We've now seen the destruction after the earthquake in Chile, and the utter devastation after the earthquake in Haiti. It is true, says a NASA geophysicist, that a major earthquake can shift the earth's axis by a couple of inches. The earthquake that struck Chile was so powerful it shifted the planet's axis enough to make it spin slightly faster, meaning our days will be shorter by 1.26 millionths of a second, according to preliminary calculations.Scientists have long noted that just about any event that shifts a large amount of mass from one part of the planet to another will have a tiny — and sometimes measurable — effect on the Earth's rotation. Such events include changes to ocean currents, big shifts in the atmosphere, earthquakes, and possibly even the creation of more and more reservoirs from the damming of rivers.
The earthquake that struck Chile was so powerful
it shifted the planet's axis enough to make
it spin slightly faster, meaning our days will
be shorter by 1.26 millionths of a second
The 8.8-magnitude temblor that struck Chile on Saturday is one of the largest quakes in a century. It was the result of an ocean tectonic plate—a shard of the earth's crust—sliding under the South American plate; over time, the two became locked together. When the pent-up energy overcame the forces of friction and the South American plate sprang upward, it unleashed a huge amount of energy in the form of the quake.
The planet was jolted to its roots. A chunk of the Earth's mass was redistributed vertically, which caused the planet's figure axis - on which the Earth's mass is balanced - to move by about three inches, or to be more technical - 2.7 milliseconds of arc, or about eight centimetres, according to calculations. The net effect of that mass redistribution made the earth spin slightly faster, just as a figure skater speeds up when she pulls in her arms.
It's also important to point out that the 8.8-magnitude Chilean earthquake released so much energy that it may have slightly shortened the length of the Earth's day, says Richard Gross, a researcher at the Jet Propulsion Laboratory. He computed how the Earth's rotation may have been affected by the Feb. 27 quake, which has killed at least 723 people.
The JPL computer model suggests that the length of the Earth day may have been shortened by millionths of a second., as reported above. The change in the length of the day came as a result of the shift in the Earth's axis that occurred because of the quake.
Dr. Gross said that though the Chilean quake was less powerful than the 2004 Indonesia temblor, it likely changed the position of the figure axis a bit more. (The planet's figure axis is separate from the north-south axis; they are about 33 feet apart.)
The vertical redistribution of mass caused by the Chilean quake had a slightly greater impact in shifting the figure axis because the quake happened near the mid-latitudes, NASA concludes. Plus, the fault in Chile dips into Earth at a slightly steeper angle, which again has a greater effect on shifting the figure axis.
The Haiti quake, by contrast, didn't make the Earth wobble in quite the same way, and it was caused by one tectonic plate sliding past another, not above or under it. There was almost no vertical movement there when the quake happened, so you wouldn't expect the same effect.
Not long after the 1989 Loma Prieta earthquake happened, California tried to get ahead of the game. Cuts would be made in the beams supporting the bridge, just beneath the roadway. Sections of steel would be pulled out, and replaced with "isolation bearings" - flexible joints that sometimes look like gargantuan shock absorbers.It was an ambitious, expensive plan, which quickly got caught in bureaucracy. Before much of the work was done, California decided it needed to replace much of the aging bridge anyhow.
Swaying, Rather Than Collapsing
But San Francisco's city hall is now on 590 isolators, installed in the hope that if the ground vibrates, the building will more gently sway back and forth above it. The building won't escape damage if The Big One hits, but engineers say it is far less likely to collapse.
These are examples of how structures in earthquake-prone areas can be designed to survive a disaster - the kind that happened Saturday in Chile.
The Chilean government got serious about earthquake protection after a disaster in 1985. It imposed some of the kinds of building codes California has. Engineers say there are some basic ideas that can help protect buildings and the people in them:
- It is better for a house to bend than to break. Wooden houses, for instance, tend to fare better than brick.
- Tall buildings can be buttressed with diagonal beams, so that they hold together. Poured concrete can be reinforced with steel rods.
- Taller buildings do best if they are firmly anchored to their foundations. Shorter buildings may do better on isolation bearings.
Chile's major cities appear to have fared far better than Port-au-Prince did after the earthquake in Haiti on Jan. 12. As seismologists told us afterward, Haiti's cinderblock buildings - common in an impoverished country - were the worst kind of construction for an earthquake zone. In many cases they turned to dust.
"Buildings - designed to withstand gravity - have to be built to withstand lateral motion," said Art Lerner-Lam of Columbia University's Lamont-Doherty Earth Observatory.
Which is where those isolators come in. They're expensive - unless a major earthquake hits. The quake in Chile is one of the very biggest magnitude quakes you can have - if you can survive that, you can survive anything.
Chile's major cities appear to have
fared far better than Port-au-Prince
did after the earthquake in Haiti on Jan. 12.
Northwest At Risk Of Megaquake Like One In Chile
Just 50 miles off the Pacific Northwest coast is an earthquake hotspot that threatens to unleash on Seattle, Portland and Vancouver the kind of damage that has shattered Chile.
The fault has been dormant for more than 300 years, but when it awakens -- tomorrow or decades from now - the consequences could be devastating.
Recent computer simulations of a hypothetical magnitude-9 quake found that shaking could last 2 to 5 minutes -- strong enough to potentially cause poorly constructed buildings from British Columbia to Northern California to collapse and severely damage highways and bridges.
Such a quake would also send powerful tsunami waves rushing to shore in minutes. While big cities such as Portland and Seattle would be protected from severe flooding, low-lying seaside communities may not be as lucky.
The Pacific Northwest "has a long geological history of doing exactly what happened in Chile," said Brian Atwater, a geologist with the U.S. Geological Survey and University of Washington. "It's not a matter of if but when the next one will happen."
The last one hit in 1700, a magnitude-9 that sent 30- to 40-foot-tall tsunami waves crashing onto the coast and racing across the Pacific, damaging Japanese coastal villages.
There's an 80 percent chance the southern end of the fault off southern Oregon and Northern California would break in the next 50 years and produce a megaquake, according to Chris Goldfinger, who heads the Active Tectonics and Seafloor Mapping Laboratory at Oregon State University.
Research presented last year at a seismology conference found that Seattle high-rises built before 1994, when stricter building codes took effect, were at high risk of collapse during a superquake.
Disaster managers in Oregon and Washington are aware of the risks, and work is ongoing to shore up schools, hospitals and other buildings to withstand a seismic jolt.
"We're definitely being proactive in trying to get those fixed, but we have a long way to go," said Yumei Wang, geohazards team leader with the Oregon Department of Geology and Mineral Industries.
Oregon has 1,300 schools and public safety buildings that are at high risk of collapse during a major quake. The state recently doled out $15 million to two dozen schools and emergency facilities to start the retrofit process. State law requires that all poorly built public safety buildings be upgraded by 2022 and public schools by 2032.
The state is also helping its coastal communities - home to 100,000 residents - plan for vertical evacuation buildings that could withstand giant tsunami waves.
Seattle plans to retrofit its 34 fire stations. The city is also working on a plan to upgrade 600 buildings considered most at risk.
"We have been preparing aggressively," said Barb Graff, who heads the city's Office of Emergency Management.
Chile and the Pacific Northwest are part of several seismic hotspots around the globe where plates of the Earth's crust grind and dive. These so-called subduction zones give rise to mountain ranges, ocean trenches and volcanic arcs, but also spawn the largest quakes on the planet.
The magnitude-8.8 Chile quake occurred in an offshore region that was under increased stress caused by a 1960 magnitude-9.5 quake - the largest recorded in history, according to geologist Jian Lin of the Woods Hole Oceanographic Institution. The temblor destroyed or badly damaged 500,000 homes and killed more than 700 people.
There's an 80 percent chance the southern
end of the fault off southern Oregon and
Northern California would break in the
next 50 years and produce a megaquake.
Chris Goldfinger
Active Tectonics and Seafloor Mapping Laboratory at Oregon State University
Similar tectonic forces are at play off the Pacific Northwest, where the Juan de Fuca plate is diving beneath North America. At some point, centuries of pent-up stress in the Cascadia subduction zone will cause the plates to slip. Scientists cannot predict when a quake will occur, only that one will happen.
The region is all too familiar with violent earthquakes. In 2001, a 6.8-magnitude quake centered near Olympia, Wash., rattled a swath of the Pacific Northwest, but remarkably caused no deaths. While it was not the type of quake that hit Chile, it was a reminder of how a big disaster could strike at any time.
To better understand megaquakes, a group of scientists planned to travel to Chile in May for a conference on giant earthquakes and their tsunamis. There are field trips planned to commemorate the 50th anniversary of the 1960 Chile quake.
Chile and Haiti: A Comparison
The Chile earthquake was 500 times stronger than the massive quake which hit Haiti in January. Here's why the impact last Saturday was so much less devastating.
There is a tendency to compare disasters, and many of us started to do that Saturday morning when we heard about the 8.8-magnitude earthquake in Chile. Another earthquake! Is it like the one in Haiti?
In places such as Haiti, where 72.1% of the population live on less than $2 a day, and in cities like Port-au-Prince, where many are housed in poor and densely-packed shantytowns and badly-constructed buildings, the devastation is always expected to be greater. The Haiti quake is making a much more significant impact on the country than the others because so much of the population was living in or near Port-au-Prince and was so severely affected, and it will have a much larger effect on the country's economy.It is hard to make v alid comparisons between such tragedies. But earthquakes have killed more people than any other disaster over the last 10 years, according to the Center for Research on Epidemiology of Disasters (CRED) in Belgium, as explained in an article on the IRIN Global news service. And an increasing proportion of those affected by earthquakes are in developing countries. The data and lessons learned from these comparisons to focus its propensity to increased vulnerability to disaster, might be worth looking into. In the end, the information it will also save lives.100% Chance of An Earthquake
According to the USGS - There's a 100 percent chance of an earthquake today. Though millions of persons may never experience an earthquake, they are very common occurrences on this planet. So today - somewhere - an earthquake will occur.
It may be so light that only sensitive instruments will perceive its motion; it may shake houses, rattle windows, and displace small objects; or it may be sufficiently strong to cause property damage, death, and injury.
It is estimated that about 700 shocks each year have this capability when centered in a populated area. But fortunately, most of these potentially destructive earthquakes center in unpopulated areas far from civilization.
Since a major portion of the world's earthquakes each year center around the rim of the Pacific Ocean (Ring of Fire), referred to by seismologists as the circum-Pacific belt, this is the most probable location for today's earthquake. But it could hit any location, because no region is entirely free of earthquakes.
Stating that an earthquake is going to occur today is not really "predicting earthquakes". To date, they cannot be predicted. But anyone, on any day, could make this statement and it would be true. This is because several million earthquakes occur annually; thereby, thousands occur each day, although most are too small to be located. The problem, however, is in pinpointing the area where a strong shock will center and when it will occur.
Earthquake prediction is a future possibility, though. Just as the Weather Bureau now predicts hurricanes, tornadoes, and other severe storms, the NEIC may one day issue forecasts on earthquakes. Earthquake research was stepped up after the Alaska shock in 1964. Today, research is being conducted by the USGS and other federal and state agencies, as well as universities and private institutions. Earthquake prediction may some day become a reality, but only after much more is learned about the earthquake mechanism.
On the Web:
For the kids - Earthquake Pathfinder
The New York Times - Two Quakes in Two Months: Comparing Haiti and Chile
The New York Times - Dot Earth: Chilean Quake is A Warning to US
USGS - Can It Happen Here?
USGS & FEMA -
- What to do before an earthquake (FEMA)
- What do to during an earthquake (FEMA)
- What to do after an earthquake (FEMA)
- Seven Steps to Earthquake Safety
- Preparedness FAQs
Posts
No comments:
Post a Comment