Why the constant earthquakes? Iceland is slowly being torn apart
In an average week Iceland's national monitoring seismic network detects around 500 earthquakes. If one of the volcanoes is hit by an earthquake swarm this number can be even higher.
Iceland is being "torn apart"
The reason for this seismic activity is the location of Iceland on top the Atlantic ridge, the divergent boundary between the Eurasian and North American tectonic plates: As the two plates drift in opposite directions Iceland is in effect slowly being split apart. The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimeters (1 inch) per year, or 25 kilometers (15.5 miles) in a million years.
The volcanic zones in Iceland are directly above the boundary of the tectonic plates. These extend east from Reykjanes peninsula in the S.W. where the Reykjanes ridge part of the North Atlantic ridge effectively rises above the sea level, and north-east from Mýrdalsjökull glacier in South Iceland, beneath the western part of Vatnajökull glacier, the Central Highlands, extending north into the Arctic.
When we compare a typical map of the seismic activity in Iceland and a map of the active volcanic zones it is immediately apparent the two overlap perfectly.
Most quakes are very small tremors
Most of the earthquakes hit without people noticing them. They don't affect daily life except when they might be a sign of an impending eruption, as happened this autumn when an earthquake swarm hit the huge Katla sub-glacial volcano, and police evacuated and closed access to nearby travel destinations.
At Þingvellir National Park, in South Iceland, you can however see clearly with your own eyes how these forces of nature are shaping Iceland's landscape. The cracks, faults and canyons which traverse the region are signs of the continental drift between the North American and Eurasian Plates. Pictured to the right is the largest one, Almannagjá canyon.
The location on the tip of the Reykjanes Peninsula is the only place in the world where you can see the Mid-Atlantic ridge rise above sea level. Close by you can walk over The Bridge Between Continents. Yes, it's a little bit of a gimmicky concept but this is indeed a location where the tectonic plates are drifting apart from each other. The bridge is located About 7km south of Hafnir by road 425.
Two types of quakes
Most of the earthquakes detected in Iceland take place in one of the active volcanic zones or along the South Iceland Seismic Zone. There are also two main important types of earthquakes in Iceland: Quakes created by volcanic activity and quakes caused by the release of tension caused by the movement of the tectonic plates. Other types include quakes caused by changes in geothermal activity.
Large quakes (3+ on the Richter scale) are most common in volcanoes. The two monster volcanoes Katla, hidden under Mýrdalsjökull glacier and Bárðarbunga, beneath Vatnajökull glacier, have been particularly active since 2010. Other volcanoes and volcanic systems have also trembled in recent years, including the Eldey system off the Reykjanes peninsula, mount Hengill south of Þingvellir National Park and mount Hekla in South Iceland.
Volcanic earthquakes tend to be smaller than earthquakes at the fault lines of continental shelves, like the San Andreas fault. The forces creating the two types of earthquakes are very different, as volcanic earthquakes are caused by magma being thrust from the mantle up into the crust.
While a 3 or 4 magnitude quake on the Richter scale might not be considered particularly powerful if it took place on a continental fault line, a 3+ quake in a volcano is considered a large quake and a sign of significant activity.
The South Iceland Seismic Zone
The other significant type of earthquakes we frequently experience in Iceland are quakes along the South Iceland Seismic Zone. The S. Iceland Seismic Zone is a transform fault between offset sections of the Mid-Atlantic Ridge which runs through Iceland. The zone is made up of a series of fracture faults which run from SW to NE. The zone extends from the volcano Hengill, the easternmost part of the Reykjanes volcanic zone to Hekla, which is the westernmost volcano in the East Volcanic Zone.
There are no active volcanoes in the South Iceland Seismic Zone, but it is extremely active. The area sits between the two volcanic zones, and is constantly being pulled in two different directions, causing tension to build up in the crust which is then periodically released in earthquakes.
Waiting for the "Big One", the massive South Iceland Quake
The zone has been the source of the most powerful earthquakes in Iceland, as the most powerful earthquake which has taken place in Iceland since the country was settled took place in the Southern Seismic Zone. In 1784 a massive earthquake, which is believed to have been 7.1 magnitude, shook all of southern Iceland, causing widespread damage to farmhouses.
The second largest earthquake, and the largest to be measured with modern equipment, was detected in 1912. This quake was 7.0 on the Richter scale. Both quakes took place close to Saturday's tremor. Quakes of this magnitude are believed to hit once every 100-150 years, and could cause significant damage. Locals in South Iceland are still waiting for the "big one".
Will we get a warning?
The Icelandic Met Office's automatic monitoring network has been in operation for two decades. Besides evaluating source function and mechanism information carried from below by micro-earthquakes, it provides near real-time information that is used as the basis for an alert system.
Scientists at the Icelandic Meteorological Office believe they can predict the onset of major quakes in the South Iceland Seismic Zone with considerable certainty. Scientists similarly believe they can anticipate volcanic eruptions. However, such predictions would only give us a one hour notice at most. Since volcanoes like Bárðarbunga and Katla are located in regions where cell-phone coverage is limited, authorities worry that evacuating people could prove difficult.
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