System Explanation of Earthquakes in Germany and Central Europe

The sudden movement of tectonic blocks along geologic faults acts as immediate trigger for an earthquake and the following spreading of seismic waves. Reasons for tectonic forces leading to widespread deformations are stresses within the brittle parts of the Earth's crust and mantle. Alongside geological faults, such as divisional planes or lines of motion, either aseismic creeping or earthquakes may happen as a consequence of this stress. As mentioned before, a first-order fault is represented by the plate boundaries. Most of the deformation energy from earthquakes is released here, rather than within the continents. The western part of the Eurasian Plate with its boundaries is shown in Figure 1. Earthquake activity is concentrated along the plate boundaries. Figure 1 shows the northern bulge of the African Plate (AP) at the southern edge of the Eurasian Plate, namely the "Apulian Spur", which once thrust the Alps while drifting northwards. This drift of the African Plate is antagonised by an extensive resistance. The equally northward drifting Arabian Plate is exposed to a smaller resistance resulting in a higher drifting velocity of that plate. The two micro plates (MP) outlined in the Anatolia region are forced to a westward drift by the Arabian Plate resulting e.g. in characteristic movements combined with heavy earthquakes along the North-Anatolian Fault (NAF). As a result of the northward drift African oceanic crust is subducted near the Aegean Sea region and southeast of the Tyrrhenian Sea. By far the most significant part of Europe's seismic activity is concentrated along this plate boundary between the Eurasian Plate and the African Plate.

Figure 1: Western part of the Eurasian Plate with the adjacent plates as well as their drift directions, after Gruenthal and Strohmeier (1992, completed): 1 direction of the vector of movement relative to Europe and/or direction of the forces being active along plate boundaries, 2 extensional tectonics, 3 main plate boundaries, 4 collision front, 5 subduction front, 6 additional first order fractures/faults and their direction of movement; MP: Anatolian Micro Plates, PB: Pannonian Basin, NAF: North Anatolian Fault The forces acting against the northward drift of the African Plate originate from the Mid-Atlantic Ridge. Here, the ridge is widened through convectional flux processes within the Earth's mantle and ascending liquid magma, which solidifies and eventually results in a widening of the Atlantic Ocean. These processes are connected to quake activities, which clearly map out the Mid-Atlantic Ridge on epicentre maps. These forces, resulting from global plate tectonics, bring about the Mid-European stress field shown in Figure 2. The lines represent the trajectories of maximum horizontal compression in the Earth's crust. This figure results from numerous stress field measurements as well as numerical models reproducing the measurement data surprisingly well. Additionally extensive forces act within the Pannonian Basin resulting in an almost radial stress field. Figure 2: Pattern of the trajectories of maximum compressive stress SHmax in the Earth's crust. Dashed lines: uncertain course, dotted lines: alternative interpretation after Gruenthal and Strohmeyer (1994) This stress field pattern may lead to quake-generating processes along fractures and faults that are aligned in a certain angle to the stress trajectories. The key to understanding the spatial distribution of seismicity is the pattern of fracture faults combined with knowledge of the stress field, since not every fault created in the geological past is predestined to generate earthquakes on condition of the current stress field of the Earth's crust.

Source:

• Grünthal, G. und Stromeyer, D. (1992): The recent crustal stress field in central Europe - trajectories and finite-element modeling. Journal of Geophysical Research 97/B8, 11.805-11.820, 1992.
• Grünthal, G. und Stromeyer, D. (1994): The recent crustal stress field in Central Europe sensu lato and its quantitative modelling. Geologie en Mijnbouw 73, 173-180.