The waters off the coast of Tallinn and around Naissaar are well known to Estonian geologists and seismologists. As the Estonian Navy regularly discovers and safely detonates old wartime sea mines still littering the Gulf of Finland, seismic signals in the area are often assumed to be caused by humans.

Monday's tiny quake, however, was recorded just off of Naissaar at 5:21 a.m., well outside typical detonation hours.

After the Navy confirmed no explosions had taken place, Estonian seismologists analyzed the data with colleagues in Helsinki and concluded the event was natural.

According to the Estonian Geological Survey (EGT), the earthquake originated in rapakivi granite buried beneath Estonia's sedimentary rock. The ancient granite, formed deep inside long-extinct volcanoes, is known to fracture relatively easily when geological stresses build up.

Such granite can be found below Southeastern Finland, where the Wiborg rapakivi granite batholith extends east well beyond the border with Russia and small, shallow earthquakes occur relatively often.

Unlike Estonia's larger historic earthquakes, these events tend to happen in clusters and are caused by the granite itself fracturing under stress rather than by a sudden release of tectonic energy along a major fault.

"We haven't observed this type of event in Estonia before, even though the same rapakivi granite lies beneath our feet," said EGT seismologist Heidi Soosalu.

Million times weaker than record Osmussaar quake

Estonia's strongest recorded earthquake to date was a magnitude 4.5 event off the northwestern coast, near the island of Osmussaar, in 1976.

That quake, which could be felt in Estonia, Finland and even Sweden, resulted from tectonic stresses transmitted across Northern Europe as the Eurasian and North American plates continue to slowly move apart beneath the Atlantic Ocean.

"The Osmussaar earthquake released a full million times more energy than this Naissaar event," Soosalu said, adding that earthquakes that strong are extremely rare in the Baltic Sea region.

Monday's quake occurred only about 3 kilometers below the surface. While Estonia's modern network of seismic stations picked up the tremor, it was far too weak to be felt by humans, even directly above the epicenter.

In general, people begin noticing earthquakes only when magnitudes approach 1.

While not dangerous, scientists say events like the Naissaar quake are still valuable because they reveal hidden fracture zones beneath Estonia's thick sedimentary layers. Each precisely recorded tremor also helps improve maps of the country's bedrock.

"Unexpected bits of information like this may one day help us locate mineral resources or better understand how potentially hazardous geological processes work elsewhere," Soosalu added.

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