The question of predicting earthquakes and how to tell if the Big One is coming remains a mystery. In recent years, several prediction models have been advanced, including the possibility of measuring thermal patterns of heat on the earth, from space, in order to serve as a predictor. Scientists have poked holes in this latest theory, and much to everyone’s dismay, no one single method is reliable for determining when an earthquake will occur. What does remain fairly constant is that most large earthquakes occur on fault lines, where constant pressure from underlying tectonic plates can make the ground suddenly shake, rattle and roll. Scientists can thus say that larger earthquakes are much more likely along fault lines, especially some noted faults like the San Andreas Fault that runs through much of California.
The question of whether little earthquakes are precursors to big earthquakes is thus a complex one. First, you’d have to define little earthquakes; are these the ones people don’t generally feel, or are they small 2.0-3.0 quakes that a few people will feel? Even then, if you do arrive at a definition you can’t say definitively that little earthquakes always come before big ones.
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For instance, in California, if you look at the US Geological Survey (USGS) website, you can count hundreds of earthquakes that aren’t even felt, occurring with great regularity. If these little earthquakes are precursors to big earthquakes, then we’d constantly be having large earthquakes. On the other hand, small earthquakes suggest a certain level of fault line activity and pressure building up, and scientists regularly suggest that we must all prepare for the Big One, since it could occur at any time. Thus you can say small earthquakes may presage big earthquakes because they do suggest that eventually, at some point in the future, a big earthquake is likely.
When a large earthquake occurs, the earthquakes in the days prior are called foreshocks. The small earthquakes that occur after a large earthquake are called aftershocks. Little earthquakes don’t necessarily have to be either, but scientists may cluster earthquake activity before and after a large earthquake in the hopes of better understanding how and why earthquakes occur and under what circumstances they are most likely. Still, using the theory that little earthquakes are precursors to big earthquakes is not sound science. Only some of them are. It’s more accurate to suggest that active fault lines that produce these tiny unfelt shakers are likely at some point to produce larger earthquakes.