Shallow swarm-type earthquakes in south-eastern Finland

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The Vyborg rapakivi batholith comprises an area of more than 20 000 km3 in south-eastern Finland and Russian Karelia. The 1.65-1.63 Ga old batholith intrudes Paleoproterozoic bedrock that was formed in the Svecofennian orogeny between 1.9 and 1.8 Ga. The batholiths area is known for shallow swarm type earthquake activity. The latest swarm has been occurring since December 1, 2011 in and around the town of Kouvola. More than 160 earthquakes, with magnitudes ranging from ML 0.4-2.8, have been recorded so far by the Finnish permanent seismic network. The swarm has been widely felt and reports on explosion- or thunder-like sounds accompanied by shaking of the ground are common. The felt effects and a strong Rg phase recorded by the nearest stations, at epicentral distances of 60-70 km, constrain the earthquake foci within the uppermost few kilometres of the crust. The events are concentrated around a major sub-vertical fault and contact zone traversing the batholith in NE- SW direction.
Inspired by the ongoing activity, four off-line seismic stations were operating in the source area between December 19, 2011 and June 6, 2012. The central station was located close to the active fault segment, while the others were deployed within a radius of 9 km from it. The stations were equipped with three-component Trillium 120 PA broadband seismometers and the sampling rate was set to 250 Hz. The response to ground velocity was flat in the frequencyband of 0.01-100 Hz.
In this study, waveform data recorded by the off-line stations were used to improve the detection capability and hypocentre locations of the events. Focal depths were constrained by using synthetic waveform-modelling of depth sensitive phases, such as Rg and sP. The hypocentres were correlated with magnetic anomalies associated with tectonized lithological contacts within the batholiths. A composite fault plane solution was computed by using the three strongest events.
Preliminary results indicate that the swarm events are concentrated around a major sub-vertical fault and contact zone traversing the batholith in NE- SW direction. Injection of fluids into the zones of crustal weakness has been suggested as the governing process behind the shallow swarm-type earthquake activity. In the next phase, a high precision relative location method will be applied to further constrain the hypocenter distribution.
Original languageEnglish
Title of host publicationThe 43rd Nordic Seismology Seminar : Programme, Abstracts, List of Participants
Number of pages1
Publication date24 Oct 2012
Publication statusPublished - 24 Oct 2012
MoE publication typeB3 Article in conference proceedings
EventNordic Seismology Seminar - Tallinn, Estonia
Duration: 24 Oct 201226 Oct 2012
Conference number: 43

Fields of Science

  • 1171 Geosciences

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