Charles Meertens

A volcano bursting at the seams: Inflation, faulting, and eruption at Sierra Negra volcano, Galápagos

The results of geodetic monitoring since 2002 at Sierra Negra volcano in the Galapagos Islands show that the filling and pressurization of an ∼2-km-deep sill eventually led to an eruption that began on 22 October 2005. Continuous global positioning system (CGPS) monitoring measured >2 m of accelerating inflation leading up to the eruption and contributed to nearly 5 m of total uplift since 1992, the largest precursory inflation ever recorded at a basaltic caldera. This extraordinary uplift was accommodated in part by repeated trapdoor faulting, and coseismic CGPS data provide strong constraints for improved deformation models. These results highlight the feedbacks between inflation, faulting, and eruption at a basaltic volcano, and demonstrate that faulting above an intruding magma body can relieve accumulated strain and effectively postpone eruption.

Monitoring selective availability dither frequencies and their effect on GPS data

SummaryThe signals transmitted by Block II satellites of the Global Positioning System (GPS) can be degraded to limit the highest accuracy of the system (10 m or better point positioning) to authorized users. This mode of degraded operation is called “Selective Availability” (S/A). S/A involves the degradation in the quality of broadcast orbits and satellite clock dithering. We monitored the dithered satellite oscillator and investigated the effect of this clock dithering on high accuracy relative positioning. The effect was studied over short 3-meter and zero-baselines with two GPS receivers. The equivalent S/A effects for baselines ranging from 0 to >10,000 km can be examined with short test baselines if the receiver clocks are deliberately mis-synchronized by a known and varying amount. Our results show that the maximum effect of satellite clock dithering on GPS double difference phase residuals grows as a function of the clock synchronization error according to: S/Aeffect=0.04 cm/msec, and it increases as a function of baseline length like: S/Aeffect =0.014 cm/100 km. These are equations for maximum observed values of post-fit residuals due to S/A. The effect on GPS baselines is likely to be smaller than the 0.14 mm for a baseline separation of 100 km. We therefore conclude, for our limited data set, and for the level of S/A during our tests, that S/A clock dithering has negligible effect on all terrestrial GPS baselines if double difference processing techniques are employed and if the GPS receivers remain synchronized to better than 10 msec. S/A may constitute a problem, however, if accurate point processing is required, or if GPS receivers are not synchronized. We suggest and test two different methods to monitor satellite frequency offsets due to S/A. S/A modulates GPS carrier frequencies in the range of-2 Hz to +2 Hz over time periods of several minutes. The methods used in this paper to measure the satellite clock dither could be applied by the civilian GPS community to continuously monitor S/A clock dithering. The monitored frequencies may aid high accuracy point positioning applications in a postprocessing mode (Malys and Ortiz 1989), and differential GPS with poorly synchronized receivers (Feigl et al. 1991).

UNAVCO Conference explores advances in volcanic geodesy

Volcanic eruptions are among Earths most spectacular surface phenomena. However, attempts to understand their basic physics face the challenge that the key processes occur at great depth and are difficult to observe. Thus volcanologists have been interested for years in using ground deformation measurements to study active volcanoes and predict their behavior during extended volcanic crises, such as the dramatic six weeks in 1980 between the initial and major eruptions of Mt. Saint Helens. The advent of new technologies in recent years—in particular the Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (INSAR)—has shown potential for significant advances in volcano studies. Progress in this direction was explored at a conference organized by the University Navstar Consortium (UNAVCO) on September 15–17, 1999, with financial support from the National Science Foundation (NSF), NASA, and the U.S. Geological Survey (USGS). The meeting aimed to assess the status of various geodetic technologies and their potential to address crucial scientific and social needs in volcanic science and monitoring, as well as to develop recommendations on ways to spur further progress in these areas.

EarthScoping the inner workings of magmatic systems

In the shadow of one of the worlds great volcanic systems, an intensive 3-day workshop was undertaken to work toward developing a scientific plan for the magmatic systems component of the U.S. National Science Foundations (NSF) EarthScope Initiative. This NSF-sponsored workshop was designed to provide direction to the EarthScope planning committee and the NSF in developing scientific, technical, deployment, and management decisions related to the magmatic systems component of EarthScope. The meeting featured a mixture of oral and poster scientific sessions, breakout group and plenary discussions, and a field trip to examine one of the targets of the EarthScope magmatic science research plan: Mount St. Helens. The 60 participants represented a broad cross-section of the volcanology community including geologists, geophysicists, geodesists, penologists, and geochemists. Details on the meeting plan can be viewed at http://www. unavco.net/earthscope.asp.