Ground effects of large morphogenic earthquakes.

Morphogenic earthquakes are those seismic events capable of generating, or modifying, the surface morphology instantaneously and permanently (Caputo, 1993). It is also of interest to distinguish between linear and areal seismogenetic features, that is, features generated by a morphogenic earthquake. The former type is represented, for example, by the classical free-faces that can be produced by the upward propagation of a co-seismic displacement and the resulting ground rupture. Based on a catalogue of welldocumented recent and historical events from the broader Aegean Region and the empirical relationships between magnitude and surface rupture, Pavlides and Caputo (2004) conclude that the minimum magnitude for a morphogenic earthquake associated with crustal normal faulting and capable to produce a linear seismogenetic feature is about 5.5. Therefore, though the proposed magnitude depends on several factors like the kinematics and the geometry of the fault, the hypocentral depth and possibly the general mechanical behaviour of the seismogenic volume, only moderate (M= 5.5–6.0), but mainly large earthquakes (M> 6.0), can produce such features. When this phenomenon occurs their recognition and full mapping, at least since the last decades, can be complete. If linear seismogenetic features are repeatedly produced insisting on the same locations, the corresponding cumulative features that form are, for example, fault scarps, linear mountain fronts, trapezoidal and triangular facets, aligned knick-points on river profiles or bending-points on water courses, etc. Accordingly, investigating the surface expressions of capable faults is a crucial tool for recognising past earthquakes and for a better understanding of the seismotectonics of a region. Indeed, linear seismogenetic features can be investigated based on remote sensing techniques (air photos and different kinds of satellite imageries), morphotectonic and hydrographic surveys, palaeoseismological trenches, high-resolution geophysical methods (electrical resistivity tomographies, seismic profiles, ground penetrating radar, etc.) and various geodetic methods. In contrast, the areal seismogenetic features are mainly caused by the elasto-plastic deformation of the entire seismogenic volume and especially of its top surface, corresponding to the Earth’s surface. For example, subsidence, uplift and tilting phenomena associated with relatively superficial events are typical areal seismogenetic features. On the other hand, all structures produced by local, though areally distributed, effects like liquefaction phenomena, seismo-induced landslides, coastal modifications by tsunamis, etc., should be also included in this category. As a rule, the amount of permanent surface motion and/or the areal extension associated with each phenomenon strongly depend on the size, the kinematics and the hypocentral depth of the causative earthquake. Also in this case, if areal seismogenetic features are repeatedly produced, the corresponding cumulative structures that are commonly generated are large sedimentary basins characterised by growth-faults geometry and uplifted hilly or mountain sectors deeply entrenched by the hydrographic network. The study of areal seismogenetic features can be based on different methodological approaches like morphotectonic mapping, hydrographic analyses, remote sensing techniques, sedimentological and palaeogeographic studies, Archaeoseismology and Historical Seismology. Following the above definitions, it is clear that seismic events deeper than few tens of kilometres can never produce linear seismogenetic features, while very deep slab-related seismic events, say more than 2–300 km depth, could rarely become morphogenic earthquakes at all.With few exceptions characterised by particularly shallowearthquakes, like in volcano-tectonic settings, or close to huge open pits and mines or surrounding deep artificial lakes, also as concerns magnitude there is a physico-mechanical threshold below which an earthquake cannot be morphogenic.