Seismological studies of the Pärvie endglacial fault system, northern Sweden

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Karlsson, Eva; Lund, Bjorn; Juhlin, Christopher; Dehghannejad, Mahdieh
Holding Date 03 September 2008

The Pärvie fault extends for over 150 km and is one of the largest known endglacial faults. The fault exhibits reverse faulting throw of more than 10 m and based on studies of Quarternary deposits, landslides and liquifaction structures it is inferred to have ruptured as a one-step event. An earthquake of this size would have had a magnitude of approximately 8. Understanding the mechanics of the Pärvie, and other large endglacial faults, requires knowledge of the prevailing stress field, rock properties and the geometry of the fault. As the causative stress field contained contributions from both tectonic and glacially induced stresses, it is non-trivial to estimate. Assuming the tectonic stress has remained constant throughout the latest glaciation, knowledge of the fault geometry would significantly contribute to our understanding of the mechanics of endglacial faulting. In an ongoing seismological study of the Pärvie fault, we have acquired a 20 km long seismic reflection profile across the fault. Using seven temporary seismic stations, in addition to the eight permanent northernmost stations of the Swedish National Seismic Network and collaborating Finnish and Norwegian stations, we are currently recording microearthquake activity along the fault. First results of the reflection seismic processing indicate that the surface supracrustals rocks extend to about 1 km depth with the base generating a strong wide-angle reflection. The profile crosses three surface mapped faults where the westernmost, main fault strand, is dipping about 50-60 degrees to the east, the middle fault dipping 70-80 degrees east and the easternmost fault dipping 50-60 degrees to the west. The faults are imaged from the near surface down to about 2 km depth. The seismic stations have recorded numerous small events, many of which are mining induced microearthquakes from the nearby Kiruna iron ore mine. Large numbers of microearthquakes originate from the vicinity of the Pärvie fault system, the current station geometry allows detection and location of events as small as magnitude -2. We present hypocenter locations and focal mechanisms of all recorded events. Clusters of closely located earthquakes have been relocated with both a joint hypocenter determination and a double difference algorithm. We will show how well the individual clusters align on fault planes and how these agree with the estimated focal mechanisms. The inferred faults are then associated with the main strands of the Pärvie fault. We invert the focal mechanisms for the causative state of stress and show how stress estimates vary along strike of the fault.