Mireille Laigle

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.

Rift structure, evolution, and earthquakes in the Gulf of Corinth, from reflection seismic images

Abstract Continental extension is forming the Gulf of Corinth across the strike of earlier Alpine evolution. Here, we present the first deep reflection sections with pre-stack depth-migration processing across the deep basin of the Corinth active rift, which image structures unpredicted by current models. Resolving the infill as a pile of layers that are broken by faults allows one to follow their subsidence and deformation history. Variation among the profiles suggests that southern normal faults control the rift in a time progression from the east towards its western tip. On the central, Derveni–Itea transect, a 3-km widening of the basin accrued since the initiation of this control that is marked by an unconformity between the two main sedimentary units. This is estimated to have occurred 0.5–0.6 Myr ago, assuming the glacio-eustatic sea-level changes have controlled the stratigraphy of sediments deposited as a succession of layers on the subsiding hangingwall, as they did on the southern footwall in forming the famous flight of marine terraces of Corinth. A roll-over anticline and crestal collapse graben are diagnostic of the control by a normal fault of dip varying with depth. The deeper low-angle part of this bi-planar fault is indeed imaged as a reflector in the basement. The occurrence of the collapse with a breakaway at the steep southern basin-bounding fault of the hangingwall slab can be estimated 0.12–0.2 Myr ago, with a marked increase in extension rate that brought it to its present fastest value over 10 mm/yr. The low-angle part of the active fault might also have controlled earlier evolution upslope and in the basin. When compared with inferences from earthquake studies, this low-angle active fault may not appear to be seismogenic but may participate to the seismic cycle, possibly in a conditionally stable regime. Active faults seen as sea-bottom scarps merely accommodate deformation of its subsiding hangingwall. The footwall of the low-angle faults, which current seismicity shows to be in extension, appears then as being pulled out from beneath the rift, in a motion towards the rolling-back slab that causes the Hellenic subduction retreat.

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.

Seismological and SAR signature of unrest at Nisyros caldera, Greece

Abstract Nisyros island, a Quaternary volcanic center located at the SE of the Aegean Volcanic Arc, has been in the past characterized by periods of intense seismic activity accompanied sometimes by hydrothermal explosions, the last one being in 1887. The recent long lasting episode of unrest (1995–1998) in the area is the first instrumentally documented providing information on the behavior of the volcano. Evidence from seismicity and SAR interferometry suggests that the presently active part of the Kos–Nisyros volcano-tectonic complex is located at the NW coast of Nisyros island defining an area much smaller than the whole volcano-tectonic area. Seismicity patterns vary both temporally and spatially consistently with different rates of vertical ground deformation inferred from SAR interferometry. These observations help us to discuss the different elements controlling the behavior of the volcanic system such as: the existence, location and timing of magma chamber inflation, the occurrence of tensile failure at the boundaries of the chamber and the possibility of magmatic fluids being expelled to form a shallow magmatic intrusion, the seismic failure and migration of hypocenters indicating shallow magma transport.

Seismic coupling and structure of the Hellenic subduction zone in the Ionian Islands region

Abstract The western Hellenic arc has been commonly considered as a largely aseismic subduction zone, from the comparison of a small rate of shortening derived from the seismic moment release, with a large rate of convergence inferred from geology. Complete seismic coupling would instead be expected from models that consider a control by plate tectonic forces, because of the trenchward velocity of the Hellenic–Aegean upper plate now confirmed with GPS measurements. In the region of the Ionian Islands, a subduction interplate boundary has been recently imaged and its seismogenic downdip width suggested to be moderate, from reflection seismic profiling and local earthquake tomography. In the appropriate model for such an earthquake source region, which considers a single interplate fault and takes into account these features, the moderate seismic moment release is found consistent with complete seismic coupling of this subduction. The shallow downdip limit of the seismogenic zone can be interpreted as due to the interplate boundary being overlain there by the ductile deeper crust of the orogenically thickened Hellenides.

Slab top dips resolved by teleseismic converted waves in the Hellenic subduction zone

The variations of the arrival times and polarities with backazimuth and distance of teleseismic P-to-S converted waves at interfaces bounding the slab crust under the upper plate mantle are used to constrain the depth, dip angle and azimuth of the slab of the Hellenic subduction zone. A grid search is designed to estimate the model parameters. Dip values of 16-18°, with an azimuth of 20° to 40°, are thus derived at 3 sites aligned over 50 km along the eastern coast of Peloponnesus. They are consistent with the variation from 54 to 61 km of the slab top depths constrained below each receiver. North of the Gulfs of Corinth and Evvia, a similar depth for the top of the slab is found at a distance from the subduction at least 100 km larger. This suggests flatter subduction of a different slab segment. Such a variation in slab attitude at depth across the region from south of the eastern Gulf of Corinth to north of Evvia is a candidate for the control of the recent or active localized crustal thinning of the upper plate we documented in earlier work, and of the surface deformation.

Explosion‐seismic tomography of a magmatic body beneath Etna: Volatile discharge and tectonic control of volcanism

To constrain the upper depth range of lateral heterogeneities beneath Mt Etna, a seismic tomography method with artificial sources is designed to undershoot the target and restrict the depth of seismic propagation by reflection at the top of the basement. A strong high velocity anomaly thus imaged southeast of the summit, interpreted as a magmatic body, is reliably constrained to reach at least as shallow as 2 km depth where exsolution of SO2 from magma may occur. The high gas output rate of Etna [Allard, 1997] hence appears as the normal complement of the huge magma body seismically imaged. This magma body is located beneath the volcanic edifice rather than within, which attests to the control of volcanism by extension. These volatile output and internal structure distinguish Mt Etna volcanism among current types and may relate to the peculiar context of a recent regional change from subduction-related compression to extension.

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.