8) The formula that describes the velocity profile across a vertical strike slip

Question

8) The formula that describes the velocity profile across a vertical strike slip fault that is slipping at rate u0 locked at the surface to depth D is u(x) = (u0/ )atan(x/D). Here x is the distance from the fault.

a) Draw a figure that shows the velocity as a function of distance from the fault x. Labels your axes with units.

b) Draw a figure that shows the velocity function if D=0.

c) This function represents the motion over interseismic time, when the fault is locked at the surface but is slipping at depth. Over longer times many earthquakes occur and the average rate over time includes the cumulative effect of earthquakes. Draw a function on the same axis that shows the relative rates over *very long* periods of time, i.e. over many seismic cycles.

d) On the same figure axes draw the interseismic strain rate as a function of distance from the fault.

Explanation / Answer

ANSWER:

In between major earthquakes, the ground deforms due to movement of tectonic plates. For strike-slip faults, such as the San Andreas Fault, the ground deforms in an 'S' shape that can be modeled as an arctangent. To better understand what is observed at the surface, imagine a fence built perpendicularly across a strike-slip fault. When the fault is first built it is nice and straight, but over time it starts to deform and look kind of like an "S". When the earthquake occurs the ground (and the fence) will snap, and the two sides of the fence will become straight again at some time after the earthquake, although displaced.

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