Rit (Vísindafélag Íslendinga) - 01.06.1971, Blaðsíða 57

CRUSTAL STRUCTURE OF ICELAND
57
Bjarnarhafnarfjall, where the depth to layer 3 is ahout 3 km. On
crossing Hraunsfjördur an abrupt decrease in delay time of about
0.20 sec occin-s, giving a depth to layer 3 of about 1.5 km. This depth
persists across Kolgrafarfjördur to Grundarfjördur where the depth
starts to increase again. The low delay time area between Hrauns-
fjördur and Grundarfjördur coincides with a geologically distinct
area which is characterized by acid volcanism and gabhroic intrusives
(Sigurdsson, 1966). Still farther to the west, near the farm Fródá
about 4 km east of Ólafsvík, a delay time anomaly is found indicat-
ing a relatively shallow depth to layer 3, hetween one and two kilo-
meters. This is in agreement with what was found on the continuous
profile between Ólafsvík and Kolheinsstadir. To the west of Ólafsvík
the depth to layer 3 increases markedly.
It is of interest to compare the depth variations to layer 3 with
existing gravity data on the Snaefellsnes peninsula. Gravity stations
are available on the northern and southern parts of the peninsula as
far west as Búdir (Einarsson, 1954), and additional data have re-
cently heen ohtained in a survey made by the National Energy
Authority, Reykjavik.
The available gravity data on the southem part of the peninsula
are shown in Fig. 17. The Bouguer anomaly is increasing to the west
along the eastem part of the peninsula and appears to level off west
of the seismic shot point S2 (Hraunsmúli). An assumed regional
trend has been plotted, which is due to density variations at depths
greater than those reached by the seismic soundings. The deviations
of the Bouguer anomaly from this regional trend are due to density
variations in the depth range covered by the seismic soundings. It is
seen that a gravity variation is associated with the change in depth
to layer 3 just west of shot point S3. The gravity change amounts to
ahout 6 milligals. For a two-dimensional step model of 1.5 km change
in depth, this would correspond to a density difference of about 0.1
g/cm3. It should be noted, however, that this is a very rough estimate
since both the magnitude of the anomaly and the assumption of two-
dimensionality are highly uncertain.
A positive gravity anomaly is associated with the low delay time
near Fródá just east of Ólafsvík. This is shown by the gravity value
in parenthesis in Fig. 17. This gravity station is about 4 km north of
the seismic profile line.
The gravity data from the northern part of the peninsula are
shown in Fig. 18. A similar regional trend is assumed here as on