Jordi Diaz

Mount Etna dense array local earthquake P and S tomography and implications for volcanic plumbing

Inversion for the three-dimensional velocity structure of Mount Etna is performed with a data set of arrival times of P and S waves of local earthquakes from temporary dense arrays of three-component seismographs. A high-V p body revealed by the original tomography without nearby stations is confirmed, and its image is sharpened using new velocity constraints provided by refraction data. Synthetic tests of V p and V p /V s , and comparison with an independent artificial source tomography with a fundamentally different geometry consistently calibrate the significance threshold of the resolution indicators. The trustworthy part of the image shows a high-V p body centered under the southern part of Valle del Bove above the 6 km below sea level deep basement, which extends towards sea level and may be rooted in or through the crust. It has a large contrast of over 1 km/s with the surrounding sediments and sharp lateral limits and can thus be regarded as made of intrusive material of magmatic origin. The massive high-V p body is heterogeneous in V p /V s . The regions inside it where V s is relatively low can then be suspected of containing a proportion of melt or be fractured and act as pressure links or transport zones. Such features may be structurally linked and appear to be activated in eruptive phenomena. By taking into account the heterogeneities in structure and physical state retrieved by seismic tomography a succession of seismic events, deformational episodes, and geochemical variation in lavas can be discussed with respect to the well-observed eruptions.

A deep seismic sounding investigation of lithospheric heterogeneity and anisotropy beneath the Iberian Peninsula

Abstract The dimensions of the Iberian Peninsula, the facility of firing large charges in the surrounding waters, the well-known and relatively uniform geology, and prior knowledge of the crustal structure, resulted in it being chosen as a study area for the investigation of the seismic structure of the lower lithosphere via refraction-wide-angle reflection seismic profiling. The Iberian lithosphere Heterogeneity and Anisotropy experiment (ILIHA), with a star-shaped arrangement of six long-range DSS profiles, was carried out in October 1989. The models derived from a first interpretation of the recorded data are presented. Three crustal profiles cover the same western and central part of the Hercynian Massif as the mantle profiles. The resulting interpretations all include a middle as well as a lower crustal layer above the mantle. The velocities of the layers in all three models are similar; however, the layer depths vary beneath the profiles. Interpretation of the mantle derived data suggests a layered lower lithosphere. One reversed line and an intersecting unreversed line indicate the layering penetrates to at least 90 km depth. The homogeneity of these layers contrasts strongly with the heterogeneous Hercynian surface geology. Velocities derived from reflected data from the deep layers suggest the constituent materials are either anisotropic or that the layers suffer a slight regional dip.

Perturbation to the lithosphere along the hotspot track of La Réunion from an offshore-onshore seismic transect

A 250 km long NE-SW lithospheric transect spanning the 40 km wide island of La Reunion and its submarine edifice is derived from lines of air gun shots at sea on either side, along the assumed hotspot trace. Seismic records were obtained from an array spanning the whole transect and including sea bottom and land receivers, providing a system of reversed and overlapping observations. Low seismic velocity, and hence density, is found on average for the whole edifice above the oceanic plate. We attribute high- velocity anomalies within the edifice to an intrusive core confined under the central northern quarter of the island-crossing segment. Unexpectedly, the main seismic interfaces, top and bottom of the prevolcanic crust, do not show significant flexural downwarping under the island. In addition, clear multipathing in the recorded wave field requires the presence of a body with a seismic velocity intermediate between the prevolcanic crustal material and the normal mantle. This lithospheric structure provides the first example where underplating occurs beneath an active volcanic island, suggesting a genetic relationship. The underplated body could represent residues of the evolution of primary picritic melts that yield erupted basalts. Evidence for reflectors deeper in the lithosphere may indicate further related heterogeneity. In the plate/hotspot model commonly assumed, the structural variation along the transect could be interpreted as a variation with time of the amount and physical state of underplated material.

Modes of raising northeastern Tibet probed by explosion seismology

Abstract New wide-angle reflection and refraction seismic data provide constraints on the structure of the upper lithosphere, and test models of its evolution to raise the northeastern part of the Tibetan Plateau. Amplitudes observed for reflections from the crust–mantle boundary are sufficiently large to suggest that there is no significant partial melt in the deep crust. The data show an increase of the crustal thickness between terranes from north of the Kun Lun Fault into the Qang Tang of central Tibet, and a contrast among their intracrustal images and compositions. In the north, P and S velocities are consistent with a dominantly felsic composition and show that only the upper crust thickened. South of the Kun Lun Fault a thicker crust made of two layers could result from the superposition of the originally thin crust of the Bayan Har terrane on the lower part of the crust of the domain to the north, which upper crust it shoved and thickened. Different modes of crustal thickening, either by thickening of individual layers or superpositions and imbrication among them appear to work jointly to raise the topography.

High-resolution imaging of the Pyrenees and Massif Central from the data of the PYROPE and IBERARRAY portable array deployments

The lithospheric structures beneath the Pyrenees, which holds the key to settle long-standing controversies regarding the opening of the Bay of Biscay and the formation of the Pyrenees, are still poorly known. The temporary PYROPE and IBERARRAY experiments have recently filled a strong deficit of seismological stations in this part of western Europe, offering a new and unique opportunity to image crustal and mantle structures with unprecedented resolution. Here we report the results of the first tomographic study of the Pyrenees relying on this rich data set. The important aspects of our tomographic study are the precision of both absolute and relative traveltime measurements obtained by a nonlinear simulated annealing waveform fit and the detailed crustal model that has been constructed to compute accurate crustal corrections. Beneath the Massif Central, the most prominent feature is a widespread slow anomaly that reflects a strong thermal anomaly resulting from the thinning of the lithosphere and upwelling of the asthenosphere. Our tomographic images clearly exclude scenarios involving subduction of oceanic lithosphere beneath the Pyrenees. In contrast, they reveal the segmentation of lithospheric structures, mainly by two major lithospheric faults, the Toulouse fault in the central Pyrenees and the Pamplona fault in the western Pyrenees. These inherited Hercynian faults were reactivated during the Cretaceous rifting of the Aquitaine and Iberian margins and during the Cenozoic Alpine convergence. Therefore, the Pyrenees can be seen as resulting from the tectonic inversion of a segmented continental rift that was buried by subduction beneath the European plate.

Lithospheric anisotropy beneath the Pyrenees from shear wave splitting

We investigate upper mantle anisotropy beneath the Pyrenean range along three N-S profiles across the mountain belt. The results of a first profile that operated in 1993 in the central part of the belt have been presented elsewhere. We present the results of two other profiles that ran in 1995-1996 and 1996-1997 in the eastern and western part of the belt, respectively and propose an interpretation of the whole results. Teleseismic shear waves (SKS, SKKS, and PKS) are used to determine splitting parameters: the fast polarization direction φ and the delay time δt. Teleseismic shear wave splitting in the eastern Pyrenees displays homogeneous φ values trending N100°E and δt values in the range 1.1 to 1.5 s. A station located in the southern Massif Central, 100 km north of the range, is characterized by different splitting parameters (φ = N70°E, δt = 0.7 s). In the western part of the belt, anisotropy parameters are similar across the whole belt (φ = N110°E and δt = 1.3 to 1.5 s). Most of the measured delay times, including those obtained in the central part of the range, are above the global average of the SKS splitting (around 1 s). At the belt scale, φ is generally poorly correlated with recent estimations of the absolute plate motion, which predicts a fast direction ranging between N50°E and N80°E. Instead, the orientation of φ (N100°E) is parallel to the trend of the Pyrenean belt but also to Hercynian preexisting structures. This parallelism supports an anisotropy primarily related to frozen or active lithospheric structures. We show that a signature related to the Pyrenean orogeny is likely for the stations located in the internal domains of the belt. By contrast, the anisotropy measured at the stations located on the external parts of the belt could reflect a pre-Pyrenean (Hercynian) deformation. We suggest that a late Hercynian strike-slip deformation is responsible for this frozen upper mantle anisotropy and that the Pyrenean tectonic fabric developped parallel to this preexisting fabric. Finally, no particularly strong splitting is related to the North Pyrenean Fault, commonly believed to represent the plate boundary between Iberia and Eurasia.

Crustal thinning in the Southwestern Iberia Margin

The mode of crustal thinning in the southwestern margin of the Iberian Peninsula is investigated along a transect that extends from onshore Iberia to the eastern end of the Horseshoe Abyssal Plain. On onshore areas, the crustal structure has been deduced using wide-angle seismic reflection data, whereas offshore we have used coincident steep and wide-angle reflection data along a NE-SW oriented seismic profile that extends from Cape San Vicente to the Horseshoe Abyssal Plain. In addition, 2D gravity modelling has been performed to validate the crustal structure deduced from seismic data. Our model results reveal that the crust undergoes a strong but continuous thinning from 31 km onshore Iberia to less than 15 km in the Horseshoe Abyssal Plain and that thinning occurs over horizontal distances of about 120 km.

Increase in melt fraction along a south-north traverse below the Tibetan Plateau: evidence from seismology

Abstract A mid-crustal low-velocity zone and crustal melt fraction contrast the Lhasa block in the Maxiang Yangbajain zone with the Tethyan Himalayas in the south. Evidence from wide-angle reflections is extended by vertical reflection and differential S to P wave teleseismic delays. The temperature and rheology implied allow phase transitions which may change crustal buoyancy and elevation (Le Pichon et al., 1997), and may allow lateral mass flow. In the uppermost mantle, particular path geometries to a tight temporary teleseismic array allow detection of a decrease in velocity northward through the Lhasa block. A larger relative variation in late teleseismic S with respect to P indicates an increase in Poissons ratio, hence of partial melt fraction. This may be seen as evidence of a higher position of the mantle asthenosphere. Level Tibet hence appears to be underlain by variation in the structure within the crust across the Indus-Tsangpo suture, and in the structure within the mantle, further northward of the middle Lhasa block. These variations are most readily attributable to spatial variation in partial melt fraction, and hence temperature, which also induces phase transformations. Mode and amount of deformation and control of buoyancy on elevation may vary accordingly. The dynamics of the system is evidenced by seismic anisotropy which we relate to ductile flow.

The deep roots of the western Pyrenees revealed by full waveform inversion of teleseismic P waves

Imaging the architecture of mountain roots is required to understand the support of topography and for kinematic reconstructions at convergent plate boundaries, but is still challenging with conventional seismic imaging approaches. Here we present a three-dimensional model of both compressional and shear velocities in the lithosphere beneath the western Pyrenees (southwest Europe), obtained by full waveform inversion of teleseismic P waves. This tomographic model reveals the subduction of the Iberian crust beneath the European plate, and the European serpentinized subcontinental mantle emplaced at shallow crustal levels beneath the Mauleon basin. The rift-inherited mantle wedge acted as an indenter during the Pyrenean convergence. These new results provide compelling evidence for the role of rift-inherited structures during mountain building in Alpine-type orogens.

Seismic monitoring of an Alpine mountain river

The Canfranc underground laboratory (LSC), excavated under the Central Pyrenees, is mainly devoted to the study of phenomena which needs “cosmic silence.” It also hosts a geodynamical facility, named Geodyn, which holds an accelerometer, a broadband seismometer, and two high-resolution laser strainmeters. During the routine processing of the seismic data, we detected an unusual spectral signature in the 2–10 Hz frequency band, which does not correspond to the typical sources of seismic noise and which can also be recognized in the strain records. After checking against meteorological and hydrological data, we can relate those signals to variations in the discharge by the Aragon River, an Alpine-style river in the southern Pyrenees, located about 400 m from the LSC Geodyn facility. Four main episodes have been identified since early 2011, each lasting 1–2 to 6–8 days. Additionally, a limited number of shorter episodes have also been detected. Three types of river-generated seismic events have been identified, related respectively to moderate rainfall, snowmelt, and flooding events associated to severe storms. Each of those types has distinctive characteristics which allow monitoring the hydrological events from the analysis of seismic and deformation data. A few previous studies have already described the seismic noise close to rivers with larger discharge or in small-scale experimental settings, and we are showing here that the so-called “fluvial seismology” can be useful to study the hydrological evolution of Alpine style streams and may have a potential interest for the civil authorities in charge of the management of hydrological basins.