Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 55

CRUSTAL STRUCTURE OF ICELAND
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Table 63. Hofsstadir-Kolbeinsstadir.
D-station No. AT sec ÖTrHo SeC X km V3 km/sec
1567 4.04 0.10 25.51 6.48
1568 4.01 9? 25.29 6.47
1569 4.00 59 25.51 6.54
Average V3 = 6.50 km/sec
These results indicate that the Hraunsmúh-Hofsstadir segment
of the hne is characterized by a somewhat higher velocity in layer 3
than the segments on both sides. This higher velocity occtn-s in the
area where the delay time also is low and the depth to layer 3 small,
between one and two kilometers.
These velocity determinations support the hypothesis that the most
probahle value of the P-wave velocity in layer 3 is about 6.50 km/sec,
but they also indicate that deviations from this velocity may occur.
On the longest profiles Ólafsvík—Kolbeinsstadir and Kolbeinsstad-
ir-Ólafsvík later arrivals occur at 45-60 km, which have an apparent
velocity of about 7.2 km/sec. This is probably the P4-wave which is
observedonotherprofilesof sufficientlength (seesection 6.10). From
the travel times an estimate can be made of the depth to layer 4 under
the Snaefellsnes peninsula. On both profiles seismometer stations are
located at a distance of 52.5 krn and the arrival times of the P4-wave
as read from the seismograms are 8.86 and 8.90 sec. Using the aver-
age value of 8.88 sec and assuming a P4-wave velocity of 7.2 km/sec
gives a total delay time of 1.58 sec. Assuming furthermore that the
3-4 boimdary is horizontal, and taking into account the effects of
layers 1 and 2, gives an estimated depth to layer 4 of about 9 km
under the central part of the Snaefellsnes peninsula (cf. Fig. 17).
The offshore profile Ll, which will be discussed later, indicates that
this depth increases to the west near the Snaefellsjökull volcano.
On the northern part of the Snaefellsnes peninsula the structure
is very irregular as clearly shown by the two unreversed profiles
Skógaströnd (61) and Stykkishólmur—Ólafsvík (60). The P3-wave
total delay times are shown in Fig. 13 and the inferred structure in
Fig. 18. One feature of this section is a change in depth to layer 3
just east of Breidabólsstadur. This amounts to about 1.1 km, the depth
being about 3.5 km to the east and about 2.4 km to the west. In the
Stykkishólmur area the depth is about 2.2 km, increasing to the south-
west toward the western end of the volcanic line on Snaefellsnes near