Pure and Applied Geophysics

Wang, Kang; Peng, Suping; Lu, Yongxu; Cui, Xiaoqin

doi: 10.1007/s00024-022-03080-2pmid: N/A

Fractures in underground media are mostly vertical and orthogonal. Based on the assumption of long wavelengths, fractures can be considered infinitely thin planes embedded in a homogeneous medium. The fracture interface satisfies the conditions of displacement discontinuity and stress continuity, i.e. a linear slip boundary. The finite difference method is typically used to simulate the propagation of seismic waves at the fracture interface. In this study, a new finite difference scheme is proposed based on the velocity-stress equation, which can be used to simulate the propagation of seismic waves in vertical and orthogonal fracture media. The new finite difference scheme more closely resembles the conditions of actual vertical and orthogonal fractures. The Lebedev grid was adopted, and no interpolation was required for the calculation, which improved the accuracy. The numerical simulation of the new finite difference scheme reveals its long-term stability and accuracy. This scheme can be used for the analysis of reservoir fracture delineation. In addition, an explicit slip interface condition and the new finite difference scheme were used to comparatively model fractured media. Virtually identical results were obtained, which verifies the effectiveness of this model. Finally, based on the boundary conditions of linear slip, an improved Zoeppritz equation was established, and the effect of fracture compliance on the reflection and transmission coefficients was studied. This scheme can be used for the analysis of reservoir fracture traps.

Ji, Zhiwei; Li, Zongchao; Chen, Xueliang; Li, Tiefei; Wu, Qing; Zhang, Bo; Li, Qi; Huang, Ting

doi: 10.1007/s00024-022-03094-wpmid: N/A

The geological activity in Sichuan and Yunnan of China is intense, and large earthquakes such as the Tonghai earthquake in 1970 have caused huge casualties and economic losses. Therefore, it is significant to reduce the long-period earthquake damage of dams and high-rise buildings to reproduce the near-fault long-period ground motion characteristics of the Tonghai earthquake. In this paper, the finite difference method is used to predict the long-period characteristics of the Tonghai Ms 7.8 earthquake. Considering the uncertainty of the initial fracture point, number of asperities, and slip angle, 18 seismic scenarios are established. The predicted results are verified by comparing intensity, attenuation relation and pulse probability distribution. At the same time, based on the velocity pulse recognition method, the pulse probability distribution of the near-field is obtained. We selected six seismic scenarios with high probability from 18 seismic scenarios. Our results show that the spatial distribution and pulse probability distribution of near-field simulated ground motion is significantly affected by the initial rupture positions. After the characteristics of the largest asperity on the fault are determined, the remaining asperities have limited influence on the spatial distribution of simulated ground motion. This study expands the application scope of the finite difference method, which can simulate the seismic intensity characteristics of destructive earthquakes in areas with few seismic records.

Najaftomaraei, Mohammadreza; Rahimi, Habib; Tanircan, G.; Shahvar, Mohammad

doi: 10.1007/s00024-022-03097-7pmid: N/A

In this study, a stochastic simulation model proposed by Yamamoto and Baker (Bulletin of the Seismological Society of America 103:3044–3056, 2013) is applied to the Iranian strong motion database, which comprises more than 3828 recordings for the period between 1975 and 2018. Each ground motion is decomposed into wavelet packets. Amplitudes of wavelet packets are divided into two groups, and for each group, model parameters are estimated using the maximum likelihood method. Regression coefficients are then obtained relating model parameters to seismic characteristics such as earthquake magnitude, distance, and site condition. Inter-event residuals of coefficients and correlation of total residuals of those parameters are also calculated. An inverse wavelet packet transform is used to reconstruct the amplitudes in the time domain and perform the simulation. Finally, a validation test is performed. The comparison of ground motion intensity measures for recorded and simulated time series shows acceptable conformity in the application. The estimated parameters using the simulated data agree with the real data, indicating the acceptable validity of the estimated stochastic simulation model. The obtained regression equations can generate ground motions for future earthquake scenarios in Iran.

Benites, Rafael; Villamor, Pilar

doi: 10.1007/s00024-022-03105-wpmid: N/A

The deformation and stress fields due to a three-dimensional, pressurized magma chamber are computed using the Indirect Boundary Integral Method (IBIM) with a numerical scheme based on point, single-force distribution over the closed surface of the chamber, and Green’s function representation of the contribution of each single-force to the overall deformation. This scheme follows on Yang et al. (1988) analytical solution of the deformation due to ellipsoidal cavities, obtained from prescribed distributions of dilatation dipoles and double-couples in and around the chamber, respectively. Our solutions for generic cases of three-dimensional magma chambers in a half-space provide excellent agreement with those from analytical or other numerical, well accepted solutions. As well, our algorithm computes the deformation derivatives and stress fields via Hooke’s law. Using the stress tensor field, we compute the Coulomb Failure Stress (CFS) on a target earthquake fault plane. We have calculated the CFS values caused by inflating spherical, prolate ellipsoidal, penny shaped and dyke chambers on a nearby fault. For all these cases, we prescribe the elastic parameters, the fault dimensions, orientation and dip, and the volumetric change of the magma chamber upon inflation. A parametric study in terms of the magma chamber dimensions shows that the CFS depends strongly on the shape and orientation of the chamber with respect to the fault. Although we do not present the results for arbitrarily shaped magma chambers other than those described above, we emphasize that the numerical formulation of our algorithm is not constrained by a particular shape, and can be applied to realistic cases of magma reservoirs. The only assumption is that the distributed single forces are perpendicular to the internal wall of the chamber, regardless of its shape. The magnitude of the forces is determined by the elastic properties of the confining rock, upon volumetric changes. The method is remarkably fast, judging from our solution of the benchmark case of an ellipsoidal magma chamber in a half-space. Using 1150 point forces to represent the chamber, it takes 5.54s CPU time to compute the ground deformation at 2500 points on the free-surface. This, we believe, can be useful in applications where the computation of many trial models are needed to converge to a final solution. Such applications could be the interpretation of surface deformation from GNSS data, and the study of volcano-fault interactions by computing the stress perturbations of volcanic episodes and nearby earthquake fault ruptures.

Li, Dong; Peng, Suping; Guo, Yinling; Lu, Yongxu; Cui, Xiaoqin; Du, Wenfeng

doi: 10.1007/s00024-022-03108-7pmid: N/A

Prediction of changes in reservoir parameters is important for understanding dynamic changes in reservoirs and guiding resource development. Time-lapse seismic exploration provides an effective means for predicting changes in reservoir parameters; however, the lack of time-lapse logging curves and prediction accuracy limits its application. To alleviate this problem, we propose a two-stage dual-network (DN) prediction method. The first stage establishes the relationship between the baseline logging curve and seismic attributes through an improved deep feedforward neural network (TP-DFNN) to predict the baseline reservoir parameters. The second stage establishes the relationship between the baseline seismic data and reservoir parameters through a fully convolutional network (FCN) to predict changes in reservoir parameters. The proposed method can calculate both reservoir elastic parameters (P- and S-wave velocities, and density) and reservoir physical parameters (porosity, permeability, Poisson’s ratio, resistivity, etc.). Only the baseline logging curve is needed, and there is no need to set up an initial model in the DN inversion. The test results of the field data show that the proposed method can efficiently and accurately predict the time-lapse changes in reservoir parameters. The DN-based method predicted P-wave velocity with higher accuracy than that presented by methods based on traditional amplitude variation with offset (AVO) inversion. Thus, the proposed DN-based method can provide good detection of reservoir parameters, and can be adopted in various fields, such as in oil and gas exploration, coal mining, and CO2 storage.

Zhou, Ying; Wang, Hongwei; Wen, Ruizhi; Miao, Tianming; Cui, Jianwen

doi: 10.1007/s00024-022-03077-xpmid: N/A

On 21 May 2021, an M 6.4 earthquake occurred in Yangbi county, west of Yunnan Province, China. The Yangbi seismic sequence was initiated on 18 May and reached its climax as the occurrence of the M 6.4 mainshock. It was a typical foreshock-mainshock-aftershock sequence. A total of 1043 strong-motion recordings during 44 events from this sequence were selected and utilized for the spectral decomposition to separate the path attenuation and source spectra. The path attenuation curves are overall close to the simplest distance decay form described by R−1. The path attenuations were further represented by the combination of the geometric spreading and anelastic attenuation. Geometric spreading of R−0.37 was retrieved, indicating slower distance decay at local distances. The larger area close to the epicenter tends to undergo strong ground shaking. The quality factors obtained were regressed as Qs(f) = 64.27f0.76 in the frequency range of 0.25–20 Hz, lower than those reported for the whole Yunnan region, indicating faster attenuation at large distances in the study region. The seismic moments, corner frequencies, and stress drops were estimated from the inverted source spectra. The stress drops for the Yangbi seismic sequence were mainly in the range of 0.5–3.0 MPa. The mainshock had the largest stress release (~ 2.7 MPa). The strong foreshocks and aftershocks with Mw ≥ 5.0 had higher stress drops (~ 1.5–2.5 MPa) than those of small events with Mw < 5.0 (~ 0.1–1.5 MPa). We inferred that the strong foreshocks and aftershocks with high stress releases may not rupture the same fault activated by the mainshock, while they may occur in various fault branches.

Urrutia-Fucugauchi, Jaime; Arellano-Catalán, Oscar; Pérez-Cruz, Ligia; Romero-Galindo, Irving A.

doi: 10.1007/s00024-022-03074-0pmid: N/A

Modeling gravity and magnetic anomalies over the Chicxulub crater are used to constrain the structure, stratigraphy, and asymmetries. Chicxulub is a multiring ~ 200 km rim diameter structure with a central uplift and well-preserved peak ring. The low relief terrain and physical property contrasts have facilitated geophysical modeling of the structure and impactite deposits. Nevertheless, contrasting models have been obtained due to data resolution limitations, uneven coverage, non-uniqueness solutions, boundary conditions, and heterogeneous/anisotropic media. We employ a multi-technique approach based on regional–residual separation, spectral analysis, first and second derivatives, upward and downward analytical continuations, horizontal gradients, analytical signal, Euler deconvolution, reduction to the pole, and forward modeling to constraint the anomaly sources, geometry and depths. Forward modeling of gravity anomaly favors central uplift flat-top models, whereas magnetic models show irregular shapes with a peak towards the NE, at 4–5 km depth. Analysis shows the effects of intersecting regional anomalies in the semicircular pattern that limit the definition of asymmetries, which constrains impact angle and trajectory, crater structure and pre-existing target features. Models link lateral–vertical density and magnetic property contrasts, distinguishing non-magnetic pre-and post-impact carbonates and carbonate-rich breccias from melt and basement rich breccias, and displaced, fractured impactites and basement uplift.

Chimoto, Kosuke

doi: 10.1007/s00024-022-03099-5pmid: N/A

Joint analysis of the receiver and autocorrelation functions was performed to estimate the structure of the P- and S-wave velocities VP and VSas well as the depth to the seismic bedrock using strong motion records obtained in Narita, Kanto basin, Japan. From data of over 1000 P-waves and their coda induced by local seismic events, the autocorrelation revealed a clear reflection in the sedimentary layer, and the receiver function revealed clear Ps and PpPs converted phases at the seismic bedrock. The sediment thickness and VS were measured from the H–κ stack of the vertical and transverse components of the autocorrelation and the Ps and PpPs phases in the receiver function. The previous VS structure model obtained with only the transverse autocorrelation in Narita was found to be appropriately evaluated, whereas the corresponding VP structure model must be improved. Because the H–κ stack requires an assumed VP value, an appropriate value of VP is crucial for accurate estimation of VS and the thickness of the sediment. Thus, the frequency analysis of the H–κ–VP stack was performed to estimate VP, VS, and the thickness of the sedimentary structure. From the results of this analysis, VS was determined to have been estimated appropriately for the present case in Narita. However, VP had been estimated to be lower than the value estimated by this study. It is clear that an appropriate assumption of VP is important to capture the Ps and PpPs phases in the receiver function.

Schattner, Uri; Segev, Amit; Mikhailov, Valentin; Rybakov, Michael; Lyakhovsky, Vladimir

doi: 10.1007/s00024-022-03100-1pmid: N/A

This study presents new regional Total Magnetic Intensity and Reduced to Pole (TMI and RTP, respectively) magnetic anomaly maps of northern Israel and NW Jordan (10,620 km2) in an unprecedented 1-km grid spacing. The maps quantitatively integrate new ground data, collected along 5000 km using the bike-mag design, with all available marine and aerial magnetic surveys previously conducted. The removal of anthropogenic influences during processing is evident from a comparison of the resulting maps with the distribution of infrastructure. The study discusses the correlation between previously mapped magmatic outcrops and the new RTP anomalies. This correlation suggests that the subsurface magmatic bodies are broader than their surficial outcrops. RTP anomalies found over areas free of surface basaltic outcrops indicate subsurface causative bodies. Some of these bodies were previously identified in wells. The complete spatial coverage of the RTP map allows linking these sporadic occurrences and placing them into the regional geological context. The detailed map breaks down the previous course anomalies and allows a better understanding of magmatic events spanning from the Jurassic to the present. This study demonstrates the robustness and efficiency of the bike-mag for ground data acquisition, and the integration procedures for producing maps from a variety of sources and parameters.

Vemula, Sreenath; Raghukanth, S. T. G.

doi: 10.1007/s00024-022-03076-ypmid: N/A

Ground motion prediction equations (GMPEs) are developed using past strong-motion records to predict the effect of future events. Often, the records in the database are incomplete, not covering all possible input scenarios or not recorded at the site of interest for performing site-specific hazards. Such cases are handled by adjusting the GMPEs to suit the required site/region characteristics. Recent studies have shown that scaling the Fourier amplitude spectrum (FAS) rather than the pseudo-spectral acceleration (PSA) is physically justifiable. The present work develops a recurrent neural network to predict the PSA ordinates from the FAS and duration (D5-95) for the New Zealand region. The developed network has no potential underfit or overfit and has a strong correlation coefficient, R > 0.97, with total sigma values in the range of 0.1–0.13 (log10 units). If the predicted FAS and duration are used as inputs, its uncertainty must be included in the final sigma, which lies from 0.25 to 0.3 (log10 units). At low frequency, scaling of FAS and PSA values is identical. In contrast, scaling of higher-frequency FAS values affects the wide range of the PSA values, with a prominent effect initially observed at lower frequencies and later at higher frequencies.