Displacement and Shear Discontinuities in GPS and Other Survey Networks

It seems we can detect the orientation and nature of faults and shear zones using a method for analyzing deformation with survey data. Survey data are in terms of arbitrary coordinate systems---the initial (X,Y) and current (x,y) systems---rather than in terms of natural coordinate systems (S,N,s,n) of the fault or shear zone. The angle α is the clockwise angle between X and S. A method for detecting faults and shear zones involves computing the deformation gradient tensor F_{X_Y} with the survey data, and then determining the natural coordinates by maximizing the partial derivative (∂ s)/(∂ N) as a function of α . Results are presented from survey measurements made on a landslide surface, near earthquake ground ruptures, and in the vicinity of a tectonic plate boundary. I studied displacements of points on the Slumgullion landslide in Colorado and contrasted the results with mapped structures. I find sudden, large, jumps in the partial derivative (∂ s)/(∂ N) where there are faults with a strike-slip component parallel to α . Such discontinuities in the derivative are absent where strike-slip shift is absent. Likewise, I studied part of the ground rupture associated with the 1999 Ducze-Bolu earthquake in Turkey. The rupture passed beneath the Bolu-Kaynasli viaduct, damaging the structure. The piers of the viaduct were permanently displaced, and were surveyed before and after the earthquake. This analysis shows that the earthquake rupture, at the ground surface, is a belt of right-lateral shear approximately 50 meters wide in the vicinity of the viaduct. This method can also be used with GPS data. Preliminary results from 16 continuously operating stations near San Francisco suggest portions of the San Andreas, Hayward and Calaveras fault zones were active at different times during 2003. For example, during the interval April to May all three fault zones were possibly active. From August to September, it seems the Hayward and Calaveras fault zones were active, but only in a region north of San Francisco Bay.