The current edition of "Seismological Research Letters" (May-June, V.78:3), has an Opinion piece by Cinna Lomnitz (a very prominent researcher; papers everywhere) of the University of Mexico (link below). In it, he presents an argument, some parts of which are over my head, suggesting that the whole theory of elastic rebound is in real doubt. Elastic Rebound theory is just about the first thing taught in Geology course sections dealing with earthquakes, and has been well accepted since not long after it was first proposed by by Harry Reid and Andrew Lawson shortly after the 1906 S.F. quake. It also seems intuitively, almost necessarily, correct, and not very surprising (assuming that rocks can store strain energy elastically, which I believe has been shown to be true).

In brief, here is a description of the theory, provided by Lomnitz:
1. Fracture is caused by elastic strains exceeding the strength of the rock.
2. These strains accumulate over a long period of time.
3. Coseismic motion represents the elastic rebound or release of the
stored elastic strain.
4. Seismic waves originate at the fracture of the rock.
5. The energy of the earthquake equals the elastic strain energy previously
stored in the rock.

I am hoping John Vidale, or other professional readers, can provide their opinion of this article, and answer some questions below. Of course, all readers are welcome to comment.

In arguing against Elastic Rebound, Lomnitz states that "nothing is stated [in the theory] about the origin of strain accumulation. In the atmosphere or in the oceans, no analogous model has been proposed."
-Lomnitz argues against mantle convection as being the source of the
input energy, stating that "no energy accumulates anywhere in a
convecting system." I find this difficult to believe. In my mind's eye,
I imagine the mantle slowly moving along horizontally beneath a
continental or oceanic plate, and "dragging" the plate along. (Or
an oceanic plate is moving due to pressure from the spreading center,
and/or being pulled by the weight of the descending slab). It is
easy to see how, as said plate experiences opposition to that movement,
via contact with another plate for instance, energy would become stored
in the rock. Also, though Lomnitz finds no analogous strain accumulation
"in the atmosphere in the oceans," isn't that irrelevant? If such
accumulations are found in other physical systems, that should be
sufficient, no? For instance, the two rubber blocks that are occasionally
used to demonstrate Elastic Rebound, and slip processes, are pressed
together, and then moved in opposite directions. Their boundaries
initially deform, and then, when strain exceeds friction, the deformation
relaxes (though may not disappear entirely). Though that example may be
faulty as regards the source of earthquakes, it provides a counter
to Lomnitz' argument regarding lack of analogs to stored strain energy,
it seems to me.

Lomnitz further states that "earthquakes never occur in the undamaged host-rock." I find this surprising, having always assumed that we do not have observations of faults forming simply because we've not been around long enough. I would have thought that the origins of most faults were to be found in fracture of previously un-fractured rock. He also seems to state that faults occur as a result of "environmental corrosion, usually because of the slow chemical reaction of water." I may be misreading him there. I hope so.

Lomnitz proposes an alternate to the Elastic Rebound Theory, which he calls "strength degradation." He futher argues that the failed Parkfield earthquake prediction experiment, particular its failure to detect any precursory signals, is strong evidence against Elastic Rebound. I see it as, at most, weak evidence. The lack of precursory signals may simply mean that the forces involved, spread over so large an area, are simply too weak to produce such signals. And, he states: "the striking absence of precursory signals agreed with the lack of success in earthquake prediction." Isn't that a classical example of a circular argument?

Mike Williams
Arroyo Grande, CA USA

• Equivalence of Tectonic Motions