The Ivrea-Verbano Zone (IVZ) located in the Italian Alps is known as one of most complete archetypes of continental crust–upper mantle section on Earth (e.g. Pistone et al., 2017). Because of its accessibility at the surface it can be used as natural laboratory to improve the understanding of the crust–mantle transition zone. Several geophysical observables indicate the presence of mantle rocks (high density, high seismic velocity) in the shallow sub-surface (~ 1 km), commonly known as the “Bird’s Head” or Ivrea body (Berckhemer, 1968; Diehl et al., 2009; Scarponi et al., 2021). The project SEIZE images and characterizes the shallow upper crust at the Balmuccia site (Italy) providing depth, extent and shape of the outcropping Ivrea body as well as its rock properties. Our tomographic study covers the crust down to about 3 km depth, while seismic reflection imaging is possible down to 6 km depth or deeper. With SEIZE we contribute to the comprehensive ICDP Drilling program in the Ivrea-Verbano ZonE (DIVE, www.dive2ivrea.org). To tackle this task, a controlled source (vibroseis) seismic experiment was carried out in the region around Balmuccia in October 2020. The seismic survey comprised two crossing profiles with a total length of 28 km which ran along (NNE-SSW) and across (W-E) the Balmuccia peridotite. In total, 432 vibro points were acquired with a nominal distance of ~60 m which were recorded using a fix-spread (110 receivers, ~250 m spacing) and a roll-along setup (330 receivers, ~20 m spacing). To obtain a structural image of the shallow upper crust various seismic techniques are applied: The fix-spread data set is used to recover the velocity structure down to 3 km depth. By using a 3D Markov chain Monte Carlo travel time tomography a shallow, distinct high velocity body is imaged in 3D near Balmuccia, at the proposed drill site. Reflection seismic processing is applied to the roll-along data set. However, the difficult terrain setting (deep mountain valleys) results in complex wave propagation that is challenging for conventional processing methods (e.g. static and dynamic corrections, CDP stacking). Therefore, pre-stack migration techniques are applied enabling the imaging of steeply dipping structures.

The seismic time term method of Gardner (Geophysics 4:247–259, 1939) has become popular in the context of seismic \(P_n\) studies, since it provides a possibility to estimate not only the Earth’s crustal thickness, but also the P-wave slowness of the uppermost mantle. In the last decades an extended form of this Time Term Method has been extensively used in order to predict a heterogeneous isotropic or anisotropic upper mantle slowness function. One of the main goals of this study is to formalize the mathematical construction for which the Time Term Equations are indeed linearized versions of the Travel Time Equations for such a complex laterally varying slowness function. For this purpose, an alternative definition of Time Terms is given that allows to consider them as parts of the exact travel time equation. In the case of constant upper mantle slowness functions these terms coincide with the classical Time Terms of Gardner. As a consequence, an alternative derivation of the extended Time Term Method can be given for heterogeneous (isotropic) upper mantle slowness functions.

The causes for the formation of large igneous provinces and hotspot trails are still a matter of considerable dispute. Seismic tomography and other studies suggest that hot mantle material rising from the core-mantle boundary (CMB) might play a significant role in the formation of such hotspot trails. An important area to verify this concept is the South Atlantic region, with hotspot trails that spatially coincide with one of the largest low-velocity regions at the CMB, the African large low shear-wave velocity province. The Walvis Ridge started to form during the separation of the South American and African continents at ca. 130 Ma as a consequence of Gondwana breakup. Here, we present the first deep-seismic sounding images of the crustal structure from the landfall area of the Walvis Ridge at the Namibian coast to constrain processes of plume-lithosphere interaction and the formation of continental flood basalts (Parana and Etendeka continental flood basalts) and associated intrusive rocks. Our study identified a narrow region (<100 km) of high-seismic-velocity anomalies in the middle and lower crust, which we interpret as a massive mafic intrusion into the northern Namibian continental crust. Seismic crustal reflection imaging shows a flat Moho as well as reflectors connecting the high-velocity body with shallow crustal structures that we speculate to mark potential feeder channels of the Etendeka continental flood basalt. We suggest that the observed massive but localized mafic intrusion into the lower crust results from similar-sized variations in the lithosphere (i.e., lithosphere thickness or preexisting structures)

Upwelling hot mantle plumes are thought to disintegrate continental lithosphere and are considered to be drivers of active continental breakup. The formation of the Walvis Ridge during the opening of the South Atlantic is related to a putative plume-induced breakup. We investigated the crustal structure of the Walvis Ridge (southeast Atlantic Ocean) at its intersection with the continental margin and searched for anomalies related to the possible plume head. The overall structure we identify suggests that no broad plume head existed during opening of the South Atlantic and anomalous mantle melting occurred only locally. We therefore question the importance of a plume head as a driver of continental breakup and further speculate that the hotspot was present before the rifting, leaving a track of kimberlites in the African craton.

In the framework of the Dead Sea Integrated Research project (DESIRE), 59 seismological stations were deployed in the region of the Dead Sea Basin. Twenty of these stations recorded data of sufficiently high quality between May and September 2007 to be used for ambient seismic noise analysis. Empirical Green’s functions are extracted from cross-correlations of long term recordings. These functions are dominated by Rayleigh waves, whose group velocities can be measured in the frequency range from 0.1 to 0.5 Hz. Analysis of positive and negative correlation lags of the Green’s functions makes it possible to identify the direction of the source of the incoming energy. Signals with frequencies higher than 0.2 Hz originate from the Mediterranean Sea, while low frequencies arrive from the direction of the Red Sea. Travel times of the extracted Rayleigh waves were measured between station pairs for different frequencies, and tomographically inverted to provide independent velocity models. Four such 2D models were computed for a set of frequencies, all corresponding to different sampling depths, and thus together giving an indication of the velocity variations in 3D extending to a depth of 10 km. The results show low velocities in the Dead Sea Basin, consistent with previous studies suggesting up to 8 km of recent sedimentary infill in the Basin. The complex structure of the western margin of the Basin is also observed, with sedimentary infill present to depths not exceeding 5 km west of the southern part of the Dead Sea. The high velocities associated with the Lisan salt diapir are also observed down to a depth of ~5 km. The reliability of the results is confirmed by checkerboard recovery tests.

The motion of tectonic plates is accommodated at fault zones. One of the unanswered questions about fault zones relates to the role they play in controlling shallow and local hydrology. This study focuses on the Arava/Araba Fault (AF) zone, the southern portion of the Dead Sea Transform (DST) in the Middle East. We combine seismic and electromagnetic methods (EM) to image the geometry and map the petro-physical properties and water occurrence in the top 100 m of this active fault. For three profiles, P-velocity and resistivity images were derived independently. Using a neural network cluster analysis three classes with similar P-velocity and resistivities could then be determined from these images. These classes correspond to spatial domains of specific material and wetness. The first class occurs primarily east of the fault consisting of 'wet' sand (dunes) and brecciated sediments, whereas the second class composed of similar material located west of the fault is 'dry'. The third class lies at depth below ca. 50 m and is composed of highly deformed and weathered Precambrian rocks that constitute the multi-branch fault zone of the AF at this location. The combination of two independent measurements like seismics and EM linked by a stringent mathematical approach has thus shown the potential to delineate the interplay of lithology and water near active faults.

Fault zones are the locations where motion of tectonic plates, often associated with earthquakes, is accommodated. Despite a rapid increase in the understanding of faults in the last decades, our knowledge of their geometry, petrophysical properties, and controlling processes remains incomplete. The central questions addressed here in our study of the Dead Sea Transform (DST) in the Middle East are as follows: (1) What are the structure and kinematics of a large fault zone? (2) What controls its structure and kinematics? (3) How does the DST compare to other plate boundary fault zones? The DST has accommodated a total of 105 km of left‐lateral transform motion between the African and Arabian plates since early Miocene (∼20 Ma). The DST segment between the Dead Sea and the Red Sea, called the Arava/Araba Fault (AF), is studied here using a multidisciplinary and multiscale approach from the μ m to the plate tectonic scale. We observe that under the DST a narrow, subvertical zone cuts through crust and lithosphere. First, from west to east the crustal thickness increases smoothly from 26 to 39 km, and a subhorizontal lower crustal reflector is detected east of the AF. Second, several faults exist in the upper crust in a 40 km wide zone centered on the AF, but none have kilometer‐size zones of decreased seismic velocities or zones of high electrical conductivities in the upper crust expected for large damage zones. Third, the AF is the main branch of the DST system, even though it has accommodated only a part (up to 60 km) of the overall 105 km of sinistral plate motion. Fourth, the AF acts as a barrier to fluids to a depth of 4 km, and the lithology changes abruptly across it. Fifth, in the top few hundred meters of the AF a locally transpressional regime is observed in a 100–300 m wide zone of deformed and displaced material, bordered by subparallel faults forming a positive flower structure. Other segments of the AF have a transtensional character with small pull‐aparts along them. The damage zones of the individual faults are only 5–20 m wide at this depth range. Sixth, two areas on the AF show mesoscale to microscale faulting and veining in limestone sequences with faulting depths between 2 and 5 km. Seventh, fluids in the AF are carried downward into the fault zone. Only a minor fraction of fluids is derived from ascending hydrothermal fluids. However, we found that on the kilometer scale the AF does not act as an important fluid conduit. Most of these findings are corroborated using thermomechanical modeling where shear deformation in the upper crust is localized in one or two major faults; at larger depth, shear deformation occurs in a 20–40 km wide zone with a mechanically weak decoupling zone extending subvertically through the entire lithosphere.

Our understanding of the tectonic evolution of the Cape Fold Belt, southern Karoo Basin and associated geophysical Beattie Magnetic Anomaly (BMA), may be challenged by the new ~ 100 km Near Vertical Reflection (NVR) seismic image which reveals ~42 to 45 km of complex crust and a sharp crust-mantle transition. The 5 to 10 km upper crust consists of dipping reflectors interpreted as the folded sequences of the Karoo Basin, underlain by continuous flat-lying reflectors that represent the Cape Supergroup (CSG) which may contain thrusts.

One of the projects within the framework of Inkaba yeAfrica, an earth system science initiative between German and South African research communities, is the Agulhas-Karoo transect. This 800 km north-south off-onshore transect runs from the offshore Agulhas Plateau onto the South African coast, across the Cape Fold Belt, Beattie Magnetic Anomaly, the Karoo Basin, the Great Escarpment and into the Kaapvaal Craton. Among the number of geophysical measurements taken along the transect are two wide-angle on-shore seismic lines collected in April and May 2005. The lines run roughly parallel to each other approximately 200 km apart, starting at Mossel Bay and St. Francis, and running about 200 km north to Fraserburg and Graaf Reinet, respectively. At each line 48 seismic receivers were used to record data from 13 shots. The profiles cross a wide variety of geological terrains, such as the siliciclastic sequences of the Paleozoic - Mesozoic Karoo and Oudtshoorn basins, the lower Paleozoic Cape Fold Belt, and the Eocambrian Kango and Kaaimans inliers. They also cross the Beattie Magnetic Anomaly, a large east-west orientated crustal feature within the upper crust, anti more than 1000 km long, first reported almost a century ago, but still not fully understood. The overall quality of seismic data is very good. First (P-wave) arrivals were manually picked on the available traces, and tomographic inversion was done using these travel times. The ray coverage made it possible to create the P-wave velocity model to depths of up to 25 km. We find excellent correlation of the shallow features with surface rock type. Deeper down we can identify both stratigraphic and tectonic contacts between geological groups. These include an inferred possible blind Paleozoic thrust fault, and the unconformity between the Cape Supergroup and the Namaqua-Natal Metamorphic Complex. The normal listric geometry of the Kango and Gamtoos Faults is clearly seen to a minimum depth of 15 km. We also observe a high velocity anomaly within the NNMC at similar to 10 km depth that we relate to the source of the Beattie Magnetic Anomaly.

Tomographic inversion techniques were applied to first‐arrival traveltimes of refracted P waves to study the shallowest part of the crust in the vicinity of the Arava Fault (AF), the Dead Sea Transform (DST) segment between the Dead Sea and the Red Sea; tomographic inversion techniques were applied to first‐arrival traveltimes of refracted P waves. A 100‐km‐long seismic line was centered on and oriented approximately perpendicular to the AF. A large number of P wave traveltimes from vibroseis and explosive shots (>280,000) were picked manually and used to invert for shallow P wave velocity structure. The regularized inversion approach (Zelt and Barton, 1998) was used for the tomographic inversion of the traveltimes. Extensive testing of model and inversion parameters was carried out to derive a reliable P wave velocity model. Complementary checker‐board tests indicate that depending on the size of velocity homogeneities, the velocity structure is well resolved down to a depth of several kilometers. This model represents the first shallow P wave velocity across the whole width of the DST system, showing features that correlate well with surface geology and also some buried structures. The model further suggests that the AF extends vertically downward to at least 3 km. The observed variation in upper‐crustal velocity implies the existence of a simple deformation compatible with a large lateral fault offset. From this model, a structural and dynamic interpretation of the DST system is then presented. The depth extension and geometry of several additional major faults and DST‐associated shallow sedimentary basins were successfully imaged through the integration of the well‐known surface geology and nearby boreholes. This work again confirms the DST as a typical transform fault system with a dominant strike‐slip motion confined to a narrow zone.

A controlled source Near Vertical Reflection (NVR) Seismic experiment along a similar to 100 km profile yields the first high quality seismic image of the crust and Moho across the southern Karoo Basin in South Africa. The highly reflective crust comprises upper, middle and lower layers. In the upper crust, folded and gently south-dipping continuous reflectors up to the Escarpment, represent the bedding of the Karoo and Cape Supergroups respectively. Decollement structures occur locally along carbonaceous shales of the Whitehill Formation. A well-defined mid-crustal layer that hosts the Beattie Magnetic Anomaly (BMA), occurs below a seismically imaged unconformity. The mid-crustal layer is similar to 20 km thick in the vicinity of the BMA and is likely to be a subsurface continuation of the 1.0 to 2.0 Ga granitoid gneisses of the Bushmanland sub-province in the 1.2 to 1.0 Ga Namaqua-Natal Orogenic Belt. The internal seismic fabric of this layer is interpreted as a tectonic fabric dipping to the north. The probable source of the BMA appears at 7 to 15 km depth, as a narrow feature in a similar to 10 km wide tectonically complex zone confined to the upper mid-crust. The underlying lower crustal layer is wedge-shaped: similar to 24 km thick in the north and decreasing to similar to 12 km thick beneath the Cape Fold Belt. This lower crustal layer may represent granulite-gneisses of the Namaqua sub-province. The internal seismic fabric in the tipper part of this layer dips both to the north and south, but a north-dipping fabric dominates the lower part. A clearly imaged undulating Moho occurs at a depth of similar to 43 km in the north, with a nick point at similar to 42 km depth, similar to 35 km along the profile, and then deepens to similar to 45 km in the south beneath the tectonic front of the Cape Fold Belt. A possible similar to 1 to 2 km thick lowermost crustal layer of high seismic reflectivity, overlies the Moho and may represent underplated mafic material. The reflectivity seen in this NVR seismic image bears similarities to seismic transects across the coeval Mesoproterozoic Grenville orogen in Canada.

The left-lateral Dead Sea Transform (DST) in the Middle East is one of the largest continental strike-slip faults of the world. The southern segment of the DST in the Arava/Araba Valley between the Dead Sea and the Red Sea, called Arava/Araba Fault (AF), has been studied in detail in the multidisciplinary DESERT (DEad SEa Rift Transect) project. Based on these results, here, the interpretations of multi-spectral (ASTER) satellite images and seismic reflection studies have been combined to analyse geologic structures. Whereas satellite images reveal neotectonic activity in shallow young sediments, reflection seismic image deep faults that are possibly inactive at present. The combination of the two methods allows putting some age constraint on the activity of individual fault strands. Although the AF is clearly the main active fault segment of the southern DST, we propose that it has accommodated only a limited (up to 60 km) part of the overall 105 km of sinistral plate motion since Miocene times. There is evidence for sinistral displacement along other faults, based on geological studies, including satellite image interpretation. Furthermore, a subsurface fault is revealed ≈4 km west of the AF on two ≈E–W running seismic reflection profiles. Whereas these seismic data show a flower structure typical for strike-slip faults, on the satellite image this fault is not expressed in the post-Miocene sediments, implying that it has been inactive for the last few million years. About 1 km to the east of the AF another, now buried fault, was detected in seismic, magnetotelluric and gravity studies of DESERT. Taking together various evidences, we suggest that at the beginning of transform motion deformation occurred in a rather wide belt, possibly with the reactivation of older ≈N–S striking structures. Later, deformation became concentrated in the region of today’s Arava Valley. Till ≈5 Ma ago there might have been other, now inactive fault traces in the vicinity of the present day AF that took up lateral motion. Together with a rearrangement of plates ≈5 Ma ago, the main fault trace shifted then to the position of today’s AF.

Cross-well seismic data from the Mallik 2002 project were analyzed using ray-based tomo- graphic reconstruction algorithms. Images of the P-wave velocity, the P-wave anisotropy, and the P-wave attenuation (Qp-1) structures are reconstructed from the picked arrival times and from ray-path-averaged attenuation values. The reconstructed horizontal velocities are generally 10 to 20% faster than the vertical velocities. The reconstructed isotropic P-wave velocities correspond to the average of the horizontal and vertical velocities. The reconstructions image the delta-front-shallow-marine Mackenzie Bay Sequence and the underlying fluviodeltaic Kugmallit Sequence. The tomograms are dominated by the effects of the gas hydrate deposits, which cause elevated seismic velocities. The location of laminated silt is correlated with increased velocity anisotropy. A combination of high velocities and strong attenuation (Qp<15) is observed in sediments with the highest gas hydrate saturation, which provides constraints on models for the microscopic structure of the deposits.

Crosswell seismic experiments were carried out using two, 1160 m observation wells (Mallik 3L- 38 and 4L-38), each located 42.5 m from the central, 1188 m production research well (5L-38). These provide images of a gas hydrate interval between 900 and 1100 m depth, and portions of the surrounding sedimentary sequences. A baseline survey was conducted to provide the background seismic structure before thermal stimulation tests were initiated to trigger gas hydrate dissociation. Following the baseline survey, three time-lapse monitor surveys were carried out during the production experiment to evaluate the seismic detection of gas hydrate dissociation. The data from the baseline survey are analysed using ray-based tomographic reconstruction algorithms. Images of the P-wave velocity, anisotropy and attenuation confirm that the first order structure is laterally continuous. The hydrate-saturated zones also appear to be anisotropic, caused potentially by fine-scale layering within the sands and silts hosting the gas hydrates. Lateral variations in these properties could indicate that the hydrates occur in more lense-like rather than stratified structures reflecting the fluvio-deltaic deposition of the host material. A combination of high velocities and strong attenuation (Quality factor < 15) is observed in sediments with the highest gas hydrate saturation, which provides constraints on models for the microscopic structure of the deposits. The tomographic results show no indications for the presence of free gas, taking into account similar velocity and attenuation values observed in the non-hydrated sediments on top of the section. The four surveys are highly repeatable, however the effects of the small, experimental thermal dissociation test are expected to be very subtle. Modelling studies were carried out to investigate the possible effects of the gas hydrate dissociation on the repeat crosswell data. The most significant effects occur when the direct P-wave is transmitted through the dissociation region, resulting in a phase shift for these arrivals. Other effects, such as diffractions and mode conversions from the top and the bottom of the anomaly are likely to be masked by other secondary arrivals, and would exhibit very weak amplitudes as a result of the strong attenuation of the gas hydrates. Advanced processing will be required to detect these effects. Full waveform inversion of the differential wavefields, as carried out by Watanabe et el (2004) is considered to be the most suitable method to make use of these observed phase changes, in order to image the dissociated regions.

Combined analysis of seismic P and S velocity information provides a reasonable and efficient basis for the petrologic interpretation of seismic cross sections. In this paper, a methodology is presented which allows extraction of prominent features related to well‐defined P velocities and Poisson's ratios from a tomographic velocity model using a classification approach. We used first‐arrival travel time data from a near‐vertical seismic experiment and independently determined P and S velocities by forward and inverse modeling. Resolution and uncertainties were estimated from inverting synthetic data. The classification procedure was carried out in two subsequent steps. First, prominent classes were identified in the parameter space spanned by Poisson's ratios and P wave velocities. For this purpose, a probability density function was calculated from the tomograms. A function measuring the topography of the probability density was then determined, and a histogram analysis was carried out to detect significant classes. In the second step, the results from principal component analysis for the identified classes were used to map their distribution along the seismic profile. We applied the method to the Messum intrusive complex of Namibia and identified three prominent classes. On the basis of the integration of petrophysical data and comparison with surface geology, we conclude that quartz‐syenite composition dominates the upper 800 m of the crust under the complex. Outside of the intrusion the upper crust has properties corresponding to felsic metasediments and granites which are abundant in the local basement. This material shows strong depth‐dependent changes of seismic properties which are ascribed to decreasing porosity and fluid saturation with depth.

Short-period array recordings from Pacific earthquakes show precursors to PP produced by underside reflections of P-waves off the discontinuities in the upper mantle. We use these events to study the structure of the transition zone discontinuities in the central and northern Pacific. The discontinuities of the mantle transition zone at depths of 410 km and 660 km are particularly interesting for the interpretation of the chemistry and temperature structure of the mantle transition zone. The PP reflections from these discontinuities are too small to be identified in unprocessed seismograms. Therefore, array methods are used to detect and identify the PP underside reflections. The data of several events show reflections from the 410-km discontinuity. The topography of the reflector can be used to study the olivine to spinel phase transition in the central and northwestern Pacific. The mean depth of the reflector is 404±16 km with topography near the Hawaiian Islands and the Kuriles. Forward modeling enables an estimate of the minimum impedance contrast and the maximum thickness of the discontinuity. This study shows that the 410-km discontinuity must be sharper than 6 km, assuming a simple linear gradient for the α→β phase change, with an impedance contrast of 8.9% as in IASP91, to be in agreement with our data. The minimum impedance contrast for a first-order discontinuity would be 6.5%. The 660-km discontinuity cannot be detected in this dataset using PP underside reflections in agreement with previous studies. Forward modeling shows that the non-detection of the 660 can be explained by a discontinuity with a thickness of more than 12 km for the IASP91 impedance contrast or by a first-order discontinuity with an impedance contrast of less than ∼9%.