Why didn't we detect any signs before Wenchuan earthquake?

The unpredictability of earthquakes is determined by three factors. The first is the inviolability of the earth. At present, human beings can only speculate on the underground changes by observing the surface, and this speculation is not unique. The second is the complexity of the law of earthquake pregnancy. Through years of research by experts, it is gradually realized that the process of earthquake preparation, occurrence and development is very complicated. In different geographical and tectonic environments and different time stages, earthquakes of different magnitudes all show quite complicated gestation process. The third is the small probability of an earthquake. "You may think that earthquakes occur every year in the world, and some are relatively large earthquakes; But for a region, the repetition time of earthquakes is very long, and there are statistical samples for scientific research for decades, hundreds and thousands of years. The acquisition of this sample is very difficult in a person's life. "