2.2.1. Seismology and seismic imaging – EUROARRAY

During the past decade, the analysis and understanding of dynamic crust-mantle processes has greatly progressed owing to major advances in the field of seismic tomography at global and regional scales (e.g. Bijwaard et al., 1998). Tomographic imaging techniques are applied to observations of body and surface waves, and provide spectacular 3-D images of mantle structures. These images can readily be linked to global plate tectonic processes, such as past and active subduction of lithospheric plates (Fukao et al., 2001). Tomographic evidence for mantle plumes originating at great depth (Bijwaard and Spakman, 1999; Goes et al., 1999; Romanowicz and Gung, 2002; Montelli et al., 2004) suggests links between mantle plumes and such surface processes as intra-plate volcanism, rifting and vertical surface motions.

For the European-Mediterranean domain, recently developed tomographic models of mantle structure (e.g. Piromallo and Morelli, 1997; Bijwaard et al., 1998; Spakman et al., 1999; Bijwaard and Spakman, 2000; Piromallo and Morelli, 2003) have greatly advanced the linking of lithosphere-mantle processes to the past and on-going tectonic evolution of the Earth’s crust (Fig. 11). Conceptual models of mantle dynamics derived from tomography and analogue lab-models emphasize the role of a variety of mantle processes as driving mechanisms of major tectonic processes, the mechanical evolution of the lithosphere, and surface deformations (e.g. Wortel and Spakman, 2000; Bellahsen et al., 2003; Faccenna et al., 2003; Faccenna et al., 2004; Funiciello et al., 2004; Spakman and Wortel, 2004; Van Hinsbergen et al., 2005a; Faccenna et al., 2006).

European-scale tomographic models are based on a global observation network of seismological stations with a very heterogeneous spatial distribution. This leads to a strongly non-uniform data density and consequently to a strong spatial variability in model resolution varying between 50 km to hundreds of km. In some regions of Europe, temporary seismological networks with a dense spatial distribution (30-60km) were installed during the past decades for relatively short periods of six months to one year in order to address specific phenomena (e.g. Massif Central [Southern France], TOR [Sweden, Denmark, northern Germany] , SVEKALOPCO [Finland], EIFEL [Eastern France, Western Germany], CALIXTO [Vrancea, Romania]). These and other successful experiments targeted important lithospheric transition zones, mantle plumes and subduction zones. Importantly, these experiments demonstrated the presence of detailed (10-30 km) crust-mantle structure associated with dynamic processes affecting surface deformation. This was only possible owing to the high resolving power that can be attained with dense observation networks. The short period of network employment, however, restricted the data volume, whilst the spatially localized nature of these experiments has prevented to obtain a contextual image of mantle structure and processes. Furthermore, by the very nature of the tomographic experiments conducted, mantle structure could only be imaged relative to an unknown background of absolute wave speed.

The shortcomings of continental-scale tomographic experiments and of local experiments with dense temporary networks can only be overcome by acquiring observations from a spatially more uniform and dense European network. A considerable densification and extension of the existing European seismological network would allow the development of a new generation of crust-mantle models based on surface wave and body wave data with a much more homogeneous spatial resolution. Models will be obtained in absolute wave speeds giving strong constraints on the temperature and compositional fields of the mantle, and on associated mantle processes. Highly improved crust-mantle models (temperature and composition) are a fundamental prerequisite for numerical modelling of crust-mantle processes and the resulting surface deformation both for reconstruction of topography and for establishing the current dynamic state of European topography (surface motions, state of stress and strain-rate build-up). Technologically, tomographic methods are advanced enough to deal with a heterogeneous crust, ray bending effects, finite frequency effects, and even 3-D reference models of the Earth’s structure. The forward leap towards much more advanced models is only hampered by the (spatial) availability of data.

A very important part of a new generation of structural crust and mantle models are the discontinuities in material properties that occur around the crust-mantle interface (the Moho; a compositional transition as well as the granulite-eclogite transition), around 410 km depth (dominated by the olivine to β-spinel transition) and around 660 km depth (dominated by the γ-spinel to lower-mantle-oxides transition). Special seismological techniques can be used, and developed, to detect the topography and sharpness of these (and related) phase transitions, the precise nature of which is still a matter of active research. Receiver Function analysis of seismic data has proved to be a powerful method for the detection of the phase transition interfaces (e.g. Vinnik et al., 1996; Kind et al., 2002). The topographic configuration of these discontinuities is in fact dynamic, owing to the interaction of mantle flow (slabs, plumes) impinging on these interfaces with the physics of phase transitions. Dynamic surface topography is strongly diagnostic for the type and local nature and thermal characteristics of mantle flow. In long-wavelength mantle flow models, the dynamic surface topography is related to the dynamic topography of the internal surfaces. One of the key problems in understanding surface topography is the interaction between the mantle induced dynamic topography and other (shallow) topography generating processes.

Studies on mantle rocks, both xenoliths and tectonically emplaced samples, show that the mantle is heterogeneous at all observable scales, down to crystal dimensions. Recent seismological studies have shown that it is possible to image crustal and mantle structures and to determine their physical properties on a kilometer scale or finer, much smaller than the seismic wavelength. Only recently, methods have been developed for extracting information on fine-scale heterogeneity of the crust and mantle from seismic data at scales which require a statistical representation of physical parameters (e.g. Holliger and Levander, 1994; Thybo and Perchuc, 1997; Ryberg et al., 2000; Nielsen et al., 2002; Baig and Dahlen, 2004; Shearer and Earle, 2004; Thybo, 2006; Thybo and Anderson, 2006). These methods push the attainable resolution below the usual detection limit, although they cannot provide unique solutions for the structure of the Earth.

New-generation models of the crust-mantle system can only result from a concerted effort of seismic tomography research, strong seismic-contrast and dynamic-topography research, and fine-scale imaging of crustal and mantle properties (Fig. 12). For TOPO-EUROPE, such seismological studies can provide the principal source of information on the detailed structure of the European crust-mantle system. Data from existing global networks (FDSN, IRIS, GEOFON, EarthSCOPE) and from existing European networks are, however, insufficient to make the necessary step forward toward the development of much more detailed crust-mantle models. This requires a much denser observation network, complementing the existing European seismological infrastructure. As a component of TOPO-EUROPE, the project EUROARRAY aims at developing such a dense network for monitoring surface deformations and acquiring Solid-Earth geophysical data all across Europe. Eventually, a spatially uniform and dense network (60 km spacing) of co-located GPS, magnetotelluric and seismological instruments will complement existing European instrumentation for a wide variety of scientific purposes. EUROARRAY aims specifically at densification of the existing European seismological network that forms the backbone of the EU-funded seismological NERIES project. In addition, a dense roving network of co-located geophysical instruments is geared to focus on surface deformations and the subsurface structure of key areas of the TOPO-EUROPE natural laboratories (section 3). In the spirit of TOPO-EUROPE, the EUROARRAY initiative will promote organisational and scientific collaboration across Europe for the benefit of the multidisciplinary Solid-Earth science community, implementing the latest in high-technology European infrastructure and providing an open data policy. The anticipated boost in quality, quantity and availability of data achieved through EUROARRAY will effectuate strong technological and methodological innovation, allowing Europe to maintain its internationally leading role in Solid-Earth sciences.

The existing international seismic network, complemented by a growing EUROARRAY of co-located instruments, combined with existing state-of-the-art analysis and new seismological very high resolution modelling techniques, will reveal the internal structure of Europe’s lithosphere and sub-lithospheric mantle with unprecedented detail. This will permit to develop advanced tomographic models for the European lithosphere and mantle, greatly improving on presently available models:

• Determination of the detailed crustal structure beneath each station will lead to a high-resolution crustal model
• Mantle heterogeneities will be revealed at scales exceeding the traditional resolution limit and with a much-improved, uniform spatial resolution
• Dynamic topography of seismic discontinuities and the thickness of transition zones will be accurately and uniformly determined, allowing for the explicit detection of vertical mantle flow and associated temperatures
• The lithosphere-asthenosphere boundary, lateral variations in lithospheric structure, and large lithospheric shear zones on which deformation concentrates, can be detected
• Subducted lithospheric slabs, mantle plumes, and their relation to crustal structure and major continental deformation zones can be delineated in detail
• Uniform sampling of the mantle permits detection of seismically anisotropic structures, which will in turn allow distinguishing between deeper mantle flow directions and anisotropy frozen in the lithospheric mantle.

Apart from its own merits, a new-generation model of crustal and mantle structure will provide the necessary input for advanced modelling of the European crust-mantle system constrained by high-resolution satellite gravity and geodetic observations of active surface deformation. This provides the “depth-to-surface” relations required for the reconstruction of mantle induced surface topography. The current generation of tomographic models can and will, within limitations, be exploited for this purpose during the early phases of TOPO-EUROPE and for the development of 4-D modelling techniques of crust-mantle dynamics. This will work out well for some selected regions where current tomographic resolution is relatively high (e.g. Apennines-Aegean-Anatolia), but will lead to ambiguous results for Western, Central and Northern Europe where the spatial resolution is much lower. In parallel, EUROARRAY will focus on developing the Earth observation data-platform required for near-future Solid-Earth science and topography research.