Geophysical Journal International

SummaryIn this study, we compare the usability of a simplified microtremor-based empirical method and a conventional microtremor method based on an inversion analysis of a subsurface velocity structure model for constructing a map of average S-wave velocity (AVS) values. In the simplified (empirical) method, the phase velocities of Rayleigh waves, which can be obtained by processing a microtremor array, at wavelengths of 13, 25, and 40 m are regarded as AVS values from the ground surface to depths of 10, 20, and 30 m (${\overline {Vs} }_{10},{\overline {Vs} }_{20},\ {\rm{and}}\ {\overline {Vs} }_{30}$), respectively. Microtremor array surveys were conducted at 173 observation points within a 15 km × 17 km area east of Aso caldera, Kyushu, Japan (target area). AVS values are obtained by applying the empirical method to the phase velocities obtained at each observation point. The AVS values at an observation point (located near the centre of the target area) with velocity logging data are verified by a comparison with those based on the velocity logging data (i.e. overestimations by 6 per cent at maximum). It is found that for the entire target area, the spatial distribution of the obtained AVS values is consistent with the geological distribution. The AVS values within areas of the Aso-3 ignimbrite are 30–40 per cent larger than those within areas of thick soil and tephra on the strongly consolidated Aso-4 ignimbrite. In addition, the AVS values of the Aso-3 deposits are more than 10 per cent larger than those of the Aso-4 deposits and about 10 per cent smaller than those of geological units older than the Aso-3 deposits. We also apply a conventional (i.e. inversion) method to the phase velocity data at each observation point to obtain a one-dimensional S-wave velocity (Vs) structure model from which we deduce AVS values. The deduced AVS values at the velocity logging point are underestimated by -8 per cent, with differences from the AVS values obtained using the empirical method reaching 13 per cent. The average systematic difference between the two methods is 15 per cent, as determined from a statistical analysis. None the less, a strong correlation is found between the methods, with an average correlation coefficient of 0.94, with no evidence showing that either method is more accurate. The empirical method can be used to construct an AVS map if overestimation is carefully considered. This analysis also reveals that the average maximum survey depths of the one-dimensional Vs structures based on the inversion method are only 23±10 m, making them often insufficient to map ${\overline {Vs} }_{20}$ and ${\overline {Vs} }_{30}$ (the ratios of the available to total numbers of data points are only 60 and 21 per cent, respectively). In contrast, the empirical method can determine ${\overline {Vs} }_{10},{\overline {Vs} }_{20},\ \ {\rm{and}}\ {\overline {Vs} }_{30}$ at more than 80 per cent of all sites. The construction of AVS maps using the empirical method is effective in terms of the simplicity and reliability of planning, observational efficiency, and simplicity of data processing, which support a practical and objective approach to seismic assessments.

SummaryIn this study we obtain 35 903 high-quality P-wave receiver functions from 1737 teleseismic events recorded at 120 dense broadband TanluArray temporary stations deployed in and around the Tanlu fault zone (TLFZ). After station azimuth and sediment correction are made, a detailed Moho depth distribution is obtained by CCP stacking. Our results show a sharp change in the Moho depth across the TLFZ from the west to east, which well corresponds to the surface geological structure. The deepest Moho (38.0 ∼ 40.0 km) occurs beneath the Dabie orogenic belt and the Sulu orogenic belt. The Moho beneath the Luxi uplift, Jiangnan orogenic belt and Jiaodong uplift is deeper (36.0 ∼ 37.0 km), whereas the Subei basin and the southern basin of the South Yellow Sea have a shallow Moho (28.0 ∼ 30.0 km). There is an obvious Moho uplift near Weifang, which corresponds to the Changle ancient volcano on the surface and may be a channel for upwelling of hot mantle material. The Moho is unclear under the fault zone near Tancheng, which is speculated to be a channel for upwelling of hot mantle material. It may be related to upwelling of hot and wet flows in the big mantle wedge above the subducted Pacific slab that is stagnant in the mantle transition zone beneath East Asia, which is a possible cause of the 1668 M8.5 Tancheng earthquake.

SummaryThe Limpopo transform margin offshore southern Mozambique results from the separation of Gondwana along the East Africa continental margin. Over the last three decades, more than thirty different reconstruction models have been proposed, sometimes contradicting each other. Here, we present results from the travel-time tomography of wide-angle seismic data acquired during the second China-Mozambique Joint Cruise, allowing the interpretation of the crustal structure and magmatism in the Limpopo Corridor and the Mozambique Basin. Using these results, we determine the extent of the Continent Ocean Transition and the location of the Continent Ocean Boundary on the southern Mozambique margin. The seismic profile is 442-km long, extending from the eastern part of the North Natal Valley in the west and crossing the Limpopo Corridor and the Mozambique Basin to the east. Based on the tomographic velocity model, we delineated three distinct domains from west to east along the profile: (1) a western transitional domain with anomalous or mixed crust, bounded by the Mozambique Fracture Zone to the east, where the crust gradually thins eastward from ∼14 km at distance 45 km to ∼10.8 km at distance 140 km; (2) a domain of thickened oceanic crust resulting from enhanced magmatism, where the crust thins eastward, from ∼10.8 km to ∼8.5 km over ∼100 km distance; and (3) an eastern domain of normal oceanic crust, where the average crustal thickness is ∼8 km. We suggest that (1) the western transitional domain roughly corresponds to the Limpopo Corridor and is of continental crustal origin but was affected and modified by strike-slip motion and magmatic activity, resulting in anomalous or mixed crust. The eastern Continent Ocean Boundary of the Limpopo Margin is close to the Mozambique Fracture Zone; (2) The thickened oceanic domain thins eastward, and the crustal velocity and thickness change dramatically compared to the oceanic domain. This domain seems to have strongly interacted and contaminated by the Limpopo Corridor during the opening of Mozambique Basin and seafloor spreading; (3) The eastern oceanic domain shows a relatively uniform oceanic crust of ∼8 km and high velocity up to 7.4 km/s in the lower crust, suggestive of a hotter mantle that produces more MgO-rich melts probably due to the influence of a thermal mantle anomaly.

SummaryThe downward continuation of gravity field can provide valuable information for 3-D gravity-field modeling, shallow-layer geological interpretation, source depth estimation, and so on. However, downward continuation is ill-posed, and traditional approaches often suffer from computational instability, poor noise resistance, and limited continuation depth, making it a longstanding challenge in gravity data processing. We present a new approach for fast, stable and large-depth downward continuation of gravity anomalies by using frequency-domain 3-D imaging. First, we utilize the frequency-domain 3-D imaging approach to invert the gravity anomalies at the original observational plane to quickly obtain the equivalent density model in the subsurface. Then, we apply the optimized strategy of frequency-domain 3-D forward calculation on the equivalent density model to rapidly obtain high-precision gravity anomalies at the downward-continuation plane. The synthetic data tests prove the effectiveness of our approach, and demonstrate that our approach enables fast, stable, robust noise resistance and large-depth downward continuation of large-scale gravity anomalies data, and has superior performance compared to the traditional regularized filtering approach and spatial-domain equivalent-source approach. The real data test of the free-air gravity anomalies data in the central South China Sea also verifies the fast, stable and reliable downward continuations of large depths by our approach. The 3-D gravity-field model built by our approach will provide significant support for the tectonic studies and resource exploration in this area.

SummaryModelling and inversion of controlled-source electromagnetic data requires elaborate numerical tools. The major challenge is the high computational cost of computing solutions to numerous forward problems (for the forward responses as well as the sensitivity matrix). Forward modelling is accomplished using either a direct or an iterative solver. Current modelling suites predominantly employ direct solution methods in the forward operator since multiple solutions are easily accessible using inexpensive and quick forward-backward substitution after an initial resource-demanding matrix factorisation step. Iterative techniques, on the other hand, require little resources for single forward solutions, and are yet very time consuming if solutions for many right-hand sides are to be computed. Evaluations of different solution techniques for modelling and inverse problems are only sparsely investigated. In light of this, we integrated an iterative solver as alternative in the forward and and inversion operators of the open-source software custEM and pyGIMLi. In particular, we implemented a two-level iterative scheme where the outer solver employs a generalised conjugate residual algorithm preconditioned with a highly efficient block-based preconditioner for square blocks. The inner-level solver is either of the same type as the outer solver, but preconditioned with the auxiliary-space Maxwell preconditioner, or may alternatively be a direct solver. In this paper, we evaluate the described iterative forward operator for forward modelling tasks for the Marlim R3D model for a single as well as numerous right-hand side vectors and compare the performance to the direct solver MUMPS. We further investigate the solver’s applicability on small and medium-sized computing platforms. We then examine the iterative solver for inversions of synthetic land-based and semi-airborne data in terms of computational requirements. Our results demonstrate that forward modelling tasks are best performed using an iterative approach for single source problems. Moreover, simulations of large and complex problems are accessible on even on small computing platforms such as laptops in very reasonable time. For inversions, the iterative forward operator, in particular the mixed iterative-direct-based one, performs equally well in terms of time as the direct one while reducing the memory demands for the computations of the forward responses and the data sensitivities.

SummaryIn a region of complex geology, we examine the influence of spatial resolution of conductivity models on Geomagnetically Induced Currents (GICs) estimations. We focus on the southern region of Portugal mainland, for which magnetotelluric (MT) sounding measurements have been obtained with lower noise from human activity. Using two conductivity models inverted from sets of MT soundings with different sampling distance, we look for an interpretation of the differences in GIC estimations at substation grounding resistances. We make use of two different proxies, the Local Effective Field (LEF) and the Regional Electromotive Source (RES), built from the electric induced field at each substation site and the sum of electromotive forces along all transmission lines connected to that substation, respectively. We compare different time signals associated to GICs using a parameter that combines Pearson correlation and linear regression slope, the Correlation Regression Coefficient (CRC). Our main conclusion is that spatially detailed information on lateral heterogeneities of the conductivity associated to complex geology is crucial for a rigorous assessment of GIC hazard, leading to relative differences in GIC standard deviation and in GIC peak values that can amount to more than 100% in certain cases. Additionally, using LEF and RES, we emphasise the non-locality of GIC drivers and bring new input concerning the choice of proxies used to monitor and forecast this kind of hazard.

SummaryThe Scandinavian Peninsula and its vicinity comprise highly tectonically diverse blocks, including the Baltic Shield, the continental margin, and the North Sea Basin. The crustal rheology is a critical constraint to understanding the tectonic evolution in this region. Based on 19 416 Lg waveforms from 233 earthquakes and 560 broadband digital stations, using an inversion method combining both single- and two-station ray paths, we constructed a broadband (0.05 and 10.0 Hz) Lg wave attenuation model in the study region, with the resolution approaches to 110 km (∼1°) or higher in areas with dense ray path coverages. The QLg distributions correlate well with regional geological features. The Baltic Shield exhibits the highest QLg, consistent with its thick Precambrian crust and high rheological rigidity developed through Archean Svecofennian orogeny. In contrast, passive margins with crustal thinning, magmatic modification, and thick sedimentary sequences exhibit strong attenuation, reflecting a reduction in rheological strength resulting from interactions with mantle plumes and extensional tectonics. The North Sea Basin exhibits the lowest QLg values and the presence of hydrocarbon-bearing sediments. The extremely high QLg distribution reveals the ancient cratonic core of the Baltic Shield, particularly in areas where the surface rock dating sample cannot be collected due to seawater coverage.

SummaryWe analysed infrasound waves associated with the Gyeongju earthquake (ML 5.8) that occurred on September 12, 2016, in the southeastern Korean Peninsula. For infrasound wave detection, the Progressive Multi-channel Correlation method was applied to the infrasound dataset recorded at 7 arrays operating in South Korea at epicentral distances ranging from 178 to 472 km. Based on the back-projection method constrained by array-dependent celerity and azimuth deviation models, the source regions were identified in both the epicentral and nonepicentral regions. Remarkably, the nonepicentral secondary sources of this earthquake were located in regions with shallow water depths: i) the western coastal area in the Yellow Sea and ii) the shallow ocean basin and bank in the East Sea. The location results obtained from the earthquake could be corroborated through its foreshock (ML 5.1), yielding location results consistent with those of the mainshock. The generation of infrasound waves over shallow water depths was fortuitously validated by direct recordings of dominant single-frequency (∼0.3 Hz) infrasound waves at close range via temporary sensors near the ocean basin and bank. We interpreted that low-frequency infrasound signals could be generated from interactions among the ocean floor, shallow seawater, and atmosphere. We performed numerical simulations of seismoacoustic fields to predict ground motions on the seafloor and acoustic transmission efficiency between the water and air interface. The simulations quantified the energy transfer through different media and clarified our observational results. We found that because this solid Earth‒water‒atmosphere coupled air wave has a relatively low frequency (∼0.3 Hz), it can survive propagation over long distances compared with high-frequency infrasound waves generated in inland and mountain regions. In this study, we extend our understanding of water‒atmosphere coupling and the monitoring framework for earthquake-associated nonepicentral infrasound waves, encompassing not only inland ground shaking but also shallow sea regions located far from the epicentre.