Jökull - 01.06.2000, Page 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