46433-G8
Shear-Wave Splitting Analysis of Stress Field Perturbations Around an Active Magmatic Conduit: Soufriere Hills Volcano, Montserrat
Results from a study of shear-wave splitting in regional and local earthquakes recorded by a broadband seismic network on the island of Montserrat have confirmed an hypothesized link between changes in fluid pressure within an active magmatic conduit system and changes in the orientation of local stresses within the rock surrounding the conduit. Montserrat is home to the Soufriere Hills Volcano, which has been erupting since 1995 in a sequence of ~1-4 year-long phases separated by periods of quiescence, making it an ideal place to study temporal changes in stress between periods of eruption and quiescence. A network of broadband seismometers has operated on Montserrat since October 1996, making it possible to analyze shear-wave splitting (from which the orientation of maximum horizontal compression can be determined) prior to and during phases of eruption from 1996-1998, 1999-2003, and 2005-present. To date, 219 high-quality measurements of stress field orientation beneath Montserrat have been obtained for the period 1996-2006. They demonstrate a systematic ~90° reorientation of local maximum compression prior to a change in eruption style in 1997, prior to the onset of eruption phases in 1999 and 2005, and prior to a major dome collapse event in 2001. These results are in strong agreement with an earlier study of changes in stress based on volcanic earthquake fault-plane solutions, which showed ~90° changes in stress prior to all phases of the eruption as well as prior to a change in eruption style in 1997.
The results obtained to date provide the first tie between two parallel (and up to now, entirely independent) lines of research concerning changes in stress around volcanic conduits. Previously, pre-eruptive changes in stress had been documented either by analysis of fault-plane solutions, or by analysis of shear-wave splitting. Both approaches suggested similar pattens of precursory stress field reorientation, which are modeled as the result of inflation of a volcanic conduit system. A critical test of both the observations and the model was to apply both techniques to a single eruption. The Soufriere Hills Volcano is now the first system to have been studied using both approaches to stress field analysis, and the results of the comparative study both validate the use of stress field analysis as a monitoring technique and highlight the relative strengths and weaknesses of each approach as a tool for volcano monitoring. From the Soufriere Hills study, we have learned that shear-wave splitting is a key tool for long-term volcano monitoring, and also for determining a 'stress baseline' at quiescent volcanoes. We have also learned that shear-wave splitting analysis may be more sensitive to minor changes in stress than fault-plane solution analysis. However, shear-wave splitting analysis, because it depends on the occurrence of moderate to large regional earthquakes in the crust beneath the area of study, is not well-suited to intermediate- or short-term monitoring, when a high sampling frequency of stress field orientation is required. Thus, the study clearly demonstrates that a combination of shear-wave splitting and fault-plane solution analysis is the best approach to detecting changes in stress that may herald changes in volcanic activity.
This study has had a major impact on my career in that the results have clearly justified a greater attempt to understand stress changes beneath active volcanoes, as well as the state of stress beneath quiescent volcanoes (a critical piece of information for using volcanic stress field analysis as a forecasting tool.) As a result of the work funded by this starter grant, I have begun a major NSF-funded study of changes in stress beneath Aleutian volcanoes, with the aim of understanding 'failed' volcanic eruptions, or stalled mid-crustal intrusions of magma. This study involves two graduate students who have been given responsibility for major components of the study. A second phase of the PRF-funded study that is now in progress is a study of background stress field orientation at Crater Lake, Oregon. The aim of this study is to critically examine the assumption that regional stress field orientation can be taken as a proxy for local background stress field orientation at a volcano, and hopefully to demonstrate the need for focused baseline stress studies at potentially active volcanoes. Finally, the work funded by my PRF Type G grant has lead to a new collaborative relationship with Martha Savage of Victoria University, a leading researcher on volcanic shear-wave splitting analysis. Prof. Savage will be visiting my lab in December 2008 to work with me on analyzing additional data from Soufriere Hills and interpreting the results, and to work on a proposal submitted to the Royal Society of New Zealand for continuing studies of precursory changes in stress at restless volcanoes.
