Jökull


Jökull - 01.06.2000, Side 32

Jökull - 01.06.2000, Side 32
Foulger and Field Figure 1. Map of Iceland showing the locations of the Hengill-Grensdalur and Kralla areas. - Yfirlitskort. conversion of one to the other uncertain. Also, LET and gravity have different sensitivities to bodies at different depths. The amplitude of the gravity field at the Earth’s surface due to a body decreases as the depth to the body increases. In the case of a spherical body it decreases as the inverse square of depth to the body. Thus the gravity field is most sensitive to shallow bodies and sensitivity decreases rapidly for deeper bodies. In contrast, the resolving power of LET is typically patchy at shallow depths because rays rise steeply beneath stations and shallow areas between stations are unsampled. Resolution is best at intermediate depths above the maximum depth of earthquakes where ray coverage is good. Comparison of LET and gravity, or joint inversi- on, has been applied to the Imperial Valley, Parkíield, The Geysers-Clear Lake area, Coalinga and Coyote Lake, CA (Evoy, 1978; Oppenheimer and Herken- hoff, 1981; Thurber, 1983; Eberhart-Phillips, 1990; 1993; Eberhart- Phillips and Michael, 1993) and Ki- lauea, Hawaii (Ellsworth and Koyanagi, 1977). In the present study gravity and LET velocity models are compared for the Hengill-Grensdalur and Krafla areas (Figure 1). The exercise extends insight into the true structures of these two areas beyond that which could be gained from either method alone (Field, 1994). 30 JÖKULL No. 48

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