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Recent Progress on GNSS Seismology
LIU JINGNAN, FANG RONGXIN, and SHI CHUANG
GNSS Research Center
Wuhan University
ABSTRACT
In this paper we introduce the background of seismology development and the
opportunities and challenges it faces. To verify the agreement of precise point posi-
tioning (PPP) results and seismometer results, we set up a Global Navigation Satel-
lite System (GNSS) seismologic testing platform. The experiments reflect that the
results by the two methods have great agreement. With regard to the applications
of GNSS seismology, we discuss the method of using GNSS to infer epicenters
of earthquakes. Eventually the perspective of GNSS seismology is put forward.
INTRODUCTION
The GNSS has been widely used to study kinematic deformation. The accu-
racy of GNSS kinematic positioning has been significantly improved and can
reach millimeter level (El�segui et al., 2006). At present, the receiver technol-
(El�segui et . technol-
ogy and data storage capabilities of GNSS have been greatly improved with the
sampling rate up to 50 Hz (Genrich and Bock, 2006). At the same time, high-rate
GNSS tracking networks have been well established, with which recent earth-
quakes have been successfully observed (Emore et al., 2007; Irwan et al., 2004;
Langbein and Bock, 2004).
When comparing GNSS with a seismometer, the differences are remark-
able (Wang et al., 2007). The digital seismometer measures the accelerations or
velocities of ground motions directly (Wang et al., 2011). By contrast, GNSS
estimates the position of an antenna by recording range measurements between the
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190 GLOBAL NAVIGATION SATELLITE SYSTEMS
antenna and satellite. Moreover, the datum of GNSS positioning is satellite constel-
lations, 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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191
RECENT PROGRESS ON GNSS SEISMOLOGY
There are some challenges for GNSS seismology. How to improve the preci-
sion of GNSS positioning? How to realize real-time processing for huge network
and ultra high-rate (20–50 Hz) GNSS data? How to explore methods of combining
GNSS data and strong motion data for earthquake investigation?
For the above problems, we made some primary studies and tests as indicated
in the following.
GNSS KINEMATIC POSITIONING AND TEST
Presently there are two approaches in GNSS kinematic positioning: relative
positioning (RP) technique (Bock and Prawirodirdjo, 2004; Larson et al., 2007)
and PPP technique (Geng et al., 2010; Kouba, 2003; Larson et al., 2003; Shi et
al., 2010). In the former approach, at least one station must be fixed or tightly
constrained to its known position, which will possibly experience displacement by
the seismic motions. Therefore, the displacements estimated for the other stations
are affected by the motions of the fixed station. In order to obtain displacements
relative to a reference frame, stations not affected by the earthquake should be
included as reference stations for data processing. As the position accuracy of rela -
tive positioning degrades along with the length of baselines, the inter-station dis -
tance is limited to hundreds of kilometers as in published studies (Ge et al., 2006).
In the PPP approach, where satellite clocks and orbits are fixed to pre-
estimated precise parameters, for example, in the International GNSS Services
(IGS) final products, the coordinates can be estimated station by station in the
reference frame defined by the orbits and clocks. However, orbit and clock errors
may affect the accuracy of estimated position. Hence we need to obtain high-
precision orbit and clock data for positioning. In our study and test, the PPP
approach was employed.
GNSS Seismometer Testing Platform
In order to compare the displacements of high-rate GNSS PPP results and
seismometers results, we set up a testing platform (Figure 1). It is made of an
aluminous board that is connected to the metal frame by eight springs. A GNSS
antenna, seismometers recording acceleration, and an inertial measurement unit
(IMU) are attached on the same board.
GNSS data were collected with a sampling rate of 50 Hz while seismometers
and IMU simultaneously worked to record data of acceleration and attitude.
Data Processing Strategy
High-rate GNSS data were processed with the PANDA (position and naviga -
tion system data analyst) software, which was developed by the GNSS Research
Center of Wuhan University as a multifunctional tool for GNSS research and
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192 GLOBAL NAVIGATION SATELLITE SYSTEMS
FIGURE 1 GNSS seismometer testing platform.
Liu-etal_Fig1.eps
bitmap
applications (Liu and Ge, 2003; Shi et al., 2006, 2008). Its performances in
precise static and kinematic positioning were demonstrated by Ge et al. (2008)
and Geng et al. (2009). In our test, PANDA software was used to compute dis-
placements recorded by a GNSS receiver with the PPP technique. In order to
correct for the tilting of the platform, IMU was used to obtain the attitude of the
platform. After attitude correction, accelerometer data was integrated twice to
obtain displacements.
Results and Analysis
Figure 2 shows a 50-second oscillation in the east component. The maximum
displacement amplitude is about 1 cm. The amplitude of the maximum error is
5.6 mm. About 98 percent of the differences are less than 4 mm. The RMS error
over the entire event is 1.8 mm.
APPLICATIONS OF GNSS SEISMOLOGY
Studies on Propagation Velocity and Earthquake Parameter Determination
Seismic waves take different times to reach various stations, and with the
signals captured by GNSS we can determine the arrival times of seismic waves.
Figure 3 shows the Wenchuan earthquake seismic waveforms of the east com-
ponent captured by GPS sites located at Shanghai, Wuhan, Xi’an, and Chongqing,
respectively (from top to bottom).
With the time that an earthquake occurs, the times that seismic waves reach
GNSS stations, and the distance between the stations and the epicenter, we can
calculate the propagation speed of seismic waves. According to our calculation,
the average speed of seismic wave propagation for the Wenchuan earthquake is
3.9 km/s.
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193
RECENT PROGRESS ON GNSS SEISMOLOGY
FIGURE 2 Comparison between GPS and seismometer (below), and their differences
Liu-etal_Fig2.eps
(upper).
bitmap
Suppose the seismic waves propagate at equal speeds along all components,
the position of the epicenter and the average speed of seismic wave propagation
can be inferred with the known coordinates of the above four stations and the
arrival times of seismic waves. Figure 4 describes the principle of epicenter deter-
mination with the coordinates of GNSS stations and the arrival times of seismic
waves. Based on this method, GPS data of the Wenchuan earthquake are used to
infer the epicenter. Compared with the result issued by the China Seismological
Bureau, the inversion error is 12.5 km. The epicenter of the Chile earthquake on
February 27, 2010, is also inferred with the inversion error of 27.4 km compared
with the result released by the U.S. Geological Survey (USGS).
CONCLUSIONS
High-rate GNSS is going to be a new method to monitor earthquakes. With
high-rate GNSS data, both seismic waveforms and the permanent offsets can be
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194 GLOBAL NAVIGATION SATELLITE SYSTEMS
FIGURE 3 Seismic waveforms of Wenchuan earthquake in the east component captured
Liu-etal_Fig3.eps
by GPS sites located at Shanghai, Wuhan, Xi’an, and Chongqing respectively (from top
bitmap
to bottom).
obtained. Combining high-rate GNSS data and seismometer data, seismologic
research on earthquake parameter determination and fault rupture modeling will
be more scientific and reliable (Bock et al., 2011). Thus the high-rate GNSS
positioning technique can be applied for real-time earthquake monitoring (Allen
and Ziv, 2011), tsunami warning (Blewitt et al., 2006; Sobolev et al., 2007), and
other engineering deformation monitoring.
There are still some problems with GNSS seismology. For instance, GNSS
is not as sensitive as seismometers, GNSS is not applicable for small or far-field
earthquakes currently, and the sampling rate is still insufficient to capture high-
frequency seismic waves. These problems need to be further studied in our future
research.
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RECENT PROGRESS ON GNSS SEISMOLOGY
Liu-etal_Fig4.eps
FIGURE 4 Epicenter determination with four GNSS stations.
bitmap
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