Pure and Applied Geophysics

Mikhailov, V. O.; Timoshkina, E. P.; Diament, M.; Smirnov, V. B.

doi: 10.1007/s00024-023-03351-6pmid: N/A

The Mw 7.6 Olyutorskii earthquake of April 20, 2006, struck the southern edge of the Koryak Highland, in a region of great complexity at the junction of the North American, Eurasian, and Pacific plates. This seismic event was notable for several remarkable features. Firstly, it had an unexpectedly large magnitude, leading to a reassessment of the seismic hazard in the Northern Kamchatka region. Secondly, the GCMT focal mechanism solution showed two nodal planes corresponding to nearly thrusting displacements on a 40–51° dipping rupture plane. However, geological field studies conducted in the epicentral area allow to map at the surface three primary segments of a 120 km long rupture zone. The central segment showed predominant right-lateral strike-slip displacements, while the NE and SW segments exhibited mostly thrusts steeply dipping SE with smaller strike-slip components. Thirdly, aftershocks were recorded in an area extending over 200 km southwestward from the surface ruptures mapped in the field, with an intense activity observed in a vast area 75 km to the NW from the surface ruptures. Fourthly, although the ruptures found during fieldwork were dipping to the SE, most aftershocks occurred NW of them, with their depth decreasing in the NW direction. SAR interferometry resolved this apparent discrepancy between seismological and field-geology data, indicating that the primary displacements occurred northwest from the central and southwest segments of the ruptures at the earth's surface. We present in our study a new rupture model based on SAR, GPS, and field geology data. The model consists of a major blind thrust extending NW and three rear subvertical ruptures that reached the earth's surface and were mapped during the field survey. Additionally, models of the main Mw 6.6 aftershocks that occurred on April 29, 2006, and May 22, 2006, are presented. Our new model reconciles all collected data, with a direction of movements on the major thrust agreeing with the rotation of the previously suggested Beringia microplate around a pole situated in the North of the Chukotka peninsula.

Shaukat, Ahmed Zeeshan; Tahir, Mohammad; Iqbal, Tahir; Iqbal, Talat; Shah, Muhammad Ali

doi: 10.1007/s00024-023-03352-5pmid: N/A

Moment tensor inversion was performed for earthquake of moderate magnitude occurring in May 2022 near Khuzdar, Pakistan. According to local news agencies, the event caused damages to engineered structures and collapse of several mud houses. Faulting style depicted from inversion implies that the event may be associated with Ornach-Nal strike-slip fault which is part of western boundary of Indian plate. To understand the tectonic setting of the area, the present study was augmented by inclusion of results from similar studies. Principal stress axis (SHmax) was determined by performing stress inversion. Based on 38 events focal mechanism solutions from the global Harvard Centroid Moment Tensor (CMT) of the Khuzdar region depict NW–SE orientation of SHmax direction. On the basis of their stress homogeneity, reduced stress tensors obtained from formal stress inversion have been divided into two subsets resulting in thrust and strike-slip faulting. The present-day stress state conforms to the oblique convergence of Indian and Arabian plates beneath Eurasian. Shear strain produced by strike-slip movement of plate boundary (Chaman fault system) is being accommodated in Kirthar range within the Indian plate. Before this event, a lower b-value (0.7) and accelerated earthquake sequence were observed in the Khuzdar region, which is (in our view evidence of presence of stress loaded asperities along this fault system) representation of stress loaded asperities exist. The spatial distribution of b-value depicts the lowest value in this region before the occurrence of Awaran earthquake of 2013 that took place about 70 km west of the recent event.

Sun, Yunqiang; Gong, Weicheng; Wei, Fuquan; Jiang, Wen

doi: 10.1007/s00024-023-03357-0pmid: N/A

There are two seismic gaps (Dayi seismic gap and Tianquan-Kangding seismic gap) on the Longmen Shan fault (LMSF), despite the successive occurrence of the 2008 Mw7.9 Wenchuan and 2013 Mw6.6 Lushan earthquakes. To analyze the effects of the Wenchuan and Lushan earthquakes on the LMSF (especially on seismic gaps along the LMSF) and regional seismic hazards, we calculate Coulomb stress changes caused by the Wenchuan and Lushan earthquakes based on a three-dimensional viscoelastic finite element model. Additionally, we calculate the spatial distribution of regional b-values based on the instrumental seismic catalog before the Wenchuan earthquake. By utilizing the inverse correlation between b-value and stress level, we infer the regional background stress level. The results show that regional earthquakes (including the 2008 Mw7.9 Wenchuan earthquake, 2013 Mw6.6 Lushan earthquake, 2014 Mw6.1 Kangding earthquake, 2017 Mw6.5 Jiuzhaigou earthquake, and 2022 Mw6.6 Luding earthquake) occurred in regions characterized by low b-values. Meanwhile, subsequent earthquakes occurred in regions where Coulomb stress changes caused by the Wenchuan and Lushan earthquakes were positive. This suggests that regions with both low b-values and positive Coulomb stress changes may pose higher seismic hazards. We found that there are four regions (southern Xianshuihe fault, Dongkunlun fault, northern Xiaojinhe fault, and Hanan-Qingshanwan fault) with both positive Coulomb stress changes caused by the Wenchuan and Lushan earthquakes and low b-values, which may indicate high stress accumulation and high seismic hazard in the future. The results also show that Coulomb stress changes caused by the Wenchuan and Lushan earthquakes increased significantly in the Dayi seismic gap (+ 0.216 ~  + 2.607 MPa) and Tianquan-Kangding seismic gap (+ 0.021 ~  + 0.211 MPa), while the result of the high b-values for the Dayi and Tianquan-Kangding seismic gaps indicate less background stress accumulation. However, with continued tectonic loading, seismic hazards on both seismic gaps should attract our attention.

Malakar, Sukanta; Rai, Abhishek K.; Kannaujiya, Vijay K.; Gupta, Arun K.

doi: 10.1007/s00024-023-03340-9pmid: N/A

In this study, we have developed new empirical relations between various source and rupture parameters such as moment magnitude (M), surface rupture length (SRL), subsurface rupture length (RLD), rupture width (RW), rupture area (RA), and average (AD) and maximum slip (MD), based on an extensive database. The study involves about 476 global earthquakes that occurred between 1857 and 2023, covering a range of magnitudes (≥ 4.5) and faulting styles. The results indicate that relations between M-SRL, M-RLD, M-RW, M-RA, M-AD and M-MD correlate well for all types of faulting compared with previous studies. However, log-linear regression may not account for the nonlinear behaviour of rupture parameters, and these equations are separately used for each fault parameter, which leads to inconsistency in magnitude prediction. Hence, machine learning technique has been used to estimate earthquake magnitudes using various fault parameters simultaneously, which ensures consistency. In this study, we have employed an artificial neural network (ANN) and gradient-boosting machine regression (GBM) and examined their performance and applicability. Our analysis shows that gradient-boosting machine learning estimates earthquake magnitude better than regression equations, but the artificial neural network outperforms both. The result of this study would be beneficial for paleoseismic studies where reliable estimates of earthquake magnitudes and other source parameters are often difficult to estimate.

Hou, Baorui; Li, Shanyou; Song, Jindong

doi: 10.1007/s00024-023-03335-6pmid: N/A

The China seismic instrumental intensity can be used to measure the level of destruction and serve as the foundation of earthquake early warning (EEW) systems. To indirectly develop the instrumental intensity estimation and its application to EEW, we estimated the on-site filtered peak ground motion velocity (PGV) of the intensity using a support vector machine (SVM)-based model with eight P-wave features at a 3-s time window. Alert thresholds were set when the PGV was ≥ 8.18 cm/s (VII on the instrumental intensity scale). Compared with two linear estimation models (IV2 and Pd), the mean absolute error (MAE) and standard deviation of the error of the SVM estimation model were less, 0.241 and 0.298, respectively, with better performance on the PGV estimation. To evaluate the feasibility of transforming the SVM estimation for EEW by issuing alerts based on the intensity scale, we used the accuracy, precision, recall, F1 score, and false-negative rate (FNR) as evaluation metrics, achieving values of 99.62%, 95.68%, 79.90%, 87.08%, and 20.10%, respectively, using 11,970 records. We also provided the ratio, maximum, and average of the true positives to evaluate the lead time performance. Meanwhile, we used six earthquakes to evaluate the performance of our approach in detail. The approach performed well on EEW applications by issuing alerts based on the China seismic instrumental intensity. The analysis of the feature importance and data balance strategy can provide the basis for improving the performance of the SVM-based PGV estimation model.

Zhang, Junqiu; Rao, Ying

doi: 10.1007/s00024-023-03338-3pmid: N/A

Seismic full waveform inversion (FWI) is a powerful technology to obtain high-precision and high-resolution images of subsurface structures. However, FWI is a data-intensive algorithm that needs to read extensive seismic data from disks, which significantly affects its performance. We proposed a portable parallel framework to improve FWI by overlapping data input and computation (ODIC). The framework is based on POSIX threads (Pthreads), which is a standard thread API library and can create a parent thread and a child thread in the FWI process. The former is used to perform computation and the latter to read data from disks, both running simultaneously. This framework has two attractive features. First, it is broadly applicable; it can run on almost any computer from a laptop to a supercomputer. Second, it is easy to implement; it can be readily applied to existing FWI programs. A 3D FWI example shows that the framework speeds up FWI considerably.

Anusha, Edara; Arora, Kusumita

doi: 10.1007/s00024-023-03333-8pmid: N/A

Magnetotelluric (MT) transfer functions, which carry information about the subsurface resistivity distribution, remain constant over time in geologically stable regions. However, recent results indicate the presence of significant seasonal changes in tipper/induction arrows irrespective of the geological processes. We have obtained the annual and seasonal variations in induction vectors at six observatories located in different geographical (island, inland, and coastal areas) and tectonic (cratonic, and passive/convergent margins) settings in India. Based on a 30-day period of data, the tipper/induction vectors are estimated by using 1-min magnetic variation data for 2 years at each site for a period range of 4–150 min. The average tipper amplitude variations range from 0.2 to 0.3 at coastal sites, from 0.1 to 0.2 at island sites, and < 0.1 at inland sites. The seasonal pattern at low-latitude sites is greatest at the summer solstice and smallest at the winter solstice, whereas at equatorial sites it is greatest at the winter solstice/autumnal equinox and smallest at the summer solstice. At low-latitude and equatorial sites, seasonal variation in the direction of the induction vectors is at the sharp conductivity contrast periods. The varying ranges of the seasonality of tipper amplitude and direction variations at different geological regimes are due to the interaction of the source field with the conductivity structure and the lateral changes in the conductivity around the particular site.

Doo, Wen-Bin

doi: 10.1007/s00024-023-03337-4pmid: N/A

A zone of significant high-amplitude magnetic anomalies is observed without a comparable gravity high along the Cascadia margin and is spatially correlated with the low-velocity fore-arc mantle wedge. This wedge is interpreted to be serpentinized fore-arc mantle and is further considered to be the main source of the high-amplitude magnetic anomalies. To test this hypothesis, the magnetization-density ratio (MDR) is estimated along the Cascadia margin to highlight the physical characteristics of serpentinization (reduced density and increased magnetization). Interestingly, high MDR values are found only in central Oregon, where slab dehydration and fore-arc mantle serpentinization (50–60% serpentinization) are inferred in conjunction with sparse seismicity. This result may indicate either poorly serpentinized fore-arc mantle (low degree of serpentinization) or that the fore-arc mantle is deeper than the Curie temperature isotherm for magnetite in northern and southern Cascadia. This finding means that magnetic anomaly highs and serpentinized fore-arc mantle may not always be correlated in subduction zones. On the other hand, the MDR pattern suggests segmentation of the Cascadia subduction zone, which is consistent with several previous geological and geophysical observations.

Zuo, Jiahui; Niu, Fenglin; Zhang, Lele; Liu, Lu; Zhang, Houzhu; Chen, Ke; Shuai, Da; Yang, Jidong; Zhao, Yang

doi: 10.1007/s00024-023-03325-8pmid: N/A

Elastic reverse-time migration (ERTM), which utilizes the advantages of both P- and S-wave modes, is a widely used application for imaging in 3D anisotropic media. However, crosstalk due to intrinsically coupled P- and S-wavefields may degrade the image quality. To solve this problem, this study presents an effective vector P- and S-wavefield decomposition scheme in ERTM that can improve the images of 3D transversely isotropic (TI) media. The proposed method consists of four steps: (1) rotating the observation coordinate system to align its vertical axis with the symmetry axis of 3D TI media; (2) deriving the formulations of the 3D TI decomposition operator by applying the VTI P/S wave-mode decomposition strategy based on eigenform analysis in the new coordinate system; (3) implementing vector P- and S-wavefield decomposition by constructing the 3D TI Poisson equation, and introducing a novel and efficient method based on the first-order Taylor expansion to accelerate the computational efficiency of the decomposition; and (4) applying a vector-based dot-product imaging condition to generate PP and PS images. Compared with previous studies, the algorithm of our proposed method in 3D TI media is both numerically stable and computationally efficient. The 3D TI decomposition operator generates vector P- and S-wavefields showing the correct amplitude/phase with the input ones. Several numerical examples illustrate the satisfactory performance of the proposed 3D TI decomposition operator and the effective image improvement.

Li, Xiangchun; Zhuo, Xuefei; Zhang, Kai; Bai, Shipeng; Zong, Chenghao; Xing, Mingxiu; Zhang, Liang; Yang, Tao

doi: 10.1007/s00024-023-03339-2pmid: N/A

Loading experiments were conducted to study the physical properties of coal samples under step loading conditions, and the physical properties of coal samples under different conditions were numerically simulated by FLAC3D software. The numerical simulation results under step loading showed that FLAC3D software can effectively simulate the strain value and overall change trend of coal samples, obtaining good simulation results. The accuracy and applicability of the numerical simulation model were verified. Based on this, the physical properties of coal samples under different conditions were further investigated. The simulation showed that the transverse strain at the top of the coal sample is more obvious than that at the bottom, which shows the process of stress transfer from top to bottom. The effect of the bulk modulus on the axial strain of the coal sample was small, and the axial strain decreased with the increase in the bulk modulus under the same conditions. The shape of the coal sample was found to have a great influence on the axial strain, and the axial strain of a cylindrical coal sample was much larger than that of a rectangular coal sample under the same conditions. An increase in the height of the coal sample resulted not only in an increase in the axial strain value, but also a gradual increase in the degree of strain. This has significant implications for future use in guiding the analysis of the physical properties of coal.