Geophysical datasets sensitive to different physical parameters can be used to improve resolution of Earth's internal structure. Herein, we jointly invert long-period magnetotelluric (MT) data and surface-wave dispersion curves. Our approach is based on a joint inversion using a genetic algorithm for a one-dimensional (1-D) isotropic structure, which we extend to 1-D anisotropic media. We apply our new anisotropic joint inversion to datasets from Central Germany demonstrating the capacity of our joint inversion algorithm to establish a 1-D anisotropic model that fits MT and seismic datasets simultaneously and providing new information regarding the deep structure in Central Germany. The lithosphere/asthenosphere boundary is found at approx. 84 km depth and two main anisotropic layers with coincident most conductive/seismic fast-axis direction are resolved at lower crustal and asthenospheric depths. We also quantify the amount of seismic and electrical anisotropy in the asthenosphere showing an emerging agreement between the two anisotropic coefficients. Citation: Roux, E., M. Moorkamp, A. G. Jones, M. Bischoff, B. Endrun, S. Lebedev, and T. Meier (2011), Joint inversion of long-period magnetotelluric data and surface-wave dispersion curves for anisotropic structure: Application to data from Central Germany, Geophys. Res. Lett., 38, L05304, doi: 10.1029/2010GL046358.

A series of investigations has been carried out over the last decade in Europe aimed at deriving quantitative information on site amplification from non-invasive techniques, based principally on surface wave interpretations of ambient noise measurements. The present paper focuses on their key outcomes regarding three main topics. First, methodological, hardware and software developments focusing on the acquisition and the processing of both single point and array microtremor measurements, led to an efficient tool with in situ control and processing, giving rise to robust and reproducible results. A special attention has been devoted to the derivation and use of the Rayleigh wave ellipticity. Second, the reliability of these new tools has been assessed through a thorough comparison with borehole measurements for a representative though limited set of sites located in Southern Europe, spanning from stiff to soft, and shallow to thick. Finally, correlations between the site parameters available from such non-invasive techniques, and the actual site amplification factors as measured with standard techniques, are derived from a comprehensive analysis of the Japanese KIKNET data. This allows to propose alternative, simple site characterization providing an improved variance reduction compared with the "classical" V-S30 classification. While these results could pave the road for the next generation of building codes, they can also be used now for regulatory site classification and microzonation studies, in view of improved mapping and estimation of site amplification factors, and for the characterization of existing strong motion sites.

Inversion of the fundamental mode of the Rayleigh wave dispersion curve does not provide a unique solution and the choice of the parameterization (number of layers, range of velocity, and thickness values for the layers) is of prime importance for obtaining reliable results. We analyzed shear-wave velocity profiles derived from borehole tests at 10 sites where soil layers overlay bedrock in various geologic contexts. One to three seismic layers with linear velocity laws could model all of them. Three synthetic models defined from this preliminary study were used to understand the influence of parameterization on the dispersion curve inversion. This analysis resulted in the definition of a two-step inversion procedure for sites exhibiting a strong impedance con-trast. In the first step, the dispersion curve is inverted with an increasing number of layers over half space. The evolution of the minimum misfit and bedrock depth with number of layers allows the estimation of the true bedrock depth range. In the second step, this information is introduced in inversions with linear velocity laws. Synthetic tests showed that applying this procedure requires the dispersion curve over a frequency range from [Formula: see text] to [Formula: see text], where [Formula: see text] is the site resonance frequency. The strategy was tested on two real cases for which Rayleigh wave dispersion curves were measured over this frequency range using passive and active seismic methods. The strategy was successful at the first site, while the bedrock depth was overestimated by 15% at the second site, probably resulting from the existence of a higher mode affecting the dispersion curve at low frequency.

The island of Crete is located in the forearc of the Hellenic subduction zone, where the African lithospheric plate is subducting beneath the Eurasian one. The depth of the plate contact as well as the internal structure of the Aegean plate in the area of Crete have been a matter of debate. In this study, seismic constrains obtained by wide-angle seismic, receiver function and surface wave studies are discussed and compared to a 3D density model of the region.The interface between the Aegean continental lithosphere and the African one is located at a depth of about 50 km below Crete. According to seismic studies, the Aegean lithosphere in the area of Crete is characterised by strong lateral, arc-parallel heterogeneity. An about 30 km thick Aegean crust is found in central Crete with a density of about 2850 kg/m(3) for the lower Aegean continental crust and a density of about 3300 kg/m(3) for the mantle wedge between the Aegean crust and the African lithosphere. For the deeper crust in the area of western Crete two alternative models have been proposed by seismic studies. One with an about 35 km thick crust and another one with crustal velocities down to the plate contact. A grid search is performed to test the consistency of these models with gravimetric constraints. For western Crete a model with a thick lower Aegean crust and a density of about 2950 kg/m(3) is favoured. The inferred density of the lower Aegean crust in the area of Crete correlates well with S-wave velocities obtained by surface wave studies.Based on the 3D density model, the weight of the Aegean lithosphere is estimated along an E-W oriented profile in the area of Crete. Low weights are found for the region of western Crete. (c) 2007 Elsevier Ltd. All rights reserved.

The island of Crete represents a horst structure located in the central forearc of the retreating Hellenic subduction zone. The structure and dynamics of the plate boundary in the area of Crete are investigated by receiver function, surface wave and microseismicity using temporary seismic networks. Here the results are summarized and implications for geodynamic models are discussed. The oceanic Moho of the subducted African plate is situated at a depth of about 50-60 km beneath Crete. The continental crust of the overriding Aegean lithosphere is about 35 km thick in eastern and central Crete, and typical crustal velocities are observed down to the upper surface of the downgoing slab beneath western Crete. A negative phase at about 4 s in receiver functions occurring in stripes parallel to the trend of the island points to low-velocity slices within the Aegean lithosphere. Interplate seismicity is spread out about 100 km updip from the southern coastline of Crete. To the south of western Crete, this seismically active zone corresponds to the inferred rupture plane of the magnitude 8 earthquake of AD 365. In contrast, interplate motion appears to be largely aseismic beneath the island. The coastline of Crete mimics the shape of a microseismically quiet realm in the Aegean lithosphere at 20-40 km depth, suggesting a relation between active processes at this depth range and uplift. The peculiar properties of the lithosphere and the plate interface beneath Crete are tentatively attributed to extrusion of material from a subduction channel, driving differential uplift of the island by several kilometres since about 4 Ma.

Pseudospectral 2D modeling of wave propagation provides a fresh look at receiver function data from the Hellenic subduction zone. It is shown that distinct Moho topography, which is suggested for this complex subduction environment from various geophysical data sets, can provide a conclusive explanation for previously observed “inverted” Moho phases in the forearc as well as for a complex sequence of apparently dipping conversions in the volcanic arc. Prominent effects of strongly dipping discontinuities include the change of sign in Ps phases refracted past the vertical and multiple P reflections that cannot be separated from Ps conversions during receiver function processing. Unambiguous identification of these phenomena can be hampered by poor azimuthal coverage of the data. Still, in regions where discontinuity topography can reasonably be expected, its effects should be considered when interpreting receiver functions, as results might be spurious otherwise.

Temporary local seismic networks were installed in western Crete, in central Crete, and on the island Gavdos south of western Crete, respectively, in order to image shallow seismically active zones of the Hellenic subduction zone.More than 4000 events in the magnitude range between -0.5 and 4.8 were detected and localized. The resulting three-dimensional hypocenter distribution allows the localization of seismically active zones in the area of western and central Crete from the Mediterranean Ridge to the Cretan Sea. Furthermore, a three-dimensional structural model of the studied region was compiled based on results of wide-angle seismics, surface wave analysis and receiver function studies. The comparison of the hypocenter distribution and the structure has allowed intraplate and interplate seismicity to be distinguished.High interplate seismicity along the interface between the subducting African lithosphere and the Aegean lithosphere was found south of western Crete where the interface is located at about 20 to 40 km depth. An offset between the southern border of the Aegean lithosphere and the southern border of active interplate seismicity is observed. In the area of Crete, the offset varies laterally along the Hellenic arc between about 50 and 70 km.A southwards dipping zone of high seismicity within the Aegean lithosphere is found south of central Crete in the region of the Ptolemy trench. It reaches from the interface between the plates at about 30 km depth towards the surface. In comparison, the Aegean lithosphere south of western Crete is seismically much less active including the region of the Ionian trench. Intraplate seismicity within the Aegean plate beneath Crete and north of Crete is confined to the upper about 20 km. Between 20 and 40 km depth beneath Crete, the Aegean lithosphere appears to be seismically inactive. In western Crete, the southern and western borders of this aseismic zone correlate strongly with the coastline of Crete. (C) 2004 Elsevier B.V. All rights reserved.

Research Article| May 01, 2004 CYC-NET: A Temporary Seismic Network on the Cyclades (Aegean Sea, Greece) Marco Bohnhoff; Marco Bohnhoff 1 GeoForschungsZentrum Potsdam Section Deformation and Rheology Telegrafenberg D424 D-14473 Potsdam Germany bohnhoff@gfz-potsdam.de 1 Corresponding author; now at GeoForschungsZentrum Potsdam, Germany. Search for other works by this author on: GSW Google Scholar Martina Rische; Martina Rische GeoForschungsZentrum Potsdam Section Deformation and Rheology Telegrafenberg D424 D-14473 Potsdam Germany bohnhoff@gfz-potsdam.de Search for other works by this author on: GSW Google Scholar Thomas Meier; Thomas Meier GeoForschungsZentrum Potsdam Section Deformation and Rheology Telegrafenberg D424 D-14473 Potsdam Germany bohnhoff@gfz-potsdam.de Search for other works by this author on: GSW Google Scholar Brigitte Endrun; Brigitte Endrun GeoForschungsZentrum Potsdam Section Deformation and Rheology Telegrafenberg D424 D-14473 Potsdam Germany bohnhoff@gfz-potsdam.de Search for other works by this author on: GSW Google Scholar Dirk Becker; Dirk Becker GeoForschungsZentrum Potsdam Section Deformation and Rheology Telegrafenberg D424 D-14473 Potsdam Germany bohnhoff@gfz-potsdam.de Search for other works by this author on: GSW Google Scholar Hans-Peter Harjes; Hans-Peter Harjes GeoForschungsZentrum Potsdam Section Deformation and Rheology Telegrafenberg D424 D-14473 Potsdam Germany bohnhoff@gfz-potsdam.de Search for other works by this author on: GSW Google Scholar George Stavrakakis George Stavrakakis GeoForschungsZentrum Potsdam Section Deformation and Rheology Telegrafenberg D424 D-14473 Potsdam Germany Search for other works by this author on: GSW Google Scholar Author and Article Information Marco Bohnhoff 1 GeoForschungsZentrum Potsdam Section Deformation and Rheology Telegrafenberg D424 D-14473 Potsdam Germany bohnhoff@gfz-potsdam.de Martina Rische GeoForschungsZentrum Potsdam Section Deformation and Rheology Telegrafenberg D424 D-14473 Potsdam Germany bohnhoff@gfz-potsdam.de Thomas Meier GeoForschungsZentrum Potsdam Section Deformation and Rheology Telegrafenberg D424 D-14473 Potsdam Germany bohnhoff@gfz-potsdam.de Brigitte Endrun GeoForschungsZentrum Potsdam Section Deformation and Rheology Telegrafenberg D424 D-14473 Potsdam Germany bohnhoff@gfz-potsdam.de Dirk Becker GeoForschungsZentrum Potsdam Section Deformation and Rheology Telegrafenberg D424 D-14473 Potsdam Germany bohnhoff@gfz-potsdam.de Hans-Peter Harjes GeoForschungsZentrum Potsdam Section Deformation and Rheology Telegrafenberg D424 D-14473 Potsdam Germany bohnhoff@gfz-potsdam.de George Stavrakakis GeoForschungsZentrum Potsdam Section Deformation and Rheology Telegrafenberg D424 D-14473 Potsdam Germany 1 Corresponding author; now at GeoForschungsZentrum Potsdam, Germany. Publisher: Seismological Society of America First Online: 09 Mar 2017 Online ISSN: 1938-2057 Print ISSN: 0895-0695 © 2004 by the Seismological Society of America Seismological Research Letters (2004) 75 (3): 352–359. https://doi.org/10.1785/gssrl.75.3.352 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Marco Bohnhoff, Martina Rische, Thomas Meier, Brigitte Endrun, Dirk Becker, Hans-Peter Harjes, George Stavrakakis; CYC-NET: A Temporary Seismic Network on the Cyclades (Aegean Sea, Greece). Seismological Research Letters 2004;; 75 (3): 352–359. doi: https://doi.org/10.1785/gssrl.75.3.352 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search Densely spaced digital recording temporary seismic networks are a fundamental and widely used tool to monitor microseismic activity at a low detection threshold. Such networks provide data for various types of investigations, such as the evaluation of seismic hazard potential, structural analysis, and stress field determination. Here, we focus on monitoring in the south Aegean region, which has the highest seismic activity in Europe. In this region, such networks have been operated previously on the island of Crete (e.g.,Harjes et al., 1997; Delibasis et al., 1999; Becker, 2000; Jost et al., 2002; Meier... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

We present a case study of lithospheric structure in the forearc of a retreating subduction zone for the Hellenic Arc. Lateral structural variations along the arc beneath the island of Crete are jointly investigated by receiver functions and Rayleigh phase velocities. Data from temporary short-period networks amend previous results from broad-band stations by broadening the frequency range available for phase-velocity determination and increasing the spatial coverage of receiver function profiles. Both receiver functions and dispersion analysis reveal distinct structural differences between western and central Crete. Western Crete is characterized by nearly constant S-velocities of 3.72-3.75 km s-1 from 10 km depth down to a depth of 50 km and no distinct continental Moho signal. Meanwhile, central Crete shows lower S-velocities equal to 3.3 km s-1 in the crust between 10 and 20 km depth which are followed by the Aegean Moho in about 30 km depth and a mantle wedge with an S-velocity of 4.35 km s-1. Both methods lead to an average depth of 55 km for the subducted oceanic African Moho beneath Crete. This means that the slab is separated from the Aegean crust by a mantle wedge beneath central Crete, while beneath western Crete the corresponding depth region is characterized by crustal velocities. This thickened crust in the forearc might be formed by crustal material of the Aegean Plate dragged down with the subducting African lithosphere. Furthermore, rocks extruded from a melange circulating in a subduction channel might accumulate between a depth of 20 and 50 km and contain low-velocity material, e.g. in the form of serpentinized Aegean mantle. In addition, the lateral extent of a prominent negative phase observed around 4 s differential time in receiver functions from western Crete is mapped. This phase might point to low-velocity material around 30 km depth which could be extruded from a subduction channel. An important property of the forearc found in this study is its strong lateral heterogeneity.