Sk. Arnott et Gr. Foulger, THE KRAFLA SPREADING SEGMENT, ICELAND .1. 3-DIMENSIONAL CRUSTAL STRUCTURE AND THE SPATIAL AND TEMPORAL DISTRIBUTION OF LOCAL EARTHQUAKES, J GEO R-SOL, 99(B12), 1994, pp. 23801-23825
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.