E. B. Storrøsten, Naveen Ragu Ramalingam, S. Lorito, M. Volpe, C. Sánchez-Linares, F. Løvholt, Steven J. Gibbons
Estimating coastal tsunami impact for early-warning or long-term hazard analysis requires the calculation of inundation metrics such as flow-depth or momentum flux. Both applications require the simulation of large numbers of scenarios to capture both the aleatory variability and the epistemic tsunami uncertainty. A computationally demanding step in simulating inundation is solving the nonlinear shallow water (NLSW) equations on meshes with sufficiently high resolution to represent the local elevation accurately enough to capture the physics governing the flow. This computational expense is particularly challenging in the context of Tsunami Early Warning where strict time constraints apply. A Machine Learning (ML) model that predicts inundation maps from offshore simulation results with acceptable accuracy, trained on an acceptably small training set of full simulations, could replace the computationally expensive NLSW part of the simulations for vast numbers of scenarios and predict inundation rapidly and with reduced computational demands. We consider the application of an encoder-decoder based neural network to predict high-resolution inundation maps based only on more cheaply calculated simulated time-series at a limited number of offshore locations. The network needs to be trained using input offshore time-series and the corresponding inundation maps from previously calculated full simulations. We develop and evaluate the ML model on a comprehensive set of inundation simulations for the coast of eastern Sicily for tens of thousands of subduction earthquake sources in the Mediterranean Sea. We find good performance for this case study even using relatively small training sets (order of hundreds) provided that appropriate choices are made in the specification of model parameters, the specification of the loss function, and the selection of training events. The uncertainty in the prediction for any given location decreases with the number of training events that inundate that location, with a good range of flow depths needed for accurate predictions. This means that care is needed to ensure that rarer high-inundation scenarios are well-represented in the training sets. The importance of applying regularization techniques increases as the size of the training sets decreases. The computational gain of the proposed methodology depends on the number of complete simulations needed to train the neural network, ranging between 164 and 4196 scenarios in this study. The cost of training the network is small in comparison with the cost of the numerical simulations and, for an ensemble of around 28000 scenarios, this represents a 6 to 170-fold reduction in computing costs.
{"title":"Machine Learning Emulation of High Resolution Inundation Maps","authors":"E. B. Storrøsten, Naveen Ragu Ramalingam, S. Lorito, M. Volpe, C. Sánchez-Linares, F. Løvholt, Steven J. Gibbons","doi":"10.1093/gji/ggae151","DOIUrl":"https://doi.org/10.1093/gji/ggae151","url":null,"abstract":"\u0000 Estimating coastal tsunami impact for early-warning or long-term hazard analysis requires the calculation of inundation metrics such as flow-depth or momentum flux. Both applications require the simulation of large numbers of scenarios to capture both the aleatory variability and the epistemic tsunami uncertainty. A computationally demanding step in simulating inundation is solving the nonlinear shallow water (NLSW) equations on meshes with sufficiently high resolution to represent the local elevation accurately enough to capture the physics governing the flow. This computational expense is particularly challenging in the context of Tsunami Early Warning where strict time constraints apply. A Machine Learning (ML) model that predicts inundation maps from offshore simulation results with acceptable accuracy, trained on an acceptably small training set of full simulations, could replace the computationally expensive NLSW part of the simulations for vast numbers of scenarios and predict inundation rapidly and with reduced computational demands. We consider the application of an encoder-decoder based neural network to predict high-resolution inundation maps based only on more cheaply calculated simulated time-series at a limited number of offshore locations. The network needs to be trained using input offshore time-series and the corresponding inundation maps from previously calculated full simulations. We develop and evaluate the ML model on a comprehensive set of inundation simulations for the coast of eastern Sicily for tens of thousands of subduction earthquake sources in the Mediterranean Sea. We find good performance for this case study even using relatively small training sets (order of hundreds) provided that appropriate choices are made in the specification of model parameters, the specification of the loss function, and the selection of training events. The uncertainty in the prediction for any given location decreases with the number of training events that inundate that location, with a good range of flow depths needed for accurate predictions. This means that care is needed to ensure that rarer high-inundation scenarios are well-represented in the training sets. The importance of applying regularization techniques increases as the size of the training sets decreases. The computational gain of the proposed methodology depends on the number of complete simulations needed to train the neural network, ranging between 164 and 4196 scenarios in this study. The cost of training the network is small in comparison with the cost of the numerical simulations and, for an ensemble of around 28000 scenarios, this represents a 6 to 170-fold reduction in computing costs.","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":"9 2","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140665522","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
N. Aravantinos-Zafiris, K. A. Chondrogiannis, H. R. Thomsen, V. Dertimanis, A. Colombi, M. M. Sigalas, E. Chatzi
In this work, the propagation and attenuation of vertically polarized surface waves when interacting with terraced slopes is studied experimentally and numerically. To validate the devised simulation, a laboratory-scale physical model is tested in order to examine the attenuation properties of this well-known artificial landform. The experiment involves formation of a terraced slope, in a laboratory setup, via use of an unconsolidated granular medium made of silica microbeads. This granular medium exhibits a gravity-induced power-law stiffness profile, resulting in a depth-dependent velocity profile. A piezoelectric actuator was used to excite vertically polarized surface acoustic modes localized near the surface of the medium. The three components of the particle velocity field of these modes were measured by means of a three-dimensional laser Doppler vibrometer. In accordance with the terraced slope, a simple inclined plane was further tested to investigate and highlight the differences in terms of wave propagation along these two different ground formations. The results of this research provide significant experimental evidence that the terraced slopes form mechanisms which attenuate low frequency surface waves, thus acting as metasurfaces. This work suggests the use of laboratory-scale physical models to investigate the wave propagation in different landforms, which extend beyond typical horizontal ground morphologies, and which could be linked to atypical wave propagation properties, possibly even influencing propagation of seismic waves.
{"title":"Terraced slope metasurface in granular media","authors":"N. Aravantinos-Zafiris, K. A. Chondrogiannis, H. R. Thomsen, V. Dertimanis, A. Colombi, M. M. Sigalas, E. Chatzi","doi":"10.1093/gji/ggae150","DOIUrl":"https://doi.org/10.1093/gji/ggae150","url":null,"abstract":"\u0000 In this work, the propagation and attenuation of vertically polarized surface waves when interacting with terraced slopes is studied experimentally and numerically. To validate the devised simulation, a laboratory-scale physical model is tested in order to examine the attenuation properties of this well-known artificial landform. The experiment involves formation of a terraced slope, in a laboratory setup, via use of an unconsolidated granular medium made of silica microbeads. This granular medium exhibits a gravity-induced power-law stiffness profile, resulting in a depth-dependent velocity profile. A piezoelectric actuator was used to excite vertically polarized surface acoustic modes localized near the surface of the medium. The three components of the particle velocity field of these modes were measured by means of a three-dimensional laser Doppler vibrometer. In accordance with the terraced slope, a simple inclined plane was further tested to investigate and highlight the differences in terms of wave propagation along these two different ground formations. The results of this research provide significant experimental evidence that the terraced slopes form mechanisms which attenuate low frequency surface waves, thus acting as metasurfaces. This work suggests the use of laboratory-scale physical models to investigate the wave propagation in different landforms, which extend beyond typical horizontal ground morphologies, and which could be linked to atypical wave propagation properties, possibly even influencing propagation of seismic waves.","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":"52 7","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140675734","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Constitutive laws are a necessary ingredient in calculations of glacial isostatic adjustment (GIA) or other surface loading problems (e.g., loading by ocean tides). An idealized constitutive law governed by the Maxwell viscoelastic model is widely used but increasing attention is being directed towards more intricate constitutive laws that, in particular, include transient rheology. In this context, transient rheology collectively refers to dissipative mechanisms activated in addition to creep modeled by the Maxwell viscoelastic model. Consideration of such viscoelastic models in GIA is in its infancy and to encourage their wider use, we present constitutive laws for several experimentally derived transient rheologies and outline a flexible method in which to incorporate them into geophysical problems, such as the viscoelastic deformation of the Earth induced by surface loading. To further motivate this need, we demonstrate, via the Love number collocation method, how predictions of crustal displacement depart significantly between Earth models that adopt only Maxwell viscoelasticity and those with transient rheology. Throughout this paper, we highlight the differences in terminology and emphases between the rock mechanics, seismology, and GIA communities, which have perhaps contributed towards the relative scarcity in integrating this broader and more realistic class of constitutive laws within GIA. We focus on transient rheology since the associated deformation has been demonstrated to operate on timescales that range from hours to decades. With ice mass loss enhanced at similar timescales as a consequence of anthropogenically caused climate change, the ability to model GIA with more accurate constitutive laws is an important tool to investigate such problems.
构造定律是计算冰川等静力调整（GIA）或其他地表加载问题（如海洋潮汐加载）的必要因素。以麦克斯韦粘弹性模型为指导的理想化构造定律被广泛使用，但人们越来越关注更复杂的构造定律，尤其是包括瞬态流变学在内的构造定律。在这里，瞬态流变统指除麦克斯韦粘弹性模型所模拟的蠕变之外的耗散机制。在 GIA 中考虑此类粘弹性模型尚处于起步阶段，为了鼓励更广泛地使用这些模型，我们提出了几种通过实验得出的瞬态流变学的构成定律，并概述了将其纳入地球物理问题（如地表荷载引起的地球粘弹性变形）的灵活方法。为了进一步激发这一需求，我们通过勒夫数配位法证明了仅采用麦克斯韦粘弹性的地球模型与采用瞬态流变学的地球模型之间对地壳位移的预测如何存在显著差异。在本文中，我们强调了岩石力学、地震学和 GIA 界在术语和侧重点上的差异，这些差异可能是导致在 GIA 中整合这一类更广泛、更现实的构造规律相对缺乏的原因。我们将重点放在瞬态流变学上，因为相关变形的时间尺度已被证明从数小时到数十年不等。由于人类活动引起的气候变化会在类似的时间尺度上加剧冰的质量损失，因此用更精确的构成定律来模拟 GIA 的能力是研究此类问题的重要工具。
{"title":"Surface Loading on a self-gravitating, linear viscoelastic Earth: moving beyond Maxwell","authors":"H. C. P. Lau","doi":"10.1093/gji/ggae149","DOIUrl":"https://doi.org/10.1093/gji/ggae149","url":null,"abstract":"\u0000 Constitutive laws are a necessary ingredient in calculations of glacial isostatic adjustment (GIA) or other surface loading problems (e.g., loading by ocean tides). An idealized constitutive law governed by the Maxwell viscoelastic model is widely used but increasing attention is being directed towards more intricate constitutive laws that, in particular, include transient rheology. In this context, transient rheology collectively refers to dissipative mechanisms activated in addition to creep modeled by the Maxwell viscoelastic model. Consideration of such viscoelastic models in GIA is in its infancy and to encourage their wider use, we present constitutive laws for several experimentally derived transient rheologies and outline a flexible method in which to incorporate them into geophysical problems, such as the viscoelastic deformation of the Earth induced by surface loading. To further motivate this need, we demonstrate, via the Love number collocation method, how predictions of crustal displacement depart significantly between Earth models that adopt only Maxwell viscoelasticity and those with transient rheology. Throughout this paper, we highlight the differences in terminology and emphases between the rock mechanics, seismology, and GIA communities, which have perhaps contributed towards the relative scarcity in integrating this broader and more realistic class of constitutive laws within GIA. We focus on transient rheology since the associated deformation has been demonstrated to operate on timescales that range from hours to decades. With ice mass loss enhanced at similar timescales as a consequence of anthropogenically caused climate change, the ability to model GIA with more accurate constitutive laws is an important tool to investigate such problems.","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":"66 17","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140675862","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
C. A. Niño, C. Duarte, W. Agudelo, D. A. Sierra, M. D. Sacchi
Shear wave velocity (Vs) is a fundamental property of elastic media whose estimation from PS converted waves is challenging and requires modeling the boundary where P to S conversion occurs. This paper presents a PS tomography where seismic wave conversion/reflection points correspond to reflectors modelled with the level-set function set to zero (φ(x, z) = 0). The proposed method aims for stable Vs inversion in a seismic acquisition setting using multicomponent receivers. Synthetic models simulating true Vs, Vp and the location of the geological reflector are used in the study. The inversion starts by locating a flat reflector, φ(x, z) = 0, which defines the zone Ω1 between the surface and the reflector, where the initial Vs and Vp fields are also set. To calculate the traveltimes of incident PT (P wave that propagates in Ω1 from source to the reflector) , converted PS, and reflected PP waves, for both observed and modelled data (forward problem), the methodology proposed by Rawlinson and Sambridge is adopted. This method uses the arrival times of the P-waves, Tpt, from the seismic source at each reflector point as secondary sources generating the times Tps and Tpp. These times are calculated as a solution to the Eikonal equation by using the Fast Marching method. The PS and PP residual times are minimized by updating Vs, Vp, and φ(x, z) = 0 through adjoint variables designed from a formulation using Lagrange Multipliers in a variational context. The performance of the algorithm is evaluated for models with synclinal, sinusoidal and monoclinal reflector geometries using numerical tests considering the inversion of: 1) φ, given the true values of Vs and Vp; 2) φ and Vs, given the true value of Vp; 3) φ and Vp, given the true value of Vs; and 4) the three parameters φ, Vs, and Vp, simultaneously. Good results are obtained by inverting Vs and φ, given the true value of Vp. The simultaneous inversion of the three parameters exhibits promising results, despite the illumination problems caused by the different distribution of the PS, PP, and PT time gradients due to the geometry of the reflectors and the acquisition setting (sources-receivers in the same plane). The proposed tomography estimates Vs and reflector positions which could help in statics corrections and improve the lithological characterization of near surface.
剪切波速度（Vs）是弹性介质的基本属性，从 PS 转换波估算剪切波速度具有挑战性，需要对发生 P 到 S 转换的边界进行建模。本文提出了一种 PS 层析成像法，其中地震波转换/反射点对应于反射体建模，水平集函数设为零（φ(x, z) = 0）。所提出的方法旨在使用多分量接收机在地震采集环境中进行稳定的 Vs 反演。研究中使用了模拟真实 Vs、Vp 和地质反射体位置的合成模型。反演开始时，首先确定一个平面反射体的位置 φ(x, z) = 0，它定义了地表和反射体之间的区域 Ω1，初始 Vs 和 Vp 场也设置在该区域。为了计算观测数据和模拟数据（前向问题）中入射 PT 波（P 波在 Ω1 内从声源传播到反射器）、转换 PS 波和反射 PP 波的传播时间，采用了 Rawlinson 和 Sambridge 提出的方法。该方法将震源的 P 波到达每个反射点的时间 Tpt 作为产生时间 Tps 和 Tpp 的次要来源。这些时间是通过快速行进法作为艾克纳方程的解计算出来的。通过在变异背景下使用拉格朗日乘法器设计的公式更新 Vs、Vp 和 φ(x, z) = 0，使 PS 和 PP 的残差时间最小化。利用数值测试评估了该算法在具有同轴、正弦和单斜反射器几何形状的模型中的性能，并考虑了以下反演情况：1) 根据 Vs 和 Vp 的真实值反演 φ；2) 根据 Vp 的真实值反演 φ 和 Vs；3) 根据 Vs 的真实值反演 φ 和 Vp；4) 同时反演 φ、Vs 和 Vp 三个参数。在给定 Vp 真实值的情况下，通过反演 Vs 和 φ 可以获得良好的结果。尽管由于反射器的几何形状和采集设置（光源-接收器在同一平面内），PS、PP 和 PT 时间梯度的分布不同，造成了光照问题，但同时反演这三个参数还是取得了很好的结果。建议的层析成像估算 Vs 和反射器位置，有助于进行静态校正，并改进近地表的岩性特征描述。
{"title":"Converted Wave Tomography Based on Inverse Level Set and Adjoint Formulation","authors":"C. A. Niño, C. Duarte, W. Agudelo, D. A. Sierra, M. D. Sacchi","doi":"10.1093/gji/ggae147","DOIUrl":"https://doi.org/10.1093/gji/ggae147","url":null,"abstract":"\u0000 Shear wave velocity (Vs) is a fundamental property of elastic media whose estimation from PS converted waves is challenging and requires modeling the boundary where P to S conversion occurs. This paper presents a PS tomography where seismic wave conversion/reflection points correspond to reflectors modelled with the level-set function set to zero (φ(x, z) = 0). The proposed method aims for stable Vs inversion in a seismic acquisition setting using multicomponent receivers. Synthetic models simulating true Vs, Vp and the location of the geological reflector are used in the study. The inversion starts by locating a flat reflector, φ(x, z) = 0, which defines the zone Ω1 between the surface and the reflector, where the initial Vs and Vp fields are also set. To calculate the traveltimes of incident PT (P wave that propagates in Ω1 from source to the reflector) , converted PS, and reflected PP waves, for both observed and modelled data (forward problem), the methodology proposed by Rawlinson and Sambridge is adopted. This method uses the arrival times of the P-waves, Tpt, from the seismic source at each reflector point as secondary sources generating the times Tps and Tpp. These times are calculated as a solution to the Eikonal equation by using the Fast Marching method. The PS and PP residual times are minimized by updating Vs, Vp, and φ(x, z) = 0 through adjoint variables designed from a formulation using Lagrange Multipliers in a variational context. The performance of the algorithm is evaluated for models with synclinal, sinusoidal and monoclinal reflector geometries using numerical tests considering the inversion of: 1) φ, given the true values of Vs and Vp; 2) φ and Vs, given the true value of Vp; 3) φ and Vp, given the true value of Vs; and 4) the three parameters φ, Vs, and Vp, simultaneously. Good results are obtained by inverting Vs and φ, given the true value of Vp. The simultaneous inversion of the three parameters exhibits promising results, despite the illumination problems caused by the different distribution of the PS, PP, and PT time gradients due to the geometry of the reflectors and the acquisition setting (sources-receivers in the same plane). The proposed tomography estimates Vs and reflector positions which could help in statics corrections and improve the lithological characterization of near surface.","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":"40 5","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-04-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140677410","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Plate-coupling estimates and previous seismicity indicate that portions of the Makran megathrust of southern Pakistan and Iran are partially coupled and have the potential to produce future magnitude 7 + earthquakes. However, the GPS observations needed to constrain coupling models are sparse and lead to an incomplete understanding of regional earthquake and tsunami hazard. In this study, we assess GPS velocities for plate-coupling of the Makran subduction zone with specific attention to model resolution and the accretionary prism rheology. We use finite element model-derived Green's functions to invert for the interseismic slip deficit under both elastic and viscoelastic Earth assumptions. We use the model resolution matrix to characterize plate-coupling scenarios that are consistent with the limited spatial resolution afforded by GPS observations. We then forward model the corresponding tsunami responses at major coastal cities within the western Indian Ocean basin. Our plate-coupling results show potential segmentation of the megathrust with varying coupling from west to east, but do not rule out a scenario where the entire length of the megathrust could rupture in a single earthquake. The full subduction zone rupture scenarios suggest the Makran may be able to produce earthquakes up to Mw 9.2. The corresponding tsunami model from the largest earthquake event (Mw 9.2) estimates maximum wave heights reaching 2 to 5 meters at major port cities in the Northern Arabian Sea region. Cities on the west coast of India are less affected (1-2 m). Coastlines bounding eastern Africa, and the Strait of Hormuz, are the least affected (< 1 m).
{"title":"Constraints from GPS measurements on plate-coupling within the Makran Subduction Zone and tsunami scenarios in the Western Indian Ocean","authors":"Guo Cheng, William D Barnhart, David Small","doi":"10.1093/gji/ggae046","DOIUrl":"https://doi.org/10.1093/gji/ggae046","url":null,"abstract":"\u0000 Plate-coupling estimates and previous seismicity indicate that portions of the Makran megathrust of southern Pakistan and Iran are partially coupled and have the potential to produce future magnitude 7 + earthquakes. However, the GPS observations needed to constrain coupling models are sparse and lead to an incomplete understanding of regional earthquake and tsunami hazard. In this study, we assess GPS velocities for plate-coupling of the Makran subduction zone with specific attention to model resolution and the accretionary prism rheology. We use finite element model-derived Green's functions to invert for the interseismic slip deficit under both elastic and viscoelastic Earth assumptions. We use the model resolution matrix to characterize plate-coupling scenarios that are consistent with the limited spatial resolution afforded by GPS observations. We then forward model the corresponding tsunami responses at major coastal cities within the western Indian Ocean basin. Our plate-coupling results show potential segmentation of the megathrust with varying coupling from west to east, but do not rule out a scenario where the entire length of the megathrust could rupture in a single earthquake. The full subduction zone rupture scenarios suggest the Makran may be able to produce earthquakes up to Mw 9.2. The corresponding tsunami model from the largest earthquake event (Mw 9.2) estimates maximum wave heights reaching 2 to 5 meters at major port cities in the Northern Arabian Sea region. Cities on the west coast of India are less affected (1-2 m). Coastlines bounding eastern Africa, and the Strait of Hormuz, are the least affected (< 1 m).","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":"6 18","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139803850","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Plate-coupling estimates and previous seismicity indicate that portions of the Makran megathrust of southern Pakistan and Iran are partially coupled and have the potential to produce future magnitude 7 + earthquakes. However, the GPS observations needed to constrain coupling models are sparse and lead to an incomplete understanding of regional earthquake and tsunami hazard. In this study, we assess GPS velocities for plate-coupling of the Makran subduction zone with specific attention to model resolution and the accretionary prism rheology. We use finite element model-derived Green's functions to invert for the interseismic slip deficit under both elastic and viscoelastic Earth assumptions. We use the model resolution matrix to characterize plate-coupling scenarios that are consistent with the limited spatial resolution afforded by GPS observations. We then forward model the corresponding tsunami responses at major coastal cities within the western Indian Ocean basin. Our plate-coupling results show potential segmentation of the megathrust with varying coupling from west to east, but do not rule out a scenario where the entire length of the megathrust could rupture in a single earthquake. The full subduction zone rupture scenarios suggest the Makran may be able to produce earthquakes up to Mw 9.2. The corresponding tsunami model from the largest earthquake event (Mw 9.2) estimates maximum wave heights reaching 2 to 5 meters at major port cities in the Northern Arabian Sea region. Cities on the west coast of India are less affected (1-2 m). Coastlines bounding eastern Africa, and the Strait of Hormuz, are the least affected (< 1 m).
{"title":"Constraints from GPS measurements on plate-coupling within the Makran Subduction Zone and tsunami scenarios in the Western Indian Ocean","authors":"Guo Cheng, William D Barnhart, David Small","doi":"10.1093/gji/ggae046","DOIUrl":"https://doi.org/10.1093/gji/ggae046","url":null,"abstract":"\u0000 Plate-coupling estimates and previous seismicity indicate that portions of the Makran megathrust of southern Pakistan and Iran are partially coupled and have the potential to produce future magnitude 7 + earthquakes. However, the GPS observations needed to constrain coupling models are sparse and lead to an incomplete understanding of regional earthquake and tsunami hazard. In this study, we assess GPS velocities for plate-coupling of the Makran subduction zone with specific attention to model resolution and the accretionary prism rheology. We use finite element model-derived Green's functions to invert for the interseismic slip deficit under both elastic and viscoelastic Earth assumptions. We use the model resolution matrix to characterize plate-coupling scenarios that are consistent with the limited spatial resolution afforded by GPS observations. We then forward model the corresponding tsunami responses at major coastal cities within the western Indian Ocean basin. Our plate-coupling results show potential segmentation of the megathrust with varying coupling from west to east, but do not rule out a scenario where the entire length of the megathrust could rupture in a single earthquake. The full subduction zone rupture scenarios suggest the Makran may be able to produce earthquakes up to Mw 9.2. The corresponding tsunami model from the largest earthquake event (Mw 9.2) estimates maximum wave heights reaching 2 to 5 meters at major port cities in the Northern Arabian Sea region. Cities on the west coast of India are less affected (1-2 m). Coastlines bounding eastern Africa, and the Strait of Hormuz, are the least affected (< 1 m).","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":"5 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-02-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139863731","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Stefan Lüth, Florian Steegborn, F. Heberling, T. Beilecke, Dirk Bosbach, G. Deissmann, Horst Geckeis, Claudia Joseph, Axel Liebscher, Volker Metz, Dorothee Rebscher, Karsten Rink, Trond Ryberg, Stephan Schennen
This contribution is presenting a multidisciplinary investigation of heterogeneities in a clay rock formation, based on seismic tomography, logging, and core analysis, as a reconnaissance study for a diffusion experiment. Diffusion experiments in clay rock formations provide crucial experimental data on diffusive transport of radionuclides (RN) in extremely low hydraulic conductivity media. Previous diffusion experiments, conducted, e.g. in the Mont Terri underground rock laboratory within the relatively homogeneous shaly facies of Opalinus Clay, and modelling studies of these experiments have demonstrated that the clay rock could sufficiently well be described as a homogeneous anisotropic medium. For other lithofacies, characterised by larger heterogeneity, such simplification may be unsuitable, and the description of heterogeneity over a range of scales will be important. The sandy facies of the Opalinus Clay exhibits a significantly more pronounced heterogeneity compared to the shaly facies, and a combined characterisation and RN diffusion study has been initiated to investigate various approaches of heterogeneity characterisation and subsequent diffusion in a heterogeneous environment. As an initial step, two inclined exploratory boreholes have been drilled to access the margins of the experiment location. These boreholes have been used to acquire a cross-hole tomographic seismic data set. Optical, natural gamma and backscattering logging were applied and rock cores were analysed. The integrated results of these investigations allowed the identification of an anomalous brighter layer within the investigated area of the sandy facies of approximately 1 m thickness and with its upper bound at roughly 10 m depth within the inclined exploratory wells. Mineralogical analyses revealed only slight variations throughout the rock cores and indicated that the anomalous layer exhibited a slightly higher quartz content, and locally significantly higher calcite contents, accompanied by a lower content of clay minerals. The anomalous layer was characterised by reduced natural gamma emissions, due to the lower clay content, and increased neutron backscattering likely indicating an increased porosity. Seismic P-wave velocities, derived from anisotropic tomography, exhibited a maximal gradient near the top of this layer. The transition from the overlaying darker rock matrix into this layer has been identified as an appropriate location for the setup of a tracer diffusion experiment in a heterogeneous environment.
本文介绍了一项基于地震层析成像、测井和岩心分析的粘土岩层异质性多学科调查，作为扩散实验的勘测研究。粘土岩层中的扩散实验为放射性核素（RN）在极低导水介质中的扩散迁移提供了重要的实验数据。以前在蒙特泰里地下岩石实验室（Mont Terri）相对均质的奥帕林纳斯粘土页岩岩层中进行的扩散实验，以及对这些实验进行的建模研究都表明，粘土岩可以充分地描述为均质各向异性介质。对于异质性较大的其他岩性来说，这种简化可能并不合适，因此对不同尺度的异质性进行描述非常重要。奥帕利努斯粘土的砂质岩相与鳞片岩相相比，具有明显的异质性，因此我们启动了一项综合特征描述和 RN 扩散研究，以研究异质性特征描述和异质性环境中的后续扩散的各种方法。第一步是钻探两个倾斜的探井，以进入实验地点的边缘。这些钻孔用于获取跨孔层析地震数据集。还采用了光学、自然伽马和反向散射测井技术，并对岩心进行了分析。根据这些调查的综合结果，在所调查的砂质岩层区域内发现了一个厚度约为 1 米的异常较亮层，其上限位于倾斜探井内约 10 米深处。矿物分析表明，整个岩心只有轻微的变化，异常层的石英含量略高，局部方解石含量明显偏高，粘土矿物含量较低。由于粘土含量较低，异常层的特征是天然伽马射线发射减少，中子反向散射增加，这可能表明孔隙率增加。通过各向异性层析成像得出的地震 P 波速度在该层顶部附近显示出最大梯度。从上覆深色岩石基质向该层的过渡被确定为在异质环境中设置示踪剂扩散实验的适当位置。
{"title":"Characterisation of heterogeneities in the sandy facies of the Opalinus Clay (Mont Terri underground rock laboratory, Switzerland)","authors":"Stefan Lüth, Florian Steegborn, F. Heberling, T. Beilecke, Dirk Bosbach, G. Deissmann, Horst Geckeis, Claudia Joseph, Axel Liebscher, Volker Metz, Dorothee Rebscher, Karsten Rink, Trond Ryberg, Stephan Schennen","doi":"10.1093/gji/ggad494","DOIUrl":"https://doi.org/10.1093/gji/ggad494","url":null,"abstract":"\u0000 This contribution is presenting a multidisciplinary investigation of heterogeneities in a clay rock formation, based on seismic tomography, logging, and core analysis, as a reconnaissance study for a diffusion experiment. Diffusion experiments in clay rock formations provide crucial experimental data on diffusive transport of radionuclides (RN) in extremely low hydraulic conductivity media. Previous diffusion experiments, conducted, e.g. in the Mont Terri underground rock laboratory within the relatively homogeneous shaly facies of Opalinus Clay, and modelling studies of these experiments have demonstrated that the clay rock could sufficiently well be described as a homogeneous anisotropic medium. For other lithofacies, characterised by larger heterogeneity, such simplification may be unsuitable, and the description of heterogeneity over a range of scales will be important. The sandy facies of the Opalinus Clay exhibits a significantly more pronounced heterogeneity compared to the shaly facies, and a combined characterisation and RN diffusion study has been initiated to investigate various approaches of heterogeneity characterisation and subsequent diffusion in a heterogeneous environment. As an initial step, two inclined exploratory boreholes have been drilled to access the margins of the experiment location. These boreholes have been used to acquire a cross-hole tomographic seismic data set. Optical, natural gamma and backscattering logging were applied and rock cores were analysed. The integrated results of these investigations allowed the identification of an anomalous brighter layer within the investigated area of the sandy facies of approximately 1 m thickness and with its upper bound at roughly 10 m depth within the inclined exploratory wells. Mineralogical analyses revealed only slight variations throughout the rock cores and indicated that the anomalous layer exhibited a slightly higher quartz content, and locally significantly higher calcite contents, accompanied by a lower content of clay minerals. The anomalous layer was characterised by reduced natural gamma emissions, due to the lower clay content, and increased neutron backscattering likely indicating an increased porosity. Seismic P-wave velocities, derived from anisotropic tomography, exhibited a maximal gradient near the top of this layer. The transition from the overlaying darker rock matrix into this layer has been identified as an appropriate location for the setup of a tracer diffusion experiment in a heterogeneous environment.","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":"16 20","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139382974","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Epidemic-Type Aftershock Sequence (ETAS) model is the state-of-the-art approach for modeling short-term earthquake clustering and is preferable for short-term aftershock forecasting. However, due to the large variability of different earthquake sequences, the model parameters must be adjusted to the local seismicity for accurate forecasting. Such an adjustment based on the first aftershocks is hampered by the incompleteness of earthquake catalogs after a mainshock, which can be explained by a blind period of the seismic networks after each earthquake, during which smaller events with lower magnitudes cannot be detected. Assuming a constant blind time, direct relationships based only on this additional parameter can be established between the actual seismicity rate and magnitude distributions and those that can be detected. The ETAS-Incomplete (ETASI) model uses these relationships to estimate the true ETAS parameters and the catalog incompleteness jointly. In this study, we apply the ETASI model to the SE Türkiye earthquake sequence, consisting of a doublet of M7.7 and M7.6 earthquakes that occurred within less than half a day of each other on February 6, 2023. We show that the ETASI model can explain the catalog incompleteness and fits the observed earthquake numbers and magnitudes well. A pseudo-prospective forecasting experiment shows that the daily number of detectable m ≥ 2 can be well predicted based on minimal and incomplete information from early aftershocks. However, the maximum magnitude (Mmax ) of the next day’s aftershocks would have been overestimated due to the highly variable b value within the sequence. Instead, using the regional b value estimated for 2000-2022 would have well predicted the observed Mmax values.
{"title":"Aftershock forecasts based on incomplete earthquake catalogs: ETASI model application to the 2023 SE Türkiye earthquake sequence","authors":"S. Hainzl, T. Kumazawa, Y. Ogata","doi":"10.1093/gji/ggae006","DOIUrl":"https://doi.org/10.1093/gji/ggae006","url":null,"abstract":"\u0000 The Epidemic-Type Aftershock Sequence (ETAS) model is the state-of-the-art approach for modeling short-term earthquake clustering and is preferable for short-term aftershock forecasting. However, due to the large variability of different earthquake sequences, the model parameters must be adjusted to the local seismicity for accurate forecasting. Such an adjustment based on the first aftershocks is hampered by the incompleteness of earthquake catalogs after a mainshock, which can be explained by a blind period of the seismic networks after each earthquake, during which smaller events with lower magnitudes cannot be detected. Assuming a constant blind time, direct relationships based only on this additional parameter can be established between the actual seismicity rate and magnitude distributions and those that can be detected. The ETAS-Incomplete (ETASI) model uses these relationships to estimate the true ETAS parameters and the catalog incompleteness jointly. In this study, we apply the ETASI model to the SE Türkiye earthquake sequence, consisting of a doublet of M7.7 and M7.6 earthquakes that occurred within less than half a day of each other on February 6, 2023. We show that the ETASI model can explain the catalog incompleteness and fits the observed earthquake numbers and magnitudes well. A pseudo-prospective forecasting experiment shows that the daily number of detectable m ≥ 2 can be well predicted based on minimal and incomplete information from early aftershocks. However, the maximum magnitude (Mmax ) of the next day’s aftershocks would have been overestimated due to the highly variable b value within the sequence. Instead, using the regional b value estimated for 2000-2022 would have well predicted the observed Mmax values.","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":"92 20","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139381214","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Satellite observations of the geomagnetic field contain signals generated in Earth’s interior by electrical currents in the core and by magnetized rocks in the lithosphere. At short wavelengths the lithospheric signal dominates, obscuring the signal from the core. Here we present details of a method to co-estimate separate models for the core and lithospheric fields, which are allowed to overlap in spherical harmonic degree, that makes use of prior information to aid the separation. Using a maximum entropy method we estimate probabilistic models for the time-dependent core field and the static lithospheric field that satisfy constraints provided by satellite observations while being consistent with prior knowledge of the spatial covariance and expected magnitude of each field at its source surface. For the core field, we find that between spherical harmonic degree 13 and 22 power adds coherently to the established structures, and present a synthetic test that illustrates the aspects of the small scale core field that can reliably be retrieved. For the large scale lithospheric field we also find encouraging results, with the strongest signatures below spherical harmonic degree 13 occurring at locations of known prominent lithospheric field anomalies in north-Eastern Europe, Australia and eastern North America. Although the amplitudes of the small scale core field and large scale lithospheric field are underestimated we find no evidence that obvious artefacts are introduced. Compared with conventional maps of the core-mantle boundary field our results suggest more localized normal flux concentrations close to the tangent cylinder, and that low latitude flux concentrations occur in pairs of opposite polarity. Future improvements in the recovery of the small scale core field and large scale lithospheric field will depend on whether more detailed prior information can be reliably extracted from core dynamo and lithospheric magnetisation simulations.
{"title":"Co-estimation of core and lithospheric magnetic fields by a maximum entropy method","authors":"Mikkel Otzen, Christopher C Finlay, C. Kloss","doi":"10.1093/gji/ggae008","DOIUrl":"https://doi.org/10.1093/gji/ggae008","url":null,"abstract":"\u0000 Satellite observations of the geomagnetic field contain signals generated in Earth’s interior by electrical currents in the core and by magnetized rocks in the lithosphere. At short wavelengths the lithospheric signal dominates, obscuring the signal from the core. Here we present details of a method to co-estimate separate models for the core and lithospheric fields, which are allowed to overlap in spherical harmonic degree, that makes use of prior information to aid the separation. Using a maximum entropy method we estimate probabilistic models for the time-dependent core field and the static lithospheric field that satisfy constraints provided by satellite observations while being consistent with prior knowledge of the spatial covariance and expected magnitude of each field at its source surface. For the core field, we find that between spherical harmonic degree 13 and 22 power adds coherently to the established structures, and present a synthetic test that illustrates the aspects of the small scale core field that can reliably be retrieved. For the large scale lithospheric field we also find encouraging results, with the strongest signatures below spherical harmonic degree 13 occurring at locations of known prominent lithospheric field anomalies in north-Eastern Europe, Australia and eastern North America. Although the amplitudes of the small scale core field and large scale lithospheric field are underestimated we find no evidence that obvious artefacts are introduced. Compared with conventional maps of the core-mantle boundary field our results suggest more localized normal flux concentrations close to the tangent cylinder, and that low latitude flux concentrations occur in pairs of opposite polarity. Future improvements in the recovery of the small scale core field and large scale lithospheric field will depend on whether more detailed prior information can be reliably extracted from core dynamo and lithospheric magnetisation simulations.","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":"87 4","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139381587","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Atmospheric and oceanic pressure perturbations deform the ground surface and the seafloor, respectively. This mechanical deformation, where the fluid perturbations propagate as plane waves, occurs not only on Earth but also on other planets/bodies with atmospheres, such as Mars, Titan, and Venus. Studying this type of deformation improves our understanding of the mechanical interaction between the fluid layer (atmosphere/ocean) and the underlying solid planet/body, and aids investigation of subsurface structures. In this study, we utilize eigenfunction theory to unify existing theories for modelling this deformation and to comprehensively demonstrate possible scenarios of this deformation in homogeneous and 1D elastic media, including static loading, air-coupled Rayleigh waves, and leaky-mode surface waves. Our computations quantitatively reveal that the deformation amplitude generally decays with depth and that reducing seismic noise due to Martian atmosphere requires deploying seismometers at least 1 m beneath Martian surface. We also apply our theory to illustrate how this deformation and the corresponding air-to-solid energy conversion vary on different planetary bodies. Finally, we discuss how medium anelasticity and other factors affect this deformation.
{"title":"A comprehensive theory for 1D (an)elastic medium deformation due to plane-wave fluid pressure perturbation","authors":"Zongbo Xu, Philippe Lognonné","doi":"10.1093/gji/ggae005","DOIUrl":"https://doi.org/10.1093/gji/ggae005","url":null,"abstract":"\u0000 Atmospheric and oceanic pressure perturbations deform the ground surface and the seafloor, respectively. This mechanical deformation, where the fluid perturbations propagate as plane waves, occurs not only on Earth but also on other planets/bodies with atmospheres, such as Mars, Titan, and Venus. Studying this type of deformation improves our understanding of the mechanical interaction between the fluid layer (atmosphere/ocean) and the underlying solid planet/body, and aids investigation of subsurface structures. In this study, we utilize eigenfunction theory to unify existing theories for modelling this deformation and to comprehensively demonstrate possible scenarios of this deformation in homogeneous and 1D elastic media, including static loading, air-coupled Rayleigh waves, and leaky-mode surface waves. Our computations quantitatively reveal that the deformation amplitude generally decays with depth and that reducing seismic noise due to Martian atmosphere requires deploying seismometers at least 1 m beneath Martian surface. We also apply our theory to illustrate how this deformation and the corresponding air-to-solid energy conversion vary on different planetary bodies. Finally, we discuss how medium anelasticity and other factors affect this deformation.","PeriodicalId":502458,"journal":{"name":"Geophysical Journal International","volume":"8 3","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139384226","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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