P-wave Travel-time Residuals Research Articles

Bimodality of P-wave travel time residuals found in the CTBTO seismological data (REB-bulletins)

Bimodality of P-wave travel time residuals found in the CTBTO seismological data (REB-bulletins)

Shallow and deep lithosphere slabs beneath the Dinarides from teleseismic tomography as the result of the Adriatic lithosphere downwelling

The study area covers the Dinarides and southwestern part of the Pannonian basin as the marginal zone between the Adriatic microplate (African plate) and the Pannonian tectonic segment (Eurasian plate). We created a three-dimensional seismic velocity model to 450km depth using teleseismic tomography. Our travel-time dataset was collected by means of 40 seismic stations from the ORFEUS database and Croatian Seismological Survey database. A set of 90 teleseismic earthquakes were selected in the time range 2014–2015, and relative P-wave travel-time residuals were calculated. For the first time the seismic P-wave velocity model of a relatively high resolution on the entire Dinaridic mountain belt was obtained. Based on this model, a more reliable insight in the relations of the lithosphere plates has been achieved.We imaged a fast velocity anomaly extending underneath the entire Dinaridic mountain belt which indicates cold, rigid materials. The anomaly is steeply sloping towards the northeast and directly indicates the sinking of the Adriatic microplate underneath the Pannonian tectonic segment. In the Northern Dinarides the anomaly extends to the depth of 250km, whereas in the Southern Dinarides it covers greater depths, up to 450km. The shallow Adriatic slab extends along the External Dinarides, while the deep Adriatic slab extends beneath the Internal Dinarides and ophiolite zones in the area of central and southern Dinarides. Different slab depths are interpreted as the faster convergence of the plate in the southern Dinarides than in the northern, or the convergence of the plates had started in the southern part and systematically developed to the north.

Regional travel-time residual studies and station correction from 1-D velocity models for some stations around Peninsular Malaysia and Singapore

We have investigated the average P-wave travel-time residuals for some stations around Southern Thailand, Peninsular Malaysia and Singapore at regional distances. Six years (January, 2010–December, 2015) record of events from central and northern Sumatra was obtained from the digital seismic archives of Integrated Research Institute for Seismology (IRIS). The criteria used for the data selection are designed to be above the magnitude of mb 4.5, depth less than 200km and an epicentral distance shorter than 1000km. Within this window a total number of 152 earthquakes were obtained. Furthermore, data were filtered based on the clarity of the seismic phases that are manually picked. A total of 1088 P-wave arrivals and 962 S-wave arrivals were hand-picked from 10 seismic stations around the Peninsula. Three stations IPM, KUM, and KOM from Peninsular Malaysia, four stations BTDF, NTU, BESC and KAPK from Singapore and three stations SURA, SRIT and SKLT located in the southern part of Thailand are used. Station NTU was chosen as the Ref. station because it recorded the large number of events. Travel-times were calculated using three 1-D models (Preliminary Ref. Earth Model PREM (Dziewonski and Anderson, 1981, IASP91, and Lienert et al., 1986) and an adopted two-point ray tracing algorithm. For the three models, we corroborate our calculated travel-times with the results from the use of TAUP travel-time calculation software. Relative to station NTU, our results show that the average P wave travel-time residual for PREM model ranges from −0.16 to 0.45s for BESC and IPM respectively. For IASP91 model, the average residual ranges from −0.25 to 0.24s for SRIT and SKLT respectively, and ranges from −0.22 to 0.30s for KAPK and IPM respectively for Lienert et al. (1986) model. Generally, most stations have slightly positive residuals relative to station NTU. These corrections reflect the difference between actual and estimated model velocities along ray paths to stations and can compensate for heterogeneous velocity structure near individual stations. The computed average travel-time residuals can reduce errors attributable to station correction in the inversion of hypocentral parameters around the Peninsula. Due to the heterogeneity occasioned by the numerous fault systems, a better 1-D velocity model for the Peninsula is desired for more reliable hypocentral inversion and other seismic investigations.

Descending lithosphere slab beneath the Northwest Dinarides from teleseismic tomography

The area of study covers the marginal zone between the Adriatic microplate (African plate) and the Pannonian segment (Eurasian plate). We present a tomography model for this area, with special emphasis on the northwest Dinarides. A dense distribution of temporary seismic stations in the area of the Northern Dinarides along with permanent seismic stations located in the area, allowed us to construct this P-wave tomographic model. We assembled our travel-time dataset based on 26 seismic stations were used to collect the dataset. Teleseismic events were recorded for a period of 18 months and a set of 76 distant earthquakes were used to calculate the P-wave travel-time residuals. We calculated relative rather than absolute arrival-time residuals in the inversion to obtain depths of 0–400km.We imaged a pronounced fast velocity anomaly below the NW Dinarides which directly indicates a lithosphere slab downgoing beneath the Dinarides. This fast anomaly extends towards the NW direction to at least 250km depth, and we interpreted it as a descending lithosphere slab. The thrusting of the Adriatic microplate may be brought about by sub-lithosphere rising movement beneath the Pannonian region, along with a push from African plate. In our interpretation, the Adriatic lower lithosphere has been detached from the crust, and steeply sinks beneath the Dinarides. A lithosphere model of the contact between the Adriatic microplate and Pannonian tectonic segment was constructed based on the tomographic velocity model and results of previous crustal studies.

Density and P‐wave velocity structure beneath the Paraná Magmatic Province: Refertilization of an ancient lithospheric mantle

AbstractWe estimate density and P‐wave velocity perturbations in the mantle beneath the southeastern South America plate from geoid anomalies and P‐wave traveltime residuals to constrain the structure of the lithosphere underneath the Paraná Magmatic Province (PMP) and conterminous geological provinces. Our analysis shows a consistent correlation between density and velocity anomalies. The P‐wave speed and density are 1% and 15 kg/m3 lower, respectively, in the upper mantle under the Late Cretaceous to Cenozoic alkaline provinces, except beneath the Goiás Alkaline Province (GAP), where density (+20 kg/m3) and velocity (+0.5%) are relatively high. Underneath the PMP, the density is higher by about 50 kg/m3 in the north and 25 kg/m3 in the south, to a depth of 250 − 300 km. These values correlate with high‐velocity perturbations of +0.5% and +0.3%, respectively. Profiles of density perturbation versus depth in the upper mantle are different for the PMP and the adjacent Archean São Francisco (SFC) and Amazonian (AC) cratons. The Paleoproterozoic PMP basement has a high‐density root. The density is relatively low in the SFC and AC lithospheres. A reduction of density is a typical characteristic of chemically depleted Archean cratons. A more fertile Proterozoic and Phanerozoic subcontinental lithospheric mantle has a higher density, as deduced from density estimates of mantle xenoliths of different ages and composition. In conjunction with Re‐Os isotopic studies of the PMP basalts, chemical and isotopic analyses of peridodite xenoliths from the GAP in the northern PMP, and electromagnetic induction experiments of the PMP lithosphere, our density and P‐wave speed models suggest that the densification of the PMP lithosphere and flood basalt generation are related to mantle refertilization. Metasomatic refertilization resulted from the introduction of asthenospheric components from the mantle wedge above Proterozoic subduction zones, which surrounded the Paraná lithosphere. The high‐density PMP lithosphere is presently gravitationally unstable and prone to delamination.

P-wave velocity structure beneath the northern Antarctic Peninsula: evidence of a steeply subducting slab and a deep-rooted low-velocity anomaly beneath the central Bransfield Basin

Upper-mantle structure between 100 and 300 km depth below the northern Antarctic Peninsula is imaged by modelling P-wave traveltime residuals from teleseismic events recorded on the King Sejong Station (KSJ), the Argentinean/Italian stations (JUBA and ESPZ), an IRIS/GSN Station (PMSA) and the Seismic Experiment in Patagonia and Antarctica (SEPA) broad-band stations. For measuring traveltime residuals, we applied a multichannel cross-correlation method and inverted for upper-mantle structure using VanDecar's method. The new 3-D velocity model reveals a subducted slab with a ∼70° dip angle at 100–300 km depth and a strong low-velocity anomaly confined below the SE flank of the central Bransfield Basin. The low velocity is attributed to a thermal anomaly in the mantle that could be as large as 350–560 K and which is associated with high heat flow and volcanism in the central Bransfield Basin. The low-velocity zone imaged below the SE flank of the central Bransfield Basin does not extend under the northern Bransfield Basin, suggesting that the rifting process in that area likely involves different geodynamic processes.

Integrated seismic model of the crust and upper mantle of the Trans-European Suture zone between the Precambrian craton and Phanerozoic terranes in Central Europe

The structure and evolution of the Trans-European Suture zone (TESZ), contact between Precambrian Europe to the northeast and Phanerozoic terranes to the southwest is one of the main tectonic questions in Europe. The knowledge of the crustal structure, lithosphere–asthenosphere boundary and mantle transition zone between two seismic discontinuities at depths “410” and “660” km, is one of the most important issues to understand the Earth's dynamics. To create a mantle model of the TESZ and surroundings we used different seismic data collected along the 950 km long POLONAISE'97 profile P4. Previous results of 2-D ray-tracing and P-wave travel time modelling and new results of P-wave travel time residuals methods and receiver function sections provide facts about the seismic structure from the surface down to 900 km depth. In the TESZ a large basin, about 125 km wide, is filled with sedimentary strata (Vp < 6.0 km s − 1 ) to about 20 km depth. This basin is asymmetric with its northeast margin being most abrupt. The crystalline crust under this basin is only about 20 km thick today indicating that the lithosphere of Baltica was either thinned drastically or terminated along the northeast margin of the basin. The East European craton (EEC) has a ~ 45 km thick three-layered crust. The crust of the accreted terranes to the southwest is relatively thin (~ 30 km) and similar to that found in other non-cratonal areas of Western Europe. The lower crust is relatively fast (Vp > 7.0 km s − 1 ) along most of the P4 profile. However, lower values to the southwest may indicate the termination of Baltica. High velocity (~ 8.35 km s − 1 ) uppermost mantle lies beneath the Avalonia/Variscan terranes, and may be due to rifting and/or subduction. The seismic lithosphere thickness for the EEC is about 200 km, while it is only 90 km in the Palaeozoic platform (PP). The mantle transition zone is shallower and about 30 km thicker under the EEC, which could be due to thermal conditions (lower temperature) and/or the presence of water and FeO. The result of this paper is a new compiled and integrated seismic velocity model, available in digital form down to 900 km depth ( http://www.igf.fuw.edu.pl/p4-mantle), which can be used as a preliminary model of the crust and upper mantle in the TESZ area in Central Europe.

A strong seismic velocity contrast in the shallow mantle across the Zagros collision zone (Iran)

SUMMARY Surface wave dispersion measurements are interpreted jointly with the inversion of teleseismic P-wave traveltime residuals along a dense 620-km long temporary seismic profile across the Zagros to investigate its upper-mantle structure. The S-wave model determined from Rayleigh wave dispersion in the Zagros fold and thrust belt has high velocities from 4.5 ± 0.2 km s −1 below the Moho to 4.9 ± 0.25 km s −1 at 200 km depth, which are comparable to a shield-like structure. Beneath the suture region from the Main Zagros Thrust (MZT) to the UrumiehDokhtar volcanic arc, S-wave velocities are lower than beneath the Zagros in the top 50 km of the upper mantle, with a minimum of 4.4 ± 0.2 km s −1 at 80 km depth. From 150 km and deeper, S velocities are as high as beneath the Zagros. We suggest that part of the velocity difference at shallow depth is due to higher mantle temperatures and/or higher fluid content beneath the northern half of the profile, but that velocities are too high to support the hypothesis of mantle lid delamination under the suture zone. Teleseismic P traveltime relative residuals display a long-wavelength variation along the transect, with a difference of 1.1 s between negative residuals in the Zagros Simple Folded Belt and positive residuals in Central Iran. This difference backprojects into a 6‐7 per cent lateral variation of P-wave velocity in the shallow upper mantle, with higher VP beneath Zagros and lower VP beneath Central Iran. The main short wavelength variation of the residual is located in the suture region, with late P arrivals in the region of the MZT and early arrivals in the Sanandaj-Sirjan zone (SSZ). Using synthetic models of VP perturbations, we show that the high velocities of the Arabian platform have to extend laterally at least to the SSZ to fit the observed P delays. This model also predicts Rayleigh wave phase velocities, which are within the error bars of the observed dispersion. It supports the model of crustal-scale overthrusting at the MZT.

Variations of P-Wave Travel-Time Residuals before and after the 1999 Chi-Chi, Taiwan, Earthquake

P -wave velocity variations may provide precursory information before large earthquakes. We present results from a simple study on variations of P -wave travel-time residuals before and after the 1999 Chi-Chi, Taiwan, earthquake. The short-period seismic network deployed in Taiwan by the Central Weather Bureau (cwb) is routinely used for earthquake location. The accuracy of earthquake location is often judged by the travel-time residual of seismic waves. Because the velocity model used by the cwb was one-dimensional, the travel-time residuals will show lateral inhomogeneity of the velocity structures and unmodeled vertical structures. Also, the travel-time residuals may change with time if the velocity structure changes temporally. We used the data of the Central Weather Bureau Seismic Network (cwbsn) from 1991 to 2002 to obtain the difference between observed and calculated P -wave travel times, which is defined as the P -wave travel-time residual. The purpose of this study is to find the mean value of the P -wave travel-time residuals at each station and its possible temporal changes at stations near the Chelungpu fault. Our results show that the mean P -wave travel-time residuals at stations 40 km east of the Chelungpu fault changed very little before and after the Chi-Chi earthquake. But the mean P -wave travel-time residuals at the stations immediately west of the Chelungpu fault clearly changed before and after the Chi-Chi earthquake. The anomalous zone is bounded by stations NSY, TWQ1, TCU, WNT, WGK, CHN5, YUS, WDT, and WHF. These temporal changes of P -wave travel-time residuals can be attributed to changes in the velocity structure, which in turn might be caused by crustal deformation east of the Chelungpu fault beginning about 6 years before the Chi-Chi earthquake. In other words, before the Chi-Chi earthquake, there was a long-term 6-year precursor of P -wave velocity decrease possibly due to dilatancy or development of cracks. The variations of P -wave travel-time residuals with time at the anomalous stations are also confirmed by statistical tests that show clear significance. This method can be used to monitor and identify variations of P -wave travel-time residuals which may be precursors to a large earthquake.

Directional Variations in Travel-Time Residuals of Teleseismic P Waves in the Crust and Mantle beneath Northern Tien Shan

We study the directional variation in travel-time residuals using 13,820 P-wave arrivals from 1,998 teleseismic events (15 D 98, 4.1 mb 7.3) recorded in 1991-1997 by the Kyrgyz Digital Seismic Network (KNET). Based on a modified version of the iasp91 model that accounts for the Kyrgyz crustal thickness beneath KNET, we convert P-wave travel times to travel-time residuals dt. The de- pendence of dt on backazimuth is modeled as one-, two-, and four-lobed variations in a horizontal plane (Backus, 1965). A least-squares fit of the azimuthal variation of dt indicates that the crust in the northern Tien Shan is about 11-15 km thicker than it is in the Kazakh Shield and the Chu Depression. From nine KNET stations, the one-lobe model estimates that the slowest P-wave travel-time direction is 5.0 4.8 (almost directly north) and the magnitude of variation is 1.71 0.13 sec. This result is consistent with an upwelling lower mantle plume. For the two-lobe model, the slowest P-wave travel- time directions (anisotropy term) are 89.7 and 269.7 4.7 (i.e., trending east- west). We find P-wave velocity anisotropy of 2.0%-2.9% associated with a layer with a thickness of 440 km at the top of the lower mantle. The fast direction of the P-wave travel-time (north-south) azimuthal anisotropy at the top of the lower mantle is (1) parallel to the absolute motion of the India plate and (2) close to the direction of the upwelling hot mantle flow. The last result suggests that the azimuthal aniso- tropy of the travel-time residuals is due to the shape-preferred orientation of middle- mantle material that results from plume intrusion. Shear-wave splitting studies (Mak- eyeva et al., 1992; Wolfe and Vernon, 1998) estimated the fast polarization direction to be parallel to the strike of the geological structures of the northern Tien Shan (71 29). Thus, the fast polarization direction determined from these shear-wave splitting studies using KNET data contradicts (differs by 90) the fast travel-time direction (0.3 and 179.7 4.7) we determine here using P-wave travel-time residuals using KNET data. This suggests that the azimuthal anisotropy determined from P-wave travel-time variations and from shear-wave splitting in SKS and SKKS have different sources.

Three-dimensional velocity images beneath the Kang–Dian Tethyan tectonic zone of China

Three-dimensional velocity images of the crust and upper mantle beneath the Kang–Dian Tethyan tectonic zone in China are constructed using P-wave travel-time residuals of earthquakes. The Kang–Dian Tethyan tectonic zone is a transitional zone in tectonic structures and an important topographic border line. It is also a zone of concentration of shallow-focus earthquakes. The imaging results indicate that there is a significant lateral heterogeneity in the crust and upper mantle beneath the Kang–Dian Tethyan tectonic zone in China. The velocity images of the upper crust show features closely related to the tectonic features on the surface. A low-velocity layer exists in a very wide range of the mid-crust. Almost all of the major earthquakes took place in the transition strips between high- and low-velocity zones in the crust above 20 km depth. From the velocity images at 20+0 and 50+0 km depth, respectively, we find that the epicenters of strong earthquakes with magnitude larger than 6.0 are almost entirely distributed in the low-velocity zones or on their boundaries.

3–D Tomographic Images and Deep Ore‐Control in Northern Margin of the North China Plate

Abstract: 3–D velocity images of the crust beneath the northern margin of the North China Plate have been constructed using P‐wave travel time residuals of the latest earthquakes, with the data supplied by Chinese seismic networks.The seismic image results indicate that there is a lateral heterogeneity in the crust beneath the northern part of the North China block. The velocity images of the upper crust show features closely related to the tectonic features on the surface. It can be seen from these velocity images of the vertical sections, and from the horizontal slice images at depths of 11 and 16 km that there exist East‐West and North‐East structures. The images indicate that the juncture zone of basin–and–range terrain is between the blue‐colored high–velocity block corresponding to the Yanshan mountain range that developed during the Yanshan period in northwest Beijing and the green low‐velocity area corresponding to the North China basin in southeast Beijing (Fig. 5). The juncture zone between high‐velocity and low–velocity, and EW and NE fault zones have significant ore‐control effects. From the chart of epicenters in the northern region of North China, we find that the epicenters of earthquakes are almost entirely distributed within the NE strip. Almost all major earthquakes took place in the transition strip between the high and low‐velocity zones in the crust. The distribution of epicenters also reflects the strikes of known NE–faults. From the image sections along the latitude, we find that in the area between 114.0E–118.0E, there is a blue high‐velocity block standing upright from the Moho to the upper crust (Fig. 6), from which can be deduced that some materials such as magma moved upward from the upper mantle during the history of its geological development.

Determination of 3‐D Velocity Anomalies of the Nanbei Tectonic Zone of China Based on Local Earthquakes

Abstract: In this paper, 3‐D velocity images of the crust and upper mantle beneath the Nanbei tectonic zone of China are constructed using P‐wave travel time residuals of earthquakes, with the data supplied by China's seismic networks and the International Seismic Centre.During the model parameterization in the tomographic inversion the interpolation function of grid node velocities is used as the space function of velocity, and the velocity function is allowed to be discontinuous. The difficulties caused by large memory demand and high computing cost in solving the system of equations are avoided by utilizing the least squares QR decomposition algorithm. Thus, the stability of the algorithm is ensured. Though resolution images are not derived in the inversion process based on the resolution theory of Backus and Gilbert, a covariance resolution method is put forward by the authors. The resolution analyses have proved the reliability of the image results. 3‐D ray tracing is conducted to obtain the ray paths in a spherical coordinate system.The imaging results indicate that there is a significant lateral heterogeneity in the crust and upper mantle beneath the North‐South tectonic zone of China persisting down to 200 km in depth. The velocity images of the upper crust show features closely related to the tectonic features on the surface. A low‐velocity layer exists in a very wide range of the mid‐crust. The lowest velocity value is about 5.60 km/s. In a long narrow area between 25.0°N‐38.0°N and 100.0°E‐103.2°E, a low‐velocity anomaly of about 7.49 km/s appears at the top of the upper mantle. Almost all of the major earthquakes (ML≥6.0) took place in the transitional strips between high and low velocity zones at depths above 20 km in the crust. From the velocity images at 20+0 km and 50+0 km depth respectively, we find that the epicentres of strong earthquakes with magnitudes larger than 6.0 are almost entirely distributed in the low‐velocity zones or on their boundaries.

Massif central: détermination et modélisation de l'atténuation des ondes P télésismiques

Estimations of teleseismic P-wave attenuation beneath the French Massif Central allow two regions of strong attenuation to be identified, in the Devès-Velay and Cantal areas. These two abnormal regions of propagation are clearly correlated with lateral heterogeneities observed with the inversion of P-wave travel time residuals (Granet et al., 1995). Our thermomechanical modelisation shows that the thermal effect can explain only 8 % of the observed attenuation. Only the presence of fluids could explain the Q factor value lower than 100. The persistence of strong crustal perturbation shows that these two regions were key areas for the circulation of magmas in the crust during recent volcanic events.

Three-dimensional velocity images beneath the Chinese continent

In this paper three-dimensional velocity images beneath the Chinese continent and adjacent regions are reconstructed, using P-wave travel-time residuals of regional and distant earthquakes in seismic tomography method, supplied by Chinese seismic networks and ISC (International Seismic Center). The velocity images show lateral heterogeneity of the crust and upper mantle beneath the Chinese continent and adjacent regions, which persists down to at least 1100 km deep. The correlation between the velocity images and the known surface geological features can be traced down to a depth of 110 km. Below 220 km this correlation is not obvious. The images at 45−0 km (immediately above the Moho) and 45+0 km (below the Moho) show clearly that the crust of the Chinese continent can be divided into two parts approximately along the 102.5°E longitude. The eastern part of the crust is thinner than the western part. The velocity images at 400 km and 600 km also display differences between the eastern and western parts. This means that the tectonic differences between eastern and western China may exist in the whole upper mantle.

Teleseismic P-residual study in the Italian region - inferences on large scale anisotropic structure of the subcrustal lithosphere

Jeffreys-Bullen (absolute) and relative P-wave travel-time residuals were analyzed over Italy and its surrounding using P arrival times from the ISC bulletins supplemented by the data from local observatories. We analyzed the travel-time station corrections by two independent methods to obtain information on lateral variations of the velocity structure over the area and a view of possible upper mantle anisotropy. In the first method, the station corrections are computed as a constant and two cosine terms with appropriate phase shifts. Besides a static term, the second method allows us to study the relative residuals in dependence both on azimuths and incidence angles and thus to investigate their spatial variations and to map lateral variations of anisotropic structure of the subcrustal lithosphere. The high and low-velocity directions inferred from the spatial distribution of the relative residuals as well as the high- and low-velocity upper mantle heterogeneities reflect the geodynamic development of the region, governed by the collision between the African and Eurasian plates

Teleseismic tomography across the middle Urals: lithospheric trace of an ancient continental collision

The Urals are a major collision belt between two continental plates separated by island arcs: the East European plate and the Siberian plate. Their linear structure is preserved on a length of about 3000 km. A seismic array of 61 stations has been deployed during more than 3 months on a linear profile across the middle Urals, north of Ekaterinburg. P-wave travel-time residuals are inverted and provide a tomographic cross-section of the lithosphere on a 600-km-long profile across the Urals. An east-west asymmetry is observed both in the crust and in the lithosphere: the western limit between these two different lithospheres corresponds to the western front of the Uralian orogen from the surface to a depth greater than 100 km. Several crustal studies were performed by other teams along the same profile, and we compare our teleseismic cross-section to common-depth-point seismics, to deep seismic soundings and to a wide-angle-reflection fan profile. East of the main Uralian fault, the teleseismic inversion shows high-velocity bodies corresponding to well-known volcanic/mafic and ultramafic rocks in the Tagil syncline. These high-velocity bodies do not appear to be rooted in the lower crust. As a whole, the teleseismic tomogram comforts the Moho imbrication model proposed by Juhlin et al. (1995).

Litho—asthenospheric structures of northern Italy as inferred from teleseismic P-wave tomography

Regional three-dimensional inversions of teleseismic P-wave travel time residuals recorded by high-frequency regional and local seismic networks operating along the Western Alps and surrounding regions were carried out and lithosphere and upper mantle P-wave velocity models down to 300 km were obtained. Residuals of more than 500 teleseismic events, recorded by 98 fixed and temporary seismic stations, have been inverted. The comparison between real residuals and the ones obtained from tomographic model indicates that the method is able to solve the feature of the regional heterogeneities. Where the resolution is good, coherent lithospheric and upper mantle structures are imaged. In the shallower layers, high- and low-velocity anomalies follow the structural behaviour of the Alpine-Apenninic chains showing the existence of very strong velocity contrasts. In the deepest layers, velocity contrast decreases however two deep-seated high-velocity structures are observed. The most extended in depth and approximately trending NE-SW has been interpreted as a wreck of the oldest subduction responsible of the Alpine orogenesis. The second one, connected to the northwestern sector of the Apenninic chain, appears to vanish at depths greater than 180 km and is probably due to still active Apenninic roots. Cross-sections depict the spatial trend of perturbations and in particular outline the sub-vertical character of the Alpine and Apenninic anomalies. Under the Ligurian Sea, the 3-D inversion confirms the uplift of the asthenosphere in agreement with the tectonic evolution of the basin.

Massif Central (France): new constraints on the geodynamical evolution from teleseismic tomography

The Massif Central, the most significant geomorphological unit of the Hercynian belt in France, is characterized by graben structures which are part of the European Cenozoic Rift System (ECRIS) and also by distinct volcanic episodes, the most recent dated at 20 Ma to 4000 years BP. In order to study the lithosphere-asthenosphere system beneath this volcanic area, we performed a teleseismic field experiment. During a six-month period, a joint French-German team operated a network of 79 mobile short-period seismic stations in addition to the 14 permanent stations. Inversion of P-wave traveltime residuals of teleseismic events recorded by this dense array yielded a detailed image of the 3-D velocity structure beneath the Massif Central down to 180 km depth. The upper 60 km of the lithosphere displays strong lateral heterogeneities and shows a remarkable correlation between the volcanic provinces and the negative velocity perturbations. The 3-D model reveals two channels of low velocities, interpreted as the remaining thermal signature of magma ascent following large lithospheric fractures inherited from Hercynian time and reactivated during Oligocene times. The teleseismic inversion model yields no indication of a low-velocity zone in the mantle associated with the graben structures proper. The observation of smaller velocity perturbations and a change in the shape of the velocity pattern in the 60–100 km depth range indicates a smooth transition from the lithosphere to the asthenosphere, thus giving an idea of the lithosphere thickness. A broad volume of low velocities having a diameter of about 200 km from 100 km depth to the bottom of the model is present beneath the Massif Central. This body is likely to be the source responsible for the volcanism. It could be interpreted as the top of a plume-type structure which is now in its cooling phase.

Southern Rhine Graben: small-wavelength tomographic study and implications for the dynamic evolution of the graben

SUMMARY Inversion of P-wave traveltime residuals of teleseismic events recorded by a dense network of short-period stations yielded a detailed image of the 3-D velocity structure of the lithosphere-asthenosphere system beneath the southern Rhine Graben area. The upper part of the lithosphere, marked by velocity anomalies orientated along the graben strike, can be related to the development of the graben. These upper 50 km display stronger lateral heterogeneities than the weak anomalies observed at deeper levels which show roughtly an E-W orientation. The teleseismic inversion models yield no indications for significantly reduced seismic velocities in the upper mantle as could be expected from an actively upwelling mantle diapir located beneath the southern Rhine Graben area. The observation of generally weak velocity variations at greater depth indicates a rather smooth transition from the lower lithosphere to the upper asthenosphere. Deeper parts of the lithosphere were probably altered by metasomatic processes as a consequence of the intrusion of melts during middle and late Miocene times. These melts cooled and solidified from the end of the Miocene when rifting and thermal doming ceased in the southern Rhine Graben area.