Geologists reported on Sunday that a swarm of small earthquakes that struck western Saudi Arabia last year was actually the rumbling of a volcano.
Over 30,000 minor quakes took place between April and June 2009 within an ancient solidified lava field called Harrat Lunayyir, which damaged some buildings near Al Ays and prompted the authorities to evacuate the 40,000 residents from the region.
Most of the quakes measured less than two on the scale of magnitude, but several were hefty, delivering a jolt of up to 5.4.
Geologists concluded by using satellite radar that the shockwaves’ seismic signature and depth all point to a cause that is volcanic.
They found that the ground ruptured along five miles and ripped open about one and a half feet as a tentacle of magma probed forward just beneath the surface.
However, the experts say that the hazard is low, given the remoteness of the site and the type of eruption they expect.
Saudi Arabia’s geology is known for the oil-drenched sedimentary rocks of the east that are the source of its bounty in hydrocarbons.
Volcanic eruptions in Saudi Arabia are rare and only occur every few hundred years.
According to contemporary accounts, the best known volcanic event in the region occurred in 1256, which sent flows of lava “like a red-blue boiling river” for 52 days into the holy city of Medina.
The lead researcher John Pallister of the Volcano Disaster Assistance Program at the U.S. Geological Survey (USGS) told AFP that on a geological scale, volcanism in Saudi Arabia is contemporary.
“Several of the lava fields have ‘young-looking’ features (to a geologist) and even have deposits that overlie Neolithic [Stone Age] sites,” Pallister said in an email exchange with AFP.
Last year’s event occurred about 120 miles from the main area of geological spreading, which is happening beneath the Red Sea.
Pallister told AFP that despite this distance, the intrusion of magma points to an “increased chance” of eruptions “in the next several decades.”
However, he cautioned against fear.
“An eruption, at Lunayyir, if it were to occur, would pose little hazard due to the type of volcanism expected at the site and the remoteness of the vent areas,” he told AFP.
“There (is) a low probability of large damaging earthquakes related to this type of activity.”
“However, urban development is encroaching on other areas in Saudi Arabia where an eruption would be more serious.”
He talked positively about the Saudi Geological Survey for its rapid response.
“They quickly recognized the hazard and deployed a first-class seismic monitoring network and advised their government and citizens of the status of the unrest and the potential hazards,” Pallister told AFP.
“The Saudi Survey now has a very good network for seismic monitoring of Lunayyir. Consequently, I expect that they should be able to make a robust forecast in the case of renewed unrest.”
The study is published in the journal Nature Geoscience.
Image Caption: Lava flows radiate down desert valleys away from the center of Harrat Lunayyir, a basaltic volcanic field in NW Saudi Arabia, east of the Red Sea port of Umm Lajj. Harrat Lunayyir contains about 50 volcanic cones that were constructed along a N-S axis. Harrat Lunayyir is one of the smallest of the Holocene lava fields of Saudi Arabia, but individual flow lobes extend up to about 30 km from the center of the Harrat. One of the cones may have erupted around the 10th century AD or earlier. NASA Space Shuttle image STS26-41-61, 1988
On the Net: