Shear Wave Splitting Tomography at Kilauea

Seismic anisotropy is the variation of seismic wave speed with direction, which is most frequently observed using shear-wave splitting (SWS). Seismic anisotropy in the crust arises when microcracks in subsurface rocks are aligned, for example when the rock is under differential stress. When this occurs, the rock displays a directional variation in seismic velocity, which can be used as a proxy for maximum compressive stress and is also affected by the type of fluid filling the microcracks. Therefore, SWS analysis can be used to detect changes in stress and pore-fluid movement during volcanic activity.

Kilauea Volcano is currently producing lava eruptions in its Lower East Rift Zone and collapse events at its summit. The rate at which the summit lava lake and shallow magma reservoir drained has caused incremental caldera collapse with very high rates of seismicity and measured deformation. This suggests that during this new eruptive phase, large amounts of magmatic fluids have been mobilised, and the rate of change of crustal stresses are high. Even though similar activity has been seen before, it has never been monitored with modern equipment and so this activity provides a completely new data set and presents a unique opportunity to learn about the subsurface processes at Kilauea.

We have deployed an additional four seismic stations to complement the existing USGS network. These seismometers add coverage in the East Rift Zone but are further away from the axis of activity to achieve a wider coverage of the region. We will use data from the USGS network as well as our additional stations to carry out four-dimensional SWS tomography at Kilauea using the ongoing elevated rates of seismicity. SWS tomography overcomes the problems of heterogeneous anisotropy with migrating seismicity. This has been used to map areas of high stress and subsurface fluid throughout Kilauea, including regions in which the anisotropy has not previously been investigated due to lack of data. The current elevated rates of seismicity and additional seismic stations provide enough data to observe temporal variations in anisotropy associated with the volcanic activity, which, when combined with other observations such as ground deformation, can be interpreted in terms of migrating fluids and changing stresses.