In June and July, 1995 we installed continuously operating GPS trackers at the University of Washington campus in Seattle (SEAT) and at Cheeka Peak (NEAH) on the coast of Washington. The GPS receivers at these sites have been operating continuously since that time. Daily processing of these data together with data from US Coast Guard and Canadian trackers indicates that the UW trackers are moving to the northeast (up to 13 mm/year at NEAH) relative to the interior of the US or Canada, consistent with a simple model of the convergence between the Juan de Fuca and North America plates. In this model the plates are locked together without slipping but they steadily deform. Details of the operation of the GPS stations as well as raw and RINEX data can be found at the web site http://www.geophys.washington.edu/GPS/. A copy of this report is available at http://www.geophys.washington.edu/SEIS/PNSN/REPTS/Summaries96/G2582.htm.

In June and July 1995 UW installed two continuously recording stations, at Cheeka Peak (NEAH), near Neah Bay, Washington, and Seattle (SEAT), at the campus of the University of Washington. Both receivers are ASHTECH Z-XII instruments with Dorne Margolin Choke Ring style antennas. At Cheeka peak the antenna is mounted on a 2.4 meter high, 0.6 meter diameter cylindrical concrete pillar secured to bedrock via grouted rebar in holes that extend 2 meters below the rock surface. The antenna at the University of Washington is mounted on the roof of the Atmospheric/Geophysics building on a massive steel "quadrupod" attached to the building. Locations of these sites together with sites operated by Geological Survey of Canada that are included in the daily analysis are given in Table 1.

---------------------------------------------------------------------
Table 1. Operational Continuously Recording GPS Sites 
---------------------------------------------------------------------
 
 Location           ID    Latitude  Longitude  Start       Operating
                             (N)       (W)     Date        Agency
 -----------------  ----  -------   --------   ------      ---
 Neah Bay, WA       NEAH  48.2979   124.6249   Jul-95      UW
 Seattle, WA        SEAT  47.6550   122.3083   Jun-95      UW
 Penticton, B.C.    DRAO  49.3226   119.625    Feb-91      GSC
 Victoria, B.C.     ALBH  48.3898   123.4875   May-92      GSC

GSC=Geological Survey of Canada; UW=University of Washington.

UW has analyzed data from the GPS sites using GIPSY/OASIS II software together with data from selected IGS sites and US Coast Guard CORS sites at Whidbey Island, Washington (WHD1), Point Robinson, Washington (RPT1), and Fort Stevens, Oregon (FTS1). Raw and RINEX format data from the UW sites NEAH and SEAT are available via computer for 15 days from the the date of recording at http://www.geophys.washington.edu/GPS/.

Results

Station positions for several continuously recording IGS GPS sites plus NEAH, SEAT, and US Coast Guard stations at Whidbey Island (WHD1) and Point Robinson (RPT1) in the Puget Sound and Fort Stevens (FTS1) at the mouth of the Columbia river are computed daily using GIPSY/OASIS II software. The results for 4 stations are shown in Figure 1 for a one year period.

Figure 1. Deviations of the daily positions of several sites with respect to Canadian station DRAO in Penticton, BC. No outliers in the data have been removed. Velocities were computed using weighted least-squares regression. Errors represent 95% confidence limits.

The motions indicate that deformation is dominated by the interaction between the Juan de Fuca and North America plates. Computed Station velocities are to the northeast relative to the stable interior of British Columbia, roughly parallel to the relative motion between the Juan de Fuca and North America plates (Figure 2 and Table 2).

Figure 2. Computed station velocity (mm/yr) with respect to Canadian station DRAO, located at Penticton, BC in the Canadian interior. Results are based on 1 year of data (1996). No outliers have been removed. Velocities were computed using weighted least-squares regression. Ellipses around the tips of each arrow indicate 95% confidence limits on the velocity.

---------------------------------------------------------------------
Table 2. Estimated Station Velocities (mm/yr) relative to DRAO based 
         on GPS data through 1996
---------------------------------------------------------------------

Sta   Vnorth  2s     Veast    2s       Vup    2s    Vhoriz    2s   Az
ALBH   2.9   1.1       9.4   1.2      -2.5   1.6       9.8   1.6   73
FTS1   6.7   1.8       2.6   1.8      -8.8   3.5       7.2   2.5   21
NEAH   5.2   1.3      11.5   1.6      -4.6   3.2      12.6   2.1   66
RPT1   3.2   1.5       1.6   1.6      -2.9   3.2       3.6   2.2   27
SEAT   1.9   1.1       0.8   1.4      -5.5   2.3       2.1   1.8   23
WHD1   4.5   1.5       3.7   1.7       6.9   3.5       5.8   2.3   39
---------------------------------------------------------------------

North, east, and vertical velocity is given together with the 2 standard
deviation uncertainty. Azimuth is the direction of the horizontal 
component of  velocity.

The largest horizontal velocity reaches 13mm/yr near the Washington coast. The component of relative motion in a north-south direction between any two stations in Figure 1 does not exceed 1 mm/year. This result is interesting because it is well known that crustal earthquakes in the Puget sound have fault plane solutions indicating a roughly north-south direction of maximum principal stress. Apparently, the direction of maximum principal stress is not aligned with the direction of maximum (compressive) strain rate.

1997 GPS Campaign Surveys

In the summer of 1997 we plan to conduct campaign surveys of 52 survey marks in western Washington (Table 2), principally in the Puget Sound region. Each site will be occupied for two 8-hour periods on successive days. Many of these sites have been previously occupied by UW and the the USGS, including occupations in 1995 and 1996. For a history of the occupation of these sites see http://www.geophys.washington.edu/GPS/. The data for these sites will processed together with data from continuously recording stations. The results will be compared with data from previous surveys in order to provide strain rate information at higher spatial resolution than can be obtained by the continuously recording stations.

---------------------------------------------
Table 2. GPS Campaign sites Monitored  in 
         Western Washington
---------------------------------------------
 Lat     Long   Sta     Lat     Long   Sta
------ -------- ----   ------ -------- ----
48.162 -122.158 ARLI | 46.300 -124.076 NHD4
48.170 -122.154 ARLO | 46.967 -122.908 OLYM
47.686 -122.249 AVIA | 47.636 -122.142 OVER
47.501 -122.755 B197 | 47.908 -122.272 PAIN
47.855 -123.260 BLUE | 48.017 -123.495 PTAN
48.513 -122.599 CAPS | 48.098 -124.119 PYST
46.973 -123.703 CENT | 47.837 -124.561 QUI2
47.232 -121.171 EAST | 47.837 -124.561 QUIL
46.631 -123.275 F064 | 48.054 -123.011 SAN2
47.937 -124.396 FORK | 47.237 -123.141 SAND
46.541 -123.500 FRAN | 46.602 -120.477 SARG
47.738 -121.986 FUNK | 47.824 -122.875 SCHO
47.549 -122.786 GLDH | 48.508 -122.228 SEDR
47.536 -124.333 GP14 | 47.571 -122.553 SITA
46.903 -124.100 GRAY | 46.663 -123.810 SOBE
47.304 -124.279 GREN | 47.401 -122.164 SPOO
47.485 -122.176 HAFF | 48.239 -122.348 STAN
48.055 -122.809 JCIA | 47.273 -122.573 TACO
47.212 -121.998 KIBL | 47.387 -123.113 TAHU
47.735 -122.648 KING | 47.479 -123.986 TENO
47.094 -122.621 LEWI | 47.103 -122.290 THUN
47.922 -122.683 LUDL | 47.509 -121.985 TIGE
47.660 -122.111 MARY | 47.012 -120.521 WILS
46.938 -122.554 MCKE | 48.141 -122.768 WORD
46.551 -122.270 MORT | 46.516 -124.057 X537
48.370 -124.598 NEA2 | 47.033 -123.112 Z478

Abstracts

G. Khazaradze, A. Qamar, and H. Dragert, Permanent GPS Network in Washington, EOS V76, N46, supplement pg. F150, (abstract) 1995.

G. Khazaradze, A. Qamar, E. Endo, H. Dragert, and N. King, Preliminary Results from New GPS Measurements in Washington, Annales Geophysicae V14, supplement 1, pg. 251, (abstract), 1996.

G. Khazaradze and A. Qamar, GPS Based Crustal Deformation Studies in Washington, Abstracts from the 8th Annual IRIS Workshop, pg. 39, 1996.

Back to:... UW Dept. of Earth and Space Sciences ..... SEISMOLOGY INFO..... PNW EARTHQUAKES