Jökull - 01.12.1992, Page 17

Figure 2. A map showing the four OBS stations and
lhe 35 explosion sites used in the refraction survey.
Kort af rannsóknarsvœðinu. Skotstaðir eru tákn-
aðir með stjörnu en staðsetning neðansjávarskjálfta-
tnœla með þríhyrningi.
as low as 2 km/s at the surface down to 6.5±0.2 km/s,
whereas the lower crust (Layer 3) is characterized by
a nearly constant P-wave velocity and may be equated
to Layer 3 in the oceanic crust. Various seismic studies
throughout Iceland show that the upper crustal thick-
ness is highly variable, from 2-11 km, whereas the
total crustal thickness has been debated in the liter-
ature during the last 30 years (Báth 1960; Pálmason
'971; Zverev et al. 1976; Flóvenz 1980; Gebrande et
ai- 1980; Flóvenz and Gunnarsson 1991; Bjarnason
et al. 1993). A tomographic image of a 170 km long
wide-angle reflection/refraction profile in southwest-
ern Iceland revealed a sharp Moho reflection at a depth
ot 20-24 km beneath the south Iceland seismic zone
(Bjamason et al. 1993). No comparable data exist
from northern Iceland.
This paper describes the results from refraction
survey performed in the Tjömes fracture zone in Au-
gust 1991. Refraction data from a 70 km long profile
along Axarfjarðardjúp, the easternmost trough of the
TFZ (Figure 2) is analysed here. The data consists
°f 32 shots recorded at 4 OBS stations situated along
the profile. The profile was shot within the trough
in order to avoid major faults and other large topo-
graphic variations. Also, our data coverage precluded
higher-resolution analysis needed in order to image
more complicated tectonic features such as basement
highs and escarpments. The southem tip of the profile
is at the junction between the northern volcanic zone
and the Grímsey seismic lineament.
The purpose of the present study was to gener-
ate a new crustal model for Axarfjörður. The model
enables us to determine the average sediment thick-
ness in a region within the fracture zone which has
not been severely distorted by complicated tectonic
processes. The model also provides information on
the increase in sediment thickness offshore compared
with the northern volcanic zone (Flóvenz and Gunn-
arsson 1991) which is important in order to improve
earthquake locations in the region.
THE SEISMIC SURVEY
Eleven digital, three-component, portable record-
ing stations (PRS4’s) and eighteen ocean bottom seis-
mometers (OBS's) were operated along the northem
coast and in the Tjömes fracture zone between Axar-
fjörður, Eyjafjarðaráll and Grímsey, in August 1991.
The seismometers collected both earthquake and ex-
plosion refraction data during their 25 days of de-
ployment. During the initial stage of the project a
70 km refraction profile was shot in the Axarfjarðar-
djúp trough. The OBS-project was carried out by the
Laboratory of Ocean Bottom Seismology, Hokkaido
University, Japan, the Icelandic Meteorological Office
and the Iceland Coast Guard, whereas the land work
was carried out by the Nordic Volcanological Institute
and the Science Institute of the University of Iceland.
The present paper focuses on the refraction data
from the 4 OBS’s (number 12, 13, 17, and 18) on
the refraction line in the Axarfjarðardjúp trough (Fig-
ure 2). A total of 35 dynamite explosions were det-
onated in the sea within the study region. Two large
shots (100 kg) were detonated at each end of the profile
at a depth of 100 m (E01 and E35 on Figure 3). About
30 small shots (1.8 kg) were fired along the profile at a
depth of 10 m with an average spacing of about 2 km.
In addition, three shots (each 40 kg) were fired at a
JÖKULL,No. 42, 1992 15