M. Laigle

Roots of Etna volcano in faults of great earthquakes

Abstract Results from several seismic methods allow us to sketch the deep structure of Etna and its Ionian margin. Under Etna a volume of high velocity material is found in a structurally high position; the emplacement of this suggests spreading of the surrounding medium. Just offshore, down-to-the-east normal faults penetrate through the upper crust. The deeper crustal structure beneath appears upwarped from the basin towards Etna. Juxtaposed with the crust of Sicily, a thinner crust reaches from the Ionian Basin under Etna, and the mantle is upwarped. In such a structure, magma can then be viewed as a melted lens capping a mantle upwarp at shallow depth, rather than in an intracrustal chamber. This reduces the conflict between estimates of its volume from excess output of volatiles and short residence times. A link in time is indicated between volcanic and seismic activity at a large scale: over the millennium the reported ends of episodes of high output rates of magma are followed by the reported occurrences of magnitude 7 + earthquakes which caused destruction in southeastern Sicily. Several steep active normal fault have been imaged to a depth of 10 km the crust up to 30 km offshore of the cities of Catania and Augusta, which may be fault planes for such large earthquakes. They expand and prolongate the system of the Timpe faults on the eastern flank of Etna, thus linking large-scale tectonics offshore with the volcano. Etna developed together with normal faulting, upwarp, and spreading during the recent evolution of the former Ionian subduction. Activation of the material at depth at the lateral edge of the slab, by vertical motion with extension above, could produce the peculiar type of Etna magmatism.

Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution

Abstract Crustal imaging could be achieved on normal-incidence reflection profiles offshore eastern Sicily by using industrial-grade reflection seismic with improved marine sources. Thick recent sediments, a reflective pile including the Mesozoic deposits, a transparent upper crust, and a band of low frequency reflections attributed to the lower crust, are imaged in the seismic sections. The structure of the crust and its thickness show features inherited from the Mesozoic evolution as a passive margin, by which Ionian basin crust was formed around the Hyblean continental promontory of Africa to constitute the southern plate in the later convergence with Europe. The seismic images are also marked by the lithospheric deformation due to the Neogene overriding of the northern part of this paleomargin by the Calabro-Peloritan block of European continental crust. This transpressive motion may have been guided along the northern part of this paleomargin where the seismic profiles evidence a hinge line between the northward upslope of the Moho of that old passive margin and its downslope to the present slab under the Tyrrhenian Sea. Etna volcano is located at the intersection of this mantle upwarp by a zone of active sea-bottom normal-faults, which cut across the formerly constructed compressional belt. The onset of its volcanic activity is roughly coeval with that of the cessation of interplate thrusting and could hence be related to a change of the coupling of the Ionian slab. This slab is now probably disconnected from the overriding plate and rolled back in front of the expanding hot Tyrrhenian asthenospheric dome with the mobilisation of a viscous mantle material at depth. An active lithospheric fault is here imaged which cuts over more than 100xa0km into the Ionian basin. The fault runs from the Tyrrhenian margin in the SSE direction of Etna updip of the southwestern lateral edge of this slab, leaving north-eastward the extruded Calabrian block and on its south-westward edge the uplifted crust and mantle structures of Etna. Along it, the crust, including the Mesozoic and deeper layers, has sagged vertically in the segment in front of the slab, with a finite throw across the fault increasing from the basin towards Etna.

North Aegean crustal deformation: An active fault imaged to 10 km depth by reflection seismic data

Three multichannel seismic profiles imaged a normal fault to at least 10 km depth in the North Aegean Trough and Thermaikos basin. The fault is active and recent, forming a scarp at sea bottom and crossing the Quaternary deltaic front on the northern slope of the trough controlled to the south by the North Anatolian fault. Prestack depth migration imaged the fault as a seismic reflector cutting steeply across the sedimentary rocks and flattening in the basement. From the seismic image, the N100°E strike of the fault scarp, and the orientations of the three profiles, the true fault dip is constrained to an average 20° in the basement, a low-angle dip. The throw and age estimated from the geometry in the sedimentary rocks document recent onset of the motion that must have occurred at a high rate. Both the direction and the rate of slip are consistent with the instantaneous motion as measured by space-based geodesy, which shows the fault to be forming by pure normal slip. Large earthquakes that have occurred in the basin may relate to such normal faults, whereas the North Anatolian fault with its current strike-slip earthquakes appears to slip here under low resolved shear stress.

Segmented Hellenic slab rollback driving Aegean deformation and seismicity

The NE dipping slab of the Hellenic subduction is imaged in unprecedented detail using teleseismic receiver function analysis on a dense 2-D seismic array. Mapping of slab geometry for over 300u2009km along strike and down to 100u2009km depth reveals a segmentation into dipping panels by along-dip faults. Resolved intermediate-depth seismicity commonly attributed to dehydration embrittlement is shown to be clustered along these faults. Large earthquakes occurrence within the upper and lower plate and at the interplate megathrust boundary show a striking correlation with the slab faults suggesting high mechanical coupling between the two plates. Our results imply that the general slab rollback occurs here in a differential piecewise manner imposing its specific stress and deformation pattern onto the overriding Aegean plate.

Slab segmentation controls the interplate slip motion in the SW Hellenic subduction: New insight from the 2008 Mw 6.8 Methoni interplate earthquake

We present an integrated approach of the seismic structure and activity along the offshore SW Hellenic subduction from combined observations of marine and land seismic stations. Our imaging of the slab top topography from teleseismic receiver function analysis at ocean bottom seismometers supports a trenchward continuation of the along-dip slab faults beneath the Peloponnesus. We further show that their morphostructural control accounts for the backstepping of the thrust contact of the Mediterranean Ridge accretionary wedge over the upper plate. Local seismic activity offshore SW Peloponnesus constrained by ocean bottom seismometer observations reveals a correlation with specific features of the forearc: the Matapan Troughs. We study the Mw6.8 14.02.2008 interplate earthquake offshore SW Peloponnesus and show that its nucleation, rupture zone, and aftershocks sequence are confined to one slab panel between two adjacent along-dip faults and are thus controlled by not only the offshore slab top segmentation but also the upper plate sea-bottom morphology.