Rit (Vísindafélag Íslendinga) - 01.06.1971, Síða 98

98
GUÐMUNDUR PÁLMASON
Iceland. On the offshore profiles no S-wave velocities have heen de-
termined.
On several profiles later arrivals were observed with apparent
velocities corresponding to one of the P head waves. The amplitudes
of these phases were sometimes relatively strong compared with the
first arrivals. An example of such arrivals, denoted by P3’, is given
in the travel time diagram of Fig. 33. No systematic studies have
heen made of these waves, but it appears from the travel times that
they may be reflected refractions, i.e. head waves which have suf-
fered a reflection from the surface or from an interface above the
refractor. Such waves have been described e.g. by Meissner (1965)
and studied experimentally by Nakamura (1966), who found that
they can have larger amplitudes than the corresponding head waves.
A closer study of these waves could he of value for the interpretation
of the refraction data.
10. SEISMIC STRUCTURE OF ICELAND FROM THE
REFRACTION DATA
The seismic refraction data that have been collected so far in Ice-
land have given a fairly detailed representation of crustal layering in
terms of seismic velocities to a depth of about 20 km. This includes a
a part of the island shelf off the south and west coasts. The only major
gap still to be filled is the depth to layer 4 in northem and eastem
Iceland. Some additional data are also needed in the Vatnajökull
area to elucidate the change in stmcture which apparently must take
place between southern and southeastern Iceland.
Layer 0 occurs as a surface layer everywhere in the Neovolcanic
zone. It is thickest in the zone of active volcanism, as for instance on
the southem part of the Reykjanes peninsula (profiles 43, 44 and
46b), to the east of Thingvellir (profiles 39 and 40), east of Hekla
(profile 33), and in the volcanic zone in NE-Iceland (profiles 13a,
15, 16 and 19). The largest thickness is found on the southern part
of the Reykjanes peninsula, up to 1.0 km. The velocity values found
for this layer show a fairly large variation (cf. Fig. 5), indicating a
very varying degree of compaction. This variation is to be expected
from the nature of the surface rocks in the volcanic zone.
Layers 1 and 2 will be discussed together as both are known to he
composed of Tertiary flood basalts mainly. Layer 1 forms the surface