Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 32

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GUÐMUNDUR PÁLMASON
6.2. S-Iceland.
The main characteristic of the seismic structure of S-Iceland is
a relatively large increase in depth to layers 3 and 4 in the eastem
part of the area. This change appears to be associated with the
eastern branch of the Neovolcanic zone, although lack of data from
the area covered by the Vatnajökull ice cap makes it somewhat dif-
ficult to establish a firm relationship between the subsurface seismic
structure and the surface geology.
A total of 12 profiles have been measmed in S-Iceland. Their
length is between 22 and 110 km. Three of the profiles (31, 32, 34)
are of insufficient length to record the P3-wave as a first arrival, but
three of them (L3, L7, L8) are sufficiently long to record the P4-
wave as a first arrival.
A summary of the total delay time of the P3-wave in S-Iceland
is given in Fig. 7. Delay times of the P4-wave are shown in Fig. 28.
The easternmost profile in this area is the Sída-Fljótshverfi profile
(31). At its shot point near Kirkjuhaejarklaustur layer 0 has a velo-
city of 3.3 km/sec and a thickness of 0.2 km. It is underlain by layer
1 with an apparent P-wave velocity of 4.50 km/sec. At a distance of
20-30 km the apparent velocity increases to over 6 km/sec. This may
be indicative of that the depth to layer 3 is decreasing to the east, but
more measurements are needed here to establish this conclusively.
The profile L8, which is about 90 km long, partly runs along the
same road as profile 31. At the shot point in lake Heidarvatn the
surface layer is about 0.5 km thick, taking the average of profiles L8
and 34, which run in opposite directions from the same shot point.
Layer 1 on profile L8 has a rather high P-wave velocity of 4.34
km/sec. Layer 2 also has a rather high apparent velocity of 5.52
km/sec. The thicknesses are about 2.9 km and 5.0 km, giving a total
depth of 8.4 km to layer 3. This depth is much greater than found in
other parts of the country, but it is confirmed by the neighbouring
offshore profile L9 (cf. sect. 6.10). The total delay time of the P4-
wave, which has an apparent velocity of 7.22 km/sec, is 3.5 sec. Di-
viding this equally between shot and seismometer stations gives a
thickness of 6.1 km for layer 3 and a total depth of 14.5 km to layer 4.
Profile 34 runs westwards from the same shot point as on L8, to
a distance of about 41 km. Layer 1 is here calculated to be about 2.2
km thick. The thickness of layer 2 cannot be calculated since the P3-
wave does not appear as a first arrival at distances less than 41 km.