Naissaar quake reveals seismic activity in buried rapakivi

A magnitude 0.5 earthquake recorded early Monday near Naissaar in Tallinn Bay was a natural event caused by stresses in a buried rapakivi granite block, ERR reported.

The area around Naissaar is better known to Estonia’s Geological Survey and seismologists for artificial events. The Baltic Sea, particularly the Gulf of Finland, contains numerous wartime sea mines that the Estonian Navy regularly finds and disposes of, so vibrations in the region are initially often attributed to demining.

This event drew attention because it occurred at 5.21 a.m. on Monday, outside the military’s usual afternoon work schedule. The Navy confirmed to seismologists that it had carried out no explosions at sea at that time. Analysis of the data with colleagues in Helsinki eventually confirmed that the tremor was a natural earthquake.

Stress in the bedrock

The earthquake originated in the bedrock beneath Estonia’s sedimentary rocks. A massive granite block extends beneath the Naissaar region. Geologists call the rock rapakivi, following the terminology used by their Finnish colleagues. The igneous rock formed inside ancient volcanoes and tends to fracture easily under geomechanical stress.

A large rapakivi massif with similar properties, the Vyborg rapakivi massif, extends from southeastern Finland far across the Russian border. Southeastern Finland has therefore attracted close scientific attention because of its frequent seismic activity. The tremors there are generally weak and often occur in clusters in the upper layers of the Earth’s crust.

“We have not previously observed such events in Estonia, even though we have exactly the same rapakivi beneath our feet,” Heidi Soosalu, a seismologist at the Estonian Geological Survey, said.

Most of the world’s major earthquakes occur through a different process, mainly as a result of global plate tectonics. The Eurasian and North American plates move apart in the mid Atlantic near Iceland, but this movement can also build up stresses in Estonia’s crust. The accumulated energy is eventually released along existing and fractured fault zones, producing classic earthquakes such as the one recorded near Osmussaar in 1976.

In southeastern Finland and now near Naissaar, the Earth cracks in a different way. Seismic events form series rather than having one main shock. However, not all earthquakes in the Nordic region are harmless microtremors. In the Tornio and Kuusamo regions of Finland, the crust is much more seismically active, and tremors can cause significant damage even to buildings.