Seismic Imaging of the Nesjavellir geothermal field, SW-Iceland

The harnessed Nesjavellir geothermal area is one of several geothermal fields on the flanks of the Hengill volcano, SW-Iceland. In this study, we present a detailed seismic imaging of the area through the mapping of the V_P, V_S and V_P/V_S ratio using seismic data recorded from 2016 to 2020 and compare them to a resistivity model from the same area and rock temperature measured in boreholes. To obtain reliable initial hypocenter locations and a reference seismic velocity, we solve the coupled hypocenter-velocity problem and obtain a reliable minimum 1D P-wave velocity model for the study area. First, we performed the relocation of all the events in the catalogue using the new 1D velocity model and the estimated  V_P/V_S value of 1.77. We chose the highest quality events based on the quality of the relocations and used them to perform the 3D tomographic inversion. We used an iterative linearized delay-time inversion to estimate both the 3D P- and S-wave velocity models and earthquake locations.

The results highlight that at depths less than 1 km the crust has a high V_P/V_S ratio (around 1.9) and low V_P and V_S values. Low resistivity at comparable depths in the same region has been explained as being due to the smectite clay cap. The observed low V_P/V_S ratio of 1.64 to 1.70 for depths between 1 and 3 km coincides with high resistivity values. The seismicity in this region, where temperatures often exceed 240°C, seems to be sparse and concentrated near the production wells. This seismicity has been explained as being caused by both production and tectonic activity.  At depths larger than 3 km significant high V_P/V_S ratio anomaly (>1.9) is observed and coincides spatially with a deep-seated conductive body that domes up at about 4.500 m b.sl. Elevated temperatures are observed above this structure in borehole temperature data. We propose that these signals reflect hot 600-900°C cooling intrusives, close to the brittle ductile transition - possibly the heat source(s) of the geothermal field above. These anomalies are at the same location as the last fissure eruption in Hengill almost 2,000 years ago. A deeper NNE-SSW trending seismic cluster at 3-6 km depth, likely outlining an active fault, is observed at the edge of this high V_P/V_S anomaly. The heat source of the Nesjavellir geothermal field is most likely connected to this most recent volcanism as reflected by the deep-seated low resistivity body and high V_P/V_S ratio, located beneath the deep fault that connects the flow path of the high temperature geothermal fluid, resulting in an actively producing reservoir.

The availability of a 3D model represents a starting point for 4D tomography study which will allow us to track changes in crustal properties over time and the estimation of fault mechanisms and kinematic source parameters.

This work has been partially supported by PRIN-2017 MATISSE project, No 20177EPPN2, funded by the Italian Ministry of Education and Research.