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The elastic rebound theory (6)

Well before the introduction of the plate tectonic theory, Reid hypothesised that the forces that cause an earthquake are not near the earthquake but very far away. (For the sake of this explanation assume that the road in the diagram is a Roman road - very straight, very old.)

Over a period of hundreds to thousands of years these distant forces cause a gradual build up elastic energy. (The distortion in the following diagram is greatly exaggerated, and a more recently built fence has been added.)

The ground continues to slowly distort until an existing weakness in the Earth (a fault) cannot take the strain and breaking point is reached. Then within a short period of time (approximately 40 seconds for the Izmit earthquake) the elastic energy is suddenly and catastrophically released resulting in an earthquake. The regions on either side of the fault rebound and the ground breaks along the fault, in this case causing five meters of fault offset. (The distortion of the fence is what Reid observed from the survey data taken just before and after the San Francisco earthquake.)

Over long periods this repeated cycle results in a large fault offset. For example, the North Anatolian Fault has a fault offset of around 75 kilometers. This offset would take something like 15 000 Izmit-like earthquakes and over three million years to accrue at present rates of deformation.

We now know that Reid's elastic-rebound model is too simplistic. It ignores changes in the properties of rocks with depth, and therefore cannot explain why interseismic deformation is focused on the fault that eventually ruptures or why rapid deformation often occurs in the immediate aftermath of an earthquake (postseismic deformation). There is broad agreement that most continental earthquakes occur in the so-called seismogenic crust, typically the upper 10 to 20 kilometers, and that this behaves elastically, just as in Reid's model. Below this, where rocks are hotter, the material properties and behaviour of continental crust are still controversial. COMET scientists are using observations of surface deformation at various stages of the earthquake cycle to place bounds on competing models.

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