The University of Washington (UW) operates four continuously recording GPS receivers as part of a Pacific Northwest Geodetic Array designed to monitor deformation of the earth's crust and its impact on earthquake hazards in the region. In 1997 the UW added two new permanent GPS stations north and south of the Puget Sound. Geodetic observations indicate that existing UW sites at Neah Bay (NEAH) and Seattle (SEAT) are moving to the northeast relative to sites further east by up to 13 mm/year (at NEAH). In addition to the continuously recording stations, UW surveyed 52 sites in western Washington using temporary "campaign" techniques. These sites will yield greater spatial resolution to the analysis of deformation.

In 1997 we made geodetic measurements using Global Positioning System (GPS) technology in order to study deformation in the Puget Sound region, an area that has major crustal faults and which is subject to stress from the Pacific/Juan de Fuca/North America system of plates. We carried out two major field efforts:

• installation of two new permanent GPS tracking stations.
• a campaign survey of 52 stations in the summer of 1997. Many of these stations had been previously surveyed by the UW in 1995 or 1996 and by the US Geological Survey in 1991 and 1993.

Continuously Recording GPS Stations

UW installed two new continuously recording stations, Sedro Woolley (SEDR) and Kelso (KELS), Washington this summer. These sites are included in the 1997 expansion (5 sites) of the Pacific Northwest Geodetic Array (PANGA) by the University of Washington and Central Washington University (CWU) - see Table 1. PANGA represents a cooperative effort between several agencies operating GPS receivers in the Pacific Northwest including the University of Washington, Central Washington University, Oregon State University, University of Oregon, the US Geological Survey, and the Geological Survey of Canada. Permanent stations now operating that can be considered a part of PANGA are listed in Table 1:

---------------------------------------------------------------------
Table 1. Operational Continuously Recording GPS Sites 
             Ordered by Operating Agency 
---------------------------------------------------------------------
 
 Location           ID    Latitude  Longitude  Start       Operating
                             (N)       (W)     Date        Agency
 -----------------  ----  -------   --------   ------      ---
 Williams Lk, B.C.  WILL  52.2369   122.1678   Oct-93      GSC
 Holberg, B.C.      HOLB  50.6404   128.135    Jul-92      GSC
 Whistler, B.C.     WSLR  50.1265   122.9212   Sep-96      GSC
 Penticton, B.C.    DRAO  49.3226   119.625    Feb-91      GSC
 Nanoose, B.C.      NANO  49.2948   124.0865   May-95      GSC
 Uclulet, B.C.      UCLU  48.9256   125.5416   May-94      GSC
 Sidney, B.C.       PGC1  48.6486   123.4511   Dec-89      GSC
 Victoria, B.C.     ALBH  48.3898   123.4875   May-92      GSC
 Neah Bay, WA       NEAH  48.2979   124.6249   Jul-95      UW
 Seattle, WA        SEAT  47.6550   122.3083   Jun-95      UW
 Sedro Woolley, WA  SDWL  48.5216   122.2238   Oct-97      UW
 Kelso, WA          KELS  46.15     122.91     Oct-97      UW
 Pacific Beach, WA  PABC  47.2      124.22     Aug-97      CWU
 Goldendale, WA     GOBS  45.82     120.825    Aug-97      CWU
 Cape Blanco, OR    CABL  42.84     124.58     Aug-97      CWU
 Satsop, WA         SATS  46.9657   123.5412   May-97      CVO
 Johnston Ridge, WA JRO1  46.2751   122.2176   May-97      CVO
 Newport, OR        NEWP  44.64     124.07     Jun-96      OSU
 Corvallis, OR      CORV  44.575    123.23     May-96      OSU

GSC=Geological Survey of Canada; UW=University of Washington;
CWU= Central Washington University; CVO=Cascade Volcano Observatory (USGS);
OSU=Oregon State University

The five sites installed by UW and CWU in 1997 consist of 5-leg braced monuments drilled to a depth of 10 meters, Trimble 4000 SSI receivers, and choke ring antennas. Data from the new UW sites SEDR and KELS are being downloaded by Central Washington University where they are analyzed. For the time being the UW is also analyzing the new sites using GIPSY/OASIS software together with other sites we have been processing for several years. UW continues to download the Ashtech Z-12 data from Neah Bay (NEAH) and Seattle (SEAT). These data are transferred to a data center at Central Washington University. They are also available on line for 15 days from the the date of recording at http://www.geophys.washington.edu/GPS/.

The site at Sedro Woolley is located at a regional office of the Washington Department of Natural Resources. The Kelso site is located on land owned by Cowlitz County at the Kelso airport. It is 20 meters from a GPS site that has been operated as a GPS survey reference site by Cowlitz county for several years using an antenna monument consisting of an 8 foot high tripod staked to the ground. We expect that the more stable monument we have installed will eventually serve as a replacement for the Cowlitz County reference site.

Results - Continuously Recording GPS Stations

Preliminary results from UW analysis of several continuously recording GPS stations in the Pacific Northwest including US Coast Guard stations at Point Robinson and Whidbey Island in the Puget Sound, and Fort Stevens at the mouth of the Columbia river northwest of Portland, Oregon indicate deformation dominated by the interaction between the Juan de Fuca and North America plates. 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. The largest 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.

Figure 1. Station velocities with respect to the stable part of the North America plate (station DRAO in Canada). Error ellipses represent 95% confidence limits on velocity. Velocities based on daily analysis of more than two years of data using GIPSY/OASIS.

1997 GPS Campaign Survey

In the summer of 1997 we conducted campaign surveys of 52 survey marks in western Washington (Table 2), principally in the Puget Sound region. Each site was occupied for two 8-hour periods on successive days. The data for these sites has been processed together with available data from continuously recording stations. Comparison with data from previous surveys are currently being analyzed. At many of the sites we hope to be able to compare the computed positions with positions derived from USGS data obtained prior to 1995.

---------------------------------------------
Table 2. GPS Campaign sites surveyed in 1997.
---------------------------------------------
 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

Figure 2. Campaign GPS stations surveyed by UW in June-July 1997. Data from continuously recording stations (diamond symbols) are also used in the analysis.

Publications

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.

G. Khazaradze and A. Qamar, Strain Accumulation Rates Inferred from GPS Measurements in Western Washington, Seismological Research Letters, Vol. 68, No. 2, pg. 316, (abstract), 1997.

G. Khazaradze, A. Qamar, and H. Dragert, New Results from the Pacific Northwest Geodetic Array , (figure). Accepted for a presentation at the AGU Annual Meeting in San Francisco, December 8-13, 1997.

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