THE GEOLOGICAL COMPONENT OF EARTHSCOPE'S PLATE BOUNDARY OBSERVATORY

The diffuse plate boundary zone of the western US and Alaska consists of hundreds of active tectonic elements that collectively accommodate large-scale relative motions between the North American, Pacific and Juan de Fuca plates. The life-cycle of an individual fault zone lasts on the order of 1 to 10 Myr, and the repeat times of major earthquakes (durations of the seismic cycle) are on the order of hundreds to thousands of years or more. Hence any attempt to understand tectonic systems must include observations on these time scales. The geodetic component of the PBO will provide a "backbone" of GPS sites, targeted "clusters" of GPS sites, and borehole and laser strainmeter arrays that will define the strain field of the upper continental crust on the decadal timescale, including transient modes of deformation. The geological component of the PBO will establish the context of these geologically "instantaneous" observations, by examining the strain field at longer time scales. This effort is particularly timely because of the development over the last decade of new imaging and analytical tools that are revolutionizing neotectonic investigations. New hyperspectral landscape imaging systems, such as ASTER and Landsat7, will make large-scale, high-resolution tonal images easily accessible to the entire geological community. New laser ranging systems are able to penetrate the forest canopy and thereby enable identification of structures that would otherwise remain unknown over the much of the active plate boundary deformation zone. Analytical methods, especially the determination of the surface exposure age of most geological materials using cosmogenic nuclides, enable precise dating of geomorphic surfaces.