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, w as monitored for 3 months in 1985 using a dense, local seismometer net work. The seismicity was continuous, and the spatial and temporal dist ributions were roughly known prior to monitoring. The instruments coul d thereby be deployed in a well-positioned array. A total of 489 locat able events were recorded within the network, and 1771 arrival times w ere inverted to calculate the three-dimensional P-wave velocity struct ure and hypocentral locations. Low-velocity volumes were imaged beneat h the Krafla and Namafjall geothermal areas and indicate zones of hydr othermal alteration. High-velocity bodies beneath the Krafla caldera r im are interpreted as gabbroic intrusions. Using a three-dimensional v elocity structure instead of a refraction-based one-dimensional model to locate the hypocenters significantly improves their location qualit y and illustrates the shortcomings of using refraction-based models to locate earthquakes in local, anomalous areas. Seismic activity was co ncentrated within the Krafla and Namafjall geothermal areas and in a n arrow zone where dike injections had occurred 8 and 5 years earlier. T he activity occurred in the depth range 0-3 km. The seismic rate for t he 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 geotherm al processes in the manner proposed for other Icelandic areas (Foulger and Long, 1984; Foulger, 1988b). Seismicity directly beneath the Bjam arflag well field within the Nandmafjall area is probably induced part ly by geothermal exploitation. Considerable seismicity also occurred i mmediately below Leirhnjlikur, a site of intense geothermal activity i n the center of the Krafla caldera that overlies a roof pendant in the magma chamber below. This volume of high seismicity is probably highl y fractured and may provide a conduit for magma escaping from the magm a chamber.