SUMMARY In the past two decades or so, ambient noise tomography (ANT) has emerged as a powerful tool for investigating high-resolution crustal and upper-mantle structures. A crucial step in the ANT involves extracting phase velocities from cross-correlation functions (CCFs). However, obtaining precise phase velocities can be a formidable challenge, particularly when significant lateral velocity variations exist in shallow subsurface imaging that relies on short-period surface waves from ambient noise. To address this challenge, we propose an unwrapping correction method that enables the accurate extraction of short-period dispersion curves. Our method relies on the examination of the continuity of phase velocities extracted from CCFs between a common station and other neighbouring stations along a linear array. We demonstrate the effectiveness of our approach by applying our method to both synthetic and field data. Both applications suggest our unwrapping correction method can identify and correct unwrapping errors in phase velocity measurements, ensuring the extraction of accurate and reliable dispersion curves at short periods from ambient noise, which is essential for subsequent inversion for subsurface structures.
{"title":"A phase unwrapping approach in measuring surface wave phase velocities from ambient noise","authors":"Yanan Xie, Yinhe Luo, Yingjie Yang","doi":"10.1093/gji/ggae288","DOIUrl":"https://doi.org/10.1093/gji/ggae288","url":null,"abstract":"SUMMARY In the past two decades or so, ambient noise tomography (ANT) has emerged as a powerful tool for investigating high-resolution crustal and upper-mantle structures. A crucial step in the ANT involves extracting phase velocities from cross-correlation functions (CCFs). However, obtaining precise phase velocities can be a formidable challenge, particularly when significant lateral velocity variations exist in shallow subsurface imaging that relies on short-period surface waves from ambient noise. To address this challenge, we propose an unwrapping correction method that enables the accurate extraction of short-period dispersion curves. Our method relies on the examination of the continuity of phase velocities extracted from CCFs between a common station and other neighbouring stations along a linear array. We demonstrate the effectiveness of our approach by applying our method to both synthetic and field data. Both applications suggest our unwrapping correction method can identify and correct unwrapping errors in phase velocity measurements, ensuring the extraction of accurate and reliable dispersion curves at short periods from ambient noise, which is essential for subsequent inversion for subsurface structures.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-09-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142177392","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 Three-dimensional shear wave velocity and density models are important for understanding the structures, material composition, tectonic deformations, and dynamical mechanisms of the Earth's crust. Such models are usually based on surface-wave tomography and gravity inversion, which provide high resolution in the vertical and horizontal directions, respectively. The joint inversion of surface-wave and gravity methods can promote the mutual constraints and complementary advantages of seismic and gravity information, improving the imaging resolution and reducing the uncertainty in the individual methods. However, the traditional joint inversion methods directly construct only the shear wave velocity model, excluding density models. We present a joint inversion method of surface-wave and gravity data that simultaneously constructs both crustal shear wave velocity and density models. Unlike the previous studies, we reconfigure the seismic kernels of surface-wave tomography to preserve the seismic kernel of density. Moreover, the gravity kernel of density is combined with the seismic kernels to establish an objective function of simultaneous joint inversion. Consequently, the imaging resolution of density structure is improved. Our method is validated on the northeastern Tibetan plateau. The inversion results show that the Dingxi, Jiuzhaigou, and Jishishan earthquakes occurred in the high–low anomaly transition zones of shear wave velocity or density, implying that they were induced by accumulation of strain energy in the upper crust of the northeastern Tibetan plateau when the surrounding harder blocks extruded during the tectonic deformation process.
{"title":"Simultaneous Joint Inversion of Surface-Wave and Gravity Data for Revealing 3-D Crustal S-wave Velocity and Density Structures: A Case Study of the NE Tibetan Plateau","authors":"Xiang Wang, Lianghui Guo, Xueyang Bao, Yang Chen","doi":"10.1093/gji/ggae317","DOIUrl":"https://doi.org/10.1093/gji/ggae317","url":null,"abstract":"Summary Three-dimensional shear wave velocity and density models are important for understanding the structures, material composition, tectonic deformations, and dynamical mechanisms of the Earth's crust. Such models are usually based on surface-wave tomography and gravity inversion, which provide high resolution in the vertical and horizontal directions, respectively. The joint inversion of surface-wave and gravity methods can promote the mutual constraints and complementary advantages of seismic and gravity information, improving the imaging resolution and reducing the uncertainty in the individual methods. However, the traditional joint inversion methods directly construct only the shear wave velocity model, excluding density models. We present a joint inversion method of surface-wave and gravity data that simultaneously constructs both crustal shear wave velocity and density models. Unlike the previous studies, we reconfigure the seismic kernels of surface-wave tomography to preserve the seismic kernel of density. Moreover, the gravity kernel of density is combined with the seismic kernels to establish an objective function of simultaneous joint inversion. Consequently, the imaging resolution of density structure is improved. Our method is validated on the northeastern Tibetan plateau. The inversion results show that the Dingxi, Jiuzhaigou, and Jishishan earthquakes occurred in the high–low anomaly transition zones of shear wave velocity or density, implying that they were induced by accumulation of strain energy in the upper crust of the northeastern Tibetan plateau when the surrounding harder blocks extruded during the tectonic deformation process.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-09-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142177395","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 A new automated algorithm for picking the arrival times of the global P-, SH- and SV-wave phases from multi-component seismic waveform data is presented. This picker is based on a sequential approach using autoregressive prediction of the filtered waveform in a sliding time window, the Akaike-Information-Criterion and the Hilbert transform of the original waveform. The quality of the individual picks is computed by combining signal-to-noise ratios and higher order statistics into a single measure. Synthetic tests are used to find values for high and low quality thresholds. The algorithm is applied to a global data set of waveforms from teleseismic events with magnitude 6 or higher that occurred between 1990 and 2019. This resulted in approximately 4 million P-phase arrival times as well as approximately 3 million SH- and SV-phase arrival times each. These automatic picks are compared to approximately 830 000 manual P-picks as well as approximately 70 000 manual S-picks from the ISC-EHB catalogue. An upper bound for the picking errors of the automatic picks is estimated by using high quality picks of neighbouring stations. This upper bound is found to be 0.55s for the P-picks and 4.3s for the S-picks. If only high quality picks are considered, this represents 50 per cent of the P-picks and 25 per cent of the S-picks, then these errors decrease to 0.35s for the P-picks, and 1.5s for the S-picks, respectively. As a by-product of the picking, the dominant periods of the arriving signals are determined as well.
摘要 介绍了一种从多分量地震波形数据中提取全局 P 波、SH 波和 SV 波相位到达时间的新自动算法。该拾取器基于一种顺序方法，使用滑动时间窗中滤波波形的自回归预测、阿凯克信息准则和原始波形的希尔伯特变换。通过将信噪比和高阶统计量合并为一个单一指标来计算单个拾取的质量。合成测试用于找到高质量和低质量阈值。该算法适用于 1990 年至 2019 年期间发生的 6 级或以上远震事件波形的全球数据集。这产生了约 400 万个 P 相到达时间以及约 300 万个 SH 相和 SV 相到达时间。这些自动选取时间与 ISC-EHB 目录中的约 830 000 个人工 P 相选取时间和约 70 000 个人工 S 相选取时间进行了比较。通过使用邻近台站的高质量采样，估算出了自动采样的采样误差上限。结果发现，P 选区的误差上限为 0.55 秒，S 选区的误差上限为 4.3 秒。如果只考虑高质量采样，即 50%的 P-采样和 25%的 S-采样，那么 P-采样的误差将分别降至 0.35s 和 1.5s。作为采样的副产品，到达信号的主周期也被确定下来。
{"title":"Automated measurement of teleseismic P-, SH-, and SV-wave arrival times using autoregressive prediction and the instantaneous phase of multicomponent waveforms","authors":"J Stampa, F Eckel, H Keers, S Lebedev, T Meier","doi":"10.1093/gji/ggae307","DOIUrl":"https://doi.org/10.1093/gji/ggae307","url":null,"abstract":"Summary A new automated algorithm for picking the arrival times of the global P-, SH- and SV-wave phases from multi-component seismic waveform data is presented. This picker is based on a sequential approach using autoregressive prediction of the filtered waveform in a sliding time window, the Akaike-Information-Criterion and the Hilbert transform of the original waveform. The quality of the individual picks is computed by combining signal-to-noise ratios and higher order statistics into a single measure. Synthetic tests are used to find values for high and low quality thresholds. The algorithm is applied to a global data set of waveforms from teleseismic events with magnitude 6 or higher that occurred between 1990 and 2019. This resulted in approximately 4 million P-phase arrival times as well as approximately 3 million SH- and SV-phase arrival times each. These automatic picks are compared to approximately 830 000 manual P-picks as well as approximately 70 000 manual S-picks from the ISC-EHB catalogue. An upper bound for the picking errors of the automatic picks is estimated by using high quality picks of neighbouring stations. This upper bound is found to be 0.55s for the P-picks and 4.3s for the S-picks. If only high quality picks are considered, this represents 50 per cent of the P-picks and 25 per cent of the S-picks, then these errors decrease to 0.35s for the P-picks, and 1.5s for the S-picks, respectively. As a by-product of the picking, the dominant periods of the arriving signals are determined as well.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-08-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142177424","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Summary The left-lateral Xianshuihe fault is a seismically active fault system located at the eastern boundary of the Tibetan Plateau. We analyzed the Sentinel InSAR data from 2014 to 2021 to study the temporal and spatial variation of fault creep along the Xianshuihe fault. We applied the InSAR stacking method and the coherence-based SBAS method to derive the Line-Of-Sight (LOS) velocity map and time-series from both the ascending and descending orbits. We studied both the secular component and the time-dependent component of surface deformation from InSAR. We compare the InSAR-derived velocity maps with the GPS-derived velocity field and found that these two independent measurements are consistent. A 200 km long creeping section is identified along the central segment of the Xianshuihe fault. The surface creep rate is measured to be ranging from 0 to 6 mm yr−1. We combined the elastic dislocation model and the InSAR velocity maps to invert for the geodetic fault slip rate and the aseismic slip distribution in the upper crust. The secular fault creep model shows that most of the Xianshuihe fault is creeping at depth. The time-dependent fault creep model indicates that the maximum aseismic slip rate from Bamei to Kangding accelerated from 30 mm yr−1 to 40 mm yr−1 and then decayed to 5 mm yr−1 from 2014 to 2021. The fully creeping segment of the Xianshuihe fault seems to become a partially locked segment in a short time period (a couple of years). We suspect that the acceleration of fault creep from 2017 to 2019 is linked to dynamic triggering by passing seismic waves or fluid migration. Finally, we compare the temporal variation of fault creep with previous studies and discuss the earthquake hazard implications.
{"title":"Temporal and Spatial Variation of Fault Creep along the Xianshuihe Fault from InSAR Stacking","authors":"Xiaopeng Tong, Shi Chen","doi":"10.1093/gji/ggae310","DOIUrl":"https://doi.org/10.1093/gji/ggae310","url":null,"abstract":"Summary The left-lateral Xianshuihe fault is a seismically active fault system located at the eastern boundary of the Tibetan Plateau. We analyzed the Sentinel InSAR data from 2014 to 2021 to study the temporal and spatial variation of fault creep along the Xianshuihe fault. We applied the InSAR stacking method and the coherence-based SBAS method to derive the Line-Of-Sight (LOS) velocity map and time-series from both the ascending and descending orbits. We studied both the secular component and the time-dependent component of surface deformation from InSAR. We compare the InSAR-derived velocity maps with the GPS-derived velocity field and found that these two independent measurements are consistent. A 200 km long creeping section is identified along the central segment of the Xianshuihe fault. The surface creep rate is measured to be ranging from 0 to 6 mm yr−1. We combined the elastic dislocation model and the InSAR velocity maps to invert for the geodetic fault slip rate and the aseismic slip distribution in the upper crust. The secular fault creep model shows that most of the Xianshuihe fault is creeping at depth. The time-dependent fault creep model indicates that the maximum aseismic slip rate from Bamei to Kangding accelerated from 30 mm yr−1 to 40 mm yr−1 and then decayed to 5 mm yr−1 from 2014 to 2021. The fully creeping segment of the Xianshuihe fault seems to become a partially locked segment in a short time period (a couple of years). We suspect that the acceleration of fault creep from 2017 to 2019 is linked to dynamic triggering by passing seismic waves or fluid migration. Finally, we compare the temporal variation of fault creep with previous studies and discuss the earthquake hazard implications.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-08-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142177420","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Summary The recent developments in array-based surface-wave tomography have made it possible to directly measure apparent phase velocities through wavefront tracking. While directionally dependent measurements have been used to infer intrinsic $2psi $ azimuthal anisotropy (with a 180° periodicity), a few studies have also demonstrated strong but spurious $1psi $ azimuthal anisotropy (360° periodicity) near major structure boundaries particularly for long period surface waves. In such observations, Rayleigh waves propagating in the direction perpendicular to the boundary from the slow to the fast side persistently show a higher apparent velocity compared to waves propagating in the opposite direction. In this study, we conduct numerical and theoretical investigations to explore the effect of scattering on the apparent Rayleigh-wave phase velocity measurement. Using two-dimensional spectral-element numerical wavefield simulations, we first reproduce the observation that waves propagating in opposite directions show different apparent phase velocities when passing through a major velocity contrast. Based on mode coupling theory and the locked mode approximation, we then investigate the effect of the scattered fundamental-mode Rayleigh wave and body waves interfering with the incident Rayleigh wave separately. We show that scattered fundamental-mode Rayleigh waves, while dominating the scattered wavefield, mostly cause short wavelength apparent phase velocity variations that could only be studied if the station spacing is less than about one tenth of the surface wave wavelength. Scattered body waves, on the other hand, cause longer wavelength velocity variations that correspond to the existing real data observations. Because of the sensitivity of the $1psi $ apparent anisotropy to velocity contrasts, incorporating such measurements in surface wave tomography could improve the resolution and sharpen the structural boundaries of the inverted model.
{"title":"Spurious Rayleigh-wave Apparent Anisotropy Near Major Structural Boundaries: A Numerical and Theoretical Investigation","authors":"Qicheng Zeng, Fan-Chi Lin, Victor C Tsai","doi":"10.1093/gji/ggae305","DOIUrl":"https://doi.org/10.1093/gji/ggae305","url":null,"abstract":"Summary The recent developments in array-based surface-wave tomography have made it possible to directly measure apparent phase velocities through wavefront tracking. While directionally dependent measurements have been used to infer intrinsic $2psi $ azimuthal anisotropy (with a 180° periodicity), a few studies have also demonstrated strong but spurious $1psi $ azimuthal anisotropy (360° periodicity) near major structure boundaries particularly for long period surface waves. In such observations, Rayleigh waves propagating in the direction perpendicular to the boundary from the slow to the fast side persistently show a higher apparent velocity compared to waves propagating in the opposite direction. In this study, we conduct numerical and theoretical investigations to explore the effect of scattering on the apparent Rayleigh-wave phase velocity measurement. Using two-dimensional spectral-element numerical wavefield simulations, we first reproduce the observation that waves propagating in opposite directions show different apparent phase velocities when passing through a major velocity contrast. Based on mode coupling theory and the locked mode approximation, we then investigate the effect of the scattered fundamental-mode Rayleigh wave and body waves interfering with the incident Rayleigh wave separately. We show that scattered fundamental-mode Rayleigh waves, while dominating the scattered wavefield, mostly cause short wavelength apparent phase velocity variations that could only be studied if the station spacing is less than about one tenth of the surface wave wavelength. Scattered body waves, on the other hand, cause longer wavelength velocity variations that correspond to the existing real data observations. Because of the sensitivity of the $1psi $ apparent anisotropy to velocity contrasts, incorporating such measurements in surface wave tomography could improve the resolution and sharpen the structural boundaries of the inverted model.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-08-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142177422","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}
Yehezkiel Halauwet, Afnimar Pagacancang, Wahyu Triyoso, Jiří Vackář, Daryono Daryono, Pepen Supendi, Gatut Daniarsyad, Andrean V H Simanjuntak, Bayu Pranata, Herlina A A M Narwadan, Muhammad L Hakim
Summary The complete catalog of moment tensor (MT) solutions is essential for a wide range of research in solid earth science. However, the number of reliable MT solutions for small to moderate earthquakes (3.0 ≤ M ≤ 5.5) is limited due to uncertainties arising from data and theoretical errors. In this study, we develop a new procedure to enhance the resolvability of MT solutions and provide more reliable uncertainty estimates for these smaller to moderate earthquakes. This procedure is fully automatic and efficiently accounts for both data and theoretical errors through two sets of hybrid linear-nonlinear Bayesian inversions. In the inversion process, the covariance matrix is estimated using an empirical approach: the data covariance matrix is derived from the pre-event noise and the theoretical covariance matrix is derived from the residuals of the initial solution. We conducted tests using synthetic data generated from the 3D velocity model and interference from background seismic noise. The tests found that using a combination of the non-Toeplitz data covariance matrix and the Toeplitz theoretical covariance matrix improves the solution and its uncertainties. Test results also suggest that including a theoretical covariance matrix when analyzing MT in complex tectonic regions is essential, even if we have the best 1D velocity model. The application to earthquakes in the northern region of the Banda Arc resulted in the first published Regional Moment Tensor (RMT) catalog, containing more than three times the number of trusted solutions compared to the Global Centroid Moment Tensor (GCMT) and the Indonesian Agency for Meteorology Climatology and Geophysics Moment Tensor (BMKG-MT) catalog. The comparison shows that the trusted solutions align well with the focal mechanism of the GCMT and BMKG-MT, as well as with the maximum horizontal stress of the World Stress Map, and tectonic conditions in the study area. The newly obtained focal mechanisms provide several key findings: (i) They confirm that the deformation in the northern and eastern parts of Seram Island is influenced by oblique intraplate convergence rather than by the subduction process; (ii) They validate the newly identified Amahai Fault with a greater number of focal mechanisms; (iii) They reveal an earthquake Mw 4.7 with the same location and source mechanism six years before the 2019 Ambon-Kairatu earthquake (Mw 6.5) which occurred on a previously unidentified fault.
{"title":"A new automated procedure to obtain reliable moment tensor solutions of small to moderate earthquakes (3.0 ≤ M ≤ 5.5) in the Bayesian framework","authors":"Yehezkiel Halauwet, Afnimar Pagacancang, Wahyu Triyoso, Jiří Vackář, Daryono Daryono, Pepen Supendi, Gatut Daniarsyad, Andrean V H Simanjuntak, Bayu Pranata, Herlina A A M Narwadan, Muhammad L Hakim","doi":"10.1093/gji/ggae309","DOIUrl":"https://doi.org/10.1093/gji/ggae309","url":null,"abstract":"Summary The complete catalog of moment tensor (MT) solutions is essential for a wide range of research in solid earth science. However, the number of reliable MT solutions for small to moderate earthquakes (3.0 ≤ M ≤ 5.5) is limited due to uncertainties arising from data and theoretical errors. In this study, we develop a new procedure to enhance the resolvability of MT solutions and provide more reliable uncertainty estimates for these smaller to moderate earthquakes. This procedure is fully automatic and efficiently accounts for both data and theoretical errors through two sets of hybrid linear-nonlinear Bayesian inversions. In the inversion process, the covariance matrix is estimated using an empirical approach: the data covariance matrix is derived from the pre-event noise and the theoretical covariance matrix is derived from the residuals of the initial solution. We conducted tests using synthetic data generated from the 3D velocity model and interference from background seismic noise. The tests found that using a combination of the non-Toeplitz data covariance matrix and the Toeplitz theoretical covariance matrix improves the solution and its uncertainties. Test results also suggest that including a theoretical covariance matrix when analyzing MT in complex tectonic regions is essential, even if we have the best 1D velocity model. The application to earthquakes in the northern region of the Banda Arc resulted in the first published Regional Moment Tensor (RMT) catalog, containing more than three times the number of trusted solutions compared to the Global Centroid Moment Tensor (GCMT) and the Indonesian Agency for Meteorology Climatology and Geophysics Moment Tensor (BMKG-MT) catalog. The comparison shows that the trusted solutions align well with the focal mechanism of the GCMT and BMKG-MT, as well as with the maximum horizontal stress of the World Stress Map, and tectonic conditions in the study area. The newly obtained focal mechanisms provide several key findings: (i) They confirm that the deformation in the northern and eastern parts of Seram Island is influenced by oblique intraplate convergence rather than by the subduction process; (ii) They validate the newly identified Amahai Fault with a greater number of focal mechanisms; (iii) They reveal an earthquake Mw 4.7 with the same location and source mechanism six years before the 2019 Ambon-Kairatu earthquake (Mw 6.5) which occurred on a previously unidentified fault.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-08-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142177421","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 Within the last decade, thin ultra-low velocity zone (ULVZ) layering, sitting directly on top of the core-mantle boundary (CMB), has begun to be investigated using the flip-reverse-stack (FRS) method. In this method, pre- and post-cursor arrivals that are symmetrical in time about the ScS arrival, but with opposite polarities, are stacked. This same methodology has also been applied to high velocity layering, with indications that ultra-high velocity zones (UHVZs) may also exist. Thus far, studies using the FRS technique have relied on 1-D synthetic predictions to infer material properties of ULVZs. 1-D ULVZ models predominantly show a SdS precursor that reflects off the top of the ULVZ and an ScscS reverberation within the ULVZ that arrives as a postcursor. 1-D UHVZ models are more complex and have a different number of arrivals depending on a variety of factors including UHVZ thickness, velocity contrast, and lateral extent. 1-D modeling approaches assume that lower mantle heterogeneity is constant and continuous everywhere across the lower mantle. However, lower mantle features display lateral heterogeneity and are either finite in extent or display local thickness variations. We examine the interaction of the ScS wavefield with ULVZs and UHVZs in 2.5-D geometries of finite extent. We show that multiple additional arrivals exist that are not present in 1-D predictions. In particular, multipath ScS arrivals as well as additional postcursor arrivals are generated. Subsequent processing by the FRS method generates complicated FRS traces with multiple peaks. Furthermore, post-cursor arrivals can be generated even when the ScS ray path does not directly strike the heterogeneity from above. Analyzing these predictions for 2.5-D models using 1-D modeling techniques demonstrates that a cautious approach must be adopted in utilization and interpretion of FRS traces to determine if the ScS wavefield is interacting with a ULVZ or UHVZ through a direct strike on the top of the feature. In particular, travel-time delays or advances of the ScS arrival should be documented and symmetrical opposite polarity arrivals should be demonstrated to exist around ScS. The latter can be quantified by calculation of a time domain multiplication trace. Because multiple postcursor arrivals are generated by finite length heterogeneities, interpretation should be confined to single layer models rather than to interpret the additional peaks as internal layering. Furthermore, strong tradeoffs exist between S-wave velocity perturbation and thickness making estimations of ULVZ or UHVZ elastic parameters highly uncertain. We test our analysis methods using data from an event occurring in the Fiji-Tonga region recorded in North America. The ScS bounce points for this event sample the CMB region to the southeast of Hawaii, in a region where ULVZs have been identified in several recent studies. We see additional evidence for a ULVZ in this region centered at 14° N and 153°
{"title":"Effects of 2.5-D ultra-low and ultra-high velocity zones on flip-reverse-stacking (FRS) of the ScS wavefield","authors":"Michael S Thorne, Surya Pachhai, Edward J Garnero","doi":"10.1093/gji/ggae315","DOIUrl":"https://doi.org/10.1093/gji/ggae315","url":null,"abstract":"Summary Within the last decade, thin ultra-low velocity zone (ULVZ) layering, sitting directly on top of the core-mantle boundary (CMB), has begun to be investigated using the flip-reverse-stack (FRS) method. In this method, pre- and post-cursor arrivals that are symmetrical in time about the ScS arrival, but with opposite polarities, are stacked. This same methodology has also been applied to high velocity layering, with indications that ultra-high velocity zones (UHVZs) may also exist. Thus far, studies using the FRS technique have relied on 1-D synthetic predictions to infer material properties of ULVZs. 1-D ULVZ models predominantly show a SdS precursor that reflects off the top of the ULVZ and an ScscS reverberation within the ULVZ that arrives as a postcursor. 1-D UHVZ models are more complex and have a different number of arrivals depending on a variety of factors including UHVZ thickness, velocity contrast, and lateral extent. 1-D modeling approaches assume that lower mantle heterogeneity is constant and continuous everywhere across the lower mantle. However, lower mantle features display lateral heterogeneity and are either finite in extent or display local thickness variations. We examine the interaction of the ScS wavefield with ULVZs and UHVZs in 2.5-D geometries of finite extent. We show that multiple additional arrivals exist that are not present in 1-D predictions. In particular, multipath ScS arrivals as well as additional postcursor arrivals are generated. Subsequent processing by the FRS method generates complicated FRS traces with multiple peaks. Furthermore, post-cursor arrivals can be generated even when the ScS ray path does not directly strike the heterogeneity from above. Analyzing these predictions for 2.5-D models using 1-D modeling techniques demonstrates that a cautious approach must be adopted in utilization and interpretion of FRS traces to determine if the ScS wavefield is interacting with a ULVZ or UHVZ through a direct strike on the top of the feature. In particular, travel-time delays or advances of the ScS arrival should be documented and symmetrical opposite polarity arrivals should be demonstrated to exist around ScS. The latter can be quantified by calculation of a time domain multiplication trace. Because multiple postcursor arrivals are generated by finite length heterogeneities, interpretation should be confined to single layer models rather than to interpret the additional peaks as internal layering. Furthermore, strong tradeoffs exist between S-wave velocity perturbation and thickness making estimations of ULVZ or UHVZ elastic parameters highly uncertain. We test our analysis methods using data from an event occurring in the Fiji-Tonga region recorded in North America. The ScS bounce points for this event sample the CMB region to the southeast of Hawaii, in a region where ULVZs have been identified in several recent studies. We see additional evidence for a ULVZ in this region centered at 14° N and 153° ","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-08-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142177419","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Summary The Baihetan Reservoir, the second largest in the world, is located at the intersection of multiple large active fault zones on the eastern boundary of the Sichuan-Yunnan rhombic block. After impoundment on April 6, 2021, many earthquakes occurred around the reservoir area submerged by water. The largest ML 4.7 earthquake in the reservoir area occurred after the water level reached its highest point. But the seismogenic structures and mechanisms of earthquakes in the reservoir area are still unclear. Based on dense array data from the reservoir area, this paper uses the experimental site sub-model (CSES) of USTC-Pickers, transfer learned with “DiTing” dataset of China to obtain a high-precision earthquake catalog that is twice as large as that the manual catalog. This study show that earthquakes in the reservoir region primarily occur on secondary faults of pre-existing ones, characterized by a prominent feature of high dip angles trending northwest to southeast. Combined with the spatiotemperal migration characteristics of earthquakes and the relationship between earthquakes and water levels, we infer that most earthquakes are rapid response type and may be induced by rapid increase in elastic stress. Only the spatiotemporal distribution image of the ML 3.2 earthquakes sequence in the dam site-Toudaogou section conforms to the law of pore pressure diffusion, and belongs to the fast response type, which may be induced by the poroelasiticity coupling mechanism. The ML 3.0 earthquake swarm with deep depths in the Heishui River section belongs to the delayed response type and may be induced by the poroelasiticity coupling mechanism.
摘要 白鹤滩水库是世界第二大水库，位于四川-云南菱形地块东部边界多条大型活动断层带的交汇处。2021 年 4 月 6 日蓄水后，被水淹没的库区周围发生了多次地震。库区最大的 ML 4.7 地震发生在水位达到最高点之后。但库区地震的震源结构和机制尚不清楚。本文基于库区密集阵列数据，利用中国科学技术大学-皮克斯实验场子模型（CSES），通过与中国 "地动仪 "数据集的转移学习，获得了比人工地震目录大一倍的高精度地震目录。该研究表明，库区地震主要发生在原有断层的次级断层上，其突出特点是高倾角，呈西北至东南走向。结合地震的时空迁移特征以及地震与水位的关系，我们推断大多数地震属于快速响应型，可能是由弹性应力的快速增加诱发的。只有坝址-头道沟断面 ML 3.2 地震序列的时空分布图符合孔隙压力扩散规律，属于快速响应型，可能由孔隙耦合机制诱发。黑水河段深层 ML 3.0 地震群属于延迟反应型，可能由孔隙耦合机制诱发。
{"title":"Earthquake Migration Characteristics and Triggering Mechanisms in the Baihetan Reservoir Area based on Machine-Learning Microseismic Detection","authors":"Mengqiao Duan, Lianqing Zhou, Longfei Duan, Ziyi Li, Cuiping Zhao, Xiaodong Zhang","doi":"10.1093/gji/ggae304","DOIUrl":"https://doi.org/10.1093/gji/ggae304","url":null,"abstract":"Summary The Baihetan Reservoir, the second largest in the world, is located at the intersection of multiple large active fault zones on the eastern boundary of the Sichuan-Yunnan rhombic block. After impoundment on April 6, 2021, many earthquakes occurred around the reservoir area submerged by water. The largest ML 4.7 earthquake in the reservoir area occurred after the water level reached its highest point. But the seismogenic structures and mechanisms of earthquakes in the reservoir area are still unclear. Based on dense array data from the reservoir area, this paper uses the experimental site sub-model (CSES) of USTC-Pickers, transfer learned with “DiTing” dataset of China to obtain a high-precision earthquake catalog that is twice as large as that the manual catalog. This study show that earthquakes in the reservoir region primarily occur on secondary faults of pre-existing ones, characterized by a prominent feature of high dip angles trending northwest to southeast. Combined with the spatiotemperal migration characteristics of earthquakes and the relationship between earthquakes and water levels, we infer that most earthquakes are rapid response type and may be induced by rapid increase in elastic stress. Only the spatiotemporal distribution image of the ML 3.2 earthquakes sequence in the dam site-Toudaogou section conforms to the law of pore pressure diffusion, and belongs to the fast response type, which may be induced by the poroelasiticity coupling mechanism. The ML 3.0 earthquake swarm with deep depths in the Heishui River section belongs to the delayed response type and may be induced by the poroelasiticity coupling mechanism.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-08-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142177423","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 Antarctica has been proposed as a significant source of the meltwater that entered the oceans during Meltwater Pulse 1B (MWP1B) approximately 11,500 years ago. Support for this scenario has been provided by evidence that the deep fjords of coastal Antarctica, which were heavily glaciated at the maximum of glaciation, were deglaciated at this time. Further support for this scenario was provided by the observation that the inter-hemispheric sea level teleconnection associated with significant southern hemisphere deglaciation at this time provided an explanation of the highly non-monotonic relative sea level histories recorded at sites on the coast of Scotland, a region which had also been heavily glaciated at the last glacial maximum. Furthermore, it has been argued that a significant contribution to MWP1B must have also been delivered to the oceans by the abrupt northern hemisphere warming that occurred at the end of the Younger Dryas (YD) cold reversal, which also occurred approximately 11,500 years ago. Our focus in the present paper is to distinguish between these two possible primary sources of MWP1B. The investigation of how local alterations to ice thicknesses are able to explain evidence which has previously been used to argue for an Antarctic dominant MWP1B will lead us to the conclusion that the Laurentide may be primary source of MWP1B.
{"title":"The hemispheric origins of meltwater pulse 1B","authors":"J Velay-Vitow, W R Peltier, G R Stuhne, T Li","doi":"10.1093/gji/ggae311","DOIUrl":"https://doi.org/10.1093/gji/ggae311","url":null,"abstract":"Summary Antarctica has been proposed as a significant source of the meltwater that entered the oceans during Meltwater Pulse 1B (MWP1B) approximately 11,500 years ago. Support for this scenario has been provided by evidence that the deep fjords of coastal Antarctica, which were heavily glaciated at the maximum of glaciation, were deglaciated at this time. Further support for this scenario was provided by the observation that the inter-hemispheric sea level teleconnection associated with significant southern hemisphere deglaciation at this time provided an explanation of the highly non-monotonic relative sea level histories recorded at sites on the coast of Scotland, a region which had also been heavily glaciated at the last glacial maximum. Furthermore, it has been argued that a significant contribution to MWP1B must have also been delivered to the oceans by the abrupt northern hemisphere warming that occurred at the end of the Younger Dryas (YD) cold reversal, which also occurred approximately 11,500 years ago. Our focus in the present paper is to distinguish between these two possible primary sources of MWP1B. The investigation of how local alterations to ice thicknesses are able to explain evidence which has previously been used to argue for an Antarctic dominant MWP1B will lead us to the conclusion that the Laurentide may be primary source of MWP1B.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-08-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142177394","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 Understanding the crustal seismic characteristics of tectonically active regions is crucial for seismic hazard assessment. The study conducted in NW Iran utilized surface wave tomography, radial anisotropy, and density information to analyze the complex crustal structure of the region, which is outstanding because of diverse tectonic features, sedimentary basins, and volcanic formations. By selecting a dataset of 1243 events out of over 3,500 earthquakes with M>4, and employing strict data selection criteria (such as SNR, M, Δ), the researchers calculated Rayleigh and Love wave group velocity dispersion curves using Gaussian multiple filters and phase-matched filtering. The tomographic procedure was initiated by excluding data with residuals > 2σ for enhanced stability. Individual inversions were then carried out for local Rayleigh and Love wave dispersion measurements to obtain 1D VSV and VSH models. Radial anisotropy and VS iso were determined through a discrepancy and averaging of the obtained VSH and VSV, respectively. Gravity modeling was also employed alongside surface wave analysis to understand the region's complex geology, revealing insights into upper-middle-lower crust boundaries, subsurface structures, and Moho depths. The study's velocity maps reveal significant findings related to geological units and tectonic features in various regions based on the provided results. Low velocities in the South Caspian Basin (SCB) and Kura Depression (KD) regions are attributed to substantial sedimentary layers, while low velocities, and depth of VS in NW Iran and Eastern Anatolian Accretionary Complex (EAAC) regions suggest the presence of partially molten materials in the upper and middle crust. The Sanandaj-Sirjan Zone (SSZ) region shows a low velocity anomaly in longer periods and greater depths of VS, surrounded by normal to high velocities, indicating a thick middle crust. Analyzing radial anisotropy and VS iso profiles offers insights into upper-middle-lower crust boundaries, subsurface structures, and Moho depths, highlighting middle crust thickening and lower crust thinning beneath the SSZ. The study confirms the gentle subduction of the SCB oceanic-like lower crust beneath NW Iran in the Talesh (TAL) region, with a rigid middle crust. Additionally, cross-sections reveal igneous laccoliths underplate with a VS iso of 3.7 km/s in the volcanic region. The difference observed by subtracting the velocity models at two adjacent depths, combined with parametric test results, indicates that the Sahand volcanic system is clearly identifiable, while the influence of subtle subduction on the Sabalan volcano at depths up to 30 km remains less distinct. The magma chamber beneath Sahand is situated at depths ranging from 18 to 25 km.
{"title":"The Crustal and Upper Mantle Structure beneath NW Iran: An Integrated Analysis of Surface Waves and Gravity Data","authors":"Taghi Shirzad, Shaghayegh VahidRavesh, Gholamreza Mortezanejad, Somayeh Abdollahi, Mohsen Kazemnia Kakhki, Mojtaba Naghavi, Habib Rahimi, Mohammad Reza Hatami","doi":"10.1093/gji/ggae306","DOIUrl":"https://doi.org/10.1093/gji/ggae306","url":null,"abstract":"Summary Understanding the crustal seismic characteristics of tectonically active regions is crucial for seismic hazard assessment. The study conducted in NW Iran utilized surface wave tomography, radial anisotropy, and density information to analyze the complex crustal structure of the region, which is outstanding because of diverse tectonic features, sedimentary basins, and volcanic formations. By selecting a dataset of 1243 events out of over 3,500 earthquakes with M>4, and employing strict data selection criteria (such as SNR, M, Δ), the researchers calculated Rayleigh and Love wave group velocity dispersion curves using Gaussian multiple filters and phase-matched filtering. The tomographic procedure was initiated by excluding data with residuals > 2σ for enhanced stability. Individual inversions were then carried out for local Rayleigh and Love wave dispersion measurements to obtain 1D VSV and VSH models. Radial anisotropy and VS iso were determined through a discrepancy and averaging of the obtained VSH and VSV, respectively. Gravity modeling was also employed alongside surface wave analysis to understand the region's complex geology, revealing insights into upper-middle-lower crust boundaries, subsurface structures, and Moho depths. The study's velocity maps reveal significant findings related to geological units and tectonic features in various regions based on the provided results. Low velocities in the South Caspian Basin (SCB) and Kura Depression (KD) regions are attributed to substantial sedimentary layers, while low velocities, and depth of VS in NW Iran and Eastern Anatolian Accretionary Complex (EAAC) regions suggest the presence of partially molten materials in the upper and middle crust. The Sanandaj-Sirjan Zone (SSZ) region shows a low velocity anomaly in longer periods and greater depths of VS, surrounded by normal to high velocities, indicating a thick middle crust. Analyzing radial anisotropy and VS iso profiles offers insights into upper-middle-lower crust boundaries, subsurface structures, and Moho depths, highlighting middle crust thickening and lower crust thinning beneath the SSZ. The study confirms the gentle subduction of the SCB oceanic-like lower crust beneath NW Iran in the Talesh (TAL) region, with a rigid middle crust. Additionally, cross-sections reveal igneous laccoliths underplate with a VS iso of 3.7 km/s in the volcanic region. The difference observed by subtracting the velocity models at two adjacent depths, combined with parametric test results, indicates that the Sahand volcanic system is clearly identifiable, while the influence of subtle subduction on the Sabalan volcano at depths up to 30 km remains less distinct. The magma chamber beneath Sahand is situated at depths ranging from 18 to 25 km.","PeriodicalId":12519,"journal":{"name":"Geophysical Journal International","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-08-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142177447","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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