- © 2014 by the Seismological Society of America
Online Material: Tables of statistical indexes; figures comparing displacement solutions.
INTRODUCTION: THE STATE OF THE ART AND OUR CONTRIBUTION
The Global Positioning System (GPS) has been repeatedly proven to be a powerful tool to estimate coseismic displacements and waveforms, with accuracies ranging from few millimeters to few centimeters. These promising results were achieved following two main strategies: differential positioning (DP) and precise point positioning (PPP; Bock et al. , Kouba , Larson et al. , Larson , Ohta et al. , Xu et al. , and Hung and Rau ). In particular, both the modeling of fault rupture and the seismic moment estimation could benefit from GPS‐derived displacements, because GPS is not affected by the saturation problems experienced by seismometers located near the epicenters of strong earthquakes. Thanks to the robustness of the GPS‐derived displacement waveforms, in the last years some authors (Bock et al., 2000; Langbein and Bock, 2004; Blewitt et al., 2006; Bock and Genrich, 2006) addressed the problem to retrieve them in real time, with accuracies of a few centimeters, from GPS high‐rate observations (1 Hz or more). In this context, the Variometric Approach for Displacements Analysis Standalone Engine (VADASE) has been proposed (Colosimo et al. [2011a], Colosimo ). The approach is based on time single differences of carrier phase observations continuously collected using a standalone GPS receiver and on standard GPS broadcast products (orbits and clocks) that are available in real time. Therefore, one receiver works in standalone mode and the epoch‐by‐epoch displacements (equivalent to velocities) are estimated. Then, they are summed over the time interval when the earthquake occurred to retrieve displacements. Because VADASE does not require either additional technological complexity or a centralized data analysis, in principle, it can be embedded into the GPS receiver firmware and therefore can work in real time. Moreover, differently from DP and PPP, …