Earthquake is usually described with the data set of its start time, epicenter, depth, magnitude, and seismic intensity.But,the epicenter/depth is just only a starting point of cracking area or block.Earthquake phenomenon is a cracking/slipping in Earth crust.For example,Great East Japan Earthquake in 2011 had the broken area with almost 450km length and 200km width according with Japan Meteorological Agency's analysis.Earthquake energy is depended on the length of the earthquake fault.Earthquake energy is defined as magnitude scale.Seismic intensity is depended on the distance between the observation point and the epicenter including the depth.So,we need to predict the magnitude before the seismic intensity.
In general,cracking starts from the weakest points.Folded paper can be cut along with the folding line easily.Light selects the route with the shortest time required according with Fermat's principle.As same as Light, crack selects the route with the minimum work according with principle of least action.This may be the reason why there are some epicenters are on a line and others are not on a line against the complicated geological structure.One of the weakest line in the area should be the earthquake fault as the scar of the ground.The large fault can be the cause of big earthquake.
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