Waiting for the big one: two new studies show that when it comes to a major earthquake, California could be a time bomb.
In the last 15 years, two damaging tremors shook California: In 1989, the Loma Prieta earthquake south of San Francisco registered 7.1 on the Richter scale, a scientific measure of energy released during an earthquake. The toll: 63 dead and $6 billion in damage. In 1994, the Northridge quake in the San Fernando Valley shook L.A. with a magnitude 6.7, killing 58 and causing $20 billion worth of damage. Now two studies show the clock may be ticking for even more powerful quakes to pummel California in the future.
Geologists at the U.S. Geological Survey (USGS) and the University of Oregon have dug trenches to study and date layers of sand, gravel, and peat, in order to obtain a detailed earthquake "record" that stretches back 1,500 years. They found that San Andreas quakes with a magnitude 7.5 or higher have struck Southern California on average once in 105 years. The last Big Shaker occurred 146 years ago--which suggests that L.A. could soon experience another major hit. "That quake will occur within the lifetimes of many people living in California today," claims USGS research geologist Tom Fumal.
San Francisco isn't off the hook either. The most extensive USGS study to date estimates a 62 percent chance that a major quake will rattle the Bay Area within the next 30 years. "We can't say for certain when," Fumal says. "We just want people to be prepared."
What causes earthquakes in the first place? Earth's surface, a 5-to 100-kilometer (about 3-to 62-mile) deep layer of rock called the crust, resembles a broken eggshell: It consists of jagged pieces, or tectonic plates. These plates "float" on top of the mantle--a hot, soft layer of rock--on which the plates slowly move and collide against each other.
Faults like the San Andreas mark the boundaries between two plates moving in different directions. In this case, the oceanic crust of the Pacific Plate grinds northward past the continental crust that makes up North America. The two plates creep past each other at a snail's pace--about 50 millimeters per year, or as fast as your fingernails grow--but are locked along the fault zones. When the stress becomes too much, watch out: Earthquake!
Huge waves of energy spring from the point that breaks first, called the epicenter. The waves quickly radiate from the fault, violently vibrating and shaking surrounding rocks. Whether the ground shakes up and down, back and forth, or both, depends on the fault's motion. And the waves' amplitude and frequency (height and timing) depend on the type and location of the rocks or soil. Hard rocks like granite don't shake much; but soft soil or mud jiggles like jello. And anything built on top of shaking ground--from a highway to a skyscraper--is in for one rough ride.
While Earth's surface is etched with faults, few rival the size of the San Andreas system. Only those faults in Alaska, Turkey, China, and Sumatra compare, which is why these regions often produce headline-grabbing--and incredibly destructive--earthquakes.
While scientists still can't predict exactly when and where a Big One will strike, they're trying to develop early-warning systems (see "20 Seconds Notice," right), as well as find ways to lessen quake damage. NASA, for example, is working on satellite technologies that can detect ground motions around faults as tiny as 1 millimeter per year. And in the Southern California desert, engineers at the University of California at San Diego (UCSD) are erecting a huge outdoor shake table that mimics the effect of earthquakes on full-scale buildings and structures (see below).
Measuring about 8-by 12-meters (25-by 40-feet), the table will support structures as tall as 60 feet that weigh more than 2,000 tons. With the flip of a switch, the table will rock and shake at speeds of up to 1.8 m (6 feet) per second. And the structures sitting on it will experience the shudders of the world's most catastrophic earthquakes. The purpose: Engineers will use data from the table experiments to help build stronger, safer buildings for people living in quake-prone regions.
What shake tables test, explains Frieder Seible, director of UCSD's Powell Structural Research Laboratories, is acceleration--the rate of an object's increased speed: "In a quake, the ground moves in different directions--horizontally, vertically, and back and forth." As a quake's energy waves accelerate the earth, the mass of a building atop the ground accelerates too. And that acceleration thrusts a powerful moving force on the building. Newton's second law of motion describes that force in physical terms: Force mass x acceleration (F = m x a). For a building, more mass = higher forces and potentially more damage.
How to construct buildings that can withstand a Big One? One way is to reduce an earthquake's force by changing the mass of a building, says Seible. "We can't do much to alter the way the ground moves. But we can build lighter structures." What types of buildings and bridges are most damage-proof? That's what Seible's team is trying to find out. When completed in late 2004, UCSD's giant shake table should be able to simulate earthquakes for a whole minute. "Most earthquakes last much shorter than that," says Seible.
The very first object slated for testing is a barrel full of nuclear waste from the proposed Yucca Mountain storage facility in Nevada. Before radioactive waste is placed underground, scientists want to be sure the containers can resist a huge quake. To date, the containers' strength has only been modeled on computers. Seible and his colleagues will place the data from the first shake-table test on the Internet for use by other research labs. And the findings will be useful in earthquake-prone regions worldwide, not just California.
Meanwhile, the clock keeps ticking.
QUAKE, ROCK, 'N' ROLL
To help design quake-proof buildings, engineers in California are erecting the world's first outdoor shake the table. (1) Structures six or sevens-stories high will be built on top of the 8- by 12-meter steel table (2), or planten. To simulate the violent rocking of an earthquake, engineers flip a switch, and giant mechanical arms (3) shove the table from side to side at speeds as fast as 6 feet per second. All that rocking means that table has to be secured by a huge bathtub-shaped mass of steel and concrete (4) dug deep into the ground. And while the table quakes, engineers will watch effects from a control room (5) at a safe distance.
IT'S YOUR CHOICE
After reading the article, choose the correct answer to these questions.
1 A layer of rock on Earth's surface is a
C. tectonic plate.
2 A shake table tests a structure's
3 Which statement most accurately defines the San Andreas Fault system?
A. It's a comparatively minor series of faults.
B. It's one of the world's largest, strongest fault systems.
C. It sits on the boundary of three major tectonic plates.
D. it causes massive earthquakes every few years.
4 Which of the following substances is most susceptible to the impact of an earthquake?
20 SECONDS NOTICE
ElarmS' warning time depends on the distance from the quake's epicenter. Areas farther away would have more time between the warning and the start of shaking ground. People 60 miles away from an epicenter could have up to 20 seconds notice.
Geologists are designing a system called ElarmS that could give Southern Californians a few seconds notice during a quake. An alarm would sound with the first P-waves, a few crucial moments before ground-shaking S-waves arrive. Even seconds can mean the difference between life and death, allowing people to take shelter under a desk or door frame.
TWO TYPES OF ENERGY WAVES
P "PRIMARY" WAVES
P-waves are the fastest waves released in an earthquake, reaching areas around an epicenter first. But they're low-energy: the slight vibrations are rarely felt by humans.
S-waves travel more slowly bat carry a quake's fierce energy. Buildings and other structures can buckle as S-waves pass beneath, violently shaking the ground.
Did You Know?
* Geologists estimate there has been about 350 miles of movement along the San Andreas fault since its formation 15 to 20 million years ago. If movement along the fault continues at its current rate (about 50 mm/yr), Los Angeles and San Francisco will be adjacent to one another in approximately 15 million years.
* The Richter scale is logarithmic. An increase of one whole unit on the scale represents 10 times more ground shaking. So an earthquake of magnitude 7.0 shakes the ground 10 times harder than a magnitude 6.0 quake--and 100 times harder than a magnitude 5.0 quake. See TE 7 for a related activity.
* How much energy does an earthquake release? A magnitude 1.0 earthquake is equivalent to setting off about 6 ounces of dynamite, similar to what is used on construction sites. The magnitude 7.1 Loma Prieta earthquake, which shook California in 1989, was equivalent to the explosion of about 280,000 tons of dynamite.
History: Research the 1906 San Francisco earthquake and compare it with the 1989 quake in the same area. What factors contributed to the different levels of destruction between the two quakes? What changes can humans make to decrease the devastation caused by a future quake?
Typically, many more people die when an earthquake strikes a less-developed country, like Turkey, compared with a similar magnitude earthquake in the U. S. Why? What could be done to keep people in other quake-prone regions safe?
The United States Geological Survey has a great Web site about earthquakes. It includes science project ideas, fun earthquake facts, and maps: earthquake.usgs.gov/4kids/
Visit the Museum of the City of San Francisco online to see a detailed history of the earthquakes that have struck the city. Includes a timeline and photographs of earthquake damage: www.sfmuseum.org/alm/quakes0.html
Check out the bulletin from the National Earthquake Information Center to see a listing of all the recent earthquakes around the world. Includes locations and magnitudes and is updated daily: neic.usgs.gov/neis/bulleting.html
Directions: Match the word(s) in the left column with the correct phrase in the right column. --1. epicenter a. timing of waves --2. amplitude b. point where an earthquake first breaks --3. frequency c. slower but more destructive waves released in an earthquake --4. acceleration d. rate of an object's increased speed --5. primary waves e. fastest waves released in an earthquake --6. shear waves f. height of waves --7. tectonic plates g. large pieces of rock that make up Earth's crust --8. faults h. boundaries between Earth's crust Essay Writing: You are a scientist in an earthquake movie and are about to address the press on an impending disaster. Write a speech that includes all eight vocabulary words above.
1. b 2. f 3. a 4. d 5. e 6. c 7. g 8. h
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|Title Annotation:||Earth/physical: quakes/waves|
|Date:||Oct 13, 2003|
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