The response to precipitation of a hydrothermal system: seasonal seismicity and vertical deformation at Aluto-Langano, Ethiopia.

Vertical GPS displacement has been used to measure and quantify the elastic response of the solid earth due to the seasonal variation of precipitation in many settings, including Ethiopia, where a 1-2 month phase lag after the onset of precipitation is observed. The Aluto-Langano hydrothermal system is situated between the Lakes Ziway and Langano in the Main Ethiopian Rift and the hydrothermal system and lake level are all expected to respond to seasonal patterns of precipitation. Precipitation in this area is bimodal with one short rainy season from February to May and a long and heavy rainy season from June to September. We use a network of continuous GPS stations and seismometers to look for seasonal patterns in vertical displacement and shallow seismicity. Long-term trends in displacement associated with regional tectonic and magmatic processes are removed from the data. The number of earthquakes within the Aluto-Langano hydrothermal system (depth < 2km) peaks 1-3 months after the heavy rainy season; the effect is particularly clear after an episode of heavy rainfall. The detrended GPS time series show seasonal variations in vertical displacement of about + 10 mm, which we attribute to the hydrologic loading and changes in the hydrothermal system. The maximum precipitation and minimum vertical displacement shows a phase lag of 1 -3 months in most of the GPS sites, which we consider to be the time taken for recharge of the vadose zone, and is comparable to sites elsewhere in Ethiopia. The exception is the site at the center of the caldera which shows an instantaneous response. These observations are important for understanding the interplay between hydrological and magmatic processes within hydrothermal systems. Keywords: vertical GPS displacement, shallow seismicity, precipitation, phase lag