Publisher： John Wiley & Sons Inc
E-ISSN： 2156-2202|99|B12|23801-23825
ISSN： 0148-0227
Source： Journal Of Geophysical Research, Vol.99, Iss.B12, 1994-12, pp. : 23801-23825
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Abstract
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 Námafjall 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 Námafjall 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 Bjarnarflag well field within the Námafjall area is probably induced partly by geothermal exploitation. Considerable seismicity also occurred immediately below Leirhnjúkur, 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.
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