G. Neri, B. Orecchio, P. Pino, D. Presti, S. Scolaro, Antonino Torre
We have investigated the seismicity occurred during 2000-2021 in the area of Messina, the town which suffered greatest loss of human lives over the territory devastated by the magnitude 7.1 earthquake of December 28, 1908. We have found that most of recent activity was located beneath the historical centre of the town, in and near the very peculiar sickle-shaped harbor zone which prompted the Greek colonizers in the VII Century B.C. to give Messina the old name of “Zancle”(“Sickle” in the ancient Greek language). Extracting from the whole dataset (consisting of hundred earthquakes of maximum magnitude 3.8) the data relative to a small sequence of 28 events concentrated in a few days at the end of 2013, and performing high-quality Bayesian hypocenter locations of these events, we have found very clear epi-hypocentral trends suitable for comparison with the local structural scenario. The joint analysis of seismic, geological and geomorphological data including morphobathymetric curves of the sea bottom in the study area, has brought us to propose that the small sequence in question (and probably most of activity recorded during the whole 22-years period) may have been generated by internal dynamics of a local horst/graben system, the position of which (i) appears to correspond to one of the minor horsts documented in the Messina Strait basin area and (ii) is very close to the upper edge of the 1908 earthquake blind source reported in the Database of Individual Seismogenic Sources of the Italian National Institute of Geophysics and Volcanology.
{"title":"Recent seismicity in the Messina area, southern Italy, and comparison to the local geology and tectonics","authors":"G. Neri, B. Orecchio, P. Pino, D. Presti, S. Scolaro, Antonino Torre","doi":"10.4401/ag-8974","DOIUrl":"https://doi.org/10.4401/ag-8974","url":null,"abstract":"We have investigated the seismicity occurred during 2000-2021 in the area of Messina, the town which suffered greatest loss of human lives over the territory devastated by the magnitude 7.1 earthquake of December 28, 1908. We have found that most of recent activity was located beneath the historical centre of the town, in and near the very peculiar sickle-shaped harbor zone which prompted the Greek colonizers in the VII Century B.C. to give Messina the old name of “Zancle”(“Sickle” in the ancient Greek language). Extracting from the whole dataset (consisting of hundred earthquakes of maximum magnitude 3.8) the data relative to a small sequence of 28 events concentrated in a few days at the end of 2013, and performing high-quality Bayesian hypocenter locations of these events, we have found very clear epi-hypocentral trends suitable for comparison with the local structural scenario. The joint analysis of seismic, geological and geomorphological data including morphobathymetric curves of the sea bottom in the study area, has brought us to propose that the small sequence in question (and probably most of activity recorded during the whole 22-years period) may have been generated by internal dynamics of a local horst/graben system, the position of which (i) appears to correspond to one of the minor horsts documented in the Messina Strait basin area and (ii) is very close to the upper edge of the 1908 earthquake blind source reported in the Database of Individual Seismogenic Sources of the Italian National Institute of Geophysics and Volcanology.","PeriodicalId":50766,"journal":{"name":"Annals of Geophysics","volume":"22 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2023-08-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"74617713","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
P. Pino, S. Scolaro, Antonino Torre, S. D’Amico, G. Neri, D. Presti
Analysis of seismic noise measurements shows a clear change of the Site Resonant Frequency across a NW-SE segment cutting the historic center of the town of Messina. This change indicates strong lateral heterogeneity of the sediment cover going from southwest to northeast across the segment and suggests the existence of a fault never reported by previous investigators, oriented differently from the NNE-SSW main structural system which is widely believed to have produced the magnitude 7.1 earthquake of 1908. Additional evidence of such a NW-SE fault has been obtained by surface geology and analysis of morphological and Digital Terrain Model data. Geologic observations clearly indicate normal faulting but are not able to identify eventual strike-slip components. Activity of this fault is documented at least until Middle Pleistocene, with likely prosecution during Upper Pleistocene. The new detected fault requires deeper investigation in the near future for evaluation of its real extent and present dynamics including eventual seismogenic attitude.
{"title":"Geophysical and geological signatures of an unknown fault in the historic center of Messina (Sicily, south Italy)","authors":"P. Pino, S. Scolaro, Antonino Torre, S. D’Amico, G. Neri, D. Presti","doi":"10.4401/ag-8950","DOIUrl":"https://doi.org/10.4401/ag-8950","url":null,"abstract":"Analysis of seismic noise measurements shows a clear change of the Site Resonant Frequency across a NW-SE segment cutting the historic center of the town of Messina. This change indicates strong lateral heterogeneity of the sediment cover going from southwest to northeast across the segment and suggests the existence of a fault never reported by previous investigators, oriented differently from the NNE-SSW main structural system which is widely believed to have produced the magnitude 7.1 earthquake of 1908. Additional evidence of such a NW-SE fault has been obtained by surface geology and analysis of morphological and Digital Terrain Model data. Geologic observations clearly indicate normal faulting but are not able to identify eventual strike-slip components. Activity of this fault is documented at least until Middle Pleistocene, with likely prosecution during Upper Pleistocene. The new detected fault requires deeper investigation in the near future for evaluation of its real extent and present dynamics including eventual seismogenic attitude.","PeriodicalId":50766,"journal":{"name":"Annals of Geophysics","volume":"26 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2023-08-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"82268016","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We propose a MATLAB toolbox for the computation of the strain rate field from the coordinate time series of some continuous GNSS stations. It consists of several functions, also compatible with GNU Octave, implementing the following steps: (i) time series download from a data repository (e.g., the Nevada Geodetic Laboratory database); (ii) calculation of velocities of the selected stations by means of the Maximum Likelihood Estimation (MLE) method implemented in the external package Hector, including modeling of offsets, outliers, noise and periodic components; (iii) (optional) filtering of Common Mode Errors; (iv) calculation of the strain rate field with the modified least squares method, in which a scale factor can be introduced to define the locality of the deformation analysis and, besides uncertainty estimation, a geometric evaluation of the significance of the results is provided; (v) visualization of the results for immediate use and easy interpretation for scientific purposes. The toolbox is divided into two components: the first one, named StaVel, performs the steps (i)-(iii) and the second component, GridStrain, performs the steps (iv) and (v). The potential of the toolbox is demonstrated on a real dataset. Time series from several continuous GNSS stations in South-Eastern Sicily (Southern Italy) are processed by means of StaVel and GridStrain in order to provide the strain rate field.
{"title":"A MATLAB toolbox for computation of velocity and strain rate field from GNSS coordinate time series","authors":"G. Teza, A. Pesci, Marco Meschis","doi":"10.4401/ag-8933","DOIUrl":"https://doi.org/10.4401/ag-8933","url":null,"abstract":"We propose a MATLAB toolbox for the computation of the strain rate field from the coordinate time series of some continuous GNSS stations. It consists of several functions, also compatible with GNU Octave, implementing the following steps: (i) time series download from a data repository (e.g., the Nevada Geodetic Laboratory database); (ii) calculation of velocities of the selected stations by means of the Maximum Likelihood Estimation (MLE) method implemented in the external package Hector, including modeling of offsets, outliers, noise and periodic components; (iii) (optional) filtering of Common Mode Errors; (iv) calculation of the strain rate field with the modified least squares method, in which a scale factor can be introduced to define the locality of the deformation analysis and, besides uncertainty estimation, a geometric evaluation of the significance of the results is provided; (v) visualization of the results for immediate use and easy interpretation for scientific purposes. The toolbox is divided into two components: the first one, named StaVel, performs the steps (i)-(iii) and the second component, GridStrain, performs the steps (iv) and (v). The potential of the toolbox is demonstrated on a real dataset. Time series from several continuous GNSS stations in South-Eastern Sicily (Southern Italy) are processed by means of StaVel and GridStrain in order to provide the strain rate field.","PeriodicalId":50766,"journal":{"name":"Annals of Geophysics","volume":"40 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2023-08-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"75171849","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
G. Mancino, R. Console, M. Greco, M. Trivigno, Antonio Falciano
The study aimed at verifying the existence of stress induced on the functionality of natural ecosystems by particularly impacting anthropogenic activities. In detail, a methodology has been developed to evaluate any alterations in the phenology of plant species in areas surrounding sites defined by Italian legislation as “potentially polluted”. Specifically, the study areas located in Basilicata (southern Italy) were intended for municipal solid waste management activities and, at some stage of their management, Potential Toxic Elements (PTEs) concentrations were recorded above the thresholds permitted by the current legislation. The phenological trends of the vegetation were analyzed at gradually increasing distances from the centroid of the sites and then compared with points of the same type of vegetation, very distant from the sites, in areas that were not reasonably impacted by any contamination. The reconstruction of the phenological trends was carried out using Sentinel-2 images approximately on a monthly basis from which the Normalized Difference Vegetation Index (NDVI) was evaluated. Finally, the trends between areas adjacent the sites and unpolluted ones were statistically analyzed using dissimilarity indices which led to the conclusion of the non-existence of effects induced by PTEs on the functionality of the vegetation.
{"title":"Monitoring the vegetation stress coming from anthropogenic activities by modeling phenology using Sentinel-2 data","authors":"G. Mancino, R. Console, M. Greco, M. Trivigno, Antonio Falciano","doi":"10.4401/ag-8837","DOIUrl":"https://doi.org/10.4401/ag-8837","url":null,"abstract":"The study aimed at verifying the existence of stress induced on the functionality of natural ecosystems by particularly impacting anthropogenic activities. In detail, a methodology has been developed to evaluate any alterations in the phenology of plant species in areas surrounding sites defined by Italian legislation as “potentially polluted”. Specifically, the study areas located in Basilicata (southern Italy) were intended for municipal solid waste management activities and, at some stage of their management, Potential Toxic Elements (PTEs) concentrations were recorded above the thresholds permitted by the current legislation. The phenological trends of the vegetation were analyzed at gradually increasing distances from the centroid of the sites and then compared with points of the same type of vegetation, very distant from the sites, in areas that were not reasonably impacted by any contamination. The reconstruction of the phenological trends was carried out using Sentinel-2 images approximately on a monthly basis from which the Normalized Difference Vegetation Index (NDVI) was evaluated. Finally, the trends between areas adjacent the sites and unpolluted ones were statistically analyzed using dissimilarity indices which led to the conclusion of the non-existence of effects induced by PTEs on the functionality of the vegetation.","PeriodicalId":50766,"journal":{"name":"Annals of Geophysics","volume":"13 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2023-07-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"91288541","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
This paper is intended as a short presentation of the main limitations affecting seismic hazard assessment, revisiting possible methods available in the literature to be applied for this purpose. The convergence of the African Plate with the Eurasian Plate is the cause of the high seismic activity characterizing the Mediterranean region, with particular intensity in its eastern part. It is clear that the associated seismic risk requires appropriate measures for its mitigation. Seismic risk, the amount of resources that the community is expected to pay to earthquakes in the long term, is the product of three factors, such as seismic hazard, vulnerability and value of the exposed goods. As earthquakes cannot be prevented, seismic risk can be mitigated by improving our knowledge of seismic hazard, which is largely based on statistical analysis of historical earthquake catalogs. Nevertheless, historical records are affected by problems of reliability, completeness and shortness, as they commonly span time lengths of the same order of magnitude or even shorter than the inter-event time of the strongest earthquakes produced by specific seismic sources. In this respect, alternative methods can be proposed for integrating and improving our knowledge of seismogenic processes, and estimating both time-independent and time-dependent occurrence rates of strong earthquakes. We propose the application of physics-based earthquake simulators, requiring the knowledge of a robust geological-geophysical seismogenic model.
{"title":"Improving seimic hazard assessment in the Mediterranean Region","authors":"R. Console, P. Vannoli","doi":"10.4401/ag-8881","DOIUrl":"https://doi.org/10.4401/ag-8881","url":null,"abstract":"This paper is intended as a short presentation of the main limitations affecting seismic hazard assessment, revisiting possible methods available in the literature to be applied for this purpose. The convergence of the African Plate with the Eurasian Plate is the cause of the high seismic activity characterizing the Mediterranean region, with particular intensity in its eastern part. It is clear that the associated seismic risk requires appropriate measures for its mitigation. Seismic risk, the amount of resources that the community is expected to pay to earthquakes in the long term, is the product of three factors, such as seismic hazard, vulnerability and value of the exposed goods. As earthquakes cannot be prevented, seismic risk can be mitigated by improving our knowledge of seismic hazard, which is largely based on statistical analysis of historical earthquake catalogs. Nevertheless, historical records are affected by problems of reliability, completeness and shortness, as they commonly span time lengths of the same order of magnitude or even shorter than the inter-event time of the strongest earthquakes produced by specific seismic sources. In this respect, alternative methods can be proposed for integrating and improving our knowledge of seismogenic processes, and estimating both time-independent and time-dependent occurrence rates of strong earthquakes. We propose the application of physics-based earthquake simulators, requiring the knowledge of a robust geological-geophysical seismogenic model.","PeriodicalId":50766,"journal":{"name":"Annals of Geophysics","volume":"8 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2023-07-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"78614881","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Sanjiang lateral collision zone is a key region to understand the Tibetan Plateau’s tectonic structure and the Tethys-Himalayan’s tectonic evolution. Complex tectonic structures, intense crustal deformation, frequent seismicity, and abundant metal deposits are all present. With the seismic data recorded by a temporary array (SJ-Array) and permanent stations (Nov. 2018 ~ Dec. 2020), this paper adopts the S wave splitting technique to obtain the essential properties of upper crustal anisotropy. In the interested area, it is shown that the dominant polarization of the fast S wave is NNW, with a mean polarization direction of 167.9°. In addition, the study area can be divided into three subzones from the west to the east: A, B, and C, according to the various mean polarizations varying from NNW, NS to NNE. The mean normalized time delay between the two split S waves is 4.0 ms/km, and the range of time delay is from 2.0 to 6.3 ms/km. The largest time delay is located at the east side of the western boundary of the Sichuan-Yunnan rhombus block. Furthermore, there is a strip area of strong anisotropy stretching along the western segment of the Lijiang-Xiaojinhe fault. These all demonstrate the local tectonic differences and indicate that the crustal structure may be strongly controlled by the fault and block boundary strike.
{"title":"Seismic Anisotropy in the upper crust beneath the Sanjiang lateral collision zone in the southeastern margin of the Tibetan Plateau revealed by S wave splitting from a temporary array","authors":"Xinyi Li, Yuan Gao","doi":"10.4401/ag-8867","DOIUrl":"https://doi.org/10.4401/ag-8867","url":null,"abstract":"The Sanjiang lateral collision zone is a key region to understand the Tibetan Plateau’s tectonic structure and the Tethys-Himalayan’s tectonic evolution. Complex tectonic structures, intense crustal deformation, frequent seismicity, and abundant metal deposits are all present. With the seismic data recorded by a temporary array (SJ-Array) and permanent stations (Nov. 2018 ~ Dec. 2020), this paper adopts the S wave splitting technique to obtain the essential properties of upper crustal anisotropy. In the interested area, it is shown that the dominant polarization of the fast S wave is NNW, with a mean polarization direction of 167.9°. In addition, the study area can be divided into three subzones from the west to the east: A, B, and C, according to the various mean polarizations varying from NNW, NS to NNE. The mean normalized time delay between the two split S waves is 4.0 ms/km, and the range of time delay is from 2.0 to 6.3 ms/km. The largest time delay is located at the east side of the western boundary of the Sichuan-Yunnan rhombus block. Furthermore, there is a strip area of strong anisotropy stretching along the western segment of the Lijiang-Xiaojinhe fault. These all demonstrate the local tectonic differences and indicate that the crustal structure may be strongly controlled by the fault and block boundary strike.","PeriodicalId":50766,"journal":{"name":"Annals of Geophysics","volume":"114 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2023-06-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"74545408","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
This special issue of Annals of Geophysics “Seismic anisotropy and shear-wave splitting: Achievements and perspectives” originates from a session (S10) of the 37th General assembly of the European Seismological commission ESC 2021 Conference which was planned to take place on 21 September 2021, in Corfu Greece, but due to the Covid19 pandemic was Virtual. The main theme of the session and of this special issue was the crucial role of seismic anisotropy in investigating the Earth’s interior from the upper crust to the inner core. Shear-wave splitting, one of the most effective ways to study seismic anisotropy, can identify the properties and the geodynamics of the upper mantle, and identify the presence of fluid-saturated microcracks, oriented according to the stress regime, in the upper crust. Azimuthal anisotropy and radial anisotropy can be assessed from earthquake or ambient noise recordings to detect the seismic layered features and to rebuild the 3D seismic structure
{"title":"Seismic anisotropy and shear-wave splitting: Achievements and perspectives: foreword","authors":"G. Kaviris, Yuan Gao, L. Margheriti","doi":"10.4401/ag-8997","DOIUrl":"https://doi.org/10.4401/ag-8997","url":null,"abstract":"This special issue of Annals of Geophysics “Seismic anisotropy and shear-wave splitting: Achievements and perspectives” originates from a session (S10) of the 37th General assembly of the European Seismological commission ESC 2021 Conference which was planned to take place on 21 September 2021, in Corfu Greece, but due to the Covid19 pandemic was Virtual. The main theme of the session and of this special issue was the crucial role of seismic anisotropy in investigating the Earth’s interior from the upper crust to the inner core. Shear-wave splitting, one of the most effective ways to study seismic anisotropy, can identify the properties and the geodynamics of the upper mantle, and identify the presence of fluid-saturated microcracks, oriented according to the stress regime, in the upper crust. Azimuthal anisotropy and radial anisotropy can be assessed from earthquake or ambient noise recordings to detect the seismic layered features and to rebuild the 3D seismic structure","PeriodicalId":50766,"journal":{"name":"Annals of Geophysics","volume":"7 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2023-06-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"85496523","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Many researchers have used the birefringence of P‑to‑S converted waves from the Moho discontinuity to constrain the anisotropy of Earth’s crust. However, this practice ignores the substantial influence that anisotropy has on the initial amplitude of the converted wave, which adds to the splitting acquired during its propagation from Moho to the seismometer. We find that large variations in Ps birefringence estimates with back-azimuth occur theoretically in the presence of P‑wave anisotropy, which normally accompanies S‑wave anisotropy. The variations are largest for crustal anisotropy with a tilted axis of symmetry, a geometry that is often neglected in birefringence interpretations, but is commonly found in Earth’s crust. We simulated globally-distributed P‑coda datasets for 36 distinct 4‑layer crustal models with combinations of elliptical shear anisotropy or compressional anisotropy, and also incorporated the higher-order anisotropic Backus parameter C. We tested both horizontal and tilted symmetry-axis geometries and tested the birefringence tradeoff associated with Ps converted phases at the top and bottom of a thin high‑ or low‑velocity basal layer. We computed composite receiver functions (RFs) with harmonic regression over back azimuth, using multipletaper correlation with moveout corrections for the epicentral distances of 471 events, to simulate a realistic data set. We estimate Ps birefringence from the radial and transverse RFs, a strategy that is similar to previous studies. We find that Ps splitting can be a useful indicator of bulk crustal anisotropy only under restricted circumstance, either in media with no compressional anisotropy, or if the symmetry axis is horizontal throughout. In other, more-realistic cases, the inferred fast polarization of Ps birefringence estimated from synthetic RFs tends either to drift with back-azimuth, form weak penalty-function minima, or return splitting times that depend on the thickness of an anisotropic layer, rather than the birefringence accumulated within it.
许多研究人员利用来自莫霍不连续的P - to - S转换波的双折射来约束地壳的各向异性。然而，这种做法忽略了各向异性对转换波的初始振幅的实质性影响，这增加了转换波从莫霍向地震仪传播过程中获得的分裂。我们发现，在P波各向异性存在的情况下，理论上Ps双折射估计的反向变化很大，而P波各向异性通常伴随着S波各向异性。这种变化在对称轴倾斜的地壳各向异性中最大，这种几何形状在双折射解释中经常被忽略，但在地壳中很常见。我们模拟了36个不同的4层地壳模型的全球分布的P - coda数据集，这些模型结合了椭圆剪切各向异性或挤压各向异性，并结合了高阶各向异性Backus参数c。我们测试了水平和倾斜对称轴几何形状，并测试了在薄的高速或低速基底层的顶部和底部与P转换相相关的双折射权衡。我们计算了复合接收函数(rf)，在反向方位角上进行谐波回归，使用471个事件的震中距离的多锥度相关和移出校正，以模拟真实的数据集。我们从径向和横向rf中估计Ps双折射，这是一种类似于先前研究的策略。我们发现，只有在有限的情况下，无论是在没有挤压各向异性的介质中，还是在对称轴自始至终是水平的情况下，Ps分裂才能成为一个有用的地壳各向异性指标。在其他更现实的情况下，从合成rf估计的Ps双折射的推断快速偏振倾向于随反向方向漂移，形成弱惩罚函数最小值，或者返回分裂时间，这取决于各向异性层的厚度，而不是其中积累的双折射。
{"title":"Crustal Anisotropy from the Birefringence of P-to-S Converted Waves: Bias Associated with P-Wave Anisotropy","authors":"Jeffrey Park, Xiaoran Chen, V. Levin","doi":"10.4401/ag-8882","DOIUrl":"https://doi.org/10.4401/ag-8882","url":null,"abstract":"Many researchers have used the birefringence of P‑to‑S converted waves from the Moho discontinuity to constrain the anisotropy of Earth’s crust. However, this practice ignores the substantial influence that anisotropy has on the initial amplitude of the converted wave, which adds to the splitting acquired during its propagation from Moho to the seismometer. We find that large variations in Ps birefringence estimates with back-azimuth occur theoretically in the presence of P‑wave anisotropy, which normally accompanies S‑wave anisotropy. The variations are largest for crustal anisotropy with a tilted axis of symmetry, a geometry that is often neglected in birefringence interpretations, but is commonly found in Earth’s crust. We simulated globally-distributed P‑coda datasets for 36 distinct 4‑layer crustal models with combinations of elliptical shear anisotropy or compressional anisotropy, and also incorporated the higher-order anisotropic Backus parameter C. We tested both horizontal and tilted symmetry-axis geometries and tested the birefringence tradeoff associated with Ps converted phases at the top and bottom of a thin high‑ or low‑velocity basal layer. We computed composite receiver functions (RFs) with harmonic regression over back azimuth, using multipletaper correlation with moveout corrections for the epicentral distances of 471 events, to simulate a realistic data set. We estimate Ps birefringence from the radial and transverse RFs, a strategy that is similar to previous studies. We find that Ps splitting can be a useful indicator of bulk crustal anisotropy only under restricted circumstance, either in media with no compressional anisotropy, or if the symmetry axis is horizontal throughout. In other, more-realistic cases, the inferred fast polarization of Ps birefringence estimated from synthetic RFs tends either to drift with back-azimuth, form weak penalty-function minima, or return splitting times that depend on the thickness of an anisotropic layer, rather than the birefringence accumulated within it. ","PeriodicalId":50766,"journal":{"name":"Annals of Geophysics","volume":"96 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2023-06-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"73024520","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Yu-tao Shi, Yuan Gao, Ziqi Zhang, Yongqian Zhang, Guohui Li
By the H-к stacking of the receiver functions and the splitting of the Pms phases, using seismic data from the Regional Seismic Network and the Huanan Seismic Array, a high-resolution temporary seismic array deployed for 2 years in the study area. This study revealed the strong lateral heterogeneity in crustal structures in the central South China block. Crustal thickness reduces from northwest to southeast, with significant differences across the boundary of sub-blocks. The average crustal Vp/Vs ratio gradually increases from west to east, leading to high values in the coastal region, which suggests that the subduction of the Pacific plate has possibly caused the underplating of magma or the upwelling of upper mantle material. The crustal azimuthal anisotropy of the Dabie orogen and the Jiangnan orogen is generally consistent with the strike of the tectonic belt as well as with the orientation of the absolute plate movement. We suggest that the crustal azimuthal anisotropy of the orogen is related to the extension and deformation of the lithosphere. The anisotropy in the crust is close related to crustal deformation. The orientation in the crust and the upper mantle in the Cathaysia block are generally consistent with the orientation of the absolute plate motion, indicating that the azimuthal anisotropy of the Cathaysia block is related to lithospheric deformation and the under-invasion of upper mantle material.
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M. Pastori, P. Baccheschi, D. Piccinini, L. Margheriti
In this work, we reviewed the main anisotropic results obtained in the last two decades along the Central Apennines. Moreover, we improved this database, with new results coming from the seismicity that occurred in the Montereale area, between 2009 and 2017, which corresponds to a spatio-temporal gap in the previously analyzed datasets. The examined papers concerned both seismic sequences (as Colfiorito in 1997, Pietralunga in 2010, L’Aquila in 2009, Amatrice in 2016) and background seismicity (as the 2000-2001 Città di Castello experiment). The whole of the collected results shows a general NW-SE fast shear wave direction consistent with both the orientation of the extensional active Quaternary and inherited compressive fault systems, focal mechanisms and local stress field. Also, we observed a more intense anisotropy strength (normalized delay time > 0.006 s/km) nearby the strongest events (M > 5), all concentrated in the hanging-wall of the activated fault systems. In fact, this area is deeply affected by the surrounding rock volume perturbations that, in turn, have altered both the local stress field and crustal fracturing network. The most common anisotropic interpretative models that could explain our results are 1) the stress-induced anisotropy according to the Extensive-Dilatancy Anisotropy (EDA) model where the anisotropic pattern is related to the local stress variation and most of the variability is visible in time; 2) the tectonic-controlled anisotropy according to the Structural-Induced Anisotropy (SIA) model where the anisotropic pattern is related to the major structural features and most of the variability is visible only in space. As reported by the examined studies in Central Apennines the possibility to discriminate between stress and structural anisotropy is quite complex in a region where the directions of the extensional regime, the in situ horizontal maximum stress, the strike of major faults, both active and inherited coincide. Generally, in this review, we noted an overlap and mixture of the two aforementioned mechanisms and, just through a temporal analysis, made in the Montereale area, we supposed a predominant stressinduced anisotropy only in rock volumes where anisotropic parameter variations have been detected.
在这项工作中，我们回顾了近二十年来沿亚平宁山脉中部获得的主要各向异性结果。此外，我们利用蒙特雷地区2009年至2017年间发生的地震活动的新结果改进了该数据库，这与之前分析的数据集中的时空差距相对应。被审查的论文涉及地震序列(1997年的Colfiorito, 2010年的Pietralunga, 2009年的拉奎拉，2016年的阿马特里切)和背景地震活动(2000-2001年的citt di Castello实验)。总体上显示出北西-东西的快速横波方向与伸展活动第四纪的走向一致，与继承的压缩断裂系统、震源机制和局部应力场一致。在最强事件(M > 5)附近，各向异性强度更强(归一化延迟时间> 0.006 s/km)，均集中在活动断层系统的上盘。事实上，该地区深受围岩体积扰动的影响，这些扰动反过来又改变了局部应力场和地壳破裂网络。最常见的各向异性解释模型是:1)广义-剪胀各向异性(EDA)模型的应力诱导各向异性，各向异性模式与局部应力变化有关，大部分变化在时间上是可见的;2)构造控制的各向异性，根据构造诱导各向异性(SIA)模型，各向异性模式与主要的构造特征有关，大部分变异只在空间上可见。正如在亚平宁中部所作的研究报告所述，在一个伸展制度的方向、原地水平最大应力、活动和继承的主要断层的走向一致的地区，区分应力和结构各向异性的可能性是相当复杂的。总的来说，在这篇综述中，我们注意到上述两种机制的重叠和混合，仅通过在蒙特雷地区进行的时间分析，我们假设只有在检测到各向异性参数变化的岩石体积中，应力诱导的各向异性才占主导地位。
{"title":"S wave Splitting in Central Apennines (Italy): anisotropic parameters in the crust during seismic sequences","authors":"M. Pastori, P. Baccheschi, D. Piccinini, L. Margheriti","doi":"10.4401/ag-8844","DOIUrl":"https://doi.org/10.4401/ag-8844","url":null,"abstract":"In this work, we reviewed the main anisotropic results obtained in the last two decades along the Central Apennines. Moreover, we improved this database, with new results coming from the seismicity that occurred in the Montereale area, between 2009 and 2017, which corresponds to a spatio-temporal gap in the previously analyzed datasets. The examined papers concerned both seismic sequences (as Colfiorito in 1997, Pietralunga in 2010, L’Aquila in 2009, Amatrice in 2016) and background seismicity (as the 2000-2001 Città di Castello experiment). The whole of the collected results shows a general NW-SE fast shear wave direction consistent with both the orientation of the extensional active Quaternary and inherited compressive fault systems, focal mechanisms and local stress field. Also, we observed a more intense anisotropy strength (normalized delay time > 0.006 s/km) nearby the strongest events (M > 5), all concentrated in the hanging-wall of the activated fault systems. In fact, this area is deeply affected by the surrounding rock volume perturbations that, in turn, have altered both the local stress field and crustal fracturing network. The most common anisotropic interpretative models that could explain our results are 1) the stress-induced anisotropy according to the Extensive-Dilatancy Anisotropy (EDA) model where the anisotropic pattern is related to the local stress variation and most of the variability is visible in time; 2) the tectonic-controlled anisotropy according to the Structural-Induced Anisotropy (SIA) model where the anisotropic pattern is related to the major structural features and most of the variability is visible only in space. As reported by the examined studies in Central Apennines the possibility to discriminate between stress and structural anisotropy is quite complex in a region where the directions of the extensional regime, the in situ horizontal maximum stress, the strike of major faults, both active and inherited coincide. Generally, in this review, we noted an overlap and mixture of the two aforementioned mechanisms and, just through a temporal analysis, made in the Montereale area, we supposed a predominant stressinduced anisotropy only in rock volumes where anisotropic parameter variations have been detected.","PeriodicalId":50766,"journal":{"name":"Annals of Geophysics","volume":"229 1","pages":""},"PeriodicalIF":1.0,"publicationDate":"2023-05-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"87599117","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"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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