1 December 2015

Seismic risk in eastern Mediterranean higher than previously estimated

Posted by llipuma

by Franz Ossing and Lauren Lipuma

This map shows the geological features of the eastern Mediterranean caused by subduction of the African plate under the Aegean microplate. Credit: Wikimedia Commons

This map shows the geological features of the Hellenic margin, caused by subduction of the African plate under the Aegean microplate.
Credit: Wikimedia Commons

The eastern Mediterranean is more seismically active than previously assumed, a new study finds. On a geological time scale, seismic activity around the island of Crete has generated large earthquakes in bursts, potentially increasing the future risk for earthquakes and tsunamis in the region, finds the new study accepted for publication in Geophysical Research Letters, a publication of the American Geophysical Union.

The Mediterranean basin consists of several tectonic plates, a result of the collision between the African and the Eurasian Plates. The way these plates interact makes the eastern Mediterranean particularly prone to earthquakes, yet scientists have been puzzled by the fact that the region has experienced only two known earthquakes with magnitudes larger than 8 in the past 4,000 years.

South of the island of Crete, the African Plate subducts under the Aegean microplate in an arc-shaped region known as the Hellenic margin. In the new study, researchers explored the earthquake history of this subduction system to gain a better understanding of the key processes that drive the generation of mega-earthquakes in the area and how often these events occur.

“We study the Hellenic subduction margin going back to about 50,000 years, which is about 10 times the time window of paleo-earthquake observations in the eastern Mediterranean that we had before,” said Vasiliki Mouslopoulou, a geoscientist at the GFZ German Research Centre for Geosciences in Potsdam, Germany, lead author of the study. “For the first time ever, we were able to chart the spatial and temporal pattern with which mega-earthquakes rupture the Hellenic margin.”

In this study, researchers integrated field studies with radiometric dating and numerical models for a section of the Hellenic margin that straddles the island of Crete. The scientists identified relics of shorelines along Crete, which are today elevated up to 23 meters (75 feet) above sea level. Each shoreline is thought to represent the sea level at the time of its formation. Each shoreline’s altitude relative to sea level reflects the total vertical movement it experienced due to earthquake motion, according to the study’s authors.

Scientists look for remnants of paleo shorelines on western Crete. Red and blue arrows indicate paleo shorelines formed during the last 2,000 years and are today elevated up to 8 meters (26 feet). Credit: Vasiliki Mouslopoulou, GFZ

Scientists look for remnants of paleo shorelines on western Crete. Red and blue arrows indicate paleo shorelines formed during the last 2,000 years and are today elevated up to 8 meters (26 feet).
Credit: Vasiliki Mouslopoulou, GFZ

By radiocarbon dating the fossilised marine fauna trapped in each paleo shoreline, the researchers found that during the last 50,000 years, both western and eastern Crete have been lifted uniformly about 100 meters (328 feet) above sea level due to at least 40 earthquakes of magnitudes greater than 8. The origin of these quakes was offshore, on three seismic faults that extend along the western and eastern sections of the Hellenic margin, according to the new study.

Typical marine fauna (Dendropoma) used to date the formation of the elevated paleo shorelines on Crete. Credit: Vasiliki Mouslopoulou, GFZ

Typical marine fauna (Dendropoma) used to date the formation of the elevated paleo shorelines on Crete.
Credit: Vasiliki Mouslopoulou, GFZ

The new study shows that the faults on the eastern segment of the margin are capable of producing large earthquakes, instead of slipping without producing any seismic shock, as was previously thought. This implies that the seismic hazard in the eastern Mediterranean is significantly higher than previously assumed, according to the study’s authors.

Yet another surprise was the strong clustering of earthquakes in the time period analyzed. “We also found that, in contrast to most subduction margins globally, great earthquakes in the Hellenic margin are strongly clustered in time,” Mouslopoulou said. “The data show that most of these paleo-earthquakes occurred over a time period of about 10,000 years, while there were long periods (up to 20,000 years) of relative seismic quiescence.”

For example, the new study finds that the fault beneath western Crete appears to have ruptured every 4,500 years when its activity is averaged over the last 50,000 years, as frequently as every 1,500 years between 5,000 and 20,000 years ago, and not at all between 0 and 3000 years BCE. The other two faults exhibited a similar pattern, according to the study.

The high variability in the occurrence of large earthquakes in this region makes calculating their recurrence a great challenge, according to the study’s authors. Therefore, while the Hellenic margin is currently experiencing a seismically quiet period, the study suggests that future great earthquakes can be expected beneath both eastern and western Crete, and precautions such as tsunami early warning systems and earthquake-resistant construction will have to consider this increased potential risk.

– Franz Ossing is the head of public relations at GFZ German Research Centre for Geosciences. Lauren Lipuma is a public information specialist at the American Geophysical Union. This post originally appeared as a GFZ press release