Insights on the structure of the Soufrière Hills volcano and the associated geothermal field using joint inversion of passive P- and S-wave travel time data

The Soufrière Hills Volcano on the island of Montserrat in the Lesser Antilles was activated in 1995 after centuries of dormancy. Since then and until 2010, five major phases of activity led to explosive eruptions, dome building and collapse events, pyroclastic flows and ash clouds, which buried the capital, Plymouth, caused loss of life and extensive destruction of infrastructure, rendering the southern part of the island uninhabitable. Montserrat is currently rebuilding, with efforts focusing among others in the importance of better understanding the volcanic system and its associated geothermal field for both risk management and potential resource development.

In this work we jointly invert 7,112 P-wave and 1,376 S-wave arrival times recorded on 18 seismic stations from 1039 local events, classified by the Montserrat Volcano Observatory as tectonic or volcano-tectonic earthquakes, to determine the three-dimensional compressional and shear wave velocities and to simultaneously improve the event locations. Finite frequency sensitivity kernels are used instead of geometrical rays-paths to account for finite frequency effects, providing a more realistic representation of data’s sensitivity. To evaluate the quality and the uncertainty of the tomographic images, we utilize high performance direct sparse algorithms and graph-theory based techniques that allow the efficient calculation of the model’s resolution and posterior covariance matrices, allowing to assess the robustness of the features revealed in the tomographic models.

Our results show significant velocity anomalies associated with the volcanic centres and the associated geothermal field. Fast seismic P- and S- wave velocities are imaged beneath the active volcano of Soufrière Hills and the older dormant Central Hills volcanic centre. The fast velocity anomaly becomes more prominent and wider beneath Soufrière Hills at depths approximately between 1 and 3 km below sea level. These regions correspond to present and older volcanic cores and are possibly comprised from andesitic crystalized rocks of dome cores and intrusive magmatic bodies such as dikes and sills. The surrounding regions appear slower, possibly associated with deposits of volcanic ash, lava fragments, pyroclastic flows and lahars. A significant P- and S-wave low velocity anomaly is observed to the western side of the Island to the area of a high-temperature geothermal field that was inferred from previous geophysical studies and confirmed by three geothermal wells. In our models this feature is dipping towards E/SE, and extends down to ~3.5 km. These results are in good agreement with previous studies in the region. The Vp/Vs ratio map derived from the two velocity models reveals low Vp/Vs ratio at the volcanic centre, and positive anomalies at the flanks of Soufrière Hills. The low-velocity region that is associated with the geothermal field is characterized by low Vp/Vs ratio values too.