Last Friday, at 11:11 PM local time, the city of Marrakech in Morocco was hit by a 6.8-magnitude earthquake. Within a day, the fatality count exceeded 2,000 and continues to rise as this is written (Sunday morning). Photos from Marrakech and surrounding cities near the Atlas Mountains show cracked and toppled towers and piles of rubble, especially in small villages where enforcement of building codes is lax.
According to an Associated Press report of the disaster, Morocco lies on the intersection of the African and Eurasian tectonic plates. Earthquakes tend to cluster where plates collide, but compared to some plate-intersection regions, strong earthquakes in northwest Africa are relatively rare. When they occur, though, they tend to cause massive destruction. In 1960, a Moroccan earthquake of magnitude 5.8 caused thousands of deaths as well.
As earthquakes go, a magnitude 5.8 event is not the worst possible. The world's record quake in terms of magnitude was a 9.5-magnitude event along a long fault in Chile in 1960. Bearing in mind that quake magnitudes are logarithmic—a 6.0-magnitude quake has ten times the amplitude of movement of a 5.0-magnitude one—a magnitude-5.8 quake in the U. S. might not even cause any fatalities, depending on how densely populated the region was and the nature of the buildings involved. For example, in 1973 the 5.8-magnitude Point Mugu earthquake in Ventura County, California caused several injuries and over $1 million in damage, but no one died as a result.
Just quoting the magnitude of an earthquake doesn't tell you everything about how it will affect a given building. For that, you need something called the modified Mercatili intensity scale. This scale runs from I (not felt) to XII (third stage of extreme), and why Mercatili chose to use Roman numerals is not known (unless he felt a patriotic need as an Italian to use Roman numerals). For a given magnitude of quake, there is a maximum Mercatili number it can cause on the surface, because obviously the farther away you are from the epicenter, the less shaking you will experience. According to Wikipedia, a magnitude-6.8 quake can cause a Mercatili shaking of up to IX (nine, for those unfamiliar with Roman numerals). This can cause damage even in wood-frame structures, which are much more tolerant of earthquakes than unreinforced masonry and rock buildings. But typically, such a quake causes only shaking to Mercatili number VII (seven), which according to geophysicist Kelly Martin can be withstood by most small U. S. residential buildings, although with some minor damage but no fatalities.
And that is one of the key reasons why Morocco suffered so much damage in last week's magnitude-6.8 quake. By its nature, masonry not reinforced with steel reinforcement bars ("rebar"), which is a very traditional form of building, is poorly adapted to resist earthquakes. If the wall or building is basically one heavy block put on top of another with only gravity to hold them down, the tensile stress that occurs when the bottom layer of masonry moves under the huge inertia of the masonry above makes the joints between the blocks fail. It's not much different than piling up a pile of children's blocks on a towel, and then pulling the towel. It doesn't take much of a pull to make the whole pile come apart and fall down.
That being the case, why don't countries like Morocco enforce building codes so that most structures can withstand a Mercatili shaking of VII or more? The authorities in Morocco are in the best position to answer that question, but I can speculate.
One factor is money, or more specifically, per-capita GDP. Making a masonry building resistant to earthquakes is not a simple undertaking. It requires expertise, skilled craftsmen who know how to translate the engineering requirements into reality, and money to pay for extra things like braces, rebar, and other features that will not add anything to the perceived value of the structure unless and until an earthquake occurs.
For numerous reasons, these factors all militate against the likelihood of your average citizen moving into an earthquake-resistant house in a place like Morocco. If the citizen has a certain amount of money, and one contractor says he can build a 1,500-square-foot place for that amount, and a second contractor says he can build an earthquake-resistant building for the same amount, only it will be 1,000 square feet, well, most people are going to go for the bigger building. Much has been made about fatalism in Muslim countries, which is allegedly the habit of attributing disaster to God's will. By my judgment, there is at least as much fatalism in the U. S. concerning disasters, but what we have here is building codes that are actually enforced, plus a tendency not to make houses out of pure masonry, brick, or stone.
Asking Moroccans to build wooden houses would be like asking to go waterskiing in Death Valley: the requisite materials are simply not available. So why aren't building codes for masonry buildings enforced to make them more resistant to earthquakes?
One big problem in very old countries like Morocco is that a lot of the buildings are decades or centuries old. One famous structure that was damaged in last week's earthquake, Marrakesh's Koutoubia Mosque with a 226-foot-high tower, was built in the 12th century A. D. It reportedly suffered damage, but the extent is not known and the tower apparently did not fall down. Even U. S. building codes typically do not require retrofitting of existing structures until they are sold or extensively modified. If an old structure of importance is judged to be an earthquake hazard in the U. S., more typically it is just torn down and replaced. But you don't just tear down a historic mosque.
Our sympathies are with the survivors of the Moroccan earthquake, who will no doubt plea for the government and King Mohammed VI to do something to prevent such fatalities next time. But as next time may not be for a long time, it's quite possible that any efforts to improve building codes, or to remedy hazards in existing buildings, may be forgotten before the next major quake comes along.
Sources: I referred to an Associated Press article by Sam Metz and Mosa'ab Elshamy that was carried in the Los Angeles Times at https://www.latimes.com/world-nation/story/2023-09-09/morocco-earthquake-kills-600-damages-historic-buildings. I also referred to articles at https://www.quora.com/What-magnitude-earthquake-can-an-average-building-withstand,
https://www.usgs.gov/faqs/can-megaquakes-really-happen-magnitude-10-or-larger, and the Wikipedia articles on specific earthquakes mentioned and the modified Mercatili intensity scale.
No comments:
Post a Comment