The Mount Erebus GPS Array: In Search of Volcanic Deformation

P.R. Kyle¹, C. Meertens², B. Johns², E. Desmarais¹, C. Kurnik², R. Aster¹

¹Dept. of Earth & Environmental Science, N.M. Institute of Mining and Technology, Socorro, NM 87801 - USA

²University NAVSTAR Consortium, Boulder, CO 80301 - USA

Mt. Erebus on Ross Island is the most active volcano in Antarctica. It has a persistent convecting lava lake of anorthoclase phonolite magma, which typically has 2 to 6 small Strombolian eruptions a day. Occasionally small ash eruptions have been noted from vents adjacent to the lava lake. Deformation studies are an important tool in understanding active volcanoes. Such studies can show the influx of new magma into a volcanic conduit and herald an increase in eruptive activity. Even in a continuously erupting volcano such as Mt. Erebus eruption of bombs and other ejecta can result in withdrawal of magma from the underlying magma system and lead to deflation of the volcano. Injection of magma from the magma chamber along a dike may also result in changes of surface elevation.

Scientists from New Zealand undertook a deformation study of the summit crater of Mt. Erebus during the early 1980's (Otway et al., 1994). They conducted a program of precise triangulation, trilateration and tilt-level surveys over 6 years between 1980 and 1985. Most of the geodetic observations were made using theodolites and electronic distance measuring instruments and were laborious and depended on clear atmospheric conditions to allow sightings across the crater. These studies coincided with a significant increase in activity of the volcano, which commenced on 13 September 1994 and lasted over 3 months. They measured deformation of 50±20 mm across the 500 m wide crater.

Over the intervening 14 years from 1985 until 1999 no geodetic studies were undertaken at Mt. Erebus. We therefore started a new deformation study using the global positioning satellite (GPS) array in episodic and continuously modes. In the 1999/2000 field season we established a network of geodetic stations to monitor deformation at three different scales. Four monuments (ABBZ, BOMZ, HOOZ, and SISZ) surround the volcano at the 1800-meter elevation level, and are collocated with permanent short-period continuous monitoring seismic stations. Five more monuments (CONZ, EAST, ELHT, HELZ, and NAUS) circle the summit crater on outcrops in the summit caldera. An array of 4 closely spaced monuments (ERE0, ERE1, ERE2, ERE3) installed by the Ohio State University/US Geological Survey Trans-Antarctic Deformation Network (TAMDEF) is used as a tiltmeter.

During the 1999/2000 field season two continuously operating single frequency L1 GPS stations were established at HOOZ and CONZ. These were reconfigured in the 2000/2001 field season when a dual frequency station was established at CONZ and L1's were placed at HOOZ, E1 and NAUS. All stations are telemetered by 900MHz spread spectrum radio modems to McMurdo Station. Additionally an electronic tiltmeter with telemetry was established at E1 in association with a broadband seismic station and other environmental sensors.

The episodic monuments were reoccupied in 2000/2001 for a minimum of three full (UTC) days per visit. ELHT, at the Erebus lower hut, was occupied continuously and served as a local reference station. The IGS reference station MCM4 at McMurdo Station provided continuous reference data to isolate Mt. Erebus volcanic deformation from other deformation processes affecting Ross Island as a whole. TAMDEF monuments ROY0 at Cape Royds, and ARR6 at the Hut Point Astro pier provide commonality between the networks, and will relate any observed Mt. Erebus deformation to larger scale deformation results measured by the TAMDEF network. The ARR6 monument also serves as a reference mark to monitor MCM4 stability. These episodic measurements are currently being processed.

Over 60 microradians of tilt was observed at E1 in January 2001, which incidentally coincided closely with a small lava flow on the floor of the Inner Crater. The tilt may be real or related to settling in of the tiltmeter.

The continuous stations recorded for about 3 months in 2000 and again in 2001 and ceased operating because of instrument and power failures. In 2000 no significant deformation was seen between the L1's at HOOZ and CONZ but they did show that single frequency L1's could provide useful data for monitoring volcanoes. In 2001 there appears to be measurable deformation between NAUS and CONZ (2.1km), E1 and NAUS (0.9 km) and several other stations. Along several lines there are gradual changes over 70 days with ~0.5-1.0 cm of deformation in horizontal components.

REFERENCES

Otway P.M., Blick G.H. & Scott B.J., 1994. Volcanic deformation monitoring on Mount Erebus: Methods and results of geodetic surveys, 1980-1985. Amer. Geop. Un., Ant. Res. Series v.66 (ed. Kyle, P.R), 57-68.