GPS Measurements of Crustal Deformation in the Pacific Northwest 1434-95-G-2582 - A.I. Qamar, P.I. Dept. of Earth and Space Sciences, University of Washington Seattle, Washington 98195 ph: (206) 685-7563; e-mail: tony@ess.washington.edu

Feb. 1, 1995 - Jan. 31, 1996 Program Element: II.3

Investigations

The purpose of this project is to install a permanent GPS base station on the northwest coast of Washington and develop the infrastructure for downloading and processing the data in cooperation with the Geological Survey of Canada (GSC). The long term goal is to expand the Canadian GPS monitoring of crustal deformation in the Pacific Northwest into the US. These data will be used to improve earthquake risk assessments in Washington and Oregon.

Results

On July 19th, 1995 we installed the first GPS station in Washington designed to continuously monitor crustal deformation associated with the oblique convergence between the Juan de Fuca and North America plates. This is a cooperative project with Canadian colleagues from the Pacific Geoscience Centre (PGC) of the GSC, who pioneered a continuously operating GPS network called the Western Canada Deformation Array (WCDA) in 1990 (Dragert and Hyndman, 1995). The WCDA currently comprises 6 GPS receivers. PGC has assisted us with the selection, installation, and initial operation of our site, 5 km south of Neah Bay, WA, called NEAH (Figure 1). The station is located on bedrock at the summit of Cheeka Peak (Latitude 48.298N, Longitude 124.625W) next to an existing University of Washington permanent meteorological research station where there is power, telephone, and an air conditioned instrument shelter. In the future we will be able to combine GPS and meteorological data to monitor precipitable water vapor in the atmosphere. Use of the site was approved by the Makah Tribal Council whose reservation encompasses the northwest tip of the Olympic Peninsula. To mount the antenna we constructed an 8-foot high concrete pillar, 2-feet in diameter, that was anchored to bedrock by grouted rebar extending 6 feet into the rock.

Figure 1) NEAH, the first continuously recording GPS station in Washington, and the Western Canada Deformation Array (WCDA) stations: ALBH, UCLU, NANO, and HOLB. US Coast Guard stations are shown in dark triangles.

Initially, we utilized a Rogue SNR-800 receiver with an external quartz oscillator borrowed from PGC. The Rogue was easily integrated into the standard downloading and data processing procedure at PGC. In the interim, we worked at UW to test the system consisting of an Ashtech Z-12 receiver, an uninterruptible power supply, Supra 288 modem, and a mechanism to remotely reset power to equipment via modem. We also purchased a PC and installed the OS/2 operating system together with the Data Acquisition Software Suite (DASS) obtained from Scripps Institution of Oceanography (Yehuda Bock).

On October 5, 1995 we replaced the SNR 800 with a Turbo Rogue SNR 8000 and an Ashtech Z-12 running in parallel at the site. Both are now recording from the same Ashtech choke ring antenna. In early December we plan to replace the Ashtech Z-12 with one having upgraded firmware that will allow us to reduce daily data download time from about 40 minutes to 12 minutes. The signal to noise ratio in the data has improved significantly since we replaced the older Rogue with the Turbo Rogue and Ashtech Z-12 receivers.

Currently we analyze data from NEAH together with other selected global GPS stations using the GIPSY processing package developed at the Jet Propulsion Laboratory (Webb and Zumberge, 1993). The scatter in our daily solutions is of the order of several millimeters (Figure 2) indicating satisfactory short-term precision for these preliminary analyses. We check for the accuracy of our analyses by comparing our results with the results obtained by PGC based on Canadian processing software CGPS22 (Khazaradze, Qamar, and Dragert, 1995). Based on our initial modeling results and the observed 7 mm/year shortening rate between the WCDA stations ALBH and DRAO (Dragert and Hyndman, 1993), we expect to detect even higher strain rates in the vicinity of NEAH.

Figure 2) Deviations of daily solutions for the relative E-W, N-S, vertical components, and baseline lengths at NEAH with respect to ALBH (left) and DRAO (right). Error bars represent 2 sigma uncertainties. GIPSY analysis package from Jet Propulsion Laboratory was used.

We have purchased another Ashtech Z-12 GPS receiver using University of Washington funds. In the spring of 1996 we plan to establish another permanent site on the coast of Washington using this instrument. Until that time the extra receiver is installed on the roof of the Geophysics building on University of Washington campus in Seattle where it records data continuously. UW will obtain 3 additional receivers via funding from UNAVCO NSF/ARI that will be used to expand GPS coverage of the Pacific Northwest.

On November 3, 1995 the UW hosted a meeting of institutions interested in geodetic monitoring in the Pacific Northwest using GPS. Participants in the 1995 meeting included Herb Dragert (PGC), Meghan Miller (CWU), Ken Hurst (JPL), and representatives from the USGS (Craig Weaver, Will Prescott, Dan Dzurisin, and Elliot Endo). As a result of this meeting, the participants formed a consortium to coordinate GPS monitoring of crustal deformation in the Northwest. The combined permanent GPS stations of the consortium is called the Pacific Northwest Geodetic Array (PANGA), and we agreed to cooperate in the siting and operating of new receivers as well as processing the data and interpreting the results. With 9 new receivers to be purchased under UNAVCO NSF/ARI grants to Central Washington University and the University of Washington and with the expected new permanent receivers to be added by PGC and the USGS Cascade Volcano Observatory, we hope to be monitoring strain in the Pacific Northwest with an international PANGA network of about 20 GPS receivers in the near future.

Dragert, H. and R. D. Hyndman, 1995, Continuous GPS monitoring of elastic strain in the northern Cascadia subduction zone, Geophys. Res. Letters, 22, 755-758.

Webb F. H. and J. F. Zumberge, 1993, An introduction to GIPSY/OASIS-II, Jet Propulsion Laboratory, Pasadena.

Publications

Khazaradze, G., A. Qamar, and H. Dragert, 1995, Permanent GPS network in Washington, abstr., EOS Transactions , 76, p. F150.