John Mischler Abstract

GPS Geodetic Measurements of Surface Deformation on the Island of Dominica, Lesser Antilles

John Mischler, Dept. of Geology, Augustana College; 639 38th Street Box 1346; Rock Island, IL. 61201; john-mischler@augustana.edu
Dr. Glenn Mattioli, Dept. of Geology, University of Arkansas; 113 Ozark Hall; Fayettville AK; 72701; mattioli@uark.edu

Dominica is an island with between 4 to 8 potentially active volcanic centers. The island is located in the Lesser Antilles between the islands of Martinique and Guadeloupe. GPS observations on nine points were made in June of 2003 that were first measured in a campaign in June of 2001. Absolute point positions were obtained in 2001 using GPS. New data were collected on each of the nine previous points and three new points were installed all in June of 2003. Previous to the measurements taken in 2001, a seismic swarm occurred in the south of the island between September of 1998 and May of 2000. Another swarm occurred in the north following the 2001 set of measurements in April of 2003. These swarms may indicate the motion of magma in the subsurface of the island. This makes Dominica a prime candidate for the study of such processes and for its use as a natural lab to test existing models of volcanology and the initiation of volcanic activity on subduction zone island arc volcanoes.

Equipment used included Ashtech GPS receivers, Dorn-Margolin choke ring Ashtech GPS Antennae, and three different mounting apparatus depending on the site, (1.5m fixed height tripods, standard geodetic tripods with an optical plummet, and 0.5m spike mounts). Antennae were set up over existing benchmarks and where necessary, the slant height to the base of the choke ring antennae was measured. Power was obtained from 12 V batteries in concert with solar cells to recharge the batteries. At a few sites AC powered the units. Each package of GPS equipment and batteries was placed at a site to collect three days of uninterrupted data in order to obtain a precise position for that site. The raw observation data from the Ashtech receivers was then downloaded onto laptops in the field.

Once back at the lab in Arkansas, the data were processed using final precise orbits and clocks from JPL using the GIPSY-OASISII software environment. Data were processed to yield absolute point positions and transformed into ITRF 2000. Velocity vectors were resolved for each of the nine original points measured. The Caribbean plate velocity at each point was removed so as to get accurate vectors for the movement of individual points independent of plate motion.
Once the data were processed, many sites showed local deformation at the several millimeters per year level. In the southern part of the island, the velocity vectors were consistently larger than in the north. The vectors at WOTT and GOMM point toward each other across an area of recent seismicity. The same effect can be seen at CABR and CNCD.

These local deformation measurements, when coupled with the recent seismicity seen on the island, suggests some interesting mechanisms behind the deformation. One working hypothesis is that there is a shallow magma source causing the earthquakes in the south. The largest concentrations of epicenters are located between the city of Roseau and Morne Plat Pays.

When the data from the GPS measurements in the area is analyzed, the velocity vectors of two locations (WOTT and GOMM) on each side of the earthquake epicenter concentration point in toward the center of this swarm cluster. This could suggest a possible caldera collapse in the area. Similar deformation is seen in the north with the velocity vectors of two other locations (CABR and CNCD) pointing in towards the center of the seismic activity in that area. Much more modeling and research must be done before any certain conclusions can be reached.