Stord earthquakes December 2010 (04.03.2011)

Seismograms of the 14 earthquakes detected Stord on the station KMY. The seismograms are aligned on the P arrival time.

Locations of Stord earthquakes in December 2010. Red cirlcles give bulletin locations, while blue circles give the locations based in cross-correlation measurement and double difference location.

A total of 14 earthquakes were detected in the Stord area between 20 December 2010, 00:43 and 21 December 2010, 04:33. The two largest of these with magnitude 3.0 (00:43) and 3.3 (12:30), respectively, were felt. Comparing the seismograms reveals that all the events are very similar in terms of P and S wave arrivals, but also the amplitude ratios between P and S (Figure 1). This already means that the earthquakes had a similar mechanism and were located close to each other. This is similar to the Stord-Bømlo earthquakes in 2000 (Hicks and Ottemöller, 2002). The largest earthquake in 2000 had a magnitude of ML=4.5 and was followed by 35 locatable aftershocks over 72 hours. The earthquakes in 2000 were located between the islands of Stord and Bømlo, and possibly attributed to the Hardangerfjord Shear Zone.

While the waveform similarity suggests a small source volume, standard manual locations are distributed over several kilometres. However, by using precise relative arrival times between events based on cross-correlation measurements, and using the double-difference location technique (Waldhauser, 2000), it can be confirmed that the spatial extent of this earthquake cluster is less than 400 metres in diameter. The result in comparison to the standard locations is shown in Figure 2. The 2010 earthquakes are located about 20 km north of the cluster in 2000.

References:

Hicks, E.C. and L. Ottemöller, The M_L Stord/Bømlo, southwestern Norway, earthquake of August 12, 2000, Norsk Geologisk Tidsskrift (Norwegian Journal of Geology), 81, 293-304, 2002.

Waldhauser F. and W.L. Ellsworth, A double-difference earthquake location algorithm: Method and application to the northern Hayward fault, Bull. Seism. Soc. Am., 90, 1353-1368, 2000.