Seismisyyttä Viipurin rapakivialueella

We discuss earthquake data available for the Vyborg rapakivi granite batholith in southeastern Finland. A specific feature of rapakivi seismicity is prolonged earthquake sequences, or swarms, but single earthquakes occur as well. Pre-instrumental earthquake sequences are known from the municipality of Ruotsinpyhtää in 1751, Miehikkälä in 1899-1900 and Lapinjärvi in the early 1950s. They are taken to be of seismic origin because of similarity to instrumental occurrences. The Lapinjärvi sequence was the longest and strongest so far, causing some non-structural damage to masonry structures. A dense network of instruments was deployed in the batholith after an earthquake sequence began close to Kuusaanlampi on the 1st of December 2011, resulting in hundreds of micro-earthquakes registered in the area. Other recent swarms occurred in Viuhkola in 2014, in the vicinity of Vyborg in 2014-2016 and in Miehikkälä and Koria in 2021. Investigations of instrumental data have shown that seismicity is confined to the uppermost 1-3 km of the crust. The shallow focal depths explain why the events are widely observed by local residents despite the small magnitudes, typically far below local magnitude ML3.