Microearthquakes on the Mid-Atlantic Plate Boundary on the Reykjanes Peninsula in Iceland

Seismometer arrays were operated during the summers of 1971 and 1972 in two adjacent areas of the Reykjanes Peninsula in Iceland. Up to 13 and up to 23 stations, respectively, covered areas of about 10 × 15 km. The hypocenters of 300 microearthquakes were located with typical accuracies of 0.6 km. This sample of epicenters defines a zone of seismicity 2–5 km wide extending from the tip of the peninsula about 40 km eastward to Lake Kleifarvatn. The trend of the seismic zone is N70°E in the westernmost part of the peninsula but is nearly E-W farther east. This seismic zone is intepreted as the plate boundary between the North American and Eurasian plates. Most of the earthquakes occur at 2- to 5-km depth, a few events being as deep as 9 km. The normal level of microearthquake activity was about 20 events per day. Focal mechanism solutions were obtained for 40 earthquakes. The common feature of the solutions is the horizontal NW trend of the tensional axes. Faulting is of the normal and strike slip types, often both types of solutions occurring close to one another. Thus the tectonic picture on the Reykjanes Peninsula is one of an active plate boundary with both ridge and transform fault characteristics. A large microearthquake swarm lasting for 8 days and with approximately 14, 600 recorded events occurred within the array in 1972, During this 8-day swarm most seismic activity was concentrated into subswarms lasting up to several hours. Small swarms were frequently recorded at other times during the field seasons.