| Abstract |
The seismic wavefield contains rich information about geothermal reservoir dynamics that is only beginning to be understood. The ambient wavefield, coherent noise, and seismicity all reflect the structure and processes occurring within a volume of crust. In Iceland, the geothermal resources are natural shallow hydrothermal systems, which are tapped for hot liquid and steam. Below ∼ 2.5-3 km depth, the fluid becomes supercritical and the permeability structure may also change substantially. We present results from a new study of seismic data from the COSEISMIQ broadband array in the northern part of the Hengill Geothermal Area in Southwestern Iceland, with a focus on the Nesjavellir field and the adjacent but hydrologically disconnected, Mosfellsheiði area. Our workflow consists of (1) producing high-resolution micro-earthquake catalogs, (2) analyzing the temporal clustering and magnitude statistics, and (3) characterizing the coherent seismic energy sources in the ambient wavefield. We identify interesting spatial variations in the earthquake statistics both within and between the two geothermal areas and propose a link to stress state and fracture network morphology. We are able to detect coherent seismic energy in the frequency band 1-20 Hz that appears intermittently and is most frequently located within the Nesjavellir area at ∼ 1-2 km depth, although it moves in depth and sometimes appears further south or in the Mosfellsheiði area. |