Pure and Applied Geophysics

Liu, Shuangqing; Xue, Yan; Chen, Song; Yao, Huiqin; Jin, Dali; Wang, Yixi; Li, Yue

doi: 10.1007/s00024-023-03307-wpmid: N/A

Based on the barometric data recorded by the seismic monitoring network on the Chinese mainland, Lamb wave periods and relative arrival times of 462 stations are obtained with the aid of Meyer’s wavelet decomposition, Welch’s periodogram spectrum estimation and waveform cross-correlation. By extracting the seismic Rayleigh waves of two seismic stations in the South Pacific and comparing them with the synthetic seismograms, the occurrence time of the first two large volcanic eruptions and the largest volcanic eruption are credibly deduced, and then the travel times and propagation speeds of the Lamb waves are obtained. In order to further explain the attributes of the pressure disturbance associated with the complex acoustic gravity wave (AGW) phases on pressure, a series of numerical simulations with the QSSP program are applied to yield highly similar waves corresponding to the barometric records, which indicates that the eruption source may contain more than 10 sub-events within an hour. According to our detailed analyses, major conclusions are obtained as follows: (1) Although the primary pressure disturbance of the Tonga volcanic eruption appears to be a simple bulge, it is in fact a complex wave composed of multiple eruptions. The largest eruption occurred about 13 min after the first large eruption. (2) In addition to the propagation of the traditional Lamb mode, the Pekeris mode phase that propagates with a lower speed is also observed in some stations, and its amplitude is about a fifth of the Lamb wave. The Pekeris waves may have been more significantly delayed as they traveled against the westerly wind due to their much shorter periods and lower velocities. (3) The group velocity of the Lamb wave is about 308 m/s. Its average period is 70 min, and the wavelength is about 1300 km. The arrival time deviation at each station is negatively correlated with the difference in the near-surface air temperature between North and South China. However, accurately estimating the parameters of the Pekeris waves or the antipodal Lamb waves is challenging due to their low signal-to-noise ratio (SNR), even though the horizontal propagation speed of the Pekeris wave toward China is estimated to be approximately 225 m/s. (4) Much shorter periods and later arrivals at the stations within 200 km of Beijing may be related to the significant cooling (i.e. 12 °C temperature drop), which occurred from Ulantoba, Outer Mongolia, to Beijing beginning at noon on January 15 (Beijing time).

Anupama, M.; Sunil, P. S.

doi: 10.1007/s00024-023-03308-9pmid: N/A

The 14 November 2016, Mw 7.8 Kaikoura, New Zealand earthquake offers an unprecedented opportunity to observe the heterogeneity in stress field over a very complex fault system where the subduction zone converges with the strike-slip faults system. Here, we report the pre- and post-seismic stress field asperity for the first time in terms of the b value variations associated to the Kaikoura earthquake. Pre-seismic disparity of b values indicates the existence of two prominent low b value clusters, one in the neighborhood of the epicenter and the other just to the northeast of the earthquake rupture zone. Owing to the co-seismic stress release near the epicentral area, the pattern of low b value has become negligible in the post-seismic period. However, the pattern of low b value in the northeast of the rupture zone remains unchanged in the post-seismic period and indicates the unreleased strain energy in the province. The stress fields inferred from the inversion of the focal mechanism during pre- and post-seismic periods of the event suggest a strike-slip mechanism with a horizontal maximum stress axis (σ1) in the WNW-ESE direction. Nevertheless, before and after the earthquake, the stress field direction did not change significantly, indicating that the energy released during the Kaikoura event was insufficient to alter the stress orientations in the complex fault system.

Huang, Kaiteng; Tang, Litao; Feng, Wanpeng

doi: 10.1007/s00024-023-03286-ypmid: N/A

The Gutenberg-Richter (GR) b value is a proxy for crustal stress that has been widely applied in earthquake prediction, regional stress analysis and earthquake physics investigation. Following the 2019 Mw 7.1 Ridgecrest earthquake sequence, a large number of aftershocks were well documented along the earthquake faults. We analyzed these aftershocks based on the GR law for the spatiotemporal b value distributions within the fault zone in three dimensions. The results show that the b values are homogeneous in space at a low level of ~ 0.6 immediately after the mainshock, while they increase rapidly within 3 months to ~ 1.0, which was thought to be an average level for California in the interseismic period. Meanwhile, in the first few weeks after the mainshock, the distribution of the b values along the faults is heterogeneous, where the recovery rates of the b values on individual fault cells vary dramatically along both the strike and dip. Considering significant postseismic geodetic observations, the afterslip processes along the faults may be responsible for the time-dependent variation in b values. Given that frictional sliding is a key mechanism for afterslip, the b values could then have potential for fault frictional properties, which deserve special care in the future.

Li, Layue; Zhan, Wei; Chen, Changyun; Zhao, Jingyang; Li, Xiaobo

doi: 10.1007/s00024-023-03314-xpmid: N/A

On May 21, 2021, an Ms 6.4 earthquake occurred in Yangbi County, Yunnan Province, China. The epicenter was located on a secondary blind fault on the southwestern boundary of the Sichuan-Yunnan Block, and this area has a complex tectonic setting and seismogenic background. To improve the understanding of the seismogenic mechanism of this earthquake, we conduct an in-depth analysis on the regional stress and strain characteristics based on geodetic data from the Global Navigation Satellite System (GNSS) and seismological data (focal mechanism solutions) and analyze the stress changes at the epicenter of this earthquake from the perspective of stress triggering. The results show that the epicenter area of the Yangbi earthquake was characterized by significant shear and extension deformation, with a dextral shear rate of 2.5 × 10−8/a and an extension rate of 3.5 × 10−8/a. The present tectonic stress of the study area is dominated by NNW-directed principal compressive stress and NEE-directed principal extension stress, and the Yangbi main shock occurred on the optimal release nodal plane of the tectonic stress field where the relative shear stress reached 0.984. Furthermore, the cumulative Coulomb stress changes caused by four strong historical earthquakes exceeded the stress trigger threshold of 0.1 bar. All of these results suggest that the Yangbi Ms 6.4 earthquake was the result of the maximum effective shear stress concentration on the blind fault on the southwest side of the Weixi-Qiaohou fault, which was mainly driven by the regional tectonic stress and the surrounding historical strong earthquakes.

Kokowski, Jakub; Rudziński, Łukasz

doi: 10.1007/s00024-023-03315-wpmid: N/A

Legnica-Głogów Copper District (LGCD) is one of the most active seismic regions in Europe. Several thousand seismic events induced by underground copper mines are recorded there each year, with the strongest reaching magnitudes above 4. Seismicity in LGCD is monitored by the LUMINEOS surface seismic network and the mine's underground networks. While the horizontal location uncertainty of the LUMINEOS network is about 300–600 m, the declared epicenter uncertainty of dense mine networks is less than 50 m. It gives us a unique opportunity to test the location accuracy of seismic events recorded by the LUMINEOS network determined with various algorithms and automatic procedure. In our work, we compared the location accuracies of traveltime-based algorithms LocSAT and NonLinLoc as well as waveform-based algorithm BackTrackBB. The iterative, Geiger-type LocSAT algorithm is used in routine daily processing for the LUMINEOS. Its downside is the need to use the S wave onset times, which in the case of the LUMINEOS network are very uncertain. As an alternative, we tested the probabilistic NonLinLoc algorithm, and the waveform based BackTrackBB algorithm. The BackTrackBB algorithm is known to work well with local, high noise seismic networks. We aimed to find out if it could be used routinely with the LUMINEOS data. In addition, we conducted a comparative analysis of the location uncertainty of these algorithms to determine the effectiveness of this parameter in assessing accuracy.

Gao, Fengxia; Wang, Yanghua

doi: 10.1007/s00024-023-03303-0pmid: N/A

Conventional impedance inversion from post-stack zero-offset seismic data is usually based on the convolution model, and wave-equation based inversion is rarely used, although it is capable to precisely describe seismic wave propagation and invert impedance model with higher resolution. That is because there are more than one physical parameters involved in the conventional wave equation, making impedance inversion complicated. In this study, a one-dimensional (1D) wave equation, containing only the impedance parameter, is adopted and applied for the inversion of 1D impedance model by seismic waveform tomography. However, for a three-dimensional (3D) model, direct application of the 1D waveform tomography may lead to lateral discontinuities. Therefore, we propose to utilize a truncated Fourier series to parameterize the 3D impedance model, and then invert for the Fourier coefficients. With this parameterization scheme, the large- and small-scale components of the impedance model can be reconstructed stepwise by gradually increasing the number of Fourier coefficients. To efficiently and effectively invert the coefficients for the 3D model with salt structure, we propose a joint strategy, in which the low-frequency seismic data is used to invert for the Fourier coefficients representing the large-scale components of the model, while the high-frequency seismic data is applied to invert for the Fourier coefficients representing the small-scale components of the model. Tests on a 3D impedance model with salt structure result in models with high resolution and good spatial continuity, proving the feasibility and stability of the impedance inversion procedure.

Jiang, Huipeng; Ma, Qiang; Zhou, Fengxi

doi: 10.1007/s00024-023-03300-3pmid: N/A

Based on the propagation theory of elastic waves in frozen poroelastic media and elastic media, a theoretical model of saturated frozen soil and elastic solid bedrock is established. The phenomena of transmission and reflection of plane-P1 waves at the plane interface between saturated frozen soil and elastic solid bedrock are analyzed. The saturated frozen soil is lying on an elastic solid bedrock and is modeled as a porous material with a three-phase medium that contains two solids (soil skeleton and ice particles) and a compressible viscous fluid. First, analytical solutions for wave transmission and reflection in saturated frozen soil and elastic solid bedrock models are obtained. Then, the theoretical formulation is derived for the characteristic equations in saturated frozen soil and the amplitude ratios and energy ratios of transmitted and reflected waves. The numerical results are obtained and used to discuss the relationship between the amplitude ratios and energy ratios for transmitted and reflected waves and the incident angle, incident frequency, saturation, cementation parameters and contact parameters. This study shows that only compressional waves are present when the incidence angle is 0. As cementation parameters, saturation, porosity and contact parameters increase, the critical angle appears earlier. The trend of the amplitude ratios is similar to that of its energy ratios, but the change in the energy ratios is larger than that of the amplitude ratios. Each wave has different degrees of pulses when reaching the critical angle, among which the transmitted P wave and the reflected P1, S1 and S2 waves are the most significant. At higher saturation, it is observed that the amplitude ratio and energy ratio of the reflected S2 wave decrease significantly.

Riccardi, U.; Carlino, S.; Pivetta, T.; Hinderer, J.; Rosat, S.; Ricciardi, G.

doi: 10.1007/s00024-023-03313-ypmid: N/A

We report on the results of about 9 months of gravimetric recordings acquired at Mt. Somma-Vesuvius (SV) volcano (Southern Italy) with the new generation relative gravimeter gPhoneX#116 (gPh#116), which is a gravimeter specifically designed for continuous gravity recording. We also present the outcomes of an intercomparison experiment of the gPhone#116 conducted at the J9 gravity observatory in Strasbourg (France). In this intercomparison, we were able to check the scale factor of the meter with a high degree of precision by means of an intercomparison with 2 superconducting gravimeters (SGs) and a FG5-type absolute ballistic gravimeter. Multiple calibration approaches allowed us to validate the manufacturer's original calibration constants to a level of 1% accuracy and 0.1% precision. Moreover, we carried out a comparative study of the noise level of the gPh#116 with respect to the SGs and other spring meters routinely used in both prospecting and time-lapse gravimetry. It turns out that gPh#116 exhibits lower levels at hourly time-scales than other compared spring gravimeters (Graviton, gPhone#054, Scintrex-CG5). It was also possible to carry out a detailed study of the instrumental drift, a crucial topic for reliable monitoring of the long-term gravity variations in active volcanic areas. In fact, a challenge in time-lapse gravimetry is the proper separation of the instrumental variations from real gravity changes eventually attributable to recharge or drainage processes of magma or fluids in the feeding systems of active volcanoes. A negative finding coming out from the intercomparison is that, even when applying the tilt correction, the gravimetric residuals obtained with the gPh#116 are an order of magnitude larger and quite inconsistent with those obtained with co-located superconducting gravimeters. We guess this problem could be overcome by installing the gravimeter on an auto-levelling platform. From the analysis of the gravity records, a reliable tidal gravity model was derived, which we believe will help to improve the accuracy of volcano monitoring, as it will allow appropriate correction of tidal effects for both relative and absolute gravity measurements acquired in the area. Two further interesting elements arose from our study: (1) a peculiar cavity effect of the SV underground laboratory that seems to influence the tilt change; (2) the small residual gravity signals are time correlated with the rainfall peaks and are compatible with gravity decreases induced by increases in soil moisture above the gravimeter.

Oliveira, Alex T.; Dantas, Renato R. S.; Medeiros, Walter E.; Costa, Jessé C.; Amaral, Victória F.

doi: 10.1007/s00024-023-03287-xpmid: N/A

Seismic illumination can be defined as the maximum angle between rays that pass through a point. Only interfaces completely contained in the angular apertures can be correctly imaged with first arrival traveltime tomography (FATTT). We investigate if 2D acoustic full-waveform inversion (FWI) can compensate for the lack of illumination of a crosshole tomography experiment. We use synthetic data generated with Ricker wavelets with peak frequencies at 100 or 500 Hz, resulting in small overlapping in the frequency bandwidths, allowing the use of a multiscale FWI approach. We investigate two FWI cases: in the first case (FWI T), just the waves recorded at the opposite borehole are used whilst, in the second case (FWI T+R), the waves recorded at the two boreholes are used. For a single interface, the shape of the transmitted waveform only varies significantly when it is contained in the angular apertures. Accordingly, shot gathers for layered models with interfaces outside the angular apertures can be roughly reproduced with equivalent homogeneous media. Thus, in comparison with FATTT, both FWI cases give mild improvements for models with interfaces inside the angular coverage, and cannot compensate for the lack of illumination of the experiment. However, in the mixed condition where layers with interfaces inside the angular coverage are cut by a fault, FWI offers substantial improvements over FATTT, even if the fault plane is outside the angular coverage. In this mixed situation, resolution increases when FWI T+R is used. However, for the studied cases, no significant improvements were obtained in the FWI stage where the 500 Hz peak frequency was used.

Luo, Weifeng; Zhou, Hui; Kong, Liyun; Liu, Haihao; Zhang, Yunxiao; Ma, Yanyan

doi: 10.1007/s00024-023-03288-wpmid: N/A

In fractured reservoirs, the seismic response characteristics are more complex than in conventional reservoirs because of their inherent properties of anisotropy and dispersion-attenuation, increasing the multiplicity of the prediction solutions and making fluid identification more difficult. Since fluid saturation is a crucial parameter that can directly affect these inherent properties, its effect on F-AVAZ (frequency-dependent amplitude versus angle-azimuth) seismic response characteristics of fractured reservoirs is studied in this article. Using Norris and KG models, an effective partially saturated fractured porous medium is established. Based on the anisotropic reflectivity algorithm, the deterministic relationship between F-AVAZ and fluid saturation is obtained. The numerical simulation results of the three-layer model show that when oil–water coexists in fractured reservoirs, F-AVAZ gradually changes with growing water saturation. While the fluid changes from fully water saturated to gas bearing, F-AVAZ suddenly jumps because of the rapidly decreasing effective P-wave modulus. This abrupt increase of P-wave amplitude is the classic 'bright spot' phenomenon, which verifies the validity of the anisotropic reflectivity algorithm based on the Norris-KG model for fractured reservoirs. This study lays a solid theoretical foundation for the analysis of seismic response characteristics of multi-phase fluid saturated fractured reservoirs and provides a reliable theoretical basis for improving reservoir prediction and fluid identification accuracy.