Definition of earthquake sources and activity rates

Starting in the mid-90s, Europe has recorded a rapid advancement in the compilation of earthquake catalogues, in the understanding of active tectonics and in the implementation of seismogenic source databases. Much of this progress has been fostered by a series of EC programs (e.g., BEECD, FAUST, EPSI, SAFE, PALEOSIS, all completed, and NERIES, ongoing) that not only funded the actual research and the development of new archiving technology, but also promoted a profitable exchange of ideas and methods across European boundaries. Even more recently, the availability of extensive GPS datasets from national or regional networks (e.g., the RING in Italy, TPGN and MAGNET in Turkey, REGAL in France) and increased modelling capabilities have made it possible to derive observations of modern tectonic strain from instrumental data. SHARE will achieve progress in all areas related to the characterization of earthquake sources and activity rates.

Earthquake datasets and magnitude calibration
The work will build on ongoing initiatives which already deal with the problem of creating homogeneous, European seismicity datasets, i.e. (1) the EC funded NERIES activity “Distributed archive of historical earthquake data“ and its extension to other European countries, currently under development in the frame of a Working Group of the European Seismological Commission; (2) the program undertaken by GFZ for homogenizing to Mw the magnitude of all Mw>3.5 European earthquakes north of 44°N and for all M>5 earthquakes in southern Europe and the Mediterranean (Grünthal and Wahlström, 2007); (3) the initiatives at European scale of the EMSC, among others in the EC EPSI project, to compile homogeneous European bulletins since 1998.

Contribtion of SHARE:

• SHARE will provide the most reliable and homogeneous seismicity dataset at the European scale, covering historical and modern instrumental seismicity. We will achieve this by assessing calibration relationships on a regional basis, in order to supply uniform and seismic hazard usable magnitudes (MS, ML) standardized to moment magnitude (MW).
• To harmonize the quantification of earthquake size, we will adopt common and homogeneous ground-motion attenuation models (NGA-type with homogeneous site corrections, WP4), leading to a close compatibility between magnitude values used in the hazard model and the ground motion model.
• For the first time, SHARE will also focus on offshore seismic areas (e.g., Mediterranean and north-east Atlantic), as well as on polar regions, which are difficult areas to deal with but are of growing importance as to seismic risk issues.

Seismogenic sources and geological constraints
In the past two decades, active fault data and geological constraints have increasingly been used in seismic hazard assessment in all most tectonically active areas of the world (New Zealand: Stirling et al., 2002; Canada: Adams and Halchuk, 2003; Italy: Meletti et al., 2008). Although the basic strategies in collecting, interpreting and using these types of data have enormously improved at a local and sometimes regional scale, the main challenge for this task is to make a step forward in using them in a continent-wide scale.

Contribtion of SHARE:

• SHARE will collect and harmonise data on seismogenic sources – active faults capable of generating earthquakes above a threshold magnitude Mw ≥ 5.5 – over all main tectonically active zones in the Euro-Mediterranean area. Major tectonic features, such as plate boundaries or main transfer faults will also be taken into consideration, even if they do not fall into the above definition. This will provide a set of fully parameterized seismogenic sources based on geological/geophysical data independent from those collected in other tasks.
• All data will be in parametric form, following a standardized scheme, and in map form. We will for the first time standardize all steps required to build a homogeneous database: the definition of the shared database platform and the standardization of the fault parameters; the collection of parametric data, the criteria to perform tectonic and seismic validation of the data and to characterize uncertainties of those parameters that most affect the hazard calculation (i.e., maximum magnitude and slip rate; the classification of documentation supporting the parametric data; the tools to visualize the database consistency, analyzing the uncertainties associated); and the criteria to integrate the identified active faults and seismogenic sources into the seismic source zones used for hazard computation.

Strain- and slip-rate model
Strain and slip rates of European faults have already been quantified based on regional geodynamic models at various levels of complexity (e.g., Barba et al., 2008). The advantages of these models are that (a) they return realistic estimates of slip rates over large regions even in the absence of direct (e.g., paleoseismological) observations of fault slip, (b) they provide upper bounds to the total strain rate and seismic moment release predicted by the seismicity model, thereby allowing the validation of the internal hazard consistency, and (c) they do so by exploiting a large number of observations that are normally not directly incorporated in seismic hazard (e.g., stress and plate motion data, geodetic data).

Contribution of SHARE: SHARE will develop a prototype reference strain and slip-rate finite-element model at the European scale. Our model will be specifically designed to serve as validation for hazard assessments, using consistent geometries of the source zones as well as homogeneous definition of the earthquake parameters (i.e., magnitude scale, regional distribution of hypocentral depths). Large-scale geological structures, seismogenic sources, geodetic vectors and subsidence/uplift rates will be incorporated into the model, serving as a benchmark for the model results thorough the investigated area. The model will integrate different tectonic settings, including subduction, plate convergence, rift systems (including aborted ones), passive continental margins, as well as glacial-isostatic adjustments.

Seismic Source Zones (SSZ)
Europe is made up of strongly varying seismogenic regions with large contrasts and complexity, and displaying quite different properties, background information, data coverage, and data availability. This has resulted in a very inhomogeneous definition of seismic source models across Europe. A first uniform SSZ model was produced in the SESAME/ESC project in 2000.

Contribution of SHARE: We will construct a homogeneous model of seismic source zones combining data on large-scale geological structure, seismotectonics, seismogenic faults, observed seismicity (instrumental, pre-instrumental, and paleoseismological), and geodesy. For the first time, all SSZs in the European model will be assigned a statistical distribution of the predominant focal mechanism orientation, of the earthquake hypocentral depth and of the Maximum magnitude Mmax, in order to provide adequate control for the selection of the ground-motion attenuation model and to improve the accuracy in the hazard computations. The unified model will consider the epistemic uncertainties in the seismic source zones by applying the logic tree approach for the first time at European scale, to encompass different interpretations. This is particularly important for fault models in terms of choosing between different interpretations of fault sources and area sources by considering the moment rate comparisons.

Earthquake activity rates
Statistical tools are commonly applied in hazard assessment, but not in a homogeneous way across Europe.

Contribution of SHARE:

• SHARE will apply for the first time common statistical techniques at European scale to derive the required parameters describing the rates at which each seismic source zone has generated earthquakes of different magnitudes in the past, which are then taken as the expected probabilities to generate future earthquakes for use in the assessment of hazard. The key parameters – the activity rate, the b-value, and the maximum magnitude Mmax distribution – will be assessed in a pan-European exercise, using harmonized methodologies. The regional earthquake catalogues and the defined seismic source zone geometries will be used to derive magnitude-dependent catalogue completeness, to de-cluster aftershocks, to fix prior-distributions of maximum magnitudes and to evaluate statistical uncertainties.
• An innovative part of the work will be the implementation of checks on (a) geological and strain-rate constraints, and (b) internal consistency with regard to historical experienced shaking, to ensure that the models used for evaluating expected seismic activity is well balanced and harmonized throughout all of the regions in Europe.