Observations and Analyses of Shear-Wave Splitting in the Geothermal Field at Hengill, Iceland

Abstract

Several volcanic centers, active and extinct, are located within Iceland. One of them is the Hengill volcanic center, which lies on the plate boundary between the North American and the European crustal plates in southwestern Iceland. The rifting of the two plates has opened a north‐northeast‐trending system of normal faults with frequent magma intrusions. The Hengill central volcano and its transecting fissure swarm, extending 70–80 km long from the coast south of Hengill to north of Lake Thingvallavatn with an associated graben structure, form the Hengill volcanic system. The Hengill central volcano is currently active and is the main volcanic production focus of the area associated with a high‐temperature geothermal field. In Nesjavellir, in the northern part of the Hengill area, a 400 MW geothermal power plant has been in operation since 1987. Another active but less pronounced volcanic system, the Hromundartindur volcanic system, lies at the eastern edge of the Hengill system, outside the Hengill fissure swarm. The area near Mount Hromundartindur can be classified as the central volcano of this system; it is a separate focus of volcanic production with high geothermal activity. Fracture detection by shear‐wave splitting (SWS) is an exploration method of proven reliability and unique imaging power. The method is based on the fact that a shear wave propagating through rocks with stress‐aligned microcracks will split into two waves: a fast one polarized parallel to the predominant crack direction, and a slow one polarized perpendicular to it (Crampin, 1981, 1984; Babuska and Cara, 1991). Recent major studies of SWS include Peng and Ben‐Zion (2004) and Johnson et al. (2011), among many others. Two important parameters are associated with an SWS event: the polarization direction of the fast shear …