Analysis of 2014–2015 on- and off-shore passive seismic data on the Reykjanes Peninsula, SW Iceland

Abstract

Passive seismic techniques are a valuable tool to map active faults, follow changes in the local stress field caused by drilling and exploitation processes in geothermal areas and estimate seismic velocity ratios. They have, however, not been used extensively in basaltic crust as we find it in Iceland due to the data processing challenges that high impedance contrasts between layers of igneous rocks pose. To improve conventional passive seismic techniques and to test new ones, a dense network of seismometers recorded seismic activity on- and off-shore the Reykjanes peninsula in southwest Iceland from spring 2014 until August 2015. In addition to the existing long-term and permanent networks, a temporary network consisting of 30 on-land stations and 24 ocean bottom seismometers (OBS) was deployed. The network was laid out as concentric circles around the Reykjanes geothermal area at the tip of the peninsula and was about 100 km in diameter. The data of the on-land stations were collected every 70 days and data recorded by the OBSs were read out after one year when they were recovered. We used the SeisComp3 software to automatically detect earthquakes and then manually revised the P and S arrival times. A total of 2066 earthquakes could be located along the mid-ocean ridge and within and around the Reykjanes geothermal system. Earthquake hypocentres associated with the geothermal area occur in the uppermost 2 km while events along the spreading axis occur at 4–6 km depth on-land and appear to deepen with increased distance from the shore. We see a cluster of about 200 events, induced by the drilling and onset of operation of a local injection borehole in the Reykjanes geothermal area. We find a vp/vs ratio from all picks of 1.78 consistent with earlier studies in the area. Focal mechanisms calculated for selected events reveal a regime dominated by strike-slip and normal faulting.