Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 34

34
GUÐMUNDUR PÁLMASON
A minimum estimate for the thickness of layer 2 can, however, be
obtained and is found to be about 4.4 km, giving a minimum depth
of 7.1 km to layer 3, or about 7.0 km below sea level.
Profile 32 in Skaftártunga runs south from a shot point near Eld-
gjá. It is rather short, about 27 km, and does not show the P3-wave
as a first arrival. Layer 0 is about 0.3 km thick near the shot point,
and layer 1 is about 1.1 km thick. Only a minimmn estimate can be
made of the thickness of layer 2, which is about 4 km. A minimum
estimate of the depth to layer 3 is thus about 5 km below sea level.
All the profiles discussed above, 31, 32, 34 and L8, show or indi-
cate an abnormally great depth to layer 3, compared to other parts
of the country. The boundary of this anomalous area to the west
appears to be under the eastern branch of the Neovolcanic one. The
northem boundary is probably partly under the Vatnajökull ice cap.
Profile 33 runs westwards across the eastem branch of the Neo-
volcanic zone, north of Hekla. The shot point is in lake Frostastada-
vatn. The surface layer is there about 0.5 km thick and has a fairly
low velocity of 2.1 km/sec. A very thin layer with velocity about 3.0
km/sec may exist above layer 1. Layers 1 and 2 have normal ap-
parent velocities. The thickness of layer 1, computed on the basis of
horizontal boundaries, is 1.3 km. The total delay time of the P3-wave
is somewhat variable, probably due to variations in the thickness of
the surface layer. An average value of the total delay time will be
taken as 1.65 sec. If this is divided equally between shot point and
seismometer station it gives a thickness of layer 2 of 2.8 km, and a
depth to layer 3 of 4.6 km or 4.0 km below sea level.
Profiles 35 and 36 which intersect, are located on the lowland
east of river Thjórsá. On the Hella-Fljótshlíd profile (35) layer 1 is
practically at the surface near the shot point, with a P-wave velocity
of 3.9 km/sec. At the shot point of the Thykkvibaer—Rangárvellir
profile (36) there appears to be a low-velocity surface layer present.
Its thickness is probably quite variable along the profile, thus distort-
ing the travel time diagram. Both layers 1 and 2 appear to be present,
but their relative thicknesses are, as computed from a horiontal layer
model, quite different under the two profiles. This apparent dis-
crepancy probably results from irregularities in the low-velocity
surface layer. The delay times of the P3-wave agree fairly well on
both profiles, considering the added time delay at the shot point of
profile 36, which may be about 0.1 sec. The depth to layer 3 inferred
from the delay times, using a probable distribution of velocity with