Rapakivi Seismicity - Shallow Swarm-type Intraplate Earthquakes in Southern Finland on the Seismically Quiescent Fennoscandian Shield

The Vyborg rapakivi granite batholith (VRGB) in Southeastern Finland has an elevated level of seismicity in comparison to the contiguous areas. The specific features of rapakivi seismicity include shallow focal depths and prolonged earthquake sequences, or swarms; however, single earthquakes occur as well. Earthquakes have been small, typically below local magnitude ML3. Seismicity is confined to the uppermost 1-3 km of the crust. Despite the small magnitudes, the earthquakes are typically observed by local residents due to the shallow origin and properties of bedrock, the absence of sedimentary layers, and the low attenuation of seismic signals in the crystalline bedrock. Whether the earthquakes had the potential to evolve into higher-magnitude events is an essential question for the analysis of the seismic hazard of the Loviisa nuclear power plant, situated in the VRGB. Also other rapakivi areas in the country, especially the Åland and Laitila batholiths, have hosted earthquakes of similar type.

Developments of the Finnish National Seismic network as well as improved detection methods have made it possible to detect very low-magnitude rapakivi earthquakes during the last two decades. Pre-instrumental earthquake sequences in the VRGB are known from the municipality of Ruotsinpyhtää in 1751, Miehikkälä in 1899-1900, and Lapinjärvi in the early 1950s. The Lapinjärvi sequence was the longest and strongest so far, causing some non-structural damage to masonry structures.

An earthquake sequence began close to Kuusaanlampi, in the city of Kouvola, on the 1st of December 2011, resulting in hundreds of micro-earthquakes registered in the area. A dense network of instruments has been deployed in Kouvola since 2011, and in 2020 the area was covered with 50 instruments. In addition to several single earthquakes, the network has recorded swarms also elsewhere. In June 2021, two earthquakes were detected in the Åland rapakivi batholith that covers most of the archipelago between Finland and Sweden. A dense research seismic network has also been installed to the archipelago which previously has had a lower detection capability than mainland Finland. In this presentation, we focus on the analysis of the Kuusaanlampi earthquake sequences.