ALBERT

Description: We use data from recently installed broad-band seismographs on the islands of Crete, Gavdos, Santorini, Naxos and Samos in the Hellenic subduction zone to construct receiver function images of the crust and upper mantle from south of Crete into the Aegean Sea. The stations are equipped with STS-2 seismometers and they are operated by GFZ Potsdam, University of Chania and ETH Zürich. Teleseismic earthquakes recorded by these stations at epicentral distances between 35° and 95° have been used to calculate receiver functions. The receiver function method is a routinely used tool to detect crustal and upper-mantle discontinuities beneath a seismic station by isolating the P–S converted waves from the coda of the P wave. Converted P–S energy from the oceanic Moho of the subducted African Plate is clearly observed beneath Gavdos and Crete at a depth ranging from 44 to 69 km. This boundary continues to the north to nearly 100 km depth beneath Santorini island. Because of a lack of data the correlation of this phase is uncertain north of Santorini beneath the Aegean Sea. Moho depths were calculated from primary converted waves and multiply reflected waves between the Moho and the Earth's surface. Beneath southern and eastern Crete the Moho lies between 31 and 34 km depth. Beneath western and northern Crete the Moho is located at 32 and 39 km depth, respectively, and behaves as a reversed crust–mantle velocity contrast, possibly caused by hydration and serpentinization of the forearc mantle peridotite. The Moho beneath Gavdos island located south of Crete in the Libyan Sea is at 26 km depth, indicating that the crust south of the Crete microcontinent is also thinning towards the Mediterranean ridge. This makes it unlikely that part of the crust in Crete consists of accreted sediments transported there during the present-day subduction process which began approximately 15 Ma because the backstop, i.e. the boundary between the current accretionary wedge of the Mediterranean ridge and the Crete microcontinent, is located approximately 100 km south of Gavdos. A seismic boundary at 32 km depth beneath Santorini island probably marks the crustal base of the Crete microcontinent. A shallower seismic interface beneath Santorini at 20–25 km depth may mark the depth of the detachment between the Crete microcontinent and the overlying Aegean subplate. The Moho in the central and northern Aegean, at Naxos and Samos, is observed at 25 and 28 km depth, respectively. Assuming a stretching factor of 1.2–1.3, crustal thickness in the Aegean was 30–35 km at the inception of the extensional regime in the Middle Miocene.

Description: The increasingly dense coverage of Europe with broad-band seismic stations makes it possible to image its lithospheric structure in great detail, provided that structural information can be extracted effectively from the very large volumes of data. We develop an automated technique for the measurement of interstation phase velocities of (earthquake-excited) fundamental-mode surface waves in very broad period ranges. We then apply the technique to all available broad-band data from permanent and temporary networks across Europe. In a new implementation of the classical two-station method, Rayleigh and Love dispersion curves are determined by cross-correlation of seismograms from a pair of stations. An elaborate filtering and windowing scheme is employed to enhance the target signal and makes possible a significantly broader frequency band of the measurements, compared to previous implementations of the method. The selection of acceptable phase-velocity measurements for each event is performed in the frequency domain, based on a number of fine-tuned quality criteria including a smoothness requirement. Between 5 and 3000 single-event dispersion measurements are averaged per interstation path in order to obtain robust, broad-band dispersion curves with error estimates. In total, around 63,000 Rayleigh- and 27,500 Love-wave dispersion curves between 10 and 350 s have been determined, with standard deviations lower than 2 per cent and standard errors lower than 0.5 per cent. Comparisons of phase-velocity measurements using events at opposite backazimuths and the examination of the variance of the phase-velocity curves are parts of the quality control. With the automated procedure, large data sets can be consistently and repeatedly measured using varying selection parameters. Comparison of average interstation dispersion curves obtained with different degrees of smoothness shows that rough perturbations do not systematically bias the average dispersion measurement. They can, therefore, be treated as random but they do need to be removed in order to reduce random errors of the measurements. Using our large new data set, we construct phase-velocity maps for central and northern Europe. According to checkerboard tests, the lateral resolution in central Europe is ≤150 km. Comparison of regional surface-wave tomography with independent data on sediment thickness in North-German Basin and Polish Trough confirms the high-resolution potential of our phase-velocity measurements. At longer periods, the structure of the lithosphere and asthenosphere around the Trans-European Suture Zone (TESZ) is seen clearly. The region of the Tornquist-Teisseyre-Zone in the southeast is associated with a stronger lateral contrast in lithospheric thickness, across the TESZ compared to the region across the Sorgenfrei-Tornquist-Zone in the northwest.