High-resolution seismic imaging of the western Hellenic subduction zone using teleseismic scattered waves

Abstract

S U M M A R YThe active Hellenic subduction system has long been considered an ideal setting for studying subduction dynamics because it is easily accessible and of limited spatial extent. It has been the focus of numerous seismological studies over the last few decades but, nonetheless, the detailed structure of both the slab and the surrounding mantle remain poorly constrained in an intermediate depth range from 30 to 150 km. The objective of this paper is to fill this gap. The intermediate depth regime is of particular interest because it is pivotal for improving our understanding of the dynamic interaction between subducting lithosphere and the surrounding mantle. An interdisciplinary effort aimed at addressing this challenge is currently undertaken by the 'Multidisciplinary Experiments for Dynamic Understanding of Subduction under the Aegean Sea' (MEDUSA) project. As part of the MEDUSA initiative, a temporary array consisting of 40 densely spaced broad-band seismometers from the IRIS-PASSCAL pool has been deployed in southern Greece. We process the teleseismic data recorded by this array with a migration algorithm based on the generalized radon transform to obtain high-resolution images of the subduction zone in 2-D. The images reveal a sharp Mohorovičić discontinuity (Moho) at depths ranging from 30 km beneath the western margin of the Aegean Sea to 40 km beneath the central Peloponnesus, where it outlines the crustal root of the Hellenides. To the west of the Hellenides, the continental Moho is not identified, but we interpret a pronounced discontinuity imaged at ∼20 km depth as the contact between low-velocity sediments and high-velocity crystalline basement. The images also show the subducted oceanic crust as a low-velocity layer that plunges at a constant angle of 21 • from west to east. The oceanic crust exhibits low velocities to at least 90 km depth, indicating that the bulk of fluid transfer from the subducted slab into the mantle wedge occurs below this depth. A detailed comparison of images constructed for distinct backazimuthal illuminations reveals deviations in the geometry of the subducted slab. These deviations are attributed to structural and/or compositional changes taking place directly to the north of the MEDUSA array, and are consistent with the existence of a slab tear beneath the Central Hellenic Shear Zone.

Receiver function images of the Hellenic subduction zone and comparison to microseismicity

“…Suckale et al, 2009;Pearce et al, 2012). Our results are also consistent with those derived from regional body-wave travel-time tomography for the western Hellenic subduction zone (e.g.…”

“…Recently, receiver function studies have obtained new constraints on the nature of the subducting African plate mainly in the western Hellenic arc (e.g. Sodoudi et al, 2006;Suckale et al, 2009;Gesret et al, 2011;Pearce et al, 2012). The base of the subducting African lithosphere has been demonstrated down to a depth of about 225 km beneath the volcanic arc using converted Sto-P waves (e.g.…”

“…Studies of converted or scattered waves in the Aegean were mainly obtained from small networks mostly covering the western part of the Hellenic arc (e.g. Suckale et al, 2009;Gesret et al, 2011;Pearce et al, 2012).…”

The dynamics of laterally variable subductions: laboratory models applied to the Hellenides

“…The situation is less clear for the Kefalonia Fault; seismic tomography (Spakman et al, 1988;Piromallo et al, 2006;Suckale et al, 2009) possibly reveals a discontinuous slab and more detailed seismic experiments suggest that the surface of the slab is offset horizontally by some 100 km across the Kefalonia Fault (Gesret et al, 2011). However, whether the mantle flows through a potential Kefalonia window remains uncertain, and it is most likely small, if any.…”

Seismic visibility of a deep subduction channel – insights from numerical simulation of high-frequency seismic waves emitted from intermediate depth earthquakes

“…Tomographic studies appear not to be suitable because their resolution is generally insufficient to image such a thin channel. Methods using scattered or converted teleseismic waves (Bostock, 2002;Suckale et al, 2009) have potential to reach a resolution on the order of 10 km, but it is questionable whether they could distinguish the subduction channel from the oceanic crust. An alternative approach could be to search for specific phases in the seismic records which may serve as indicators for the existence of a channel.…”