1.1. Topography and natural hazards

To gain a better understanding of the interrelation between topography, geohazards and the environment, the temporal evolution of topography needs to be assessed, not only during the recent past but also during the last 10 or so million years. There are however some complex problems inherent to paleo-topography analysis. Apart from dealing with topography that no longer exists, the dimensions and timing of events and the underlying dynamic processes that controlled its development, as well as the topographic life cycle, pose major challenges, the complexity of which cannot be solved by a single sub-discipline but requires support by other disciplines. The geographic scope of the proposed TOPO-EUROPE programme demands co-operation on a European scale to avoid a fragmented approach. Mountain ranges (increasing surface topography) and adjacent sedimentary basins (decreasing surface topography) record signals and proxies that tell the story of the topographic life cycle. In this, the source-to-sink relationship is of key importance. However, signals and proxies are still poorly understood and we only have started to decipher the few we are aware of. A major challenge is to extract all available information contained in the system and to interpret it in terms of processes. Innovative analytical techniques, improvement of methodologies, back-to-back with innovative conceptual and quantitative modelling, are required to resolve these problems.

The main challenge in topography-related geological hazard research is to create and verify physical models of hazardous Earth Systems that integrate all relevant data, describe hazards as a function of time, and understand them as resulting from a non-linear system evolution under which processes acting on various temporal and spatial scales can become catastrophic. In this context it must be understood that topography plays a prominent role as it results from the interaction of shallow and deep Earth processes, and as such permits, in combination with other parameters, to assess the state of stress and its change through time.

There are obvious relations between geological hazards and topography. Topography is a major factor controlling slope instabilities, which can lead to the development of landslides, both on- and offshore. Uplift of e.g. Fennoscandia and the Romanian Vrancea area has caused increased landslide and rock-fall hazards. The second important parameter for catastrophic earth movements is the internal friction of soil, which in turn depends largely on the hydrological conditions and water input by precipitation. Regional climate changes when associated with a precipitation increase tend to cause increased slope instability and corresponding landslide activity.

Fig. 2. Seismicity map of Europe, illustrating present-day active intraplate deformation. Also shown are intraplate areas of Late Neogene uplift (circles with plus symbols) and subsidence (circles with minus symbols). Background elevation images are extracted from the ETOPO2 data set. Earthquake epicentres are from the NEIC data centre, and are shown as red dots. Inset map: Cenozoic rift system of Europe (after Dèzes et al., 2004).

Fig. 3. Peak ground acceleration SESAME map (Jiménez et al., 2003) for a 90% non-exceedence probability within 50 years.

Earthquakes result from crustal-scale fault-related deformation and occur in various parts of Europe (Fig. 2). Although areas with a high frequency of large magnitude earthquakes are mostly bound to the Mediterranean domain, the strong concentration of people and high-value infrastructure in densely populated areas in Europe can in these regions turn moderate hazards into large risks. The currently used ‘3rd generation’ hazard assessment method can be coined ‘seismotectonic probabilism’. This method largely relies on historical and paleo-seismological earthquake records, and results in maps giving an annual exceedance probability of a certain damage parameter (Fig. 3). The challenge to Solid-Earth science researchers lays in developing 4th generation hazard assessment methods, relying much more on a physical understanding of processes leading to earthquakes and on assessment of the actual state of stress on faults. The state of stress is strongly influenced by surface topography, but also by the topography of lithospheric boundaries (Moho, lithosphere-asthenosphere boundary). Highly sophisticated models for time-dependent hazard assessment that link several processes, such as mantle dynamics, structure and rheology of the crust and mantle, change in topography, mass re-distribution by erosion and sedimentation, and post-seismic relaxation, can be established today. Verification of these models requires data on recent deformation (both from GPS and geological reconstruction for the Holocene/Pleistocene) and tectonic stress (e.g. through the World Stress Map project). This can lead yield substantial new insights into the stress and strain evolution of the key seismogenic areas of Europe. This type of modelling may prove a particularly valuable approach to constrain extreme events with their high societal impact.

Intraplate seismicity is still poorly understood and tends to follow episodic intermittency patterns rather than quasi-periodic earthquake activity more characteristic for plate boundaries. TOPO-EUROPE will establish a database that allows for a systematic combination of lithospheric data (e.g. geometry of boundaries, temperature, stress, structure) and recent movements, including topography changes over an area that covers all levels of seismicity, such as highly active plate boundary domains, moderate intraplate activity, and seismic quiescence.

Europe is exposed to recurrent flooding events that pose major hazards to population and industrial agglomerations. The damaging potential of floods is intrinsically linked to even minor topographic changes that control the depth of inundation. Thus, it will be a challenging task for TOPO-EUROPE to combine regional climate predictions with changes in sea and river level and topography during the Holocene to fully assess Europe’s future flood hazards.

The main risk generating factor for the human society is the increased exposure and vulnerability of its assets (buildings, infrastructure, and social systems). However, during the past 150 years anthropogenic modification of the planetary environment has caused changes in the hazard potential itself. For instance, extraction of large amounts of ground water beneath and near cities modifies surface elevations and thus their inundation potential during floods. At the same time this impacts on the stability of the subsurface with consequences for ground motion during future earthquakes, associated liquefaction potential, and landslide hazard. Again, TOPO-EUROPE opens avenues to systematically address these issues on a European scale.