NJG81-4-05
2001
The ML 4.5 Stord/Bømlo, southwestern Norway, earthquake of 12 August 2000
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At 14:27 GMT on 12 August 2000, a magnitude (ML) 4.5 earthquake occurred near the islands of Stord and Bømlo in southwestern Norway, an
area that has long been known to be seismically active. The earthquake was felt at distances up to 300 km. At a depth of 18 km, a location near the islands of Stord and Bømlo, and a reverse focal mechanism, the earthquake is quite similar to a previously studied ML 4.4 earthquake, that occurred in 1983. Both these, and an additional earthquake of similar size that occurred in 1954 are located in the vicinity of the Hardangerfjord Shear Zone, with a NW dipping plane that separates the Precambrian basement to the southeast from mainly intrusive Caledonian rocks to the northwest.
In the August 2000 earthquake, a total of 35 locatable aftershocks occurred within 72 hours after the main shock. However, in this case, the
station configuration did not provide location precision that was high enough to allow the aftershock distribution to be used in delineating the
rupture area or associating the event with known geologic structures in the region. On the other hand, a detailed analysis of the differences in
phase arrival times provided an upper limit on the relative location differences for the aftershocks of about l.S km. Since the source has an estimated size of about l km, the analysis suggested that these aftershocks originated from the source of the main event or very dose to it. Possible stress sources of importance in this area include the continental scale ridge-push force, regional stresses originating from postglacial uplift and local effects due to topography and crustal inhomogenieties.
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