Teleseismic tomography: Equation one is wrong

ABSTRACT Seismic tomography methods that use waves originating outside the volume being studied are subject to bias caused by unknown structure outside this volume. The bias is of the same mathematical order and similar magnitude as the local-structure effects being studied; failure to account for it can significantly corrupt derived structural models. This bias can be eliminated by adding to the inverse problem three unknown parameters specifying the direction and time for each incident wave, a procedure analogous to solving for event locations in local-earthquake and whole-mantle tomography. The forward problem is particularly simple: The first-order change in the arrival time at an observation point resulting from a perturbation to the incident-wave direction and time equals the change in the time of the perturbed incident wave at the point where the unperturbed ray entered the study volume. This consequence of Fermat’s principle apparently has not previously been recognized. Published teleseismic tomography models probably contain significant artifacts and need to be recomputed using the more complete theory.

Voluminous Storage and Rapid Magma Ascent Beneath La Palma Revealed by Seismic Tomography

Seismic tomography provides a window into magmatic plumbing systems; however, obtaining sufficient data for ‘real-time’ imaging is challenging. Until now, syn-eruptive tomography has not been successfully demonstrated. For the first time, we obtained high-resolution images of Earth's interior during an ongoing volcanic eruption. We used data from 11,349 earthquakes, most of which during La Palma eruption (19 September-13 December, 2021), to perform travel-time seismic tomography. We present high-precision earthquake relocations and 3D distributions of P and S-wave velocities highlighting the geometry of magma sources. We identified three distinct structures: (1) a shallow localised region (< 3 km) of hydrothermal alteration; (2) spatially extensive, consolidated, oceanic crust extending to ~10 km depth and; (3) a large (> 400 km3) sub-crustal magma-filled rock volume intrusion extending from ~7 to 25 km depth. Our results suggest that this large magma reservoir feeds the La Palma eruption continuously for almost three months. Prior to eruption onset, magma ascended from ~10 km depth to the surface in < 7 days. In the upper 3 km, melt migration is along the western contact between consolidated oceanic crust and altered hydrothermal material. Similar structural weaknesses along the eastern contact could potentially cause new eruptive centres in the future.

Seismic Evidence of Pop-up Tectonics Beneath the Shillong Plateau Area of Northeast India

Our detailed analysis of high-quality arrival time data recorded by the local seismographic network using three-dimensional seismic tomography of the Shillong Plateau region using high-quality arrival times of the body wave phases recorded at a dense temporary seismic network. This technique is used to understand the heterogeneities of the crust and its implications for pop-up tectonics characterizing evaluation the of the Shillong Plateau. We investigated an area covering ~150 ×100 km2 that revealed seismicity to be confined in a depth range ≤ 60 km. High - velocity anomalies in the upper crust appear to be responsible for intense small to moderate seismic activity in the region. Crustal seismic velocities inferred from 3-D seismic tomography showed significant lateral heterogeneities beneath the lithosphere of the Shillong Plateau. High-velocity anomalies in the uppermost crust, interpreted as the Shillong Plateau act as a geometric asperity where interseismic strain may accumulate. Low-velocity anomalies in the lower crust probably play a major role to accommodating the stresses generated due to plate separation, culminating in future sources of great earthquakes. The geological faults are well imaged in the cross-sections and support the concept of Pop-up tectonics beneath the Shillong of NE India.

Seismic Tomography of Kamchatkan Volcanoes

—The Kamchatka Peninsula is one of the most tectonically active regions in the world, where intensive and diverse modern volcanic activity takes place. In the recent decade, substantial progress in the investigation of deep structures beneath Kamchatka has been achieved owing to numerous tomography studies based on seismological data provided by permanent stations and temporary networks deployed in some key areas. The goal of this review is summarizing and systematizing dozens of separate multiscale geophysical studies in Kamchatka and constructing an integral model of volcano-feeding systems. An important part of this review contains the description of results of various seismic studies related to the Klyuchevskoy group volcanoes, which can now be considered one of the best studied volcanic areas in the world. The results of the regional-scale seismic tomography reveal the existence of the Pacific slab window, which determines the particular activity of the Klyuchevskoy group volcanoes. Middle-scale tomography studies have found traces of an ascending hot mantle flow that passes through the slab window, reaches the bottom of the crust below Shiveluch Volcano, and then propagates laterally toward the Klyuchevskoy group. Seismic models of the entire crust in the area of the Klyuchevskoy group were used to identify different mechanisms of magmatic feeding of three most active volcanoes: Klyuchevskoy, Bezymianny, and Tolbachik. The data of local networks deployed on several volcanoes of Kamchatka were used to image the magma sources in the upper crust, which are directly responsible for the current eruption activity. The comparison of the results for the Kamchatka volcanoes with tomography models of several other volcanoes of the world allowed determining some common features and differences in feeding active magmatic systems.

Two-step Gravity Inversion Reveals Variable Architecture of African Cratons

The lithospheric build-up of the African continent is still to a large extent unexplored. In this contribution, we present a new Moho depth model to discuss the architecture of the three main African cratonic units, which are: West African Craton, Congo Craton, and Kalahari Craton. Our model is based on a two-step gravity inversion approach that allows variable density contrasts across the Moho depth. In the first step, the density contrasts are varied for all non-cratonic units, in the second step for the three cratons individually. The lateral extension of the tectonic units is defined by a regionalization map, which is calculated from a recent continental seismic tomography model. Our Moho depth is independently constrained by pointwise active seismics and receiver functions. Treating the constraints separately reveals a variable range of density contrasts and different trends in the estimated Moho depth for the three cratons. Some of the estimated density contrasts vary substantially, caused by sparse data coverage of the seismic constraints. With a density contrast of Δ ρ = 200 kg/m3 the Congo Craton features a cool and undisturbed lithosphere with smooth density contrasts across the Moho. The estimated Moho depth shows a bimodal pattern with average Moho depth of 39–40 km for the Kalahari and Congo Cratons and 33–34 km for the West African Craton. We link our estimated Moho depth with the cratonic extensions, imaged by seismic tomography, and with topographic patterns. The results indicate that cratonic lithosphere is not necessarily accompanied by thick crust. For the West African Craton, the estimated thin crust, i.e. shallow Moho, contrasts to thick lithosphere. This discrepancy remains enigmatic and requires further studies.

Seismic anisotropy of Opalinus Clay: tomographic investigations using the infrastructure of an underground rock laboratory (URL)

Seismic anisotropy and attenuation make claystone formations difficult to characterize. On the other hand, in many geotechnical environments, precise knowledge of structure and elastic properties of clay formations is needed. In crystalline and rock salt underground structures, high-resolution seismic tomography and reflection imaging have proven a useful tool for structural and mechanical characterization at the scale of underground infrastructure (several deca- to hundreds of meters). This study investigates the applicability of seismic tomography for the characterization of claystone formations from an underground rock laboratory under challenging on-site conditions including anisotropy, strong attenuation and restricted acquisition geometry. The seismic tomographic survey was part of a pilot experiment in the Opalinus Clay of the Mont Terri Rock Laboratory, using 3-component geophones and rock anchors, which are installed 2 m within the rock on two levels, thus suppressing effects caused by the excavation damage zone. As a source, a pneumatic impact source was used. The survey covers two different facies types (shaly and carbonate-rich sandy), for which the elliptical anisotropy is calculated for direct ray paths by fitting an ellipse to the separated data for each facies. The tomographic inversion was done with a code providing a good grid control and enabling to take the seismic anisotropy into account. A-priori anisotropy can be attributed to the grid points, taking various facies types or other heterogeneities into account. Tomographic results, compared to computations using an isotropic velocity model, show that results are significantly enhanced by considering the anisotropy and demonstrate the ability of the approach to characterize heterogeneities of geological structures between the galleries of the rock laboratory.

Stability Assessment of a Bedding Rock Slope Using Q-slope and Seismic Tomography: A Case Study in the Ecuadorian Amazon

Roads are generally affected by slope failures, and these failures can increase when there are weathered materials and high rainfall. These circumstances occur in the sub-Andean zone of Ecuador. This is the region where the study area is located. The stability of a stratified rock slope, which is affecting a section of highway E45, was evaluated. The study slope is exposed to the road, but the upper part is covered by a soil-type material and dense vegetation that makes it challenging to study. We applied the Q-slope method and seismic tomography; these methods used together worked well, because they allowed to correlate and infer information about the quality of the rock mass, even in a fast and economical way. We also performed core drilling with core recovery in the crown of the slope and SPT test. The slope presented two well-differentiated zones; therefore, Q-slope values were calculated for each of these zones. The results show that the slope is unstable. The application of seismic tomography as an input parameter for calculating Q-slope was important because it allowed evaluating the stability where it is impossible to collect geomechanical information, correlate information taken at the foot of the slope, and define the depth of the bedrock.