Shear-wave Splitting Observations and Measurements at the Geothermal Field at Hengill, Iceland

Abstract

During the summer of 2005 the seismicity at the Hengill geothermal field in Southwestern Iceland was recorded for forty-two days with an array of twenty-one PASSCAL L-28 4.5-Hz sensors. The array was divided into two parts, Eastern and Western, covering an area approximately 5 km in N-S by 10 km in E-W. During the deployment the array recorded approximately 4 microearthquakes on average per day at a sampling rate of 500 samples per second. Most seismicity has been observed to occur within the eastern part of the array. Epicenters of the earthquakes located in the east are highly clustered, while the focal depths are mostly shallower than 6 km, consistent with the estimate of the depth to the base of the brittle crust. Shear-wave splitting (SWS) was clearly observed in the seismic data from Hengill geothermal field. Measurements and consequent inversions of the shear-wave splitting parameters have provided evidence for a predominant crack system oriented approximately NNE-SSW, also consistent with the regional tectonics in Southwestern Iceland.