Journal of the Geological Society; October 1983; v. 140; no. 5;
p. 766-767; DOI: 10.1144/gsjgs.140.5.0766
© 1983 Geological Society of London
Discussion
MR. RICHARD W. MURPHY said: In your analysis of the San Andreas fault system please do not omit the tectonic underpinning of observed variations in seismic character. You have identified four segments of the San Andreas: two without microearthquakes (the ‘1906 segment’ and the ‘1857 segment’) and two with microearthquakes (central California and the Imperial Valley). Comparison of surface trace of the fault system with slip directions as revealed by plate tectonic analysis shows that the two segments without microearthquakes are wrench-compressional zones, whereas the segments with microearthquakes are wrench-extensional or wrench-parallel. The obvious inference is that the compressional zones are locked, at least on a short-term basis, whereas the active zones are more freely releasing heat and strain energy.
These observations are intended to supplement, not detract from, a fine paper in which you have brought order and meaning to a host of rather loose and chaotic data.
In reply, the AUTHOR thanked Mr Murphy for his comments and observations. Lack of time had prevented a full analysis of the several hypotheses put forward to explain changes in seismic behaviour along the San Andreas fault, but a more lengthy discussion appears in Sibson (1982b). It does indeed seem probable that he thermo-mechanical properties of fault systems are intimately linked to fault geometry, with changes in seismic style reflecting their interplay.
DR DAVID C. RUBIE said: I wish to comment on Dr Sibson's discussion of the depression of isotherms by underthrusting. If the thermal structure of a thrust sheet is
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