Earthquake primer: What happened in Chile and Haiti?
Next time your house rattles when a big truck drives by, you might want to look outside to see if there’s really a truck out there.
Bridgewater State College Professor of Geophysics Robert Cicerone said New England experiences about half a dozen earthquakes a year in the range of a magnitude 2.0 to 3.0.
An earthquake of that size would feel roughly like a big truck rumbling by, and you might not even realize an earthquake had occurred, Cicerone said.
In comparison, the recent devastating earthquake in Haiti was a 7.0, and the quake in Chile was a whopping 8.8.
That’s not two or three times more powerful than the little tremors we experience in these parts. That’s millions of times more powerful.
That’s because the scale that measures the magnitude of earthquakes doesn’t increase arithmetically, in which case an 8.8 would be roughly 25 percent more powerful than a 7.0. The earthquake scale increases exponentially, with the seismic energy released increasing 32-fold as you move just one unit up the scale, Cicerone said.
He estimated the earthquake in Chile was roughly 500 times more powerful than the quake in Haiti.
But the Haiti quake caused more damage.
Based on official reports, the death toll in Haiti was roughly 1,000 times the death toll in Chile, with some 230,000 lost lives in Haiti as compared to 279 in Chile.
Boston College Professor of Geophysics John Ebel said several factors explain the seeming discrepancy.
First, the Haiti earthquake on Jan. 12 hit closer to a population center than the Chile earthquake of Feb. 27.
Second, the Haiti earthquake occurred closer to the surface of the earth and, therefore, did more surface damage, said Ebel, the director of the Weston Observatory at Boston College, which is a seismological observatory.
“We don’t look up, we listen down,” Ebel said.
Also, he said, the buildings in Chile, which has experienced many earthquakes, including the largest ever recorded, were designed to withstand seismic disturbances. Construction in Haiti, in contrast, was based on the necessities of poverty and in many cases was the opposite of what would be recommended to withstand an earthquake.
Wood and steel are strong. They tend to bend instead of breaking, but those are uncommon building materials in Haiti. Most of the trees have already been cut down in Haiti to make charcoal, and steel is expensive.
As a result, most of the structures in the impoverished Caribbean nation are made of brick, cinderblock and unreinforced concrete, which are cheap but weak, Ebel said.
A geographer's perspective
Bridgewater State College Professor of Geography Vernon Domingo said, “For geographers, there is no such thing as a natural disaster.
“There could be a physical phenomenon, but the way it affects vulnerable populations depends on the economic system,” Domingo said.
Chile is rich with abundant natural resources such as copper. Haiti also once enjoyed a wealth of resources, but the two countries developed along very different trajectories.
“Haiti has low levels of economic development because of poor governance, geo-politics and a history of slavery that lasted so long and crippled the country,” Domingo said.
The concentration of wealth in the hands of a very few that occurred in Haiti never happened in Chile, Domingo said.
“Chile is much more middle class than many of its neighbors. With all the hiccups, it has had much more of a democratic system and a spreading around of wealth than many other countries,” Domingo said.
The importance of these cultural differences was laid bare by the recent earthquakes.
Many of the natural aspects of the two earthquakes are different, such as the magnitude and location, but even if Chile had experienced a 7.0 quake in a population center, such as Santiago, Domingo doubts the death toll would have approached the 200,000 plus that died in Haiti.
“From a geographer’s perspective, a disaster does not affect all populations equally,” Domingo said.
Not all earthquakes are created equal
To put the earthquake scale into perspective, Ebel said damage starts around magnitude 5.0.
A 5.0 would be unlikely to knock down any buildings, but you wouldn’t be likely to mistake it for anything other than an earthquake. You might see things like cracks in brick walls and plaster, and objects knocked off shelves.
Instead of thinking to yourself, “It sounds like a truck is driving by,” you might think, “Gee, it sounds like a truck is driving into a building,” Ebel said.
The larger the magnitude of an earthquake, the larger the area it will affect and the longer the shaking will last, Ebel said.
Ebel said in New England, we average a magnitude 5.0 every 60 years or so.
The last one was in the Ossipee Mountains of New Hampshire in 1940.
“So, do the math and you’ll say we’re overdue. The problem is averages are not predictive,” Ebel said.
An average of 60 years could result from two in a row and then none for 120 years, or one every 60 years.
Ebel said the epicenter of an earthquake is not necessarily where it is most powerful. The epicenter is just the point where it started, like the point where a rock hits a windshield, which then experiences widespread cracking. Proximity to the fault, not the epicenter, determines the amount of damage an area will sustain.
New England averages a 6.0 earthquake probably about every 450 to 500 years, Ebel said.
At 4:30 a.m. on Nov. 18, 1755, Massachusetts experienced a sizable earthquake that seismologists, including Ebel, believe was centered off Cape Ann, 30 to 40 miles from downtown Boston. Ebel estimates the magnitude of the quake, the greatest ever recorded on the East Coast of the United States, at 6.0 to 6.25.
“It damaged many brick walls and about a third of the chimneys. It was felt up to Nova Scotia and down to South Carolina,” Ebel said.
A famous woodcut depicting the upheaval shows Bostonians in the streets, apparently roused from their beds by the shaking, reaching their arms up to the starry skies against a backdrop of crooked steeples and uprooted buildings.
Even though that’s the most powerful earthquake we know about in New England, Ebel wouldn’t rule out the region has experienced a 7.0 and could again. But, he said, most seismologists would say the region is not susceptible to earthquakes as large as an 8.0.
Seismologists have not found evidence of even a 7.0 in this area, but a 7.0 might be hard to detect. An 8.0, however, is so big, it would have left a smoking gun, in the form of seismological evidence that would have been uncovered, Ebel said.
“It’s not like a cut on your hand. If rock cracks, it does not heal,” Ebel said.
Ebel said Canada is a different story.
In 1663, an active seismic zone near Quebec City experienced an earthquake Ebel thinks was greater than a 7.0, or larger than the recent Haiti quake. That earthquake damaged chimneys as far away as Roxbury, Ebel said.
In a neat bit of seismological detective work, Ebel determined a Roxbury minister had noted the event in his diary, based on a 19th-century report Ebel found.
California is deservedly notorious for its earthquake susceptibility, Ebel said.
“If I recorded earthquakes in California for one year and then came to New England, I’d have to record for 100 years to get the same amount of data,” Ebel said.
The areas around the San Andreas Fault have experienced earthquakes up to around 8.0 in magnitude, he said. An 1857 earthquake in Los Angeles registered a 7.8, as did an earthquake in San Francisco in 1906.
Ebel said a 6.0 happens in California every two to three years, a 7.0 every 10 to 20 years and an 8.0 every 100 years.
Once again, California would seem overdue for an 8.0, but, once again, averages are not predictive, he said.
We don’t have evidence in most of California of earthquakes greater than 8.0, but Oregon and Washington experienced a 9.0 in 1700.
That quake, stretching from the northern tip of California to British Colombia, was the most powerful ever known to have occurred in the continental United States, according to the United States Geological Survey.
The greatest magnitude earthquakes ever recorded
1. 1960 Chile 9.5
2. 1964 Alaska 9.2
3. 2004 Indonesia 9.1 (triggered catastrophic tsunami)
4. 1952 Kamchatka 9.0; 1968 Chile 9.0; 1700 Cascadia 9.0
5. 2010 Chile 8.8; 1906 Ecuador 8.8
- U.S. Geological Survey