Teleseismic Travel-time Residuals Research Articles

Lithospheric structure of the Dharwar Craton (India) from joint analysis of gravity, topography, and teleseismic travel-time residuals

• Effective elastic thickness of the Dharwar Craton varies between 12 and 35 km. • Both crust and lithospheric mantle contribute in supporting topography load. • Western Dharwar craton is thicker than the eastern Dharwar craton. • A Jelly-Sandwich type lithospheric architecture is possibly applicable in the Dharwar craton. The evolution and persistence of an uplifted surface along a passive continental margin is a long-standing enigma. The long-term vertical strains generated by such surface topographic loads have to be supported and maintained by the lithosphere's elastic strength. We estimate the lithospheric strength indicated by its effective elastic thickness (Te) and associated regional lithospheric structure along an ENE-WSW profile across the Dharwar craton-western Ghats (elevation ∼1200 m). We employ a joint analysis of teleseismic travel time residuals, Bouguer gravity, and topographic data. Our results suggest the topographic load of the Western Ghats is supported by both the Archean Dharwar crust and the lithospheric mantle (∼40 %), and indicate the possibility of a Jelly Sandwich type of rheology operating in this region. However, towards the eastern section of the profile, where the terrain flattens out, the contribution of the lithospheric mantle in maintaining the topography is observed to be uncertain due to resolvability of the method. Further, we infer the presence of ∼20–30 km thicker lithosphere beneath the western Dharwar craton (WDC) than the eastern Dharwar craton (EDC). The obtained Te values varies from ∼12 km (in the west of the WDC) to ∼32 km (towards the east of the EDC). These Te estimates are in agreement with previous studies from the region and are much lower than those for the more recent geological terrains in the subcontinent, such as the Gangetic plains and the Himalaya orogen.

Detailed Structure of the Subducted Nazca Slab into the Lower Mantle Derived From Continent‐Scale Teleseismic P Wave Tomography

AbstractNazca subduction beneath South America is one of our best modern examples of long‐lived ocean‐continent subduction on the planet, serving as a foundation for our understanding of subduction processes. Within that framework, persistent heterogeneities at a range of scales in both the South America and Nazca plates is difficult to reconcile without detailed knowledge of the subducted Nazca slab structure. Here we use teleseismic travel time residuals from >1,000 broadband and short‐period seismic stations across South America in a single tomographic inversion to produce the highest‐resolution contiguous P wave tomography model of the subducting slab and surrounding mantle beneath South America to date. Our model reveals a continuous trench‐parallel fast seismic velocity anomaly across the majority of South America that is consistent with the subducting Nazca slab. The imaged anomaly indicates a number of robust features of the subducted slab, including variable slab dip, extensive lower mantle penetration, slab stagnation in the lower mantle, and variable slab amplitude, that are incorporated into a new, comprehensive model of the geometry of the Nazca slab surface to ~1,100 km depth. Lower mantle slab penetration along the entire margin suggests that lower mantle slab anchoring is insufficient to explain along strike upper plate variability while slab stagnation in the lower mantle indicates that the 1,000 km discontinuity is dominant beneath South America.

Seismic velocity variations beneath central Mongolia: Evidence for upper mantle plumes?

Central Mongolia is marked by wide spread recent volcanism as well as significant topographic relief even though it is far from any plate tectonic boundaries. The cause of the recent magmatism and topography remains uncertain partially because little is known of the underlying mantle seismic structure due to the lack of seismic instrumentation in the region. From August 2011 through August 2013, 69 broadband seismic stations were deployed in central Mongolia. Teleseismic traveltime residuals were measured using waveform correlation and were inverted to image upper mantle P and S velocity variations. Significant lateral variations in seismic velocity are imaged in the deep upper mantle (100 to 800 km depth). Most significant are two continuous slow anomalies from the deep upper mantle to near the surface. One slow feature has been imaged previously and may be a zone of deep upwelling bringing warm mantle to beneath the Hangay Dome resulting in uplift and magmatism including the active Khanuy Gol and Middle Gobi volcanoes. The second, deep low velocity anomaly is seen in the east from 800 to 150 km depth. The anomaly ends beneath the Gobi Desert that is found to have fast shallow mantle indicating a relatively thick lithosphere. We interpret the second deep slow anomaly as a mantle upwelling that is deflected by the thick Gobi Desert lithosphere to surrounding regions such as the Hentay Mountains to the north. The upwellings are a means of feeding warmer than normal asthenospheric mantle over a widely distributed region beneath Mongolia resulting in distributed volcanic activity and uplift. There is no indication that the upwellings are rooted in the deep lower mantle i.e. classic plumes. We speculate the upwellings may be related to deep subduction of the Pacific and Indian plates and are thus plumes anchored in the upper mantle.

Subduction and volcanism in the Iberia–North Africa collision zone from tomographic images of the upper mantle

New tomographic images of the upper mantle beneath the westernmost Mediterranean suggest that the evolution of the region experienced two subduction-related episodes. First subduction of oceanic and/or extended continental lithosphere, now located mainly beneath the Betics at depths greater than 400km, took place on a NW–SE oriented subduction zone. This was followed by a slab-tear process that initiated in the east and propagated to the west, leading to westward slab rollback and possibly lower crustal delamination. The current position of the slab tear is located approximately at 4°W, and to the west of this location the subducted lithosphere is still attached to the surface along the Gibraltar Arc. Our new P-wave velocity model is able to image the attached subducted lithosphere as a narrow high-velocity body extending to shallow depths, coinciding with the region of maximum curvature of the Gibraltar Arc, the occurrence of intermediate-depth earthquakes, and anomalously thick crust. This thick crust has a large influence in the measured teleseismic travel time residuals and therefore in the obtained P-wave tomographic model. We show that removing the effects of the thick crust significantly improves the shallow images of the slab and therefore the interpretations based on the seismic structure.

Teleseismic traveltimes residuals across the Dead Sea basin

AbstractNew findings of the structure of the Dead Sea sedimentary basin and its eastern and western bordering regions are obtained by P and PKP wave relative traveltime residuals of 644 teleseisms, as recorded by the Dead Sea Integrated Research portable seismic network in the Dead Sea basin and its neighboring regions. The Lisan Peninsula is characterized by relatively small teleseismic traveltime residuals of about 0.14 s, in the latitude range of 31.22°–31.37° and at the longitude of 35.50°, slowly decreasing toward the west. The largest teleseismic traveltime residuals are in the southern Dead Sea basin, south of the Lisan Peninsula in the latitude range of 31.05°–31.15° and along longitude 35.45° and continuing southward toward the Amaziahu Fault, reaching values of 0.4–0.5 s. There is a small positive residual at the Amaziahu Fault and a small negative residual south of it probably marking the southern end of the Dead Sea basin. East and west of the Dead Sea basin the mean teleseismic traveltime residuals are negative with overall averages of −0.35 s and −0.45 s, respectively. Using the teleseismic residuals, we estimate the horizontal dimensions of the Lisan salt diapir to be 23 km × 13 km at its widest and a maximal thickness of about 7.2 km. The thickness of the Mount Sodom salt diapir is estimated as 6.2 km.

Seismological observations in Northwestern South America: Evidence for two subduction segments, contrasting crustal thicknesses and upper mantle flow

The cause of tectonic deformation in northwestern South America and its link to upper mantle structure and flow are debated. We use a combination of broadband and short period travel time seismic data for P-waves to show that observations are consistent with the presence of two subduction segments in Colombia and contrasting values of crustal thickness. In Northern Colombia, at latitudes greater than 6°N, most of the seismic stations are associated with negative teleseismic travel time residuals, relative to a regional mean, suggesting that the upper mantle is seismically faster than predicted from global models. In particular, for the Caribbean coastal plains there are no signs of significant anomalies in the upper mantle, evidenced by the small magnitude of the travel time delays and subdued Pn speeds (~7.97km/s). To the southeast of such plains there is an increase in magnitude of the negative travel time residuals, including the Northern Eastern Cordillera, the Perija Range and the Merida Andes. An analysis of non-isostatic residual topography, based on a model of crustal thickness in northwestern South America, is consistent with a slab-associated upper mantle flow beneath the region just east of the Bucaramanga Nest. We interpret these results to indicate the presence of a Caribbean slab, initially flat beneath the Caribbean coastal plains, and steepening sharply in the southeast, including the area of Bucaramanga. For most of the western Andean region and the Pacific coast, south of 6°N, teleseismic differential travel time residuals are predominantly positive, indicating that the upper mantle is in general seismically slower than the reference model. Beneath the Central Cordillera, just to the east of this area, the residuals become smaller and predominantly negative; residual non-isostatic topography is negative as well. These features are probably related to the effect of the Nazca subduction developing an asthenospheric wedge.

Significant contribution of the shallow crust to seismic PKP travel-time residuals and implications: An example from Taiwan and nearby islands

Array observations indicate that the relative PKP travel-time residuals in the Taiwan region vary from −2.5 to+2.0s, a range greater than the largest known teleseismic travel-time residuals from deep Earth. PKP waves are generally employed to probe structures of the deep Earth; however, the contribution of shallow geological structures to PKP waves is generally undefined and the implications of such a contribution have rarely been discussed. Current travel-time tomographic models of Taiwan are able to predict less than 40% of the observed PKP residuals, and the PKP residuals are consistent with the pattern of station corrections used in determining local earthquakes. Detailed island-wide gravity measurements indicate that the distribution of Bouguer anomalies, which were previously proposed to be induced by thick sedimentary deposits in large tectonic basins, is consistent with the pattern of PKP travel-time residuals. In conclusion, the observed PKP residuals are strongly affected by shallow sediments. Therefore, the contribution of residuals from shallow crustal structures should be carefully considered when employing PKP observations for imaging structures of the deep Earth or the deep crust and upper mantle.

Complex subduction and small-scale convection revealed by body-wave tomography of the western United States upper mantle

New high-resolution P- and S-wave tomography of the United States upper mantle from the Pacific Coast to the Great Plains reveals strong multi-scale heterogeneity closely correlated with tectonic and magmatic activity. We invert teleseismic travel-time residuals from the EarthScope Transportable Array and more than 1700 additional temporary and permanent stations for 3-D velocity perturbations to a depth of 1000 km. The inversion uses recent advances in western U.S. crust models to better isolate the mantle component of travel-time residuals, and frequency-dependent 3-D sensitivity kernels to map travel-time residuals, measured in multiple frequency bands, into velocity structure. In addition to separate V P and V S models, we jointly invert the two datasets for V P/V S perturbations by imposing a smoothness constraint on the δ lnV S/δ lnV P field. The joint inversion helps us identify regions where partial melt is probable. The amplitude of V P, V S, and V P/V S variations is greatest in the upper 200 km of the mantle and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and the occurrence of widespread small-scale convection. Partially molten mantle is inferred beneath Yellowstone and the eastern Snake River Plain (SRP), the Salton Trough, and the Clear Lake volcanic field. The inferred depth extent of partial melt is consistent with a generally hydrated upper mantle and elevated temperatures beneath the eastern SRP and Yellowstone. Despite continuous subduction since the Cretaceous, the distribution of sub-lithospheric high-velocity anomalies is dissected (similar to other recent studies). Based on our new tomography models, western U.S. geologic history, and plate–tectonic reconstructions, we infer patchy and incomplete removal of the flat-subducting Laramide slab and slab tearing associated with Eocene accretion in the northwestern U.S.

Evidence of partial melting beneath a continental margin: case of Dhofar, in the Northeast Gulf of Aden (Sultanate of Oman)

SUMMARY Gravity data and P-wave teleseismic traveltime residuals from 29 temporary broad-band stations spread over the northern margin of the Gulf of Aden (Dhofar region, Oman) were used to image lithospheric structure. We apply a linear relationship between density and velocity to provide consistent density and velocity models from mid-crust down to about 250 km depth. The accuracy of the resulting models is investigated through a series of synthetic tests. The analysis of our resulting models shows: (1) crustal heterogeneities that match the main geological features at the surface; (2) the gravity edge effect and disparity in anomaly depth locations for layers at 20 and 50 km; (3) two low-velocity anomalies along the continuation of Socotra-Hadbeen and Alula-Fartak fracture zones between 60 and 200 km depth; and (4) evidence for partial melting (3–6 per cent) within these two negative anomalies. We discuss the presence of partial melting in terms of interaction between the Sheba ridge melts and its along-axis segmentation.

High-resolution teleseismic body-wave tomography beneath SE Romania - I. Implications for three-dimensional versus one-dimensional crustal correction strategies with a new crustal velocity model

SUMMARY The CALIXTO (Carpathian Arc Lithosphere X-Tomography) experiment offers a dense, high-quality data set to study the lithospheric/asthenospheric system underneath SE Romania, an earthquake-prone region in SE Europe. To increase the image resolution of structures in the uppermost mantle, the application of crustal traveltime corrections by a priori information before the teleseismic traveltime inversion has become a well-accepted procedure. For such a correction we present a regional 3-D crustal seismic velocity model that serves as the basis for a high-resolution teleseismic tomography (forthcoming paper by Martin et al.). Our 3-D crustal model is based on recent research in the region. We collect new results from two long-range seismic refraction lines, 3-D refraction tomography and teleseismic Ps conversions. Adding previously published models of the sediment distribution, Conrad and Moho depths, as well as crustal seismic P-wave velocities, we compile a 3-D crustal model for SE Romania. This 3-D model does not contain shallow small-scale heterogeneities (<10 km), but it reflects the large-scale structures such as variations in sediment thickness, average seismic velocities and 3-D Moho depth. It is well suited for the correction of teleseismic traveltime residuals, a prerequisite for a high-resolution teleseismic tomography study: for example, traveltime delays of up to 1.3 s are caused by the almost 20-km thick layer of sediments in the Focsani Basin. Such delays are comparable to or larger in size than the expected upper mantle traveltime residuals. We study the significance of 3-D crustal traveltime corrections relative to 1-D station corrections and show that the complex basin structures in SE Romania require a 3-D approach to reduce the smearing of crustal anomalies into the mantle. By modelling synthetic mantle structures with a slab, as it is expected for SE Romania, we also investigate how to adapt the inversion strategy, if crustal corrections are applied. Significant improvements are found by including the already corrected crustal layers in the inversion procedure, thereby enabling the inversion algorithm to project still remaining uncertainties in the less-resolved upper crustal layers. However, the fixing of the upper layers during the inversion due to the a priori knowledge of the crustal velocity anomalies clearly leads to smearing of uncorrected anomalies that are possibly located close to the crust–mantle boundary.

Mantle Heterogeneities and the SCEC Reference Three-Dimensional Seismic Velocity Model Version 3

We determine upper mantle seismic velocity heterogeneities below Southern California from the inversion of teleseismic travel-time residuals. Teleseismic P-wave arrival times are obtained from three temporary passive experiments and Southern California Seismic Network (SCSN) stations, producing good raypath coverage. The inversion is performed using a damped least-squares conjugate gradient method (LSQR). The inversion model element spacing is 20 km. Before the inversion, the effects of crustal velocity heterogeneities represented by the Southern California Earthquake Center (SCEC) seismic velocity model version 2 are removed from the teleseismic travel times. The P-wave inversion produces a variance reduction of 43%. S-wave velocities are determined from laboratory Vp/Vs ratios. The most prominent features imaged in the results are high P-wave velocities (+3%) in the uppermost mantle beneath the northern Los Angeles basin, and the previously reported tabular high-velocity anomaly (+3%) to depths of 200 km beneath the Transverse Ranges, crosscutting the San Andreas fault. We incorporate the upper mantle seismic velocity heterogeneities into the SCEC Southern California reference seismic velocity model. The prior accounting for the crustal velocity heterogeneity demonstrates the utility of the top-down method of the SCEC seismic velocity model development.

High-resolution teleseismic tomography of upper-mantle structure using ana priorithree-dimensional crustal model

SUMMARY The effect of an a priori known 3-D crustal model in teleseismic tomography of upper-mantle structure is investigated. We developed a 3-D crustal P-wave velocity model for the greater Alpine region, encompassing the central and western Alps and the northern Apennines, to estimate the crustal contribution to teleseismic traveltimes. The model is constructed by comparative use of published information from active and passive seismic surveys. The model components are chosen to represent the present large-scale Alpine crustal structure and for their significant effect on the propagation of seismic wavefields. They are first-order structures such as the crust‐mantle boundary, sedimentary basins and the high-velocity Ivrea body. Teleseismic traveltime residuals are calculated for a realistic distribution of azimuths and distances by coupling a finite-difference technique to the IASP91 traveltime tables. Residuals are produced for a synthetic upper-mantle model featuring two slab structures and the 3-D crustal model on top of it. The crustal model produces traveltime residuals in the range between −0.7 and 1.5 s that vary strongly as a function of backazimuth and epicentral distance. We find that the non-linear inversion of the synthetic residuals without correcting for the 3-D crustal structure erroneously maps the crustal anomalies into the upper mantle. Correction of the residuals for crustal structure before inversion properly recovers the synthetic slab structures placed in the upper mantle. We conclude that with the increasing amount of high-quality seismic traveltime data, correction for near-surface structure is essential for increasing resolution in tomographic images of upper-mantle structure.

Crustal and upper mantle structure across the Dead Sea rift and Israel from teleseismic P-wave tomography and gravity data

New findings of the structure of the crust and the upper mantle across the Dead Sea rift and Israel were obtained by applying P and PKP wave relative travel time residuals inversion of 612 teleseisms, recorded by the seismic networks in Israel and Jordan. Independently of the relative teleseismic travel time residuals, the Bouguer gravity anomalies were used as an additional aid to examine the structure of the crust and upper mantle. An examination of the teleseismic travel time residuals in Israel suggests the existence of two prominent provinces of distinct crustal structure. The northward increase of the teleseismic travel time residuals is in good agreement with a gradual thickening of the sedimentary sequence and a simultaneous thinning of the crust. Examination of the inverted velocity anomalies indicates that the upper and lower crusts are sub-divided into a number of structures. We identify several velocity anomalies that are associated with prominent geological structures. The southern Dead Sea basin is characterized with the largest decrease of velocity, in both upper and lower crustal layers, due to the infilling of light material relative to the surrounding material. The velocity anomaly in the Carmel structure is probably associated with variations in the upper part of the upper mantle. Examination of a cross-section that traverses the Dead Sea transform supports the existence of lighter mass density material on the eastern side relative to that on the western side.

Teleseismic travel time residuals in North America and anelasticity of the asthenosphere

The slope a of the relation δ t S= aδ t P+ b for the teleseismic S and P travel time residuals δ t s and δ t p is practically the only robust observational value that is equally sensitive to the lateral S and P velocity variations in the upper mantle. Published estimates of a for North America are about twice higher than those at ultrasonic frequencies in olivine at comparable pressures and temperatures. In our study, lateral variations of the S residuals in North America are found not from the S wave readings, which can be easily biased, but from accurately determined delays relative to P of the Ps phases converted from `410 km' and `660 km' discontinuities. Nevertheless, the estimates of a thus derived are close to the values obtained with the standard technique. The discrepancy between the ultrasonic and seismic estimates of a can be explained by physical velocity dispersion. On the assumption of a single absorption band, our value of a is consistent with the published values of Q in the seismic frequency band and velocities in the ultrasonic range, if the high-frequency cutoff of the absorption band in the upper mantle beneath western North America is on the order of several hundred Hertz. However, in the literature there are indications of a cutoff at around 1 Hz. Then our data imply the presence of another absorption band at high frequencies outside the seismic frequency band. This band may correspond to the viscous dissipation in the Walsh [Walsh, J.B., 1969. A new analysis of attenuation in partially melted rock. J. Geophys. Res., 74, 4333–4337] model of penny-shaped inclusions of melt.

Three-dimensional crustal structure beneath the TOR array and effects on teleseismic wavefronts

The temporary seismic station array (TOR) was designed to study the lithosphere–asthenosphere system across the northwestern part of the Trans-European Suture Zone (TESZ) by teleseismic tomography. Teleseismic wavefronts, when propagating through complex crustal structure, undergo severe distortion that may result in travel time residual anomalies of significant amplitude. The inversion of teleseismic travel time residuals for deep structures without accounting for such crustal-related anomalies may erroneously map these travel time anomalies into features at greater depth. In this study we apply a three-dimensional (3-D) technique to estimate effects of a priori known 3-D crustal structure on travel times of teleseismic waves observed at the TOR seismic array across the TESZ to correct for these effects in future tomographic studies.A uniform 3-D crustal model is developed by use of published two-dimensional crustal models from previous active seismic surveys. The parameterization of this 3-D crustal model is designed to adequately represent those crustal structures that mostly influence the propagation of teleseismic wavefronts. The 3-D model includes lateral variation in velocity structure, Moho topography, and large and deep sedimentary basins. The teleseismic forward problem for this local 3-D model is solved by calculation of travel times to the base of the model using a standard whole Earth model and by subsequent propagation of spherical wavefronts using finite difference methods. Travel time calculations for an event near Japan reveal significant lateral variations in the range between −0.3s and +0.5s due to crustal structures. Being able to obtain the full travel time field at the surface of the model has the additional advantage of improving the identification and timing of seismic phases observed at the TOR seismic array.

Inversion for interface structure using teleseismic traveltime residuals

An inversion method is presented for the reconstruction of interface geometry between two or more crustal layers from teleseismic traveltime residuals. The method is applied to 2-D models consisting of continuous interfaces separating constant-velocity layers. The forward problem of determining ray paths and traveltimes between incident wave fronts below the structure and receivers located on the Earth—s surface is solved by an efficient and robust shooting method. A conjugate gradient method is employed to solve the inverse problem of minimizing a least-squares type objective function based on the difference between observed and calculated traveltimes. Teleseismic data do not accurately constrain average vertical structure, so a priori information in the form of layer velocities and average layer thicknesses is required. Synthetic tests show that the method can be used to reconstruct interface geometry accurately, even in the presence of data noise. Tests also show that, if layer velocities and initial interface positions are poorly chosen, lateral structure is still recoverable. The inversion method was applied to previously published teleseismic data recorded by an in-line array of portable seismographs that traversed the northern margin of the Musgrave Block, central Australia. The solution based on interface parametrization is consistent with models given by other studies that used the same data but different methods, most notably the standard tomographic approach that inverts for velocity rather than interface structure.

A tomography study of the Chyulu Hills, Kenya

The Chyulu Hills, a Quaternary volcanic field on the eastern shoulder of the Kenya rift in East Africa, have been the target of a multidisciplinary research project. In this contribution teleseismic travel-time residuals from a passive seismic experiment are imaged into P-velocity perturbations in the lithosphere underneath the Chyulu Hills. The images reveal lateral velocity contrasts of about 5% with a prominent low-velocity zone which is located directly beneath the volcanic range down to 70 km depth. Due to the small size and amplitude of the anomaly this zone of decreased seismic velocity is interpreted as evidence for small magma chambers with elevated temperatures in the lower crust and uppermost mantle. A major upwelling of the asthenosphere can be excluded, because the seismic anomaly is too limited in size. However, the increased temperatures below the Chyulu Hills are interpreted as a sign for the still active magmatic processes related to the rifting in East Africa.

On the use of later teleseismic arrivals in ACH‐tomography with an application in SE‐Kenya

The ACH‐tomography is used to image teleseismic travel‐time residuals into seismic velocity perturbations beneath a recording network. Commonly, these residuals are determined solely from first arrivals of body‐wave phases (e.g., P, PKP, or PKIKP). In this contribution we outline the benefit of adding later phases (e.g., PKKP, SKP, or PcP) to increase the amount of input data and to improve the ray sampling of the target volume. As a case study, a tomographic data set from the Chyulu Hills volcanic field in SE‐Kenya, East Africa, is taken. A model (M1) inverted from first arrival residuals is compared with one (M2) based on an extended data set, including 85 later arrival residuals. Model M2 has an improved statistical basis for evaluating the quality of the results. It confirms the principal features of M1, but shows a slightly larger low‐velocity anomaly beneath the volcanic field in the uppermost mantle, indicating active magmatic processes.

Local earthquake tomography of the Altiplano and the Eastern Cordillera of northern Bolivia

The velocity structure in the crust beneath the central Andes of northern Bolivia is obtained by the simultaneous inversion of local earthquake arrival-time data for velocity and hypocentral parameters. The data were recorded between October 1990 and February 1991 by the French “Lithoscope” network; the 34 vertical, short-period seismic stations were installed along a 320-km-long profile crossing two main morphological units of the Andean chain; the Altiplano and the Eastern Cordillera. The arrival times of seismic waves generated by earthquakes located in the subduction zone of the Nazca plate underneath the South American plate were inverted using a modified version of Thurber's 3-D iterative simultaneous inversion code. The results are consistent with the inversion of teleseismic travel-time residuals from the same network and closely correlate with the mapped surface geological features. The Altiplano Basin is characterized by a low-velocity zone down to 20 km, enclosed by the San Andres and Laurani faults. At the surface, the Cordillera Real fault zone (CRFZ) bounds to the west the high P-wave velocity anomaly lying under the axial zone of the Eastern Cordillera. The velocity discontinuity dips steeply to the southwest, to the bottom of the crust. From the velocity model, the depth of the Moho is estimated to be 66 km under the Altiplano. Moreover, the different behaviour of P- and S-waves found below the Western Cordillera and above the slab is consistent with the presence of partially melted asthenosphere that may be responsible for the magmatic activity. Finally, the seismicity defines, between 90 and 225 km, a part of the slab dipping to the north-northeast with a low angle of about 30°.

Evidence for a high‐velocity slab associated with the Hindu Kush seismic zone

We used teleseismic travel time residuals to determine lateral velocity variations of the crust and upper mantle in the Pamir‐Hindu Kush region in Tadjikistan and Afghanistan. Data from 29 analog seismic stations in Tadjikistan and northern Afghanistan were used to determine travel time residuals for 210 teleseismic events ranging in distance from 28° to 87° and covering a broad range of azimuths. We inverted for velocity perturbations over a rectangular grid with a block size of 99 × 99 km. The model extended to a depth of 350 km with a 50‐km‐thick first layer and two 150‐km‐thick deeper layers. The results show a strong and well‐resolved zone of high velocities in the upper mantle at depths greater than 200 km, coincident with the location of the Hindu Kush seismic zone. No clear velocity perturbations are associated with the Pamir seismic zone. Above 200 km little correlation is observed with the seismic zone, but indications of thicker crust under the Pamir and thinner crust under the Tadjik Depression are seen. The high velocities are most likely caused by the presence of oceanic lithosphere at depth.