A single couple is sometimes proposed as a model of fault slip: two forces of magnitude

PHYSICS · Multi-part · Fermi University Physics · Earth, Climate & Energy Physics · Seismology, tectonics and geodynamics · Seismology, tectonics and geodynamics

The Solid Earth, Ch The earthquake source: moment tensor, radiation and spectra

Problem

F = 4.0\times10^{11}\,\mathrm{N}

, one applied at

(+1.5\,\mathrm{km}, 0, 0)

directed along

+\hat{\mathbf{y}}

(east) and the other at

(-1.5\,\mathrm{km}, 0, 0)

directed along

-\hat{\mathbf{y}}

, in a source-centred frame with

x

north,

y

east,

z

down. Someone argues: "the two forces are equal and opposite, so the net force is zero and this pair is a perfectly good, self-contained model of the source; nothing more is needed." ((a)) Compute the net force on this system. ((b)) Compute the net torque about the origin. ((c)) Say what is wrong with the claim, using the torque found in (b).

Browse the problem library