Detailed study of the recent seismicity of the Nesjavellir (Iceland) geothermal production area.

The Nesjavellir geothermal field is located in South West Iceland, in the northern section of the Hengill central volcano. OR-Reykjavik Energy now produces electricity and hot water for district heating at two power plants in the Hengill area. Seismicity in the area has been interpreted as a result of volcano-tectonic processes, natural geothermal activity, and geothermal operations. The goal of the study is to achieve a better knowledge of the mechanisms that cause seismicity by studying the properties of the medium and the kinematic characteristics of earthquakes. At shallow depths below Nesjavellir, low $V_p/V_s$ ratios and low $b$-values are estimated. Between 3.5 and 6 km depths, high $V_p/V_s$ ratios and high $b$-values are detected in correspondence of the deepest seismicity, which is characterized by a higher number of small events. Our results point to the coexistence of various processes that regulate seismicity. We are able to observe the space-time evolution of seismicity and the related variation in elastic media properties, through 4D analysis. This work has been supported by PRIN-2017 MATISSE project, No. 20177EPPN2, funded by Italian Ministry of Education and Research.