Rit (Vísindafélag Íslendinga) - 01.06.1971, Side 37

CRUSTAL STRUCTURE OF ICELAND 37
profile B of Tryggvason and Báth (1961), but it has denser seismo-
meter stations and is about 43 km longer.
Conditions near the shot point of prof ile L3 will be discussed in some
detail in the next section. The delay time of the P3-wave in that area
rs close to 0.53 sec and the depth to layer 3 about 2.5 km below sea
level. Farther to the east, in Flói, the P3-wave delay time is about
0-50 sec in agreement with the Thingvellir—Flói profile. At a distance
°f 60—70 km the delay time increases to about 0.60 sec in good agree-
ment with the value obtained on profile 35 in the same area. An
error of 0.05 km/sec in the value of the P3-wave velocity would give
nse to an error in the delay time at 70 km of about 0.08 sec. The
difference between the delay times which are inferred from profiles
L3 and 35 in the area of the shot point of profile 35 is less than this
■value. These considerations therefore support the hypothesis that the
P-wave velocity in layer 3 is close to 6.50 km/sec in southern Iceland.
Beyond about 70 km the P4-wave is the first arrival on profile
L3. Its total delay time is 2.2-2.3 see. The most probable delay time
at the shot point in Graenavatn is about 1.0 sec, considering the
delay times of profiles CP, WP and L3 together. This gives about 1.25
sec as the P4-wave delay time at the distant end of profile L3. The
depth to layer 4 is then calculated 10-11 km, using data from profile
35 for the shallower layers. This shows that at the eastern end of
profile L3 the depth to layer 4 is increasing compared with what it
!s farther to the west. This is in agreement with results of other pro-
files in this area.
On profile L7 the surface layer at the shot point has a P-wave
velocity of about 2.3 km/sec and a thickness of about 0.2 km. Layer
1 is very clearly shown on the travel time diagram, but layer 2 is
n°t identified with certainty by first arrivals. It is very likely that it
ls present, however, at least under most of the profile. An assumed
average velocity of 4.4 km/sec for both layers 1 and 2 is therefore
nsed to convert P3-wave delay times to depth. The total delay time
°f the P3-wave is about 1.04 sec, disregarding two irregularities in
the delay time curve, which are probably caused by varying thick-
ness of the low-velocity surface layer. This gives a depth of about 2.8
Lm to layer 3. This depth appears to be fairly constant under this
profile. The P4-wave appears as a first arrival at distances beyond
about 55 km. Its total delay time is about 1.95 sec. Assuming a uni-
form depth to layer 4 under this profile, it is found to be about 8.9 km.
Before leaving S-Iceland it is perhaps worth noting that there