Rit (Vísindafélag Íslendinga) - 01.06.1971, Síða 71

CRUSTAL STRUCTURE OF ICELAND
71
ferred depth about 4.1 km. Since the shot point is here at an elevation
of 0.4 km, this gives a depth of about 3.7 km below sea level, which
is slightly smaller than on the southern part of the Reykjadalur
profiles.
In the Mývatn area two profiles run in directions perpendicular
to each other, the Hólasandur profile (15) and the Mývatn—Jökulsá
profile (16). The Mývatn—Jökulsá profile has a slightly higher delay
time, but no major delay time anomalies are seen. Assuming that
layers 0 and 1 have the thicknesses deduced from the conventional
interpretation in terms of horizontal layers, it is found that the Hóla-
sandur profile gives a depth of 3.3 km to layer 3 and the Mývatn—
Jökulsá profile a depth of 3.9 km. Taking into account the elevation
of the shot points, these depths become 3.0 and 3.6 km below sea level.
At Tjörnes profile H of Tryggvason and BAth (1961) gives
some information on the depth to layer 3, although only one seismo-
meter station records the P3-wave as a first arrival. The total delay
time at this station is 1.24 sec, which gives 0.62 sec for each of the
shot point and the seismometer stations, assuming the delay times to
be equal. Using the vertical velocity distribution given by the first
part of the profile, this gives a depth of 3.9 km to layer 3.
In Kelduhverfi a reversed profile (13 and 13a) was measured
roughly in an east-west direction. The surface low-velocity layer is
here thicker near the eastem end of the profiles at Jökulsá, than near
the western end. The delay times for the P3-wave are, however, simi-
lar at both shot points as indicated by the travel times midway be-
tween the shot points. The total delay times between the ends of the
profiles are about 1.28 sec, giving 0.64 sec at each shot point. Using
the velocity distribution given by the first parts of the profiles this
gives for the depths to layer 3 the values 3.7 km near the westem
end and 3.1 km near the eastern end.
To the east of Kelduhverfi the unreversed profile 14 runs across
Axarfjardarheidi in a westward direction. The total delay time at a
distance of about 25 km is about 1.12 sec. At the western end it in-
creases rather abruptly to about 1.24 sec. This is most likely caused
by the low-velocity surface layer at the eastern end of the Keldu-
hverfi profile. If the delay time at a distance of 25 km is divided
equally between the shot point and the seismometer stations this
gives the value 0.56 sec. Using the vertical velocity distribution given
by the first part of the profile, gives a depth of 3.8 km, or 3.7 km
below sea level.