Source: O.Kulhånek and R. Wahlström,
Macroseismic observations in Sweden 1980-1983.
FEBRUARY 13,1981, OTTERBÄCKEN, PROVINCE OF VÄSTERGÖTLAND
On the morning of February 13, 1981, a strong earthquake took place
near Otterbäcken in northern Västergötland. The hypocentre of the earthquake
was located a few km off shore beneath Lake Vänern.
Consequently, macroseismic data could be collected only from the adjoining
shore area, covering reasonably well azimuths from NNE to SSE.
Intensity estimates are available from some 130 localities;
most of them come from direct interviews and the remainder from telephone
calls and by inspection of letters to the department.
Except for a fine crack in the plaster in a hall of one house in Gullspång,
no dammage has been reported. Within the disturbed area, buildings are properly
built brick or wooden houses.
The foundation materials consist mainly of solid rock, and to lesser extent
of clay, gravel or moraines.
During the earthquake, macroseismic area residents felt heavy shaking indoors
which lasted for several seconds, in some cases up to about one minute.
From many places audible sounds similar to those of a distant thunder,
blasting or heavy traffic were reported. The duration of these sounds varied
between several seconds and several minutes. At three localities,
household pets (dogs, cats) became uneasy during the earthquake.
Several interviews revealed that the sensation of vibration was comparable
to or even more intensive than that due to the April 11, 1973 earthquake
of magnitude ML(UPP)=3.9 from this area.
The maximum intensity was observed at Otterbäcken and northwards, where it
reached the degree IV.
Spatial distributions of intensities III and IV together with the area of
perceptibility are depicted in Fig. 5.
As can be seen in the figure, the observed macroseismic field is rather complex
with a number of exceptional pockets of high er intensities as well as of
unforeseen reports "not felt".
Isolated reports of felt intensities were not encircled by corresponding isoseismals.
Due to the location beneath Lake Vänern, the epicentral intensity of the Otterbäcken
earthquake can only be approximated. It follows from Fig. 5 that the
radius of perceptibility most likely falls between 20 and 30 km.
The effect of a rather shallow focal depth, providing a smaller radius of
perceptibility , is clearly demonstrated by macroseismic observations.
The epicentral in tensity should lie approximately between the degree IV
(maximum intensity felt during the present earthquake) and V-VI
(maximum intensity felt in Sweden during the last 30 years).
We assume a value of IV-V to be a reasonable approximation of the
otherwise inaccessible intensity lo. Employing macroseismic parameters
Io=4.5 and rp=25 km, we obtain a focal depth of 8 km.
For more details on this particular event the reader is referred to
Kulhånek et al. (1981, 1983).