Edi Kissling

3D crustal structure from local earthquake tomography around the Gulf of Arta (Ionian region, NW Greece)

During summer of 1995 local seismicity was recorded in the area around the Gulf of Arta in northwestern Greece by a dense temporary seismic network. Of the 441 local events observed at 37 stations, 232 well locatable events with a total of 2776 P-phase readings were selected applying the criteria of a minimum of 6 P-observations and an azimuthal gap less than 180°. This data set is used to compute a minimum 1D velocity model for the region. Several tests are conducted to estimate model stability and hypocenter uncertainties, leading to the conclusion that relative hypocenter location accuracy is about 500 m in latitude and longitude and 1 km in depth. The minimum 1D velocity model serves as initial model in the non-linear inversion for three-dimensional P-velocity crustal structure by iteratively solving the coupled hypocenter–velocity problem in a least-squares sense. Careful analysis of the resolution capability of our data set outlines the well resolved features for interpretation. The resulting 3D velocity model shows generally higher average crustal velocities in the east, and the well resolved area of the eastern Gulf of Arta exhibits a homogeneous velocity around 6 km/s for the whole upper crust. A pronounced north–south trending zone of low velocities in the upper 5–10 km is observed in the area of the Katouna fault zone (KFZ). At greater depths (below 10 km) the KFZ is underlain by high-velocity material. E–W profiles suggest a horst–graben structure associated with the KFZ.

Local earthquake tomography of shallow subduction in north Chile: A combined onshore and offshore study

Selected travel time data from the aftershock series of the great Antofagasta earthquake (M w =8.0) have been inverted simultaneously for both hypocenter locations and three-dimensional V P and V P / V S struc- ture. The data were collected with a dense 44-station seismic network including ocean bottom hydrophones. We performed a series of inversions with increasing complexity: 1-D, 2-D, and 3-D. This approach was cho- sen to account for the heterogeneous seismicity distribution and to obtain a smooth regional model in areas of low resolution. Special efforts were made to assess the solution quality including standard resolution esti- mates and tests with synthetic travel times. The subducted plate is imaged between 20 and 50 km in depth as an eastward dipping high- V P feature. High V P / V S ratios within the oceanic crust possibly indicate elevated fluid content. Underplating of material eroded close to the trench is found beneath the Mejillones Peninsula. The lower crust of the overlying plate is characterized by an average V P of 6.8-6.9 km/s and an average to low V P / V S ratio. Large areas of anomalously high V P are found in the lower crust south of the city of Antofagasta; they are interpreted as remants of magmatic intrusions. A zone of high V P / V S ratios is found within the rup- ture area of the Antofagasta main shock, just above the subducted slab. Its location within the region of high- est stress release from the main shock suggests that the main shock rupture causes the high V P / V S ratio. The high V P / V S ratio could indicate postseismic fluid migration from the subducted oceanic crust into the overly- ing lower crust.

Postseismic fluid flow after the large subduction earthquake of Antofagasta, Chile

We interpret the time evolution of high P-wave to S-wave velocity ( V p/ V s) ratios after the large Antofagasta subduction earthquake (M w = 8.0) as due to postseismic fluid flow into the overriding plate. We suggest that accumulation of high stress forms a permeability barrier along the plate interface, capturing the fluids in the subducting plate. This seal is broken only by large subduction earthquakes that allow the fluids to rapidly migrate into the overlying plate. Postseismic fluid flow implies a relatively high permeability of the overlying lower continental crust, which we estimate to be 10 −16 to 10 −17 m 2 .

Deep structure of the Alps—what do we really know?

Abstract In the last five decades the deep structure of the Alps has been probed by every geophysical method applicable, and the resulting amount of data is unmatched for any other orogen. In this study, an attempt is made to review the data and the proposed structural models with the aim of separating the certain from the probable and from the speculative information. This can be achieved by first reviewing the theoretical resolving power and ambiguity of the applied interpretation methods and then analysing the proposed models. The methods reviewed are inversion of surface wave data, teleseismic and local earthquake seismic tomography, near-vertical reflection seismology, wide-angle reflection and refraction seismology, and gravity modelling. All information about the Moho rated as certain is combined to give a Moho map of the Alpine area. The information rated as certain and probable, and additional qualitative arguments, are used to discuss a crustal model of the Western and Central Alps represented by two cross-sections. Major structural elements in this crustal model are a thick overthrust Penninic nappe system, wedging at mid- to lower-crustal levels, a discontinuous Moho and strong variations along the strike of the orogen. Whereas the structures of the European upper crust and of the Penninic nappe system are well constrained, only few and isolated lower-crustal structural elements are rated as certain. Finally, the shape of the lower lithosphere in the Alps is discussed by review and comparison of the results from surface-wave, teleseismic travel-time residual and tomographic studies. Qualitative arguments suggest the existence of a lithospheric root or slab beneath the Alps. Probable tomographic information suggests a south-vergent European lithospheric slab beneath the Southern Alps and the Po Plain. Despite the considerable number of studies aimed at resolving the deepest part of Alpine orogeny, the available quantitative information on the sub-Moho structure cannot be rated as certain.

Model parametrization in seismic tomography: a choice of consequence for the solution quality

Abstract To better assess quality of three-dimensional (3-D) tomographic images and to better define possible improvements to tomographic inversion procedures, one must consider not only data quality and numerical precision of forward and inverse solvers but also appropriateness of model parametrization and display of results. The quality of the forward solution, in particular, strongly depends on parametrization of the velocity field and is of great importance both for calculation of travel times and partial derivatives that characterize the inverse problem. To achieve a quality in model parametrization appropriate to high-precision forward and inverse algorithms and to high-quality data, we propose a three-grid approach encompassing a seismic, a forward, and an inversion grid. The seismic grid is set up in such a way that it may appropriately account for the highest resolution capability (i.e. optimal data) in the data set and that the 3-D velocity structure is adequately represented to the smallest resolvable detail apriori known to exist in real earth structure. Generally, the seismic grid is of uneven grid spacing and it provides the basis for later display and interpretation. The numerical grid allows a numerically stable computation of travel times and partial derivatives. Its specifications are defined by the individual forward solver and it might vary for different numerical techniques. The inversion grid is based on the seismic grid but must be large enough to guarantee uniform and fair resolution in most areas. For optimal data sets the inversion grid may eventually equal the seismic grid but in reality, the spacing of this grid will depend on the illumination qualities of our data set (ray sampling) and on the maximum matrix size we can invert for. The use of the three-grid approach in seismic tomography allows to adequately and evenly account for characteristics of forward and inverse solution algorithms, apriori knowledge of earth’s structure, and resolution capability of available data set. This results in possibly more accurate and certainly in more reliable tomographic images since the inversion process may be well-tuned to the particular application and since the three-grid approach allows better assessment of solution quality.

Rapid exhumation in the Western Alps driven by slab detachment and glacial erosion

Identifying topographic and erosion rate response to tectonic and climatic forcing remains challenging. This is in part because of the difficulty in isolating the respective roles of climate and tectonics. Here we exploit 2500 thermochronometric data points collected over several decades of research, using a new inverse technique, to image the space-time evolution of erosion rate across the European Alps over the past 35 m.y. The most striking feature of our results is a two- to three-fold increase in erosion rate over the past 2 m.y. exclusively within the Western and Central Alps. This increase appears to be controlled by the inferred high rock uplift rate due to the progressive detachment of the European slab under the Western Alps. The similarity in mean elevation between the Western and Eastern Alps indicates a surprisingly low topographic response to this differential tectonic forcing, and points to the role of enhanced glacial erosion in response to surface uplift.

Important findings expected from Europe's largest seismic array

An international, interdisciplinary project, which 2 years ago deployed the largest dense seismic antenna ever in Europe, expects in the next 2 years to present important findings on the lithosphere and asthenosphere of a portion of the Trans-European Suture Zone (TESZ). Final processing is currently under way of the data from the array of 120 seismographs along a 900-km-long by 100-km-wide strip from Gottingen, Germany, in the south, through Denmark, to Stockholm, Sweden in the north, across the northwestern part of the TESZ (Figure 1). Project Tor is a teleseismic tomography experiment with interdisciplinary data exploitation. It extends across the broad TESZ boundary between two markedly different lithospheric domains.These are (1) Proterozoic Europe, with Precambrian crust in Sweden and eastern Europe, and (2) Phanerozoic central Europe, with most of the crust influenced by the Caledonian and Variscan orogenies and only small areas of relic Precambrian crust. The project is designed to investigate the deep lithosphere traces of the broad-scale geology of the TESZ area, including the Tornquist Zone, from which Project Tor has its name. It is part of EUROPROBE, a major Earth science program of the European Science Foundation, which is run by a regional committee of the International Lithosphere Program.

ADVANCES IN TRA VEL-TIME CALCULATIONS FOR THREE-DIMENSIONAL STRUCTURES

An aeeurate and effieient teehnique for determining travel times and ray paths in a heterogeneous medium is an essential part of methods for seismie event loeation and seismic tomography using three dimensional veloeity models. A wide variety ofmethods has been developed over the last few deeades. We present a review ofthe major c1asses ofmethods for travel time and ray path calculation. They ean be eategorized as either exact or approximate, and the eomputational strategy involved usually ean be c1assified as shooting, bending, perturbation, or grid-based. Many methods determine solutions using the ray equations, but some methods rely on Fermats principle, Huygens prineiple, or the eikonal equation. Relative advantages and disadvantages ofthe various teehniques are diseussed. We conc1ude by highlighting some reeent eomparative studies ofsome ofthese methods that provide valuable information regarding their relative aecuracies.

Segmented Hellenic slab rollback driving Aegean deformation and seismicity

The NE dipping slab of the Hellenic subduction is imaged in unprecedented detail using teleseismic receiver function analysis on a dense 2-D seismic array. Mapping of slab geometry for over 300 km along strike and down to 100 km depth reveals a segmentation into dipping panels by along-dip faults. Resolved intermediate-depth seismicity commonly attributed to dehydration embrittlement is shown to be clustered along these faults. Large earthquakes occurrence within the upper and lower plate and at the interplate megathrust boundary show a striking correlation with the slab faults suggesting high mechanical coupling between the two plates. Our results imply that the general slab rollback occurs here in a differential piecewise manner imposing its specific stress and deformation pattern onto the overriding Aegean plate.

Radial anisotropy in the European mantle: Tomographic studies explored in terms of mantle flow

[1] Previous studies have shown that radial seismic anisotropy as estimated from flow models is in good agreement with results from tomography at global scale, in particular underlying oceanic basins. However, the fit is typically poor at smaller scale lengths, particularly in tectonically complex regions. We conduct a comparative analysis of tomographically mapped and dynamically modeled radial anisotropy at the scale of Europe and the Mediterranean Basin, including three tomographic models based on different observations and/or methods. We find that adaptive-grid surface-wave tomography, with parametrization density depending locally on the spatial and azimuthal density of data coverage, leads to the seismic model closest to (albeit still far from) geodynamic predictions. Theability tomap regional-scaleseismic anisotropy may provide a new constraint, complementary to isotropic tomography, to the nature of upper mantle flow. Citation: Schaefer, J. F., L. Boschi, T. W. Becker, and E. Kissling (2011), Radial anisotropy in the European mantle: Tomographic studies explored in terms of mantle flow, Geophys. Res. Lett., 38, L23304, doi:10.1029/2011GL049687.