Fróðskaparrit - 01.01.1976, Page 12

20 Seismic Refraction Measurements around the Faeroe Islands
Line C (south) gives good signals until C45 but the more
distant shots give very weak signals, and no first arrival could
be picked £or e. g. C43 and C44 which are 600 lbs. shots in the
range of 40 ... 150 km varying with the station. The stacked
seismograms of station F 6 are rather characteristic (fig. 6).
For F2 it was possible to pick very weak but distinct arri-
vals for very distant shots on line C (south) (the shots C5, C7,
C8, C13, and C15); the corresponding seismograms consist of
a surprisingly short signal (a few seconds), but nothing else is
above the noise level. The travel times seem to indicate that
these arrivals are »norma.l« Moho-arrivals (see fig. 3).
Attempts were made to use later arrivals but it is felt that
the analysis cannot be trusted because the range between the
shots is too big to permit a reliable correlation of arrivals from
one seismogram to the next. The results are, however, not
inconsistent with the previous results (Pálmason 1965, Casten
1974 and Casten & Nielsen 1975).
Crustal Structures of the Faeroe Block.
The key to the deep structure of the Faeroe Block seems to
be the apparent velocity of 6 km/s for the basement below
the basalt lavas. This velocity has been observed in all direc-
tions close to the islands, and when graphs of reduced travel
times are drawn for first arrivals from a single shot then the
velocity of 6 km/s reappears in most cases as the apparent
velocity between the stations for distances above 25 km (see
fig. 7). Only two persistent exceptions have been found, namely
the most distant shots on line B where Moho-velocities appear
in both directions and the station F6 on Suðuroy (fig. 7). The
ship station MIR(D) on the border of the shelf towards north-
west can be included in this 6 km/s area.
The conversion of waves from outside into waves of approx.
6 km/s has been discussed in details for line A by Bott et
al. (1976) and seems to indicate a deep layer of this approxi-
mate velocity below the basalt lavas.