Rit (Vísindafélag Íslendinga) - 01.06.1971, Side 36

36
GUÐMUNDUR PÁLMASON
again to about 0.07 sec relative to profile 37, indicating decreasing
thickness of the low-velocity surface layer (layer 0).
The Apavatn-Land profile (37) crosses the Hreppar anticlinal
axis (Th. Einarsson, 1967). There appears to be a weak anticlinal
structure in the upper boundary of layer 3 in the same area (cf. Fig.
37), but its axis appears to be some 20 km farther to the west.
From a common shot point in a water-filled fissure at Thingvelhr
three profiles radiate to the north, east and south approximately.
Only two of these will be discussed here, but the third one will be dis-
cussed in sect. 6.4.
Profile 40 runs east more or less in an opposite direction to profile
38. It is only about 22 km long. The travel time diagram is somewhat
complicated by the relatively large and varying thickness of the low-
velocity surface layer. At the shot point the thickness of layer 0 is
about 0.6 km, probably increasing somewhat to the east. Both layer
1 and layer 2 are probably present although they are not well defined
on the travel time diagram of this profile. Profiles 39 and 41, which
run from the same shot point show clearly both these layers. The
total delay time of the P3-wave on profile 40 is about 1.28 sec. The
shot point delay time is found to be about 0.63 sec by a comparison
of the three profiles 39, 40 and 41 with each other. Using a probable
vertical velocity distribution gives a depth of about 3.1 km to layer
3 in the Thingvellir graben.
The Thingvellir-Flói profile (39) runs to the south along the
eastern shore of lake Thingvallavatn towards Flói to a distance of
about 44 km. The above remarks on layers 0, 1 and 2 also apply to
this profile. The delay time of the P3-wave decreases somewhat to
the south along this profile as is to be expected from a decreasing
thickness of layer 0. The delay time at the distant end of the profile is
close to 0.50 sec, which is in good agreement with what is obtained
on profile L3S to be discussed later in this section.
The Thingvellir graben is thus characterized by a relatively large
thickness of layer 0, but does not appear to have an anomalous
seismic structure at greater depth. The thickness of layer 0 is prob-
ably largest near the eastern shore of the lake. This is supported by
measurements to the south of the lake also (cf. sect. 6.3).
Two long profiles in S-Iceland, L3 and L7, remain to be discussed.
L3 runs from a shot point in lake Graenavatn eastwards to a distance
of about 110 km, while L7 runs northeast from a shot point in the
crater Kerid to a distance of about 84 km. Profile L7 coincides with