HENGILL TRIPLE JUNCTION, SW ICELAND 1. TECTONIC STRUCTURE AND THE SPATIAL AND TEMPORAL DISTRIBUTION OF LOCAL EARTHQUAKES

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Abstract
The Hengill area is an unstable ridge‐ridge‐transform triple junction in SW Iceland. It contains the active central volcano Hengill, associated with the present accretionary zone, and the extinct Grensdalur central volcano, associated with an accretionary zone that became inactive at about −0.7 m.y. when the locus of spreading migrated about 5 km W to the Hengill zone. The dominant tectonic trend of the area is N25°E, parallel to the accretionary zones, but transverse tectonic/topographic features have also developed in the neighborhood of the two central volcanoes. These result from local modification of the regional stress field associated with the spreading by radially symmetric stress fields associated with the volcanoes, and the migration of volcanism. A double‐high temperature geothermal system mirrors the double volcanic system. The transform branch of the triple junction is represented by an EW striking zone of historic destructive earthquakes in the S of the area. A seismological study was conducted with the aims of studying the tectonic structure and evaluating the passive seismic method as a geothermal prospecting tool. The seismicity may be broadly divided into two groups. First, infrequent intense episodes of crustal movement occur that are associated with the release of tectonic stress along the plate boundary. Second, continuous small‐magnitude activity occurs on a day to day basis. This activity is mostly associated with the extinct Grensdalur central volcano, not the present plate boundary. Statistically significant temporal variations in b accompanied a swarm in the transform zone. The continuous small‐magnitude activity may be geothermal in origin, releasing, thermal stress, and not associated with plate boundary tectonics.