STRAIN AND ROTATION RATES FROM GPS: PITFALLS AND POSSIBILITIES IN THE SEARCH FOR A PERMANENT DEFORMATION SIGNAL

The Global Positioning System (GPS) allows the realtime sampling of growing structures but what, if anything, does this instantaneous snapshot tell us about long term geologic deformation in active orogens? A common approach is to compare idealized fault slip rates calculated from elastic block models to real slip rates from paleoseismological and neotectonic studies. We have taken an alternative approach of calculating strain, rotation, and dilatation rates from the gradients in the velocity field. This has the advantage of making no a priori assumptions about rheology or about which faults and folds define the deformation pattern in the region. The approach is not without pitfalls of its own, however: artifacts can arise due to the choice of best-fitting and smoothing algorithms employed, as well as due to irregular spacing and distribution of regional GPS networks, and the inherent length scale dependency of strain, itself, especially in regions of brittle deformation. After accounting for artifacts, an apparent paradox remains: much of the deformation signal measured by GPS is clearly elastic, as documented by unambiguous elastic rebound in diverse settings such as 1995 Antofagasta (Chile) and the 1999 Izmit (Turkey) earthquakes, and yet over vast regions, GPS measured strain is entirely consistent with long term permanent deformation. In the western United States, for example, the largest horizontal extension rates and positive 2D dilation rates coincide closely with the loci of Holocene and recent faulting and active crustal thinning in the Basin and Range province. In Tibet and Anatolia, maximum shear strain rate planes from GPS parallel closely the active strike-slip faults of the region. In fact, in all of the orogens studied, the permanent structures are consistent with the interseismic part of the seismic cycle of the major plate-bounding fault zones. The interseismic stress field is the one associated with the permanent deformation. This appears to be true everywhere except in the forearcs overlying the locked segments of subduction zone thrusts. Elsewhere, interseismic elastic and long term permanent deformation are very similar. This observation is a result of the width of the elastic deformation field for different types of faults relative to the spacing of modern, largely campaign GPS networks.