why did the earthquake happen?

First, the Indian plate subduction to Asia, resulting in rapid uplift of the Qinghai-Tibet Plateau. Slowly east plateau of material flows on the plateau along the eastern edge of the Longmen Shan belt eastward extrusion, rigid block under the Sichuan Basin, encountered stubborn blocked, resulting in a long-term accumulation of tectonic stress energy, culminating in Longmenshan Beichuan - Ying show the sudden release of the region.
Second, the thrust, D, squeeze-type fault earthquake. Seismogenic structure is the central Longmen Shan fault belt, in the extrusion under stress from the southwest to north east thrust movement; the earthquake rupture earthquakes belonging to a one-way, from southwest to north east migration, causing aftershocks north the East to the expansion; compressional thrust-fault earthquakes in the main shock, the stress propagation and release process is relatively slow, and may lead to greater intensity of aftershocks, continuing a long time.
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