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Geodynamics Measuring crustal motions in coastal British Columbia with continuous GPS
H. Dragert, M. Schmidt, J. Henton, Y. Lu
This article describes the Western Canada Deformation Array (WCDA), a
network of automated continuous Global Positioning System (GPS) stations
located in southwestern British Columbia. Started by the Geological Survey
of Canada in 1991, the network has gradually expanded to serve as the
northern portion of the Pacific Northwest Geodetic Array (PANGA). The
objectives of the WCDA are to 1) provide high-quality GPS data
for global geodynamic studies; 2) provide a precise, common reference
frame for all deformation surveys carried out in this active seismic
region; 3) serve as a strainmeter to map regional strain and monitor
possible transient strain signals. This article describes how continuous
GPS data is collected, verified, and archived, and how that data is used
to identify transient signals, and estimate relative plate velocities with
a precision of better than 1mm/yr. The data are also used to constrain
tectonic dislocation models and plate trajectories, invaluable
contributions to current studies of regional crustal dynamics.
Introduction: The WCDA Network |
![WCDA Map WCDA Map](/web/20061103005311im_/http://www.gsc.nrcan.gc.ca/geodyn/images/gpscrust_wcda_net2_.gif) WCDA Map
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The Western Canada Deformation Array (WCDA) consists of 8 continuous
GPS tracking stations located in southwestern British Columbia. The purpose
of the array is to monitor crustal motions in this most active seismic
region of Canada and thus provide constraints for the modelling of plate
interactions along the northern Cascadia margin and help in the estimation
of current seismic hazard.
![WCDA Network sites WCDA Network sites](/web/20061103005311im_/http://www.gsc.nrcan.gc.ca/geodyn/images/gpscrust_ag98tb1a_.gif) WCDA Network sites
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The first station, DRAO, was established in Feb. 1991, and one of the
latest additions was WSLR which began continuous operation in Sept. 1996 (a new Site CHWK came online November, 1998).
All sites use TurboRogue receivers with Dorne-Margolin choke-ring antennas.
![Concrete pier - schematic Concrete pier - schematic](/web/20061103005311im_/http://www.gsc.nrcan.gc.ca/geodyn/images/gpscrust_pier_col2_.gif) Concrete pier - schematic
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The GPS antennas are mounted on forced-centre monuments embedded in concrete
piers which are anchored into bedrock. Details of the antenna set-up are
shown in the schematic diagram of the pier.
See also full description of the WCDA as well as all continuous GPS data
![CGPS-22 Analysis - flow chart CGPS-22 Analysis - flow chart](/web/20061103005311im_/http://www.gsc.nrcan.gc.ca/geodyn/images/gpscrust_process_.gif) CGPS-22 Analysis - flow chart
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30-sec sample GPS data are collected automatically every 4 hours from all sites
and binned into 24-hour data sets. Using precise IGS orbits and the station
DRAO as a fixed reference site, daily solutions of relative positions
of all network sites are computed at 2 min. epochs using the CGPS22 software
developed by Jan Kouba, Geomatics Canada. This software employs double-difference,
L3-phase solutions with orbits held fixed and ambiguities not fixed. One
of the strengths of this analysis software is the use of interactive graphics
to examine and edit the L3-phase data.
The changes in latitude, longitude, and height at the
various sites relative to DRAO exhibit day-to-day variations with characteristic
sigmas of 2 to 3 mm, 3 to 4 mm, and 6 to 9 mm respectively, dependent
primarily on baseline length. The monthly scatter in each of the components
(including baseline length) are shown for two stations (HOLB and WILL)
for the most recent three years. These values provide a realistic estimate
of the accuracy achievable for baselines measuring hundreds of kilometres.
![HOLB daily variations HOLB daily variations](/web/20061103005311im_/http://www.gsc.nrcan.gc.ca/geodyn/images/gpscrust_holbscat_.gif) HOLB daily variations
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![WILL daily variations WILL daily variations](/web/20061103005311im_/http://www.gsc.nrcan.gc.ca/geodyn/images/gpscrust_willscat_.gif) WILL daily variations
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Identification of transient signals |
Apparent in the plots of the daily solutions are distinct sudden offsets
and periodic variations. Since longer period variations and step-functions
can significantly bias linear trends, especially for data lengths <2 yrs,
these effects were removed through regression techniques. It was assumed
that the longer-period sinusoidal variations were predominantly annual,
and step-functions were allowed for when antenna set-up changes or reference
frame changes were known to have occurred. Note that the estimates of
the step magnitudes are analysis dependent and cannot be calibrated in
an absolute sense.
![HOLB-DRAO HOLB-DRAO](/web/20061103005311im_/http://www.gsc.nrcan.gc.ca/geodyn/images/gpscrust_holbgrf1a_.gif) HOLB-DRAO
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Using the data for HOLB as an example, the three types of regression parameter
fits (i.e. step functions, annual variations , and linear trends) are
illustrated for three components. Steps in the horizontal components for
all sites were generally less than 3 mm although occasionally as large
as 5 mm. Steps in the vertical ranged from 3 mm to as large as 19 mm showing
the strong dependency of the vertical position of the antenna phase centre
to the near-field EM environment. Annual signals in the horizontal components
have amplitudes less than 1.6 mm while the vertical annual signal amplitudes
range from 0.5 to 4.3 mm.
![HOLB-DRAO HOLB-DRAO](/web/20061103005311im_/http://www.gsc.nrcan.gc.ca/geodyn/images/gpscrust_holbgrf2a_.gif) HOLB-DRAO
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Least Squares Spectral Analysis (LSSA) confirms that periodic energy in the
vertical component is primarily limited to the annual signal, although
at HOLB a significant semi-annual signal is also apparent. The previously
reported sharp Mf (period = 13.67 days) spectral peak has diminished and
become more diffuse for the entire 6 year data set because over the past
2 years, this spectral component has effectively disappeared. It is significant
that little energy is seen for periods exceeding a year indicating minimal,
if any, 'random walk' behavior of monuments.
HOLB-DRAO Mf Tidal Estimates
Date |
Amp. (mm) |
Error (mm) |
Phase (days) |
Error (days) |
1992-98 |
1.83 |
0.32 |
0.69 |
0.38 |
1992 |
6.28 |
2.53 |
-1.57 |
0.88 |
1993 |
5.56 |
1.07 |
-2.02 |
0.42 |
1994 |
4.41 |
0.71 |
3.81 |
0.35 |
1995 |
2.98 |
0.79 |
1.99 |
0.58 |
1996 |
4.71 |
0.76 |
1.01 |
0.35 |
1997 |
1.02 |
0.65 |
-0.85 |
1.37 |
1998 |
0.10 |
0.78 |
-5.20 |
16.67 |
![Amplitudes Amplitudes](/web/20061103005311im_/http://www.gsc.nrcan.gc.ca/geodyn/images/gpscrust_ampl_.gif) Amplitudes
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![Phases Phases](/web/20061103005311im_/http://www.gsc.nrcan.gc.ca/geodyn/images/gpscrust_phase_.gif) Phases
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![Radial components Radial components](/web/20061103005311im_/http://www.gsc.nrcan.gc.ca/geodyn/images/gpscrust_holbspec_.gif) Radial components
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Table 2: Listing of changes in the antenna set-up at WCDA sites which affected the mean phase-centre positions. Dates give year followed by the Julian day.
Date |
Site |
Description of changes |
94:041 |
DRAO |
New antenna; New antenna mount; |
94:104 |
ALBH |
New antenna; New antenna mount; |
94:124 |
HOLB |
New antenna; |
94:173 |
WILL |
Added acrylic dome & mounting ring; |
95:011 |
ALBH |
New antenna mount; |
95:103 |
DRAO |
New antenna mount; |
95:158 |
ALBH |
New antenna; |
95:189 |
WILL |
New antenna mount; Added RF screening skirt; |
95:202 |
ALBH |
Added RF screening skirt; |
95:223 |
UCLU |
Added RF screening skirt; |
96:010 |
DRAO |
Added RF screening skirt; |
96:044 |
UCLU |
New antenna mount; |
96:046 |
NANO |
Added RF screening skirt; |
96:089 |
HOLB |
New antenna; New antenna mount; Added skirt; |
97:033 |
NEAH |
New antenna; New Ashtech cone dome; |
97:330 |
HOLB |
Replaced torn screening skirt; |
![Regional velocity vectors Regional velocity vectors](/web/20061103005311im_/http://www.gsc.nrcan.gc.ca/geodyn/images/gpscrust_pang_vel_.gif) Regional velocity vectors
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In the regression, all parameters (steps, annual signals, and linear trends)
are solved for simultaneously in each component. It is of course the linear
trends that are interpreted in terms of tectonic motions. The linear trends
for the north and east components are combined to produce estimates of
horizontal site velocities with respect to DRAO which is assumed fixed
on the North American plate. These vectors along with their 95% confidence
ellipses are drawn in the map diagram. Also shown are velocities for the
longer-running PANGA sites in northwestern Washington (from G. Khazaradze,
Univ. Washington) and the velocities predicted by an elastic dislocation
model of the locked Cascadia subduction thrust.
Slip-dislocation modelling |
![Cascadia Zone - modelling Cascadia Zone - modelling](/web/20061103005311im_/http://www.gsc.nrcan.gc.ca/geodyn/images/gpscrust_caslock_.gif) Cascadia Zone - modelling
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Surface crustal deformations are interpreted to be due to strain accumulating
across a locked thrust fault at the interface of the subducting Juan de
Fuca Plate and the Cascadia margin. This locked fault comprises the seismogenic
zone which will generate the next great earthquake. To the first order,
observed deformations can be accounted for by simple 2-D and 3-D elastic
dislocation models which place the seismogenic zone offshore beneath the
continental slope and vary the fault width between 40 to 100 km along
the margin.
![Cascadia Zone thrust model Cascadia Zone thrust model](/web/20061103005311im_/http://www.gsc.nrcan.gc.ca/geodyn/images/gpscrust_cszmod_.gif) Cascadia Zone thrust model
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Refined observations of crustal motions obtained from continuous WCDA data are
beginning to reveal some limitations of such simpler models. The most
striking example is the margin-parallel motion observed for northern Vancouver
Island which cannot be replicated by our current dislocation models. It
is also noteworthy that the model calculations have been brought into
better agreement with the observed velocities by using an azimuth of convergence
of 62 degrees between the Juan de Fuca and the North American plates instead
of the nominal 69 degrees.
![Top Top](/web/20061103005311im_/http://www.gsc.nrcan.gc.ca/esst_images/_up.gif)
The Western Canada Deformation Array (WCDA) is a network of automated continuous Global Positioning System
(GPS) stations located in southwestern British Columbia. Started by the
Geological Survey of Canada in 1991, the network has gradually expanded
to the current 8 sites which now also serve as the northern portion of
the Pacific Northwest Geodetic Array (PANGA). The initial objectives of
this regional array were to 1) provide high-quality GPS data for global
geodynamic studies; 2) provide a precise, common reference frame for all
deformation surveys carried out in this active seismic region; and 3)
serve as a strainmeter to map regional strain and monitor possible transient
strain signals. As such, the WCDA was a new tool being adopted in a program
of crustal deformation studies that had utilized tide gauge, levelling,
precise gravity, and trilateration surveys.
The utility of this new tool
has proven itself several times over: The infra-structure is in place
and we currently collect, verify, distribute, and archive the continuous
GPS data in near-real time with minimal manual intervention and therefore
minimal manpower; continuous regional reference stations allow the integration
of strain results from repeated campaigns, and horizontal strain tensors
can be derived in a common regional framework; relative crustal motions
have been resolved with a precision of better than 1 mm/yr within a time
span of two to three years and these observed motions are helping to constrain
models of the Cascadia Subduction Zone; seasonal and other transient signals
have been observed emphasizing the usefulness of continuous GPS coverage;
sudden shifts in positions have been found to be caused by near-field
effects on the antenna phase-centre location and pointed out the critical
importance of antenna set-up.
Although already invaluable
to current studies of regional crustal dynamics, the sparseness and limited
extent of this array present problems. The small number of continuous
sites makes each site critical for the robust estimate of regional strain
and great care must be exercised, beginning with the installation of stable
monuments through to the elimination of non-tectonic effects in the analysis
of data. This sparseness also prevents the resolution of variations in
the strain field related to active structures of modest (~100 km) spatial
scales and it prevents the clear identification of aseismic signals. With
its current geographical coverage, the WCDA network limits deformation
monitoring to the north Cascadia margin and leaves unmeasured the crustal
motions associated with the extensive seismic activity along the Queen
Charlotte Fault and regions further north.
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