| Summary: | Nesjavellir geothermal field is located in the northern part of the Hengill central volcano in South West Iceland. Electricity and hot water for district heating is currently produced by OR-Reykjavík Energy at two power plants around Hengill, Nesjavellir to the north and in Hellisheiði to the southwest. The seismicity recorded in the area is due to volcano-tectonic processes, natural geothermal activity and geothermal production. The aim of this work is to deepen the understanding of the mechanisms of generation of seismicity through the study of the properties of the medium, it’s variations over time and the kinematic characteristics of earthquakes.Below Nesjavellir low Vp⁄Vs ratios and low b-values are observed at shallow depth (due to low Vp). High Vp⁄Vs ratios and high b-values are observed between 3.5 and 6 km depths (due to high Vp and low Vs), which is in correspondence with the deepest seismicity characterized by a higher percentage of small events. Our results suggest the coexistence of distinct mechanisms that control seismicity. From 4D analysis we observe the evolution of seismicity and a corresponding variation in the elastic medium properties as a function of the depth. This work has been supported by PRIN-2017 MATISSE project, No 20177EPPN2, funded by Italian Ministry of Education and Research.
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