High-rate GPS: How high do we need to go?

A number of large high-rate GPS (HRGPS) networks, such as the Plate Boundary Observatory (PBO), are poised to capture strong-motion displacements from significant earthquakes with data collected at 1 sec or higher sampling rates. For earthquakes within these networks\u27 footprints, HRGPS data will provide displacement time series at very small epicentral distances that may have great potential to contribute to understanding earthquake rupture processes. The conditional may in the previous sentence relates to whether or not the HRGPS data are aliased, i.e., is the sampling fast enough to capture a valid temporal history of displacement? We present analysis of strong-motion recordings in the immediate epicentral area of several large earthquakes to show that 1-Hz sampling would be aliased as suggested. For M \u3e 7.5 events, which can be reasonably expected in the PBO footprint, 5-Hz data may also be aliased. These results argue for faster data collection, at least 10 Hz, to record valid displacement time histories at sites that are likely to suffer large co-seismic offsets and large near-field seismic waves. Sampling rates higher than that required by the frequency content of the observable signal provide additional data that can be used to improve the quality of the final HRGPS displacement time series