Jökull


Jökull - 01.06.2000, Blaðsíða 33

Jökull - 01.06.2000, Blaðsíða 33
Comparison oftomographic crustal models with gravity data THE HENGILL- GRENS DALUR AND KRAFLA AREAS The structures of the Hengill-Grensdalur and Krafia areas have been described in detail previously (e.g., Sæmundsson, 1978; 1982; 1995; Foulger, 1988; Foul- ger and Toomey, 1989). Both areas contain active central volcanoes, fissure swarms and the Hengill- Grensdalur area contains at least one, and possibly two, extinct central volcanoes. The Krafla central volcano has a caldera ring fault with a throw of ~ 1 km (e.g., Björnsson et al. 1977) and is underlain by a magma chamber with its top at ~3 km (e.g., Einars- son, 1978). It is well supplied with magma and is volcanically active at intervals of ~500 years (Björns- son et al. 1977). The Hengill system is poorly supp- lied with magma and is only volcanically active on average about every 1,500 years. No evidence has yet been found for a contemporary magma chamber in the area. Both areas contain high-temperature geothermal areas that generate continuous, small- magnitude earthquake activity with stable, known spatial distri- butions. This circumstance, along with the known substantial three-dimensionality of structure, made them ideal targets for LET. TOMOGRAPHIC MODELS The experiments Several hundred small earthquakes were recorded on temporary seismic networks deployed in the two areas in 1981 (Hengill-Grensdalur) and 1985 (Krafla). Excellent datasets were obtained as the stations were sited with a foreknowledge of the spatial distribution and seismic rate in each area. LET inversions were performed using the method of Thurber (1983) which parametrises the seismic velocity field by values at the nodes of a three-dimensional grid, in between which velocity is linearly interpolated. Hengill-Grensdalur In the case of the Hengill-Grensdalur study, a volume 14 x 15 km in area and 5 km in depth was studied (Figure 2) (Toomey and Foulger, 1989; Foul- ger and Toomey, 1989). The primary structures ima- ged were high-velocity bodies beneath the Grensdal- ur, Ölkelduháls and Húsmúli extinct volcanic sites. There was a notable lack of low-velocity bodies, with the exception of a small body, near the lim- it of resolution, a few cubic km in size beneath the northern flank of Hengill and tentatively inter- preted as partial melt. Seismic velocities beneath the fissure swarm in general were average. Resolution was good in the centre of the study volumes and deter- iorated towards the perimeters and the surface. The high-velocity bodies were interpreted as high-density gabbroic intrusives. A second LET study of the area was conducted in 1991 as part of an experiment to study focal mechan- isms of the earthquakes there (Foulger et al. 1995). The repeatability of the velocity models between experiments was about 0.19 km/s and most of the difference is attributed to the use of different damp- ing values. This illustrates well the trade-off between extremity of the model and damping in LET inversi- ons. The spatial pattern of anomalies was very similar for both studies. Krafla The distribution of earthquakes measured in this stu- dy was inferior for LET to those of the Hengill- Grensdalur area. Many of the earthquakes lay in a narrow zone running NNE-SSW along the centre of the fissure zone, the locus of dyke intrusions a few ye- ars earlier, and the hypocentral depths did not exceed ~3 km b.s.l. (Arnott, 1990; Arnott and Foulger, 1994). A volume 13 x 18 km in area and 4 km in depth was studied (Figure 3). The well-resolved volume was limited to the upper 2-3 km b.s.l. of the fissure swarm, and narrowed with depth. It is instructive to compare the results to recent shallow refraction profiling in the Krafla area by Brandsdóttir et al. (1997). In the LET experiment, the very shallowest layers, between sea level and the surface (which is at 500 m a.s.l.) were only sampled in narrow cones beneath individual stations, defined by the upgoing rays. The stations were all deployed on outcrops of massive bedrock and thus the lavas, tuffs and very low-velocity unconsoli- dated material that fills the caldera were essentially unsampled. For this reason, it is not valid to extra- polate the LET model laterally away from the stati- JÖKULL No. 48 31
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