Sunday, March 20, 2016

Seismic Activity in Morocco's North Causes Concern

Over the last few months the cities of Nador and Al Hoceima have experienced repeated earthquakes and aftershocks. The effect on the local population has been, to put it mildly, unnerving

According to an earthquake tracking site Nador has experienced 2 earthquakes in the past 7 days, 11 earthquakes in the past month and a total of 25 earthquakes in the past year. Nador and Hoceima have witnessed numerous quakes since January, with the strongest measuring 6.3 on the Richter scale on Jan. 25, injuring 15 people. Following the quake there were complaints that Moroccan media were slow to release details.

A previous major earthquake, also a 6.3-magnitude one, struck Hoceima in 2004, killing 600 people and leaving thousands homeless.

The latest earthquake occurred last Wednesday (March 16th) at 4.27pm. The quake of 5.1 degrees on the Richter scale and triggered a wave of unrest, with residents calling for the implementation of emergency measures. An aftershock of magnitude 4.2 was felt the following day (Thursday). The epicentre was 56 kilometres (35 miles) from Al Hoceima.

Earlier quakes in the region

While the area is known to be seismically active, the regularity of the quakes is causing alarm and psychological distress to the extent that following Wednesday's quake many of the people of Nador and Al Hoceima took to the streets and experienced a sleepless night rather than stay in their homes.

Nador residents feel safer in the streets

According to the local newspaper Nador City, citizens took to the streets and left their houses and most of them did not return home all night. Local authorities have counted twenty-six cases of fainting in schools in the province of Al Hoceima. The students were transferred to the nearest health centres and left after receiving the necessary assistance.

Damage in Melilla following January quake

Perhaps the strangest reaction to the earthquakes came from Yahya El Mdaghri, a Moroccan imam, who attributed the January series of earthquakes in Nador and Al Hoceima, to divine wrath! El Mdaghri, the imam of the Hamza Mosque in Salé, said during traditional Friday prayer that God knows what He is doing, implying that the earthquakes were the result of God’s punishment for the region. The imam said that if the earthquakes happened in such regions, which he described as “mired in drug trafficking”, then the people deserved the disasters.

According to the Le360 news website the imam, instead of showing support for the victims of the earthquake, showed no compassion and allegedly blamed Moroccans’ behaviour for the natural disaster.

Residents of northern Morocco were outraged by the imam’s words and have described him as “ISIS-like”.

A little seismic history tells another story

Back on November 1st, 1755, a massive quake struck Portugal, Spain and northern Morocco in what came to be known as the Great Lisbon Earthquake. With an estimated magnitude of 8.5 to 9.0, the earthquake nearly destroyed the city of Lisbon. What wasn't destroyed by the quake was demolished in the ensuing tsunami and fires that raged for days. Altogether, at least 40,000 people were killed.

More than 250 years later, geologists are still piecing together the tectonic story behind that powerful earthquake. A unique subduction zone beneath Gibraltar, the southernmost tip of the Iberian Peninsula, now seems to be culprit. Subduction zones are the spots where one of Earth's tectonic plates dives beneath another, often producing some of the world's strongest earthquakes.

Marc-Andre Gutscher, a geologist at the University of Brest in France, says, "If subduction occurred, and is still occurring here, then it's highly relevant to understanding the region's seismic hazards."

Gutscher's work has shown that sunken ocean lithosphere — a layer that comprises Earth's crust and upper mantle — lies beneath Gibraltar, and that it's still attached to the northern part of the African Plate. Other teams have found crumpled ocean crust and active mud volcanoes in the Gulf of Cadiz, where water within the buried lithosphere mixes with sediments and boils up to the surface.

Altogether, these lines of evidence make a pretty convincing case for subduction, Gutscher said.

The world's tectonic plates

But unlike the textbook examples of huge subduction zones found at the Mariana Trench or under Alaska's Aleutian Islands, this subduction zone is comparatively tiny.

"Its very small size and ultra-slow motion make the Gibraltar subduction zone unique," Gutscher says. "It's probably the narrowest subduction zone in the world — about 200 kilometres [120 miles] wide at most — and it's moving at far less than a centimeter per year."

What's happening under Gibraltar is an example of something called rollback subduction: As the sliver of lithosphere sinks into the mantle, the line where it's still "hinged" to the African Plate rolls back further and further, stretching the crust above it.

If subduction under Gibraltar is a thing of the past, there's little danger of future earthquakes. But that's not true if it is still happening — as Gutscher and many others believe to be the case.

That's because subduction has already created a tiny tectonic block, or microplate, between the African and Eurasian Plates. Researchers using GPS have shown that this microplate is still moving a few millimeters westward every year, thanks to ongoing rollback subduction.

The boundaries of this microplate lie in southern Spain and northern Morocco. Like California's San Andreas Fault, they're strike-slip boundaries (but smaller and slower-moving), so they're capable of generating earthquakes every now and then, Gutscher says.

The Great Lisbon Earthquake

But as far as another Great Lisbon Earthquake, residents of this region can breathe easy — at least for another millennium or so.

"Given the very slow motion of the faults in the area, you need many centuries to build up enough slip to generate such a great earthquake," Gutscher explained. "A magnitude-8.5 or -9 earthquake is probably pretty much out of the question, since the last such tremendous event was only 250 years ago."

While the science may be correct, it is little comfort to the citizens of Nador and El Hociema, who are living on shaky ground.

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1 comment:

James T said...

Thanks, Sandy. Hope you all sleep sound at night in Fez.
James (& Samina) in Islamabad.