High-resolution surface wave tomography beneath the Aegean-Anatolia region: constraints on upper-mantle structure (original) (raw)
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S U M M A R Y This study provides new constraints on the upper-mantle structure from western Greece to central Anatolia using seismic data of permanent broad-band networks recently installed in Greece and Turkey and from a two-year temporary array (SIMBAAD experiment). We used ∼200 seismic events recorded at 146 broad-band stations with a typical interstation distance of 60–100 km across the study area. The high-resolution 3-D shear wave velocity model of the mantle is obtained by inversion of fundamental-mode Rayleigh wave phase velocity maps for periods between 20 and 195 s. The tomography is based on ray tracing in heterogeneous media taking into account external propagation effects. The horizontal resolution is approximately 100 km, however small heterogeneities may suffer from some horizontal smearing and damping. The vertical resolution is approximately 100 km. The vertical smoothing is necessary to avoid unresolved spurious shear wave velocity oscillations in the upper mantle. The errors on shear wave velocities in our 3-D model (0.02–0.1 km s −1) are significantly smaller than the amplitude of Vs variations (0.3–0.5 km s −1). In spite of the vertical and horizontal smoothing, our model shows details in the upper-mantle structure never reached at regional scale in the area. The overall structure is characterized by a low-velocity zone (80–200 km depth) reflecting a slow and warm asthenosphere underlying a thin lithosphere. The southwesternmost termination of the low-velocity anomaly corresponds to the northward dipping Hellenic slab. The detailed shear velocity structure of the upper mantle beneath Anatolia appears to be far more geometrically complex than revealed in previous tomographic studies of the area. At depths larger than or equal to 160 km, velocities are overall high beneath Anatolia, partly due to the presence of dipping high-velocity anomalies which we tentatively interpret as remnant slabs. The southernmost high-velocity anomaly beneath Anatolia is separated from the eastern edge of the Hellenic slab by a major low-velocity anomaly which we interpret as the trace of asthenospheric mantle material rising inside a vertical slab tear beneath southwestern Anatolia.
Geophysical Journal International, 2009
A vertical array of accelerometers was installed in Ataköy (western Istanbul) with the longterm aim of improving our understanding of in situ soil behaviour, to assess the modelling and parametric uncertainties associated with the employed methodologies for strong-motion site-response analysis, and for shallow geological investigations. Geotechnical and geophysical investigations were carried out to define the subsoil structure at the selected site. Data associated with 10 earthquakes (2.7 < M < 4.3) collected during the first months of operation of the array were used to image the upgoing and downgoing waves by deconvolution of waveforms recorded at different depths. Results have shown that the velocity of propagation of the imaged upgoing and downgoing waves in the borehole is consistent with that of S or P waves, depending on the component of ground acceleration analysed but independent of the chosen signal window. In particular, an excellent agreement was found between the observed upgoing and downgoing wave traveltimes and the ones calculated using a model derived by seismic noise analysis of array data. The presence of a smaller pulse on the waveforms obtained by deconvolution of the horizontal components suggests both internal S-wave reflection and S-to-P mode conversion, as well as a not normal incidence of the wavefield. The presence of a pulse propagating with S-wave velocity in the uppermost 25 m in the waveforms obtained by the deconvolution of the vertical components suggests P-to-S mode conversion. These evidences imply that, even when site amplification is mainly related to 1-D effects, the standard practice in engineering seismology of deconvolving the surface recording down to the bedrock using an approximate S-wave transfer function (generally valid for vertical incidence of SH waves) might lead to errors in the estimation of the input ground motion required in engineering calculations. Finally, downgoing waves with significant amplitudes were found down to 70 m and even to 140 m depth. This result provides a warning about the use of shallow borehole recordings as input for the numerical simulation of ground motion and for the derivation of ground motion prediction relationships.
Three-dimensional S-wave structure of the upper mantle beneath Turkey from surface wave tomography
Geophysical Journal International
A 3-D upper-mantle structure beneath Turkey is investigated using phase speeds of fundamental-mode Rayleigh waves employing a conventional two-station method with high-density seismic networks in Turkey. We analyse 289 seismic events with moment magnitude 5.5 and greater, and with focal depth shallower than 100 km between 2006 and 2008. Waveform data are derived from 164 three-component broad-band seismic stations operated by two national seismic networks. At first, Rayleigh-wave phase speed maps are obtained from the inversion of two-station phase speeds using about 1000-3000 paths, depending on the period of Rayleigh waves. The three-dimensional S-wave model is then obtained in the depth range from 40 to 180 km using the phase speed maps in the period range from 25 to 120 s. Our model reveals the fast anomalies in the north of Cyprus associated with the subducted portion of the African oceanic lithosphere from the Cyprus trench. We identify a vertical discontinuity of the fast ano...
Seismic‐Wave Propagation in Shallow Layers at the GONAF‐Tuzla Site, Istanbul, Turkey
Bulletin of the Seismological Society of America, 2016
Using the first dataset available from the downhole Geophysical Observatory of the North Anatolian Fault, we investigated near-surface seismic-wave propagation on the Tuzla Peninsula, Istanbul, Turkey. We selected a dataset of 26 seismograms recorded at Tuzla at sensor depths of 0, 71, 144, 215, and 288 m. To determine near-surface velocities and attenuation structures, the waveforms from all sensors were pairwise deconvolved and stacked. This produced low-noise empirical Green's functions for each borehole depth interval. From the Green's functions, we identified reflections from the free surface and a low-velocity layer between ∼90 and ∼140 m depth. The presence of a low-velocity zone was also confirmed by a sonic log run in the borehole. This structure, plus high near-surface P-and S-wave velocities of ∼3600-4100 and ∼1800 m=s, lead to complex interference effects between upgoing and downgoing waves. As a result, the determination of quality factors (Q) with standard spectral ratio techniques was not possible. Instead, we forward modeled the Green's functions in the time domain to determine effective Q values and to refine our velocity estimates. The effective Q P values for the depth intervals of 0-71, 0-144, 0-215, and 0-288 m were found to be 19, 35, 39, and 42, respectively. For the S waves, we obtained an effective Q S of 20 in the depth interval of 0-288 m. Considering the assumptions made in our modeling approach, it is evident that these effective quality factors are biased by impedance contrasts between our observation points. Our results show that, even after correcting for a free-surface factor of 2, the motion at the surface was found to be 1.7 times greater than that at 71 m depth. Our efforts also illustrate some of the difficulties of dealing with site effects in a strongly heterogeneous subsurface.
Determination of the Crust and Upper-Mantle Structure in Anatolia by Surface Wave Data
Natural Science, 2014
In this study, the crust and upper mantle structure of Anatolia have been investigated by measuring the group velocity dispersion data of discriminated seismic surface waves. In the scope of the study, it has selected the profiles between six stations located in western Anatolia of Bogazici University Kandilli Observatory Earthquake Research Institute, national network of Turkey, and records of an earthquake (having about 10˚ epicentral distance) occurred in the eastern of Anatolia have been used. Firstly, surface wave discrimination filter based on the polarization properties has been applied to three-component records and emphasized to surface waves. Then the group velocities have been calculated by multiple filter technique. A five-layered crustal model having total thickness of 38 -40 km and Pn-wave velocity of 8.00 km/sec in the upper-mantle has been determined through inversion of surface wave group velocity dispersion data in the period range of 10 sec to 60 sec.
Studia Geophysica et Geodaetica, 2007
A set of two hundred shear-wave velocity models of the crust and uppermost mantle in southeast Europe is determined by application of a sequence of methods for surfacewaves analysis. Group velocities for about 350 paths have been obtained after analysis of more than 600 broadband waveform records. Two-dimensional surface-wave tomography is applied to the group-velocity measurements at selected periods and after regionalisation, two sets of local dispersion curves (for Rayleigh and Love waves) are constructed in the period range 8−40 s. The shear-wave velocity models are derived by applying non-linear iterative inversion of local dispersion curves for grid cells predetermined by the resolving power of data. The period range of observations limits the velocity models to depths of 70 km in accordance to the penetration of the surface waves with a maximum period of 40 s. Maps of the Moho boundary depth, velocity distribution above and below Moho boundary, as well as velocity distribution at different depths are constructed. Well-known geomorphologic units (e.g. the Pannonian basin, southeastern Carpathians, Dinarides, Hellenides, Rodophean massif, Aegean Sea, western Turkey) are delineated in the obtained models. Specific patterns in the velocity models characterise the southeast Carpathians and adjacent areas, coast of Albania, Adriatic coast of southern Italy and the southern coast of the Black Sea. The models obtained in this study for the western Black Sea basin shows the presence of layers with shear-wave velocities of 3.5 km/s -3.7 km/s in the crust and thus do not support the hypothesis of existence of oceanic structure in this region. K e y w o r d s : surface waves, group velocity, tomography, inversion, shear-wave velocity structure, southeastern Europe R. Raykova and S. Nikolova 166 Stud. Geophys. Geod., 51 (2007)
Multiple resolution surface wave tomography: the Mediterranean basin
Geophysical Journal …, 2004
From a large set of fundamental-mode surface wave phase velocity observations, we map the transversely isotropic lateral heterogeneities in the upper-mantle shear velocity structure. We design a multiple resolution inversion procedure, which allows us to parametrize any selected region more finely than the rest of the globe. We choose, as a high-resolution region, the upper mantle underlying the Mediterranean basin. We formulate the inverse problem as in a previous paper by Boschi & Ekström, calculating regional JWKB (Jeffreys-Wentzel-Kramers-Brillouin) surface wave sensitivity kernels for each pixel of a 2 • × 2 • starting model, including the high-resolution global crustal map Crust 2.0. We find that the available surface wave data can resolve the most important geophysical features of the region of interest, providing a reliable image of intermediate spatial wavelength.