Seismic Surface Waves Research Articles

Trans-dimensional inversion of multimode seismic surface wave data from a trenched distributed acoustic sensing survey

SUMMARYSeismic data acquired from surface-deployed distributed acoustic sensing (DAS) fibre are broad band and typically dense spatially sampled. Corresponding to these features, compared with geophone data, the low-frequency components in DAS data show higher signal-to-noise ratio and multimode dispersion curves by broad-band DAS data exhibit a higher resolution, which increases the investigation depth of near-surface structures and enhances identification and picking of dispersion curves, respectively. Therefore, DAS data are ideal for the estimates of reliable and highly resolved near-surface velocity profiles. As surface-wave dispersion inversion (SWD) is a natural scheme for near-surface investigation, in this study we have formulated a DAS-SWD inversion in which multiple SWD modes are extracted from the DAS data, and are used as input to a trans-dimensional (TD) inversion procedure, in which the number of subsurface layers is treated as an unknown. Vibroseis data with a minimum frequency of 1 Hz were sensed along a horizontal surface trench as part of a baseline seismic survey carried out by the University of Calgary at the Containment and Monitoring Institute Field Research Station in Newell County, Alberta, Canada. These surface DAS data readily permit the picking of multimode dispersion curves, which are observed to enhance velocity profile resolution in both shallow and deep regions of the near-surface simultaneously, with the TD algorithm adapting the model to reflect this improved resolution. To avoid collecting abnormal model samples with thin-interleaved high- and low-velocity layers based on the known geological information of the field site, we employed constraints that preclude the structures that have velocity drops over 100 m s−1 along depth. Data errors are estimated via a non-parametric iterative process in terms of covariance matrices that include off-diagonal elements. Synthetic examples show that SWD with higher-order modes provides additional constraints on the structure and accurate noise estimation. Inversion of the field data resulted in high-resolution estimates of shear wave velocity as a function of depth throughout the top 120 m of the subsurface. The inferred structure is consistent with existing estimates of the regional lithology but resolves additional layers between 1- and 50-m depth.

A Joint Seismic and Space‐Based Investigation of the 2016 Lamplugh Glacier and 2017 Wrangell Mountains (Alaska) Landslides

AbstractLandslides commonly occur in areas with steep topography and abundant precipitation and pose a significant hazard to local communities. Some of the largest known landslides occur in Alaska, including several that caused local tsunamis. Many landslides may have gone undetected in remote areas due to lack of observations. Here, we develop a semiautomated workflow using both seismic and geodetic observations to detect, locate, validate, and characterize landslides in Alaska. Seismic observations have shown promise in continuously monitoring landslide occurrence, while remote sensing techniques are well suited for verification and high‐resolution imaging of landslides. We validate our procedure using the 28 June 2016, Lamplugh Glacier landslide. We also present observations of a previously unknown landslide occurred on 22 September 2017 in the Wrangell Mountains region. The Wrangell Mountains landslide generated a coherent surface wavefield recorded across Alaska and the contiguous United States. We used Sentinel‐1 Synthetic Aperture Radar and Sentinel‐2 optical imagery to map the respective mass deposit. To investigate the landslide dynamics, we inverted regional seismic surface wave data for a centroid single force failure model. Our model suggests that the Wrangell Mountains landslide lasted for about 140 s and had two subevents involving at least five distinct stages. We estimate that the landslide had displaced 3.1–13.4 million tons of rocks over a distance of ∼2 km. Our results suggest that combining seismic and geodetic observations can vastly improve the detection and characterization of landslides in remote areas in Alaska and elsewhere, providing new insights into the landslide dynamics.

Excitation of Back‐Arc Tsunamis From Megathrust Ruptures: Theory and Application to the Sea of Japan

AbstractLarge megathrust ruptures can create notable tsunamis in tectonic back‐arc basins as was documented during the 2011 Tohoku earthquake in the Sea of Japan. We present a physical analysis of the excitation of back‐arc tsunamis by extending the fore‐arc deformation field from the earthquake centroid into the back‐arc basin and identify fault dip as the main geometrical contributor to the propagation of these events. As such, our theoretical model along with a large number of numerical simulations reveal that the dominant period of back‐arc tsunamis is different from that of the fore‐arc waves and thus they are a new class of tsunamis. Through numerical simulations and analysis of data from the 2011 Tohoku event, we show that a combination of near‐ to intermediate‐field horizontal and vertical deformation as well as transient surface waves is necessary to reconstruct the back‐arc propagation. We find that while seismic surface waves can affect coastal tsunami amplitudes in the back‐arc, their effect comes second to that of the horizontal component of deformation as manifested via bathymetric gradient. We then simulate back‐arc tsunamis and the hazard in the Sea of Japan from several potential future earthquake scenarios in the Japan Trench and Nankai Trough. Our results show that the coseismic excitation of back‐arc tsunamis can result in considerable waves close to 1 m in the Sea of Japan, near the Niigata Prefecture from megathrust earthquakes.

Beamforming of Rayleigh and Love Waves in the Course of Atlantic Cyclones

AbstractThe main sources of the ambient seismic wavefield in the microseismic frequency band (peaking in the ∼0.04–0.5 Hz range) are earth's oceans, namely the wind‐driven surface gravity waves (SGW) that couple oscillations into the seafloor and the upper crust underneath. Cyclones (e.g., hurricanes, typhoons) and other atmospheric storms are efficient generators of high ocean waves that in turn generate distinct microseismic signatures. In this study, we perform a polarization (i.e., three‐component) beamforming analysis of microseismic (0.05–0.16 Hz) retrograde Rayleigh and Love waves during major Atlantic hurricanes using a virtual array of seismometers in Eastern Canada. Oceanic hindcasts and meteorological data are used for comparison. No continuous generation of microseism along the hurricane track is observed but rather an intermittent signal generation. Both seismic surface wave types show clear cyclone‐related microseismic signatures that are consistent with a colocated generation at near‐coastal or shallow regions, however the Love wavefield is comparatively less coherent. We identify two different kinds of intermittent signals: (a) azimuthally progressive signals that originate with a nearly constant spatial lag pointing toward the trail of the hurricanes and (b) azimuthally steady signals remaining nearly constant in direction of arrival even days after the hurricane significantly changed its azimuth. This high complexity highlights the need for further studies to unravel the interplay between site‐dependent geophysical parameters, SGW forcing at depth and microseismic wavefield radiation and propagation, as well as the potential use of cyclone microseisms as passive natural sources.

Seismological reference earth model in South China (SREM-SC): Upper mantle

This work is the mantle component of constructing the Seismological Reference Earth Model in South China (SREM-SC). Although there has been a wide range of research for imaging the upper mantle structures beneath South China, most of them focus on the large-scale features of the upper mantle, and the depth resolution is insufficient for existing surface wave tomography models to distinguish anomalies below 200 km. This study aims to develop a 3-D upper mantle Seismological Reference Earth Model in South China based on the prior tomography models. The shear wave velocity model comes from the analysis of several seismic surface wave tomography, supplemented by body wave tomography and the P-wave velocity model is constructed by the conversion from S-wave velocity. The radial anisotropy model is calculated from the SV-wave and SH-wave velocity. The Density model of the upper mantle is derived using the empirical relationship linking the density to the shear-wave velocity. The model is grid with 0.5° × 0.5° in latitude and longitude and 5 km interval in depth from 60 to 300 km. The mantle component of Seismological Reference Earth Model in South China is expected to provide a good representation of the upper mantle structures for further detailed studies. The mantle component of Seismological Reference Earth Model in South China provides new insights into upper mantle structures that should be meaningful to reveal the dynamic mechanism and tectonic evolution of South China.

Reduction of earthquake amplification by seismic metamaterials made of circular positioned concrete piles

Seismic surface waves with low frequencies results in higher effects. The aim of the utilization of seismic metamaterials is to prevent or minimize the effects of this waves particularly wide frequency ranges. This study presents the investigation of the reducing effects of seismic metamaterials composed of circular array concrete piles on surface waves. The investigation of circular array concrete piles has been carried out numerically between 5-15 Hz. The effectiveness of the periodic structure has been observed numerically for fields including metamaterials by comparing empty field. The observations have been depicted according to transmission losses for two cases by using finite element method (FEM) simulations. The wave propagation also presented at the related frequencies where the transmission losses seen. Effective dimensions of radius and length of the structures have been obtained by using parametric approach. It has been proved numerically that the suggested concrete piles reduces the hazardous effects of surface waves.

Eruption sequence of the 2022 Hunga Tonga-Hunga Ha'apai explosion from back-projection of teleseismic P waves

The explosive eruption occurred at Hunga Tonga-Hunga Ha'apai on January 15, 2022, generating atmospheric waves, meteotsunamis, tsunamis, and seismic waves recorded worldwide. Energetic signals of seismic surface waves indicate multiple explosive events, but the detailed eruption episodes are hard to unravel from dispersive long-period surface waves. In this study, we performed the back-projection (BP) analysis using the teleseismic P-waves to clarify the sequence of multiple explosive eruptions. The BP results successfully detected the locations and timing of multiple explosions even though P-waves from each eruption were overlapped and masked by other seismic signals and noises. The results suggest that the series of explosive eruptions can be divided into two parts of eruption sequences. The first sequence started at 04:02 UTC, escalated into major explosions at 04:15 UTC and 200 - 300 s afterward, and lasted at least until 04:35 UTC. The second sequence began about 4 hours later (08:26 UTC) and continued for about 6 - 7 minutes, including a massive eruption at 08:31 UTC. The estimated origin times of each eruption episode determined in this study are consistent with successive arrivals of long-period surface wave trains at frequencies higher than 10 mHz. The time intervals of explosive eruptions in the first sequence suggested that the significant explosive eruptions intermittently occurred at around 270 - 280 s intervals. This interval is nearly equivalent to the eigenfrequencies of the atmospheric resonant oscillation with the Earth at 3.7 mHz. Since the observed amplitude spectra of this eruption exhibit a conspicuous spectral peak at 3.7 mHz, our BP results indicate that the eruption cycle may be synchronized with the eigen-period of the atmosphere-lithosphere coupled mode.

Rayleigh and love surface waves with regard to seismic stress state of earth bed

This article presents the results of studies on the propagation of surface seismic waves, which directly affect the stability and strength of a high embankment of the subgrade. The subgrade is the foundation of the railway track. In an elastic half-space with a layer in which surface waves propagate, the conditions for the existence and propagation of a harmonic Rayleigh wave are considered. The equations of motion of a medium in a layer and half-space and their solutions in the form of plane waves propagating in phase along the contact boundary, some general laws of Rayleigh wave propagation are given. The accepted theories and calculation methods' main provisions are proposed for designing a durable and reliable subgrade and considering its seismic stress state.

Seismic metamaterial surface for broadband Rayleigh waves attenuation

Elastic metamaterials (EMMs) have been widely studied owing to their advantages in controlling the propagation of elastic waves. In the past decade, considerable efforts have been made to attenuate Rayleigh waves using EMMs. However, the complex nonlinear behaviour of soil renders the realisation of existing EMMs challenging in practical engineering. To overcome this limitation, we present a new seismic metamaterial surface (SMMS) to isolate Rayleigh waves over a broad frequency range of 3.2–19.2 Hz. First, we discuss the propagation velocity of Rayleigh waves in EMMs and determine a reasonable design range for the effective dynamic material properties of the SMMS for seismic surface wave attenuation. Then, we construct a unit cell and demonstrate that these properties, both in band gaps and negative bands, lie within this specified design range. The negative bands are initially introduced into Rayleigh wave mitigation. Finally, a new type of SMMS is presented and validated using numerical results and scaled experiments. The results show that the vertical deformation on the surface can be decreased by more than 96 %. The findings reported here open new avenues for protecting engineering structures from low-frequency seismic vibrations.

Soil Parameters of Creep Movement Şirinköy-Denizli Example

This study, it is aimed to determine how soil parameters affect mass movement. The study area was carried out in the Şirinköy District of Merkezefendi District of Denizli. As a basis for the prevention of mass movements; Geophysical methods are of great importance in terms of determining and monitoring the properties of the sliding forces by determining the ground conditions in the region. For this reason, in this study, the characteristics of the mass movement were determined by determining the soil properties of the study area by using the seismic multi-channel surface wave (MASW) method, which is one of the geophysical methods. With the MASW method, it was determined that the seismic velocities of the area were lower than 910 m/s. The boundaries and units of the geological formations of the ground in the region have been determined. Limestone and fill surface slip surfaces in the area were observed in MASW sections. By evaluating all these studies together, the mass movement in the region was determined as slow flow (creep). Blocky limestone detected in the study area (it is the unit that relatively forms the slip surface. For this reason, developments such as groundwater, growth of roots of plants and trees, wetting, drying and freezing of the ground with the effect of precipitation may cause an acceleration of the movement speed should be followed.

Uncertainty quantification and global sensitivity analysis of seismic metabarriers

Seismic metabarriers consist of an array of locally resonant elements (i.e., mechanical resonators) installed over the soil surface, whose design is rationally engineered to reduce ground-induced vibrations and shield vulnerable structures against seismic surface waves. Successful design and implementation of seismic metabarriers require a comprehensive knowledge and characterization of the role played by the model parameters (and their associated uncertainty) governing soil-barrier dynamic interaction. In this context, sensitivity analysis techniques allow assessing the response of a given model through the quantification of the influence and action of model inputs (and model input uncertainties) concerning a target model output. This study relies on global sensitivity analysis techniques to investigate the influence that the uncertainty associated with three key mechanical parameters of a metabarrier (i.e., soil density, soil shear modulus, and mass of mechanical resonators) has on its seismic isolation performance. The latter is measured in terms of transmission coefficient (TC). We do so by employing a two-dimensional wave finite element model developed under the plane-strain conditions to evaluate the dispersion relation and transmission coefficient of a metabarrier interacting with Rayleigh waves in the low-frequency regime (i.e., frequencies between 2 Hz and 7 Hz). Our results suggest that the shear modulus is the uncertain parameter with the most significant influence on the transmission coefficient of the metabarrier across the entire frequency range of interest. Otherwise, the resonator mass plays a substantial role in the frequency range close to the metabarrier resonant frequency.

Seismic source mapping by surface wave time reversal: application to the great 2004 Sumatra earthquake

SUMMARY Different approaches to map seismic rupture in space and time often lead to incoherent results for the same event. Building on earlier work by our team, we ‘time-reverse’ and ‘backpropagate’ seismic surface wave recordings to study the focusing of the time-reversed field at the seismic source. Currently used source-imaging methods relying on seismic recordings neglect the information carried by surface waves, and mostly focus on the P-wave arrival alone. Our new method combines seismic time reversal approach with a surface wave ray-tracing algorithm based on a generalized spherical-harmonic parametrization of surface wave phase velocity, accounting for azimuthal anisotropy. It is applied to surface wave signal filtered within narrow-frequency bands, so that the inherently 3-D problem of simulating surface wave propagation is separated into a suite of 2-D problems, each of relatively limited computational cost. We validate our method through a number of synthetic tests, then apply it to the great 2004 Sumatra–Andaman earthquake, characterized by the extremely large extent of the ruptured fault. Many studies have estimated its rupture characteristics from seismological data (e.g. Lomax, Ni et al., Guilbert et al., Ishii et al., Krüger & Ohrnberger, Jaffe et al.) and geodetic data (e.g. Banerjee et al., Catherine et al., Vigny et al., Hashimoto et al., Bletery et al.). Applying our technique to recordings from only 89 stations of the Global Seismographic Network (GSN) and bandpass filtering the corresponding surface wave signal around 80-to-120, 50-to-110 and 40-to-90 s, we reproduce the findings of earlier studies, including in particular the northward direction of rupture propagation, its approximate spatial extent and duration, and the locations of the areas where most energy appears to be released.

Artificially engineered metaconcrete with wide bandgap for seismic surface wave manipulation

The energy of ground-born vibration is transported mostly by surface waves. Therefore, engineered structures which absorb externally applied energy and improve structural performance against explosive dynamic loading are highly desired. In this paper, we demonstrate the application of designed metamaterial barriers with engineered metaconcrete for reflecting and redirecting surface Rayleigh waves to avoid disastrous consequences. The metabarrier is constructed by embedding metaconcrete cubes in a semi-infinite space. The local resonance type metaconcrete cell is composed of a heavy lead ball coated by a soft rubber layer and mortar. One significant limitation of metaconcrete is the narrow bandwidth. Here, we apply an array of metaconcretes where the radii of the lead ball and soft coating increase or decrease simultaneously. Via transmission ratio analysis, we show that the metaconcrete-based metabarrier is able to achieve broadband wave attenuation. More interestingly, the elastic wave trapping and surface wave mode conversion can be realized and controlled by the metabarrier. The mechanism of trapping and conversion is fully explained with an unconventional dispersion relation. A time history analysis based on several actual earthquake records obtained from public database is also presented. The result in this paper highlights new applications of metaconcretes and it can be applied to novel seismic shields for protecting civil infrastructures against seismic destruction.

Novel periodic pile barrier with low-frequency wide bandgap for Rayleigh waves

This paper aims to investigate the propagation of low-frequency seismic surface waves in periodic in-filled barriers from the viewpoint of bandgaps. Based on Bloch-Flouquet theory, four configurations of periodic pile barriers with the same filling rate but different cross sections are studied. The periodic boundary conditions are imposed to derive the characteristics equation. Simultaneously, the finite element method is used to calculate the numerical dispersion relation and vibration modes of four types of periodic pile barriers with various inclusions. The sound cone and the energy parameter method are introduced to identify the surface wave modes further. Additionally, the mitigation effectiveness of a finite number of periodic barriers is evaluated using a three-dimensional transmission model. The results show that the initial frequency and bandwidth of the surface bandgaps are sensitive to the shape of the cross-section and mechanical parameters of the inclusions. The surface wave bandgaps within 9.64 ∼ 13.92 Hz are observed for the periodic barrier composed of cross-like polyfoam sections. Moreover, the radiation modes formed by the leakage of some surface waves into the sound cone are expected to widen the attenuation zones of the periodic barriers. The numerical results in frequency and time domains further validate the attenuation effect of finite barrier in the bandgap and the attenuation widening phenomenon caused by the dissipation of the leak modes into the bulk. This study provides new insights into the design of periodic pile barriers. In the future, it is expected to mitigate the low-frequency ground motion by fully designing the section shape and filling materials of pile barriers.

Development and Application of a Portable Vibroseis Source for Acquisition and Analysis of Seismic Surface Wave Data

Development and Application of a Portable Vibroseis Source for Acquisition and Analysis of Seismic Surface Wave Data

Analysis of Seismic Refraction and Surface Wave Data for the Evaluation of Layers and Saturation of Solid Waste from a Landfill in Brasília, Brazil

The present work discusses the characterization of landfilled solid waste and saturated zones considering the response of P and S-wave velocities (Vp and Vs), Poisson ratio (υ), Young’s modulus (E) and shear modulus (G0), obtained from velocity models in an area located in the former Jockey Clube Controlled Landfill. The obtained Vp values ranged from 231 to 1,160 m/s, while Vs values range from 124 to 449 m/s. The calculated υ ranged from 0.11 to 0.4, while G0 and E ranged from 15 to 319 kPa and from 42 to 901 kPa, respectively. The values of G0 and E indicate that the landfilled material is poorly competent. The combined interpretation of Vp, Vs and elastic parameters allowed the definition of three main layers in the surveyed area and their respective distance from soil surface, defined as: 1) Civil construction residual material, of around 10 meters thick; 2) A solid waste layer, of around 18 meters thick, marked as a lower Vs and higher υ interval, possibly associated with saturated material; and 3) the estimated natural landfill terrain, below the depth of 28 meters, composed by the oxisol.

Investigation of Road Pavement Failures Using Active and Passive Surface Wave Methods in Parts of Calabar Municipality, Cross River State, Nigeria

The active and the passive techniques of surface wave were employed to investigate some major road pavement failures within the Calabar metropolis, Cross River State. The dispersion curves from MASW (active) and ReMi (passive) were inverted to give a 1D shear wave velocity model. The aim of this study was to comparatively investigate the major causes of the incessant road pavement failures especially few months after its construction and repairs within the study area using the shear wave velocity and blow count (N values). These roads are all located within the same geologic setting of the Calabar Coastal Plan Sand and showed evidence of road failures ranging from pot holes, alligator cracking, rutting and shear failure cracking etc. For each site, two sets of data were collected; the Passive and the Active seismic surface wave data. The survey was able to gather the following data from each location; shear wave velocity, primary wave velocity, N-values for both the active and the passive data which were both derived by the Seisimager surface wave software. The software was used to delineate low velocity layers within the depth of 4m – 6m at the failed portions of the survey sites exception of location 1 (the control site) which showed an ideal situation of increasing velocity with depth. These low velocity layers were inferred to be the major cause of the incessant failure of the pavements in the survey areas. These velocities and the N- values (blow count) were used to classify each location into sites C, D and E, according to the 1997 National Earthquake Hazards Reduction Program (NEHRP).

Shear-wave tomography of the Changbai volcanic area in NE China derived from ambient noise and seismic surface waves

A detailed 3-D shear-wave velocity (Vs) model of the crust and upper mantle beneath the Changbai volcano and adjacent areas in NE China is obtained by conducting a joint inversion of group-velocity dispersion curves at periods of 5–30 s and phase-velocity dispersion curves at periods of 5–100 s from ambient noise and earthquake surface waves. We use 1602 group-velocity dispersion curves and 3125 phase-velocity dispersion curves, which were recorded at 57 permanent stations of the China Earthquake Administration and 27 portable stations of Jilin University during 2010 to 2014. Our results reveal strong heterogeneities in the crust and upper mantle beneath the study area. A remarkable low-Vs zone is revealed in the crust beneath the Changbai volcano, indicating a hot magma chamber. The Longgang volcano is also characterized by a low-Vs anomaly reflecting partial melt in the crust. In contrast, there is no notable low-Vs anomaly in the crust beneath the Jingpohu volcano. Previous geological results and our tomographic images suggest that Changbai is a composite volcano, whereas Jingpohu and Longgang are monogenetic volcanoes. Low-Vs zones are revealed in the upper mantle beneath the three active volcanoes, which are interconnected at different depths. Combining the present results with previous findings, we deem that the three active intraplate volcanoes are caused by hot and wet upwelling flows in the big mantle wedge associated with deep subduction of the western Pacific plate beneath NE Asia.

Minkowski-like fractal seismic metamaterial with wide low-frequency band gaps on single and layered soil

In this paper, we propose a resource-efficient Minkowski-like fractal seismic metamaterial by hybridizing U-shaped and cross-shaped pillars, providing wide complete band gaps (BGs) due to its multi-scale self-similar property. Numerical modeling of seismic surface waves is used to study the band structure and transmission of our seismic metamaterial. A comparison of the band structures of the developed seismic metamaterial with various levels reveals that the iteration order itself is responsible for the increase in the number of frequency bands and the decrease in the center frequencies of the BGs. Furthermore, the vibrational modes are calculated and examined to understand the mechanism of BG generation. To demonstrate the efficiency of earthquake shielding in multiple complete BGs, investigations of seismic surface wave propagation on a 1D array of Minkowski-like fractal structure units on the surface of single and layered semi-infinite substrates are employed. The results show that the layered soil has the function of widening BGs by itself. Our proposed Minkowski-like fractal structure due to its multi-scale self-similarity mitigates the intrinsic drawback of the narrow BG of resonant metamaterial, providing a superior alternative in seismology and related areas of multi-frequency band vibration reduction. Moreover, its green design and manufacturing due to low filling rates, strong wear resistance, and ductility can realize the low-carbon and sustainable development of the construction industry.

Shear‐Velocity Structure and Dynamics Beneath the Sicily Channel and Surrounding Regions of the Central Mediterranean Inferred From Seismic Surface Waves

AbstractThe evolution of the Sicily Channel Rift Zone (SCRZ) is thought to accommodate the regional tectonic stresses of the Calabrian subduction system. Much of the observations we have today are either limited to the surface or to the upper crust or deeper from regional seismic tomography, missing important details about the lithospheric structure and dynamics. It is unclear whether the rifting is passive from far‐field extensional stresses or active from mantle upwelling beneath. We measure Rayleigh‐and Love‐wave phase velocities from ambient seismic noise and invert for 3‐D shear‐velocity and radial anisotropic models. Variations in crustal S‐velocities coincide with topographic and tectonic features. The Tyrrhenian Sea has a ∼10 km thin crust, followed by the SCRZ (∼20 km). The thickest crust is beneath the Apennine‐Maghrebian Mountains (∼55 km). Areas experiencing extension and intraplate volcanism have positive crustal radial anisotropy (VSH > VSV); areas experiencing compression and subduction‐related volcanism have negative anisotropy. The crustal anisotropy across the Channel shows the extent of the extension. Beneath the Tyrrhenian Sea, we find very low sub‐Moho S‐velocities. In contrast, the SCRZ has a thin mantle lithosphere underlain by a low‐velocity zone. The lithosphere‐asthenosphere boundary rises from 60 km depth beneath Tunisia to ∼33 km beneath the SCRZ. Negative radial anisotropy in the upper mantle beneath the SCRZ is consistent with vertical mantle flow. We hypothesize a more active mantle upwelling beneath the rift than previously thought from an interplay between poloidal and toroidal fluxes related to the Calabrian slab, which in turn produces uplift at the surface and induces volcanism.