FIGURE 4.19 Thresholds of strain-rate sensitivity are shown for borehole strainmeters, GPS, and InSAR as a function of period. The diagonal lines give GPS (green) and SAR (blue) detection thresholds for 10-kilometer baselines, assuming 2-millimeter and 2-centimeter displacement resolution, respectively. GPS and InSAR strain-rate sensitivity is better at increasing periods, allowing the detection of plate motions (dashed line). Borehole strain detection threshold reaches a minimum at a period of a week and then increases with period due to Earth noise. At longer periods (months to a decade), GPS has greater sensitivity than borehole strain by one to two orders of magnitude. At intermediate periods (weeks to months) the two are comparable, and at short periods the borehole strainmeter sensitivity is one to three orders of magnitude greater. Also shown are several types of transients observed by strainmeters (red), GPS (green), and InSAR (blue). Triangles show postseismic deformation from the Landers earthquake observed by strainmeters, GPS, and InSAR. Squares show slow earthquakes; open squares show single station observations. Red and green diamonds and open circles show long-term aseismic deformation. SOURCE: PBO Steering Committee, The Plate Boundary Observatory: Creating a four-dimensional image of the deformation of western North America, White paper providing the scientific rationale and deployment strategy for a Plate Boundary Observatory based on a workshop held October 3-5, 1999. Available at <>.

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