Martine Sapin

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.

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.

Episodes of pit-crater collapse documented by seismology at Piton de la Fournaise

Calderas are oval fault-bounded depressions in volcanic regions. The formation of a small analog, a summit pit-crater, has followed a ten-month cycle of eruptive activity at Piton de la Fournaise volcano, Reunion Island, Indian Ocean. In 1985, six short episodes of summit seismic activity, with shocks indicating shear failures, have been followed by eruptions in five cases. The eruptions occurred near the base of the summit cone which is at 2.5 km elevation, inside the Enclos Fouque, an ancient caldera 7 km in diameter. In another instance, the summit seismic crisis did not evolve to an eruption but was followed by a seismic crisis 4 km deeper. In March 1986, low-level activity both at shallow depth beneath the summit and around 4 km depth to the southeast, a simultaneous occurrence not previously observed, culminated by a summital swarm on March 18. Activity resumed its low level and five hours later a small and short-lived eruption occurred at mid-distance between the summit and the rim of Enclos Fouque. One day later a flank eruption began at 1000 m elevation, outside of the Enclos, 9 km southeast of the summit. Four days later, effusion began near sea level 14 km southeast of the summit. The shift of activity from the vent inside to that outside the Enclos occurred whilst both shallow summital and deeper flank seismicity were sustained but without notable shallow activity between one vent and the next. Then the seismic activity, which in all five previous instances stopped when the vents opened, persisted for ten days increasing in energy release rate. Activity only consisted of shallow shocks in the summit area whilst eruption was low on the flank, 9 km, then 14 km away. The focal mechanisms are different from all those previously determined. They correspond neither to shear failure nor tensile fracturing by fluid overpressure. They suggest cracking due to volume increase in a low fluid-flow environment which may be consistent with the simultaneous tapping of magma deeper in the edifice. A final paroxysmal seismic crisis lasting one and a half days led to the collapse of a 200-m diameter, 80-m-deep pit-crater inside the Dolomieu summital caldera on March 29. This crisis released an order of magnitude more energy than previous activity. The long-period signature of these seismic events indicates low stress drop with respect to source dimensions. Sources are shallow. However, rather than directly marking the progressive development of a depression at the surface, they appear to correspond to fractures opening above the retreating magma and progressive stoping of a cavity. Similarities with seismic observations during the last phase of the Fernandina, Galapagos, 8-km2 caldera collapse of June 1968 suggest that the same kind of mechanical behaviour occurred with a scaling of three orders of magnitude in seismic moments and in volumes of collapsed blocks between the two cases.

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.

Complex images of Moho and variation of Vp/Vs across the Himalaya and South Tibet, from a joint receiver‐function and wide‐angle‐reflection approach

[1] Teleseismic receiver functions (RF) allow us to image the spatial variation of the crust-mantle boundary (Moho) along a tight array spanning from south of the Himalayas to the centre of the Tibetan Plateau. This approach is crosstested with wide-angle reflection imaging (WARR). Highlighted by each of the two independent methods, a complex architecture of the Moho with dipping and overlapping segments indicating lithospheric imbrication, is confirmed. The joint use of the two methods reveals an increase of the average crustal P-to-S-wave-velocity ratio from south to the centre of the Lhasa block. This may be due to lowered S-wave velocity confined in specific layers, that may be interpreted as partial melt. This accounts for half of the relative increase in the delay of direct teleseismic S-wave arrivals with respect to P-wave arrivals from south to north, suggesting a similar anomaly in the shallower mantle. INDEX TERMS: 0935 Exploration Geophysics: Seismic methods (3025); 7218 Seismology: Lithosphere and upper mantle; 9320 Information Related to Geographic Region: Asia. Citation: Galve ´, A., M. Sapin, A. Hirn, J. Diaz, J.-C.

Triple junction and ridge hotspots: Earthquakes, faults, and volcanism in Afar, the Azores, and Iceland

The Afar depression of the Horn of Africa, the Azores Archipelago, and Iceland are zones of plate divergence and mantle plume discharge. Strike-slip focal mechanisms dominate in major earthquakes. Among nodal planes a particular fault plane can be identified by precise epicentral location, from the strike of the aftershock zone, or by relation to field evidence of active faulting. These fault planes are orthogonal to the directions expected in previous models for transform faults along a continuous divergent plate boundary. Recent volcanic features are consistently at small angles with the fault planes. Rotation of blocks within a deforming zone is implied by the direction of fault planes, the sense of motion along them, and their location on a series of parallel features transverse to the deformed zone. The drift of oceanic or continental large lithospheric plates deforms a zone of lithosphere, deriving from both the plume discharge and ridge accretion while the zone is created.

Dehydration of subducting slow-spread oceanic lithosphere in the Lesser Antilles

Subducting slabs carry water into the mantle and are a major gateway in the global geochemical water cycle. Fluid transport and release can be constrained with seismological data. Here we use joint active-source/local-earthquake seismic tomography to derive unprecedented constraints on multi-stage fluid release from subducting slow-spread oceanic lithosphere. We image the low P-wave velocity crustal layer on the slab top and show that it disappears beneath 60–100 km depth, marking the depth of dehydration metamorphism and eclogitization. Clustering of seismicity at 120–160 km depth suggests that the slab’s mantle dehydrates beneath the volcanic arc, and may be the main source of fluids triggering arc magma generation. Lateral variations in seismic properties on the slab surface suggest that serpentinized peridotite exhumed in tectonized slow-spread crust near fracture zones may increase water transport to sub-arc depths. This results in heterogeneous water release and directly impacts earthquakes generation and mantle wedge dynamics.