varies from 23.4 to 64.5 mW/m2, with a mean value of 45.1 ± 8.4 mW/m2. The overall low geothermal gradient and heat flow are attributed to the continuous cooling during the Meso-Cenozoic. (2) At basin scale, the high heat flow values are primarily concentrated in areas characterized by basement uplift, whereas the low heat flow values are mainly located in the depressions. This suggests that thermal refraction is the primary factor influencing the heat flow variations. (3) Although large-scale development and utilization of geothermal resources face challenges, certain local areas in the basin show promise for geothermal resource utilization.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":"1 1","pages":""},"PeriodicalIF":2.8,"publicationDate":"2024-09-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142177365","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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