Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 44

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GUÐMUNDUR PÁLMASON
A noteworthy result of the refraction measurements in the area
of lake Thingvallavatn is the relatively large difference hetween the
depths to layer 3 at the northem and the southem ends of the lake.
This difference is about 1.2 km and shows that in the Thingvellir
graben there are not only structural variations in an east-west direc-
tion across the graben, but also in a direction along the graben. The
Bouguer gravity anomaly map of this area (Einarsson, 1954) shows
this very well. The anomaly is about 10 mgals lower at the northem
end of the lake than at the southern end. If this is due entirely to a
density difference between layer 3 and the overlying rocks, then
this density difference would amount to about 0.2 g/cm3. This is
similar to what has been found in other places where a correlation
between seismic and gravity results has been made (cf. sect. 6.7).
6.4. W-Iceland.
In this section results from W-Iceland will be discussed, with the
exception of the Snaefellsnes peninsula, which will be treated se-
parately in the following section.
The Esja—Hvalfjördur area will be discussed first. There are three
profiles available on land in this area (47, 49a, 49b and 50), of which
one is reversed. Furthermore one offshore profile (48) runs west-
wards from a station on Kjalames, which is very close to the shot
point of profile 47. Total delay times of the P3-wave of these and
other profiles in W-Iceland are shown in Fig. 12.
Two major delay time anomalies are found in the Esja-Hval-
f jördur area. The larger one is on the Kjalarnes—Mosfellsheidi profile
(47), and the smaller one on the Akranes—Hvalfjördur profile (50).
The most probable value of the delay time of the P3-wave at the
shot point of the Kjalames—Mosfellsheidi profile is found to be about
0.30 sec, taking into account both the delay times on this profile and
those on the profile Kjalarnes-Faxaflói (48). Delay times at other
points along the profile can then be calculated. It is found that at the
minimum total delay time at a distance of approximately 14 km the
station delay time is 0.07 sec. As all arrival times have been reduced
to sea level (i.e. shot point level) this means that layer 3 should here
be 300-400 meters below sea level or at a depth of 500-600 meters
below the ground surface. This estimate is relatively inaccurate due
to the uncertainty in the shot point delay time, but is is nevertheless
clear that this is the shallowest depth to layer 3 found so far in Iceland.