Abstract Title: Present Day Continental Deformation and PBO Science Goals

Abstract Author(s): Thatcher, Wayne, Hammond, Bill, and Nyst, Marleen (all at 
U. S. Geological Survey, Menlo Park, CA 94025)  

Abstract: How continents deform has been hotly debated without definitive 
resolution since the earliest days of the development of the plate tectonic 
model. The evident wide distribution of seismicity, active faulting, and 
tectonically generated topography have suggested that active continental 
deformation takes place over broad regions and differs fundamentally from the 
narrowly focused straining occurring between plates of oceanic lithosphere. 
End-member models have postulated this deformation is either quasi-continuous 
or due to the relative motions of a small number of rigid blocks 
(ÔmicroplatesÕ). The absence of precise quantitative measures of regional 
continental deformation has been the chief obstacle in determining which, if 
either, description is correct. However, GPS methods have the singular 
capability of providing the necessary quantification, permitting accurate 
mappings of site velocities across deforming zones in a common reference frame. 
In particular, campaign GPS measurements provide an economical means of 
determining the present-day velocity field across actively deforming 
continental lithosphere on regional scales. These networks are beginning to 
define deformation kinematics within the western U. S., suggesting deformation 
is concentrated in narrow zones near the boundaries of the Great Basin, with 
boundary parallel movements in California occurring via relative motions among 
5-10 small microplate slivers oriented sub-parallel to the San Andreas fault. A 
more complete GPS velocity field in Aegean Greece and Turkey is economically 
explained by motions of 3-4 microplates. Within the context of PBO, campaign 
GPS measurements are best suited to determining the presumed steady-state 
deformation field to a precision of ~1 mm/yr. The more precious continuously 
recording sites can then be strategically deployed in dense, focused arrays to 
characterize transient deformation processes in the most active regions and 
define subtle deformation gradients across low slip rate faults.