Determining the Mechanism of Seismic Anisotropy at Volcanoes: Focus on Tungurahua Volcano, Ecuador

Abstract:

The measurement of seismic anisotropy using the method of shear wave splitting (SWS) has potential as a stress monitoring tool at volcanoes and is increasingly being used by researchers. Even though anisotropy, caused by preferentially aligned microcracks, can be a valid proxy for determining the stress regime in the subsurface, there are many other reasons that SWS may be observed. Anisotropy in the crust may be due to aligned macroscopic fractures, layering, or aligned minerals. Apparent SWS may also be observed due to site effects and phase conversions near the surface. Temporal changes in SWS may be an artefact of migrating sources passing through a heterogeneous anisotropic field.

At Tungurahua Volcano in Ecuador, we have analysed SWS from local VT earthquakes from 2008 to 2012, spanning the onset of major eruptive activity in 2010. We have found significant site effects at several of the seismic stations, and corrections indicate how influential these effects can be. We explore both lateral and vertical variation in anisotropy, before targeting temporal variations associated with volcanic activity. Comparison with local geology and ground deformation allows us to identify regions where seismic anisotropy is controlled by local stress and is likely to change due to volcanic activity. Preliminary results suggest that apparent temporal changes in SWS measurements are due to sampling regions controlled by different mechanisms of anisotropy, demonstrating how important it is to identify the cause of anisotropy before seeking temporal variations caused by changing stress.