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antenna and satellite. Moreover, the datum of GNSS positioning is satellite constellations, whereas the seismometer measures in a ground inertial reference frame.

A GNSS receiver has several advantages over a traditional seismometer. First, when the displacements of ground motions are required, GNSS can directly estimate them by range measurements and the results have no accumulated errors over time, but after integrating seismometer data to displacement, large drifts will occur. Secondly, a seismometer may be saturated in a large earthquake, in which case the instrument can not record the full amplitude of velocity or acceleration while GNSS will not be saturated in amplitude. Thirdly, a seismometer operates based on the theory of gravity, and a tilt of the instrument can bring about artificial horizontal acceleration, but a GNSS receiver will not be affected in this way. When a seismometer is saturated and unavailable to record large or nearby earthquakes, GNSS can be a feasible tool for earthquake studies.

When kinematic GNSS technology is used in seismology, it is referred to as GNSS seismology (Bock et al., 2011; Larson, 2009). In other words, using a high-rate GNSS technique to investigate the background, developing process, and explosion of earthquakes, and combining other data to determine relevant parameters can be called GNSS seismology. A high-rate GNSS receiver that is used as an instrument to capture co-seismic waves is called a GNSS seismometer.

The first experiment on GPS seismometers was reported in 1994 and was carried out by Hirahara et al. (1994) at the Disaster Prevention Research Institute of Kyoto University. In another experiment, Ge et al. (2000) fixed a GPS antenna, an accelerometer, and a velometer on a truck platform and demonstrated how GPS could recover the truck oscillation in both frequency and amplitude. They were the pioneers to reveal that GPS is able to measure large displacements with a high sampling rate.

After the 2002 Mw 7.9 Denali earthquake, Larson recovered the seismic waves with 1 Hz high-rate GPS data collected from California GPS tracking networks (Larson et al., 2003). Since then, a number of similar studies followed to use high-rate GPS technology to reconstruct waves caused by earthquakes (Bilich et al., 2008; Bock et al., 2000; Kouba, 2003; Shi et al., 2010) and to determine source parameters of earthquakes (Davis and Smalley, 2009; Ji et al., 2004; Miyazaki et al., 2004).

Kinematic high-rate GNSS positioning has been demonstrated successful for measuring seismic waves, in which relative mode is applied for GNSS data processing by assuming that one of the stations can be fixed to serve as a datum to compute absolute displacements (Bock and Prawirodirdjo, 2004; Larson et al., 2007). Obviously, a fixed datum may not be appropriate in the case of a large earthquake, and under that circumstance PPP instead of relative positioning is highly desirable. However, up to the present, there is no PPP result available to directly compare displacements from a seismometer and a GNSS receiver. The current studies of high-rate GNSS waveforms focus on the horizontal component and information on the vertical component has been ignored (Larson et al., 2007).



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