Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 56

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in the vertical direction. The alignment of the geothermal areas
therefore suggests the existence of a throughgoing faulted zone.
The evidence cited here for a major WNW trending fault through
Dalvík is rather fragmentary. The existence of the fault still re-
mains to be proven through detailed geologic mapping of north
Iceland.
The extension of the hypothetical Dalvík fault into the mouth of
Skagafjördur raises an important possibility. The magnitude 7
earthquake of March 28, 1963 (event No. 12 in Table 1) was the
largest earthquake to occur within the Tjömes Fracture Zone since
1910 (9). The calculated epicenter of that earthquake (Table 1)
is off the mouth of Skagafjördur and lies between the projected
traces of the Dalvík and the Húsavík faults at the same distance
from both faults. It would take a mislocation of only 15 km to move
the epicenter to either of those faults. As was already shown in
this study, a mislocation of 15 km is not unusual when teleseismic
data are used. For reasons given later it appears more likely that
the 1963 earthquake occurred on the Dalvík fault rather than on
the extension of the Húsavík fault. Thus there is the possibility
that the proposed Dalvik fault has had two large, destructive earth-
quakes in this century. Such a fault must be taken seriously into
account when estimating the seismic risk of the region.
A focal mechanism solution for the large 1963 earthquake was
found by Stefánsson (15) and independently by Sykes (3). The
solution has two nearly vertical P-wave nodal planes, one with
the strike of 17° and the other 106°. The tensional axis has an
east-north-easterly trend. In the light of what has been said about
the Grimsey, Húsavík and Dalvík faults the nodal plane striking
107° is the most likely fault plane, thus yielding right-lateral strike
slip motion. It was this fault plane solution fhat originally led to
the interpretation of the Tjömes Fracture Zone as a transform
fault (3).
The rate of motion along the proposed Dalvík fault is difficult
to estimate. The fault crosses numerous valleys carved out by the
glaciers of the last glacial period. These valleys are not noticeably
displaced, which implies that the total horizontal motion along the
fault hardly exceeds some hundred meters in post-glacial time, i.e.
the last 10,000 years. Even if all the transform motion between