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As aforementioned, one of the most dangerous times, in respect of dam trigged earthquakes, is when the water level is dramatically falling, which was exactly what happened at the dam one week before the Wenchuan earthquake. Note that the construction of a water reservoir may increase the likelihood of larger earthquakes in the surrounding regions. The earthquake resistance of infrastructures built before or even after the construction of the reservoir normally cannot account for this increased earthquake likelihood, which can pose an elevated risk in the affected region.

24 1 Introduction Many people misunderstand the fault rupture mechanism and believe that, since the pressure developed at tectonic plate edges (Fig. 26) and the stress on a fault will be released by earthquakes, such that frequent small magnitude earthquakes can avoid the occurrence of a major one, this also forms one of the motivations of human engineered earthquakes, as will be described in Sect. 7. In fact, the energy released by low magnitude earthquakes is almost negligible compare to a major earthquake.

But the paleomagnetic evidence found later on appeared to strongly support Wegener theory of continental drift. Based on the theory of continental drift and extensive geological research later on, it is now widely accepted that over tens of millions of years of Earth’s evolution, the surface of the lithosphere (a rigid 10–150 km thick plate composed of crust and upper mantle) ruptured into a number of pieces called tectonic plates as shown in Fig. ), plus dozens of tertiary microplates. These tectonic plates are themselves relatively rigid, relatively cool slabs that form the crust and upper mantle of the Earth with a thickness of up to 250 km.