The Seismic Monitoring of a Geothermal System on Aluto, a Deforming Stratovolcano in the Main Ethiopian Rift

Posted by pkm at Mar 17, 2015 10:23 AM |
Matthew Wilks (1), J-Michael Kendall (1), Andy Nowacki (2), James Wookey (1), Tulu Bedada (3) & Atalay Ayele (3) - (1) School of Earth Sciences, University of Bristol; (2) School of Earth & Environment, University of Leeds; (3) Institute of Geophysics, Space Sciences & Astronomy, Addis Ababa University.

The volcanic centres of the Main Ethiopian Rift (MER) are known to play a central role in facilitating continental rifting during the ongoing extension of the crust. Recent InSAR studies have shown that a number of these axial volcanoes are deforming, but the region has remained relatively understudied south of 8°N despite this activity. Furthermore, many of the volcanoes have shown potential to generate geothermal power, where subsurface magma bodies are thought to heat the hydrological system and provide an abundant, renewable energy resource.

One such volcano is Aluto, where the first geothermal power plant in Ethiopia was built in the 1990s. Aluto is located in the Central MER towards the eastern rift escarpment and between Lakes Ziway to the north and Langano to the south. Despite the known capability of geothermal and hydrothermal systems to induce seismicity, Aluto has not received any previous seismic monitoring in spite of the large, proximal population to it. As a consequence a local network of 12 three-component seismometers was deployed at Aluto in January 2012 and recorded the seismic activity for a two-year period.

Over the duration of the project, 1361 events were detected within a 15 km radius from the centre of the caldera. Events are located using a 1D velocity model where the upper 2300 m of the model is derived from borehole data. A crustal tomographic velocity model of the MER is used at greater depths. Event depths range from the near surface to a maximum of 15 km. Frequency indexing highlight that the vast majority of events are volcano-tectonic (VT) in origin with the bulk of seismic energy containing frequencies greater than 6Hz. Most of the seismicity is clustered around the edge of the caldera, suggesting ring fault patterns, but a significant amount of seismicity also follows the NNE-SSW trending Wonji Fault Belt east of Aluto. Gaps in seismicity are observed at depths between ~2 km and ~6 km that we suggest could represent a region of enhanced ductility and correspond to the hydrothermal reservoir. Deeper events are most likely associated with magmatic processes. Constraining the ratio of P-wave to S-wave velocities through Wadati analysis show that a region of high Vp/Vs is evident at shallower depths, which appear to map in accordance with the observed locations of surface CO2 flux.

A local scale developed for the MER has been used to assess event magnitudes. These range from ML=-0.40 to ML=2.98, with a b-value of 1.40, a value consistent with studies of other volcanoes around the world. Moment magnitudes, attained through spectral analysis provide a similarly high b-value of 1.55. Additionally, we observe seasonality in the shallow seismicity and suggest the occurrence of a time lag of around 128 days between the periods of peak rainfall and the induced seismicity that follows.

Focal mechanisms for 53 events are produced using P- and S-wave polarities and amplitude ratios. The majority of our obtained source solutions represent normal faulting, which agrees with the extensional setting of the rift. Many solutions however do not reflect the assumed subsurface structure and we also derive thrust and strike-slip events that further highlight the complex environment around this dynamic volcano.

At present, no outstanding patterns in seismicity emerge between periods of power generation and ambience but it is hoped that more data with respect to parameters such as the fluid volumes injected and extracted will examine any potential relationships in greater detail.

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