High Seismic Velocity Research Articles

A fresh look at the Gulf of St. Lawrence, Eastern Canada: Geophysical imaging of salt bodies and volcanic rocks in the upper Paleozoic Magdalen Basin

In the Gulf of St. Lawrence, the late Middle Devonian to early Permian Magdalen Basin hosts numerous salt bodies, but their number and geometry remained poorly constrained due to the poor quality of industry-seismic data. Salt originates from the Middle Mississippian (Visean) Windsor Group, which includes marine carbonates and evaporites. In this study, we use multibeam bathymetry as well as unpublished aerogravity and aeromagnetic data to better characterize the geometry of salt bodies. These datasets show a high degree of coherency and illustrate deformation on the seafloor, gravity lows and magnetic highs, all associated with salt bodies and overlying volcanic rocks. In map view, the geometry of individual salt bodies varies from nearly circular to ovoid and strongly elongated. Some salt bodies coalesce in complex-shaped salt walls elongated NNE, parallel to some of the faults that controlled the early stages of the Magdalen Basin development. Forward modeling indicates that low density salt bodies overlain by a 100–300 m thick volcanic pile may account for both gravity and magnetic signals. The ubiquity of mafic volcanic rocks indicates that the late Visean to Serpukhovian volcanic episode is more voluminous than previously thought. This episode is possibly associated with a series of dykes continuous for up to 225 km and with a period of underplating now recorded by a lower crustal layer with high seismic velocity.

Structure of the crust and upper mantle in Greenland and northeastern Canada: Insights from anisotropic rayleigh-wave tomography

Summary Seismic velocity models provide important constraints on Greenland’s deep structure, which, in turn, has profound implications for our understanding of the tectonic history of this region. However, the resolution of seismic models has been limited by a sparse network, particularly in northern and central Greenland. We address these limitations by generating new high-resolution Rayleigh-wave phase velocity maps encompassing Greenland and northeastern Canada by processing over three decades of teleseismic earthquake records and incorporating recently added stations in Greenland and Arctic Canada. These phase velocity maps are sensitive to structure from the lower crust down to the sub-lithospheric mantle (25-185 s period). We find significant heterogeneity and a strong correlation between isotropic and anisotropic seismic velocities with inferred geological structure. High seismic velocities associated with cratonic lithosphere are broadly divided into two regions, with a belt of reduced velocity spanning central Greenland, which we interpret as lithospheric erosion resulting from interaction between the Greenland continental keel and the Iceland plume. Within each region, we identify tectonic subdivisions that suggest fundamental differences between the blocks that make up Precambrian Greenland. In the south, the North Atlantic craton (NAC) has a high-velocity keel exhibiting anisotropic stratification. Between the NAC and the cratonic lithosphere further north, the Proterozoic Nagssugtoqidian orogenic belt shows a distinct signature of reduced seismic velocity to ∼75 s period, but then appears to pinch out at depth. The northern Greenland lithosphere exhibits significant isotropic heterogeneity, with a distinct core of high velocities in the northwest (∼55-75 s period) giving way to a set of distinct east-west trending high-velocity belts at longer periods. At all periods sensitive to the lithospheric mantle in this region, anisotropic fast orientations are E-W, consistent with a north-south Precambrian assembly of the Greenland shield. In contrast to the NAC, there is no evidence of anisotropic stratification in the northern part of the cratonic keel. Based on both isotropic and anisotropic phase-velocity anomalies, we suggest that the Phanerozoic Caledonian and Ellesmerian-Franklinian fold belts are relatively thin-skinned features onshore Greenland, though the Caledonian belt may have a stronger signature off the east coast. At the longest periods, a prominent low-velocity anomaly initially centred on Iceland migrates northwards and spreads beneath central-eastern Greenland. Coupled with NW-SE trending anisotropy, this feature is interpreted as the effect of mantle flow radiating outward from the Iceland plume and interacting with the eroded Greenland lithosphere.

Geological Mapping Optimisation Using Satellite Gravity, Satellite Imagery, and Seismic Shear Velocity (Vs30) to Monitor Lithological Condition in Geothermal Area of Mount Salak, West Java

Conventional geological mapping has several challenges such as limitation of area coverage, tough terrain, unstable weather condition, high-cost survey, also limitation of expert geologist that capable to do geological mapping. Remote sensing is an alternative solution to geological mapping through the combination of satellite gravity, satellite imagery, and seismic shear wave velocity. This research aims to optimise geological mapping activity in the Mount Salak geothermal area from ineffective area coverage mapping and inefficient budgeting allocation. Integration of rock density from satellite gravity, lithological distribution from satellite imagery, and seismic shear wave velocity from Vs30 are giving more detailed lithological units interpretation with specific physical conditions. There are two main area of Mount Salak that should be preserved geothermal resource sustainable. Northeast area needs to preserve heat source and groundwater recharge area, that consist of dense vegetation as landcover, relatively high rock density value (2.30 - 2.50 gr/cm3), high seismic shear wave velocity (600 – 900 m/s), also dominantly covers by Andesitic-Basaltic Lava. Southwest area preserve reservoir and caprock of geothermal conceptual model, where covers by dense vegetation with several bare land as geothermal wells and powerplants, relatively low rock density value (2.00 – 2.20 gr/cm3), moderate seismic shear wave velocity value (450-750 m/s), also dominated by several pyroclastic product such as Lapilli and Tuff. This research shows good indication in geothermal resource preservation in the Mount Salak, that keep natural aspect of geothermal conceptual model still preserved well.

Progressive underplating of mafic material at mid-crustal depth beneath Ischia volcano, Italy

The destructive (Mw 3.9) earthquake of 21 August 2017 re-opened the question on where magma resides at the Ischia island volcano. The peculiar complexity of the seismic source initiated the debate on the involvement of fluid-related processes, but the magmatic origin of the event remains uncertain. Here we use ground displacement and broadband seismic data to investigate the magmatic system of Ischia volcano, where progressive underplating of mafic material at mid-crustal depth feeds the deep structures, which are characterized by exceptionally high seismic velocity and are connected with a shallow crystal mush. Although no direct evidence of large molten volumes was found, strong anisotropy suggests that the crystal mush is pervaded by magma-intruded dykes. We propose that the 2017 event was due to a negative tensile deformation caused by depressurization of supercritical fluids along a shallow southwest (SW-)-dipping fault defined by receiver functions (RFs) data, which acted as a valve regulating the overpressure of deep magmatic fluids.

ArrayNet: A Combined Seismic Phase Classification and Back-Azimuth Regression Neural Network for Array Processing Pipelines

ABSTRACT Array processing is an integral part of automatic seismic event detection pipelines for measuring apparent velocity and backazimuth of seismic arrivals. Both quantities are usually measured under the plane-wave assumption, and are essential to classify the phase type and to determine the direction toward the event epicenter. However, structural inhomogeneities can lead to deviations from the plane-wave model, which must be taken into account for phase classification and back-azimuth estimation. We suggest a combined classification and regression neural network, which we call ArrayNet, to determine the phase type and backazimuth directly from the arrival-time differences between all combinations of stations of a given seismic array without assuming a plane-wave model. ArrayNet is trained using regional P- and S-wave arrivals of over 30,000 seismic events from reviewed regional bulletins in northern Europe from the past three decades. ArrayNet models are generated and trained for each of the ARCES, FINES, and SPITS seismic arrays. We observe excellent performance for the seismic phase classification (up to 99% accuracy), and the derived back-azimuth residuals are significantly improved in comparison with traditional array processing results using the plane-wave assumption. The SPITS array in Svalbard exhibits particular issues when it comes to array processing in the form of high apparent seismic velocities and a multitude of frost quake signals inside the array, and we show how our new approach better handles these obstacles. Furthermore, we demonstrate the performance of ArrayNet on 20 months of continuous phase detections from the ARCES array and investigate the results for a selection of regional seismic events of interest. Our results demonstrate that automatic event detection at seismic arrays can be further enhanced using a machine learning approach that takes advantage of the unique array data recorded at these stations.

Transition from the Continental Margin of Kamchatka to the Island Arc of the Kurile Islands: Features of Volcanism, Crustal Deformation and Geophysical Parameters of the Slab

Abstract—Magmatism manifestations in the transition zone from the continental margin of Kamchatka to the Kurile island arc and some geophysical parameters of the subducted oceanic plate of the Northwestern Pacific are considered. The presence of the Miocene coastal volcanic (Pribrezhny) complex at the base of the South Kamchatka volcanic belt contributes to intense crustal processes causing caldera-forming eruptions in the Holocene. The Northern Kuriles are characterized by initiation of areal volcanism associated with crustal fault zones. Anomalous is the absence of volcanism on Shumshu Island proximal to Kamchatka. Seismic tomography data reveal a high seismic velocity anomaly below it, which can explain this phenomenon. Based on the presented data an assumption is made about existence of a seafloor elevation on the slab, whose submergence led to disintegration of the melting regions responsible for generation of volcanism.

A Preliminary Tomography Inversion Study on the Palu Koro Fault, Central Sulawesi Using BMKG Seismic Network

The Palu Koro Fault, which frequently causes solid and destructive earthquakes, is the most exciting aspect of Central Sulawesi’s tectonics. We used local earthquake data from the area in this preliminary study. Our preliminary investigation entails analyzing local seismic data and estimating the velocity structure and hypocenter location using a tomographic approach that employs P-wave arrival times from the Agency for Meteorology, Climatology, and Geophysics earthquake observation network from 2010 to 2019. We use a 1-D initial velocity structural model in this tomographic inversion method, and the results are evaluated using the checkerboard test, derivative weight sum, and ray hit count. To the west and east of the Palu Koro Fault area, preliminary tomographic inversion results show a relatively high seismic velocity. These areas are interpreted as West Sulawesi Mandala and East Sulawesi Mandala Poso Fault and Wekuli Fault. The low-velocity anomaly is found near the Palu Koro Fault. Because of the high-speed thrusting, the relocated hypocenters also clustered around the Palu Koro Fault area. In the following activity, we will determine the speed structure of the S wave, which is expected to provide better geological information in the interpretation.

Tomographic Inversion Code Analysis Using Regularization Iteratively Reweighted Norm (Irn)

Indonesia is an area located in the ring of fire, which was formed as a result of the interaction between four plates, namely the Eurasian Plate on the south, the Eurasian Plate on the, the Philippine Plate, and the Pacific Plate on the northeast. The interaction between these plates causes Indonesia to have many active volcanoes and experience many geological natural disasters. In the tomographic inversion process, ill-condition problems are often found in matrix making. Therefore, a regularization technique is needed to obtain a consistent result. The regularization technique can be done using Matlab software with a package called IR Tools (Gazzola et al., 2018). In this research, inversion techniques are used in the form of Least Square and Iteratively Reweighted Norm (IRN). This study aims to analyze the tomographic inversion code with the regularization technique using the Iteratively Reweighted Norm (IRN). In this study, two types of data were used, synthetic data and real data. Based on the results, it shows that using the Least Square method has better where the resulting checkerboard is more than adequate to reconstruct the initial velocity and amplitude model, which is described more clearly and strongly. Based on the results of the real data inversion, it shows that there is a low Vp anomaly (red) in the lower area of Mt. Merapi and Mt Merbabu. Low seismic velocity anomalies are associated with magmatic zones, partial melting, or magma reservoirs. However, there is also a high seismic velocity anomaly (blue) associated with the subduction zone of the Indo-Australian Plate to the Eurasian Plate.

RELEVANCE OF RECEIVER FUNCTION TECHNIQUE IN SUBDUCTION ZONE (AVACHA BAY)

This paper elaborates on specific aspects of P- and S-receiver functions. The functions that are researched in this paper were calculated using waveforms obtained by three adjacent broadband seismic stations within the Avacha bay area in proximity to the subduction zone of the Pacific plate. The subduction zone in seismological context manifests as a layer of high seismic velocities, which are known to introduce a level of distortion to the receiver functions. To specify the level of this effect we parsed through two sets of P and S receiver functions in this research. The first set contains events that pass through and theoretically are affected by the subduction zone of the Pacific plate and the second set contains events that do not. The paper demonstrates that converted waves and their multiples formed at the boundaries of the high-velocity layer significantly affect P-receiver functions starting with 30-th second after the primary phase. However, no notable effects on S-receiver functions were revealed. Thus, we empirically confirm that [at least in the investigated area] local single-dimensional models are valid to be used for the inversion of the receiver functions to the depth of up to 200 km after which point the seismic noise produced by the subducting plate effectively limits the applicability of such models.

Early Result of Imaging 3D Seismic Velocity Structure In Central Java Using Double-Difference Tomography

Central Java is a part of the Sunda Arc and has relatively high seismicity due to the subduction zone between the Indo-Australian Plate and the Eurasian Plate. This study uses double-difference tomography to image the P and S wave 3D seismic velocity structures associated with tectonic patterns due to subduction zones. The data used comes from the earthquake catalogs of BMKG, BPPTKG, DOMERAPI, and MERAMEX, with a recording period from May 2004 to December 2020. The number of earthquakes that successfully relocated was 1930 from 1937 earthquakes recorded by 285 recording stations. The results of hypocentre relocation show that seismic activity in Central Java is relatively high and originates from geological structures, including subduction zones, back-thrust, and the Opak Fault. The tomogram results of the P and S wave velocity models support each other. The high seismic velocity anomaly is associated with the subduction zone of the Indo-Australian Plate to the Eurasian Plate with a maximum resolution of up to 100 km depth. Low seismic velocity anomalies are found at the Merapi-Lawu Anomaly (MLA), the Modern Volcanic Arc (MVA), the Sumbing-Sundoro-Dieng volcanic complex, the Banyumas-Cilacap sedimentary basin, and the Kendeng Basin. A low-velocity anomaly is found at a depth of about 40-50 km and a depth of 100 km, which is associated with the process of slab dehydration and partial melting. The inversion results show the impact of the Indo-Australian Plate subduction and the Eurasian Plate on volcanic activity, seismicity, and the geological structures developed in Central Java.

High seismic velocity structures control moderate to strong induced earthquake behaviors by shale gas development

Moderate to strong earthquakes have been induced worldwide by shale gas development, however, it is still unclear what factors control their behaviors. Here we use local seismic networks to reliably determine the source attributes of dozens of M > 3 earthquakes and obtain a high-resolution shear-wave velocity model using ambient noise tomography. These earthquakes are found to occur close to the target shale formations in depth and along high seismic velocity boundaries. The magnitudes and co-seismic slip distributions of the 2018 Xingwen {M}_{{{{{{rm{L}}}}}}}5.7 and 2019 Gongxian {M}_{{{{{{rm{L}}}}}}}5.3 earthquakes are further determined jointly by seismic waveforms and InSAR data, and the co-seismic slips of these two earthquakes correlate with high seismic velocity zones along the fault planes. Thus, the distribution of high velocity zones near the target shale formations, together with the stress state modulated by hydraulic fracturing controls induced earthquake behaviors and is critical for understanding the seismic potentials of hydraulic fracturing.

Quantifying amplitude-variation-with-offset uncertainties related to calcite-cemented beds using a Monte Carlo simulation

Calcite cement often occurs locally, forming thin layers of calcite-cemented sandstone characterized by high seismic velocities and densities. Because of their strong impedance contrast with the surrounding rock, calcite-cemented intervals produce detectable seismic reflection signals that may interfere with target reflections at the top of a reservoir. In this case, the amplitude-variation-with-offset (AVO) of the effective seismic signature will be altered and may even create a false hydrocarbon indication. From the Monte Carlo simulation, we find that the presence of thin calcite-cemented beds increases the uncertainty of the Bayesian pore-fluid classification based on the AVO attributes intercept and gradient. In the case example of a North Sea turbiditic oil and gas field, the probability of a false-positive hydrocarbon indication increases from 3%–5% to 18%–21% assuming an equal probability of the occurrence of brine, oil, and gas. The results confirm that calcite-cemented beds can create a pitfall in AVO analysis. Realistic estimates of the AVO uncertainty are crucial for the risk assessment of well placement decisions.

Structure and evolution of the Australian plate and underlying upper mantle from waveform tomography with massive data sets

SUMMARY We present a new S-wave velocity tomographic model of the upper mantle beneath the Australian Plate and its boundaries that we call Aus22. It includes azimuthal anisotropy and was constrained by waveforms from 0.9 million vertical-component seismograms, with the densest data sampling in the hemisphere centred on the Australian continent, using all available data covering this hemisphere. Waveform inversion extracted structural information from surface waves, S- and multiple S-waves and constrained S- and P-wave speeds and S-wave azimuthal anisotropy of the crust and upper mantle, down to the 660-km discontinuity. The model was validated by resolution tests and, for particular locations in Australia with notable differences from previous models, independent inter-station measurements of surface-wave phase velocities. Aus22 can be used to constrain the structure and evolution of the Australian Plate and its boundaries in fine detail at the regional scale. Thick, high-velocity (and, by inference, cold) cratonic lithosphere occupies nearly all of western and central Australia but shows substantial lateral heterogeneity. It extends up to the northern edge of the plate, where it collides with island arcs, without subducting. Diamondiferous kimberlites and lamproite deposits are underlain by cratonic lithosphere, except for the most recent diamondiferous lamproites in the King Leopold Orogen. The rugged eastern boundary of the cratonic lithosphere resolved by the model provides a lithospheric definition of the Tasman Line. Just east of the Tasman Line, an area of intermediate-thick lithosphere is observed in the southern part of the continent. The eastern part of Australia is underlain by thin, warm lithosphere, evidenced by low seismic velocities. All the sites of Cenozoic intraplate volcanism in eastern Australia are located on thin lithosphere. A low-velocity anomaly is present in the mantle transition zone (410–660 km depths) beneath the Lord Howe and Tasmanid hotspots, indicative of anomalously high temperature and consistent with a deep mantle upwelling feeding these hotspots and, possibly, also the East Australia hotspot. High seismic velocities at 200–410 km depth below New Guinea indicate the presence of slab fragments, probably linked to the subduction of the Australian Plate. High seismic velocities are observed in the transition zone below northeast Australia and indicate the presence of subducted lithospheric fragments trapped in the transition zone, possibly parts of the former northern continental margin of Australia.

Analysis on Prediction Methods for Formation Pore Pressure under Multiple Pressure Formation Mechanisms

There is abnormal fluid pressure widely distributed in Yinggehai-Qiongdongnan Basin, with abnormal zones extending from different depths, and the formation causes of abnormal pressure here are complicated. It is one of the three HTHP offshore regions in the world at present. Hence, drilling in this region is very difficult. It is discovered in drilling practices in recent years that, due to the multiple formation mechanisms of abnormal formation pressure in this region, the conventional pressure prediction model based on under-compaction theory is insufficient to explain the abnormal high pressure suddenly occurring in the middle shallow layer in diapiric structure of Yinggehai-Qiongdongnan Basin, which causes very large deviation of pore pressure prediction before drilling. Based on the basic principle of improved Fillippone method and with the three-dimensional high precision seismic velocity field of geologic modeling inversion, the authors build a three dimensional pore pressure model of target block, extract the pressure slice of horizon and across the target well, analyze the other source pressure transmission possibly caused by formation connectivity, and import them into the mathematical model of pore pressure prediction, so as to attain the purpose of improving the precision of pore pressure prediction. This method has been well applied in pre-drilling pressure prediction for the HTHP block in the South China Sea, reaching a prediction precisi on above 95%.

Pore morphology effect on elastic and fluid flow properties in Bakken formation using rock physics modeling

Unconventional geo-resources are critical due to their important contributions to energy production. In this energy transition and sustainability era, there is an increased focus on CO2-enhanced oil recovery (CO2-EOR) and geological CO2 storage (GCS) in unconventional hydrocarbon reservoirs, and the extraction of hot fluid for energy through enhanced geothermal systems. However, these energy solutions can only be achieved through efficient stimulation to develop a complex fracture network and pore structure in the host rocks to extract heat and hydrocarbon, or for CO2 storage. Using Bakken formation well data and rock physics models, this study aimed to identify the post-depositional effect of pore structure on seismic velocity, elastic moduli, and formation fluid; and further predict the best lithofacies interval for well landing, and the implications for fluid (gas, oil, and water) recovery in naturally- and often systematically-fractured geosystems. The KT and DEM models' predictions show distinct formation intervals exhibiting needle-like pores and having higher seismic velocities (Vp and Vs) and elastic moduli (K and µ), relative to other formation intervals that exhibit moldic pores. At the same fluid concentration, the needle-like pores (small aspect ratios) have a higher impact on elastic moduli, Vp, and Vs than on the moldic spherical pores with all other parameters held constant. Vp is affected more than Vs by the properties of the saturating fluid (gas, oil, or water) with Vp being greater in Bakken formation when it is water-saturated than when it is gas-saturated. Vs exhibit the reverse behavior, with Vs greater in the gas-saturated case than in the water-saturated case. Further, analyses suggest that the middle Bakken formation will have a higher susceptibility to fracturing and faulting, and hence will achieve greater fluid (oil and water) recovery. Our findings in this study provide insights that are relevant for fluid production and geo-storage in unconventional reservoirs.Article highlightsIntegrated well log data and rock physics models.Investigated the effect of changes in pore structure on elastic properties and fluid flow in shale.Increase in porosity causes a reduction in elastic moduli and seismic velocities.Vp is more affected by pore geometry than Vs depending on density and properties of saturating fluid.Lithofacies with needle−like pores are more susceptible to fracturing than lithofacies with intragranular pores.

Slab remnants beneath the Myanmar terrane evidencing double subduction of the Neo-Tethyan Ocean.

Closure of the Neo-Tethyan Ocean is one of the most significant tectonic events of the Cenozoic, forming the longest continental collision belt on Earth and influencing global climate and biodiversity. However, whether late Mesozoic subduction of the Neo-Tethyan Ocean occurred along one single or a double subduction system remains controversial. Here, upper mantle imaging from seismic tomography and waveform modeling in the Myanmar region reveals two prominent, parallel, slab-like structures with high seismic velocities that trend to the north-south and dip to the east. The western high-velocity zone has been observed previously and represents the modern subducting slab. The eastern zone has not been previously reported and exhibits high-velocity anomalies of 1.0 to 2.5% to a depth of ~300 km. This zone likely represents a remnant of another Neo-Tethyan oceanic slab that subducted ~40 million years ago. Double subduction of the Neo-Tethyan Ocean during the late Mesozoic to early Cenozoic requires reevaluation of previous tectonic models.

Regolith and host rock influences on CO2 leakage: Active source seismic profiling across the Little Grand Wash fault, Utah

Understanding carbon dioxide (CO2) reservoir to surface migration is crucial to successful carbon capture and sequestration approaches; especially fault/reservoir interactions under injection pressure. Through seismic imaging, we explore regolith and shallow stratigraphy across the Little Grand Wash fault. The presence of natural CO2 seeps, travertine and tufa deposits confirm modern and ancient fault-controlled CO2 leakage. We consider this an analogue for a long-failed sequestration site. We estimate bulk porosity and fracture density for host rock, regolith, and fault zone from petrophysical relationships. When combined with existing geochemical and geological data, we characterize a 60 m wide damage zone that represents the primary surface delivery channel for CO2 originating from reservoir depths. Within this damage zone, low seismic velocities suggest sediments have formed through host rock chemical dissolution or mechanical weathering. In contrast, velocities within the adjacent host rock are consistent with low fracture density clastic rocks. We measure anomalously high seismic velocities within the fault zone along one profile that best represents a sealed (cemented/plugged) low permeability, relic flow channel. This suggests that shallow fault zone permeability varies along strike. While regional stress changes may account for decadal- to millennial-scale changes in CO2 pathways, we speculate that the total fluid pressure has locally reduced the fault's minimum horizontal effective stress; thereby producing both low- and high-permeability fault segments that either block or promote fluid migration. Studying CO2 migration in this system can inform potential risks to future sequestration projects and guide monitoring efforts.

Seismic evidence for lithospheric boudinage and its implications for continental rifting

Abstract The continental rifting that precedes the breakup of a continent and the formation of a new ocean basin is one of the key processes of plate tectonics. Although often viewed as a two-dimensional process, rifted margins exhibit significant variations along strike. We document along-strike variations developed during the ca. 200–160 Ma continental rifting that formed the margins of the Gulf of Mexico ocean basin. Rayleigh-wave ambient noise tomography reveals a zone of high and low seismic velocity resembling large scale geologic boudins in the mantle lithosphere of the northwestern Gulf of Mexico margin. These features become progressively less prominent eastward following the transition from a magma-poor to a magma-rich passive margin. We infer that mantle refertilization and thickness of the pre-rift lithosphere control deformation style and the along-strike variations in continental rifting. Our results also suggest that deformation during rifting produces long-lived features that persist long after breakup and, therefore, can be used to study rifted margins globally.

Dispersion of Rayleigh Surface Waves and Electrical Resistivities Utilized to Invert Near Surface Structural Heterogeneities

The single-station Rayleigh surface wave group velocities and electrical resistivities are two data sets that we cooperatively employ to image the near surface (< 40-m) anomaly structures. We numerically simulate the corresponding field measurements where the anomaly structures are assumed to have two-dimensional (2D) variations. The surface waves are represented by fundamental mode dispersion curves, and the electrical resistivities are assumed to be measured by using direct currents. We consider two types of anomaly structures, i.e., cavity and ore body. These two heterogeneities are easily distinguished from the surrounding geomaterial by their distinct physical properties. The cavity is characterized by low seismic velocity and high electrical resistivity, while the ore body is characterized by high seismic velocity and low electrical resistivity. The Rayleigh surface wave data is assumed to be collected throughout the classical common-shot gather. Multiple electrodes, multiple core cables, and multiple arrays are assumed to be used in the electrical survey. Both surface wave group velocities and electrical resistivities are shown to properly invert the anomalous structures in the subsurface. The surface wave group velocities have good horizontal resolution, while the corresponding vertical resolution is somewhat lower. The electrical resistivities have good resolution for shallow structures, but the resolution becomes somewhat reduced with increasing depth. Doi: 10.28991/HEF-2022-03-01-01 Full Text: PDF

Shallow crustal velocity structures revealed by active source tomography and fault activities of the Mianning-Xichang segment of the Anninghe fault zone, SW China

The Anninghe fault is a large left-lateral strike-slip fault in southwestern China. It has controlled deposition and magmatic activities since the Proterozoic, and seismic activity occurs frequently. The Mianning−Xichang segment of the Anninghe fault is a seismic gap that has been locked by high stress. Many studies suggest that this segment has great potential for large earthquakes (magnitude &gt;7). We obtained three vertical velocity profiles of the Anninghe fault (between Mianning and Xichang) based on the inversion of P-wave first arrival times. The travel time data were picked from seismograms generated by methane gaseous sources and recorded by three linearly distributed across-fault dense arrays. The inversion results show that the P-wave velocity structures at depths of 0−2 km corresponds well with the local lithology. The Quaternary sediments have low seismic velocities, whereas the igneous rocks, metamorphic rocks, and bedrock have high seismic velocities. We then further discuss the fault activities of the two fault branches of the Anninghe fault in the study region based on small earthquakes (magnitudes between <inline-formula><tex-math id="M1">\begin{document}$ {M}_{L} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="RA282-shaoxihui_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="RA282-shaoxihui_M1.png"/></alternatives></inline-formula> 0.5 and <inline-formula><tex-math id="M3">\begin{document}$ {M}_{L} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="RA282-shaoxihui_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="RA282-shaoxihui_M3.png"/></alternatives></inline-formula> 2.5) detected by the Xichang array. The eastern fault branch is more active than the western branch and that the fault activities in the eastern branch are different in the northern and southern segments at the border of 28°21′N. The high-resolution models obtained are essential for future earthquake rupture simulations and hazard assessments of the Anninghe fault zone. Future studies of velocity models at greater depths may further explain the complex fault activities in the study region.