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3 figs., 4 tables, 17 refs.
DDC:
550 - Earth sciences
Abstract:
On December 23, 1979 an earthquake sequence occurred near Bergshamra-Roslagen, Sweden, about 50km northeast of Stockholm. The main shock has been assigned a magnitude M"SUB L" = 3.2. A six- week field survey in the epicentral area revealed a number of small aftershocks located close to the main shock. Pg and Pn-readings suggest for the main shock reverse faulting with a strike of N36"SUP o"E and a dip of 55"SUP o" to the SE. Spectral analysis applied to available seismograms from Finnish, Norwegian and Swedish stations gave for the dynamic source parameters a fault length of 0.8km, a seismic moment estimate of 6.6 x 10"SUP 20" dyne cm and a stress drop of 6 bars. The Bergshamra sequence took place in a zone of very low seismicity in eastern central Sweden and for Swedish earthquakes at unusual shallow depth. The observed earthquake sequence suggests that the known fault system in this area represents a seismically active region. -Authors
