THE KRAFLA SPREADING SEGMENT, ICELAND .1. 3-DIMENSIONAL CRUSTAL STRUCTURE AND THE SPATIAL AND TEMPORAL DISTRIBUTION OF LOCAL EARTHQUAKES

The geothermal seismicity of the Krafla Volcanic System, NE Iceland, was monitored for 3 months in 1985 using a dense, local seismometer network. The seismicity was continuous, and the spatial and temporal distributions were roughly known prior to monitoring. The instruments could thereby be deployed in a well-positioned array. A total of 489 locatable events were recorded within the network, and 1771 arrival times were inverted to calculate the three-dimensional P-wave velocity structure and hypocentral locations. Low-velocity volumes were imaged beneath the Krafla and Namafjall geothermal areas and indicate zones of hydrothermal alteration. High-velocity bodies beneath the Krafla caldera rim are interpreted as gabbroic intrusions. Using a three-dimensional velocity structure instead of a refraction-based one-dimensional model to locate the hypocenters significantly improves their location quality and illustrates the shortcomings of using refraction-based models to locate earthquakes in local, anomalous areas. Seismic activity was concentrated within the Krafla and Namafjall geothermal areas and in a narrow zone where dike injections had occurred 8 and 5 years earlier. The activity occurred in the depth range 0-3 km. The seismic rate for the whole area was one magnitude 3.2 event per year and the b value was 0.77 +/- 0.10. Most of the seismicity appears to result from geothermal processes in the manner proposed for other Icelandic areas (Foulger and Long, 1984; Foulger, 1988b). Seismicity directly beneath the Bjamarflag well field within the Nandmafjall area is probably induced partly by geothermal exploitation. Considerable seismicity also occurred immediately below Leirhnjlikur, a site of intense geothermal activity in the center of the Krafla caldera that overlies a roof pendant in the magma chamber below. This volume of high seismicity is probably highly fractured and may provide a conduit for magma escaping from the magma chamber.