Earthquakes occur in the crust or upper mantle. According to the Earthquake Hazards program, “shallow” earthquakes occur above a depth of 70 km in the crust.1 One simple model of earthquake behavior is called the slider-block model. Essentially, a block of mass m rests on a flat surface, and it is pulled horizontally by a spring. When the force exerted by the spring equals the frictional resistance, slip occurs. The model predicts the displacement δl of the slider block (corresponding to the fault displacement) and the maximum slip velocity vm on the fault as2 
δl=4SfSρghA(1fdfS)andvm=fSgh(2ρ/S)1/2(1fdfS),
where S is the shear modulus; fs...

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