meters, alignment arrays, short-baseline triangulation, and laser length surveys (49). Deployment of these instruments revealed both steady and episodic creep occurring at shallow depths (<4 kilometers) on some faults, often near the time of earthquakes (50). Aseismic creep at greater (seismogenic) depths, such as observed in central California, appears to be rare.

High-resolution laser strainmeters, borehole strainmeters, and tiltmeters are used to measure deformation very precisely in a small region. Their sensitivity approaches 10–12, but long-term stability is a problem because they have small footprints susceptible to very localized, nontectonic deformations, such as ground swelling in rainstorms. The most stable instruments are the laser strainmeters and water-tube tiltmeters at Piñon Flat Observatory in California, which derive their stability from their length (>500 meters) and the “optical anchors” used to couple the end monuments to rock at about 25-meter depth (51). Borehole instruments suffer drift over several months, but they are very precise over shorter times and have widespread application for measuring transient deformation, including slow and silent earthquakes (52).

Under the best conditions, these instruments are as much as two to three orders of magnitude more sensitive than GPS at short periods. At longer periods, the relative advantage declines significantly, although long-baseline instruments may retain advantages even for time scales of years. Because they measure strain, which decays as the cube of distance from a dislocation, they must be positioned in reasonably close proximity to the source. Small numbers of borehole strainmeters have been operating for years in a few select locations. The proposed Plate Boundary Observatory would deploy several hundred borehole and perhaps several long-baseline strainmeters at strategically chosen sites along the San Andreas fault system, as well as at several volcanic systems (53).

Geodetic Observations of Earthquake Processes

The increasing precision and density of geodetic measurements are furnishing new constraints on how complex fault systems are loaded, how earthquakes interact, the nature of aseismic deformation transients, and how the rheological structure of the Earth’s crust controls the earthquake process.

Plate Tectonics and Fault Motions Geodetic studies have shown that plate-tectonic models, based on data that average over thousands to millions of years, can accurately predict the short-term motions across plate boundary zones a few hundred kilometers wide. Denser measurements from geodetic networks place strong constraints on the slip rates

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