Seismic tomography of Nabro caldera, Eritrea: insights into the magmatic
and hydrothermal systems of a recently erupted volcano
Abstract
Understanding the crustal structure and the storage and movement of
fluids beneath a volcano is necessary for characterising volcanic
hazard, geothermal prospects and potential mineral resources. This study
uses local earthquake traveltime tomography to image the seismic
velocity structure beneath Nabro, an off-rift volcano located within the
central part of the Danakil microplate near the Ethiopia-Eritrea border.
Nabro underwent its first historically-documented eruption in June 2011,
thereby providing an opportunity to analyse its post-eruptive state by
mapping subsurface fluid distributions. We use a catalogue of
earthquakes detected using machine learning methods to simultaneously
relocate the seismicity and invert for the three-dimensional P- and
S-wave velocity structures (Vp, Vs) and the ratio between them (Vp/Vs).
Overall, our model shows higher than average P- and S-wave velocities,
suggesting the presence of older consolidated volcanic deposits or
intrusive magmatic rocks in the crust. We identify an aseismic region of
low Vp, low Vs and high Vp/Vs ratio at depths of 6–10 km b.s.l.,
interpreted as the primary melt storage region that fed the 2011
eruption. Above this is a zone of high Vs, low Vp and low Vp/Vs ratio,
representing an intrusive complex of fractured rocks partially-saturated
with over-pressurised gases. Our observations identify the persistence
of magma in the subsurface following the eruption, and track the
degassing of this melt through the crust to the surface. The presence of
volatiles and high temperatures within the shallow crust indicate that
Nabro is a viable candidate for geothermal exploration.