Tuesday, 1 July 2008

Tremor warning

Image is a simplified structural map of Lebanon showing major faultlines.

"Health officials in southern Lebanon and northern Israel have been warned to prepare for mass casualties from a major earthquake expected to hit the region", reports Arabian Business magazine.

"There have been around 500 minor tremors in a three-month period from February to May, which have been increasing in frequency and intensity.

"Mo’een Hamzeh, head of the Bhannes Centre for Seismic and Scientific Research, said on Monday that Lebanon could face an earthquake similar to the one that hit the nation in 1956."

According to the American University of Beirut :

Lebanon is cut by faults of every scale. The longest fault in Lebanon is the Yammouneh Fault that runs along the western margin of the Bekaa and links the major fault of the Jordan Valley to the Ghab Valley Fault of Northern Syria. This is a lateral, or strike slip fault and is the Lebanese segment of the Dead Sea Transform Fault.

It originated around 12 to 10 million years ago as the boundary between the Arabia Plate and the Levantine part of the African Plate and has ben moving since. The result of this is that the Bekaa has moved some 50 km northwards with respect to Mount Lebanon.

The evidence suggests that the Yammouneh Fault has not moved for many thousands of years; and whether it is dead or dormant is not clear. We would dearly love to know will move again.

Like many large faults the Yammouneh Fault is not actually very impressive on the ground and is often only marked by a wide breccia zone.

The Roum Fault, which runs from near Marjayoun towards Beirut is probably where most of the plate tectonic motion is going on now and may be the present plate boundary between the Arabian and the African Plate.

One model is that the plate motion has fairly recently (in geological terms) switched from the Yammouneh to the Roum Fault. The last recent earthquakes in Lebanon have been along this fault including the Chhim earthquake of 1956 that caused many deaths and much damage.

One slightly worrying point is that the Roum Fault seems to be on line for Beirut. If it does have an active fault segment near (or even under) the capital then that must raise the earthquake risk. Earthquakes are discussed below in Section 2.7.

There are other major faults particularly in the Anti-Lebanon. The main highway to Damascus shows a good deal of faulting in the road cuts as it passes through this area. The Serghaya Fault in particular is apparently another major strike slip fault.

There many other faults in Lebanon with displacements ranging from a few centimetres to several kilometres. Working out which are major faults, and which are minor, is not easy.

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