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Equivalence of Tectonic Motions

No one puts new wine into old wineskins.

— Luke 5:37

In 1920, Albert Einstein suggested a famous thought experiment. A man drops an object from his pocket while he is falling from the rooftop of his house. To him the object does not appear to be falling; rather, it remains stationary at his side. Einstein concludes that gravity and acceleration are equivalent, and that "free-fall acceleration is a powerful argument for extending the postulate of relativity to non-uniform motions between coordinate systems."

This formulation of the equivalence principle can be extended to other realms of physics. In electromagnetic induction, for example, there is equivalence between relative motion of the coil or of the magnet, yet "the theoretical interpretation of the phenomenon in each case is quite different," as Einstein points out.

Two tectonic plates are moving against each other while the boundary remains locked. Two different interpretations are possible.

 

Consider the example of earthquakes. Two tectonic plates are moving against each other while the boundary remains locked. Two different interpretations are possible: 1) The stress (t) at the boundary is an increasing function of time t while the strength _c at the same boundary remains constant; or 2) the stress at the boundary remains constant while the strength _c(t) at the same boundary is a decreasing function of time. In either case the rupture will occur when _c.

Let us call the first case "elastic rebound" and the second "strength degradation." The question I propose to address is: How can we distinguish between the two cases?

	THE GEODETIC ARGUMENT

 
Geodetic techniques for measuring displacements at the earth's surface are developing rapidly. These techniques include the Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) imaging. Geodetic evidence of systematic relative displacements between adjacent plates frequently is cited in . . . [Full Text of this Article]

Cinna Lomnitz

cinna@prodigy.net.mx