Solid knowledge from shaky ground

ALASKA -- Aftershocks are still rippling through Southcentral and Interior Alaska today, a week after Alaskans experienced the largest earthquake to hit the state in nearly 40 years.

"There were 150 aftershocks in 24 hours," said Paul Whitmore, acting geophysicist at the West Coast and Alaska Tsunami Warning Center in Palmer. "The aftershocks lasted for hours."

And they may continue for days, Whitmore said. Or as long as months or years. And there's always the chance that, like the Oct. 23 magnitude 6.7 earthquake, the Nov. 3 shaker is a foreshock to an even larger quake.

Why is Alaska unstable?

In short, seismology is still a relatively new science, seeking to understand activities that have been taking place tens and hundreds of millions of years. And Alaska is a good place to study that activity, as between 5,000 and 6,000 earthquakes happen each year in this state.

The reason for so much activity is that Alaska is at the top of what some scientists refer to as the "Ring of Fire," or Pacific Rim. That rim extends along the west coast of South America, the west coast of North America, the length of the Aleutian chain and over into Kamchatka and the Kuril Islands, Japan, the Mariana Islands and the Palau Islands. Along that area, the Pacific plate, a portion of the earth's outer shell, constantly moves upward toward the North America plate. Although it's not moving at a fast rate -- about an inch and a half each year -- the movement is the cause of a lot of geologic activity.

Scientists agree that the Pacific plate, upon meeting the North America plate, dives beneath that plate, creating what's referred to as a subduction zone. Most of Alaska's earthquakes, Whitmore said, happen along the edge of that North America plate and are rarely felt by Alaskans. The 9.2 quake that shook Anchorage in 1964 was along that gigantic ring, as was the 1958 Lituya Bay quake that registered about a magnitude 8.

But not every quake happens along the Pacific Rim. The Nov. 3 earthquake took place along the Denali fault, and the aftershocks that have continued to rattle the area have extended about 150 miles from the quake's epicenter. That particular fault is referred to as a strike-slip fault, in which two plates are moving horizontally in different directions.

"Plates are moving all the time, and at fault zones, it tends to get stuck," Whitmore said. "Rocks get strained because of the amount of torque put on them."

At a certain point, Whitmore said, the pressure becomes stronger than the forces holding the bedrock in place and the earth slips. In some places along the highways in the area, the movement resulted in the pavement being offset like squares on a checkerboard.

More than 10 faults, active and potentially active, exist across Alaska. The Denali fault, according to information from the USGS office, is one of the longest strike-slip faults in the world. It rivals the size of California's San Andreas fault, also a strike-slip fault.

Are we in Tsunami danger?

A nearly constant barrage of quakes took place between the 1940s and 1964, Whitmore said, and is the reason the West Coast Alaska Tsunami Warning Center was built in 1967. Government officials realized that some of the loss of life during the 1964 quake might have been averted, had townspeople in Valdez and other affected communities, known the tsunami generated by an underwater landslide after the quake was on its way.

Tsunami is the Japanese word for "harbor wave." The waves are different from tidal waves, in that they relate specifically to waves caused by the movement of the earth or as a result of asteroids hitting the ocean. An uplift in the seafloor during an earthquake, an underwater landslide or an above-water landslide that enters the water are all common causes of tsunamis.

The 1958 earthquake that struck Lituya Bay caused a large above-ground landslide that generated a tsunami wave. The wave, according to reports, ran up a mountainside on the opposite side of the bay and reached a height of more than 1,700 feet. Three small boats were in the bay when the quake and rockslide struck. One was swamped with waves and sank, one was swept away and never seen again. According to reports recorded at the time by Bill and Vivian Swanson, who were in the third boat anchored at the head of the bay, their boat rode out a 100-foot speeding wave and was deposited in deep water, far from land.

Although the tsunami center has not had to issue warnings for any tsunamis along the West Coast the size of the 1964 wave, the center provides an important service.

Using data from more than 100 seismometers and nearly as many tide gauges around the world, staff at the center record quake activity on a minute-by-minute basis. They monitor activity along the West Coast, from the California/Mexico border to Attu Island. Through the use of phone lines and, one day soon, satellite receivers, data are transmitted from the seismometers and wave generators to the bank of computers at the center.

Earthquakes are an everyday occurrence across the world and, as of this date, it's impossible to predict a quake. Because many earthquakes occur in the ocean, warnings about potential tsunami danger can be very important -- even if the epicenter of the quake is thousands of miles away.

According to estimates from the University of Alaska Fairbanks, a tsunami from northern Japan can reach Adak Island in four hours -- Kodiak Island in eight. A tsunami traveling from Peru or Chile would reach Alaska in 16 to 18 hours. Although it may seem unlikely that a wave traveling that far could hold its intensity, Whitmore said in deep water, the energy transmits very well. Not so in shallow water, he said, which means Valley residents have very little to worry about in relation to tsunamis.

"The one thing that kills the energy of waves is long stretches of shallow water," Whitmore said. "[In order to generate a tsunami in Mat-Su] it'd have to be something that was locally generated."

Even the tsunami generated in 1964, which was 20 feet tall by the time it traveled to Homer, barely registered in Anchorage and carried little amplitude by the time it traveled up Cook Inlet.

How are quakes recorded?

Today's earthquakes are, by and large, recorded using similar methods to those developed by Dr. Charles F. Richter in 1934. According to information from UAF, each whole-number designation represents a 10-fold increase in the size of seismic waves, or how much the ground moves in an earthquake or other activity, measured on a seismograph. Seismographs are the charting equipment used to monitor earthquake activity, generating the lined paper crossed by pen marks generally depicted as showing the intensity of a quake. Although a single step on the Richter scale means a 10-fold increase in the seismic wave, a single step on the Richter scale correlates to a lot more energy released from the ground -- 30 times the energy, in fact.

But the Richter scale has its limits. The energy recorded by seismometers in the field is transmitted through phone lines and converted again to energy and transmitted through a seismograph. Unfortunately, Whitmore said, only so much energy can be recorded using the Richter method.

"When you get up to about a magnitude 8, that frequency band is saturated," Whitmore said. "For big earthquakes, it often underestimated the size."

The 1964 earthquake, for example, was initially recorded as a magnitude 8.3, but is now considered to have had a magnitude of 9.2.

Using information from the seismic waves, seismologists still use the Richter scale for monitoring smaller quakes, but Whitmore said the warning center uses a moment magnitude method of measuring earthquake activity in larger quakes.

Another important factor of measurement is intensity, or how strongly the earthquake is felt. This factor can vary widely from place to place, and is dependent on several factors.

In Alaska, damage from earthquakes is much less destructive than in other areas simply because we have fewer long-span bridges, fewer high-rise buildings, fewer structures and, simply, fewer people. But another important factor in determining intensity and resulting damage, according to information from the U.S. Geological Service office, is the type of ground underlying areas affected by the earthquake. Staff cited the 1989 Loma Prieta earthquake that happened along the San Andreas fault, the largest quake to have happened along that fault in recent years.

"It was particularly bad in areas that had poorly solidified ground underneath them," said USGS geologist Allison Till.

Materials used for fill, for example, under bridges in the Oakland area simply liquefied during the quake.

"Things that are built on bedrock tend to do better than things that are on non-solid material," Till said.

The measured intensity of a quake, as determined by the Mercalli intensity scale, varies from place to place. While residents in Mentasta, near the epicenter of the Nov. 3 quake, felt a much more intense quake -- with falling shelves and destruction to buildings and utilities -- residents in the Valley had widely differing reports, from little or no sensation by people outdoors to objects such as pictures falling off walls in other locations. On the Mercalli scale, Mentasta residents likely felt an intensity of between six and eight, while Valley residents felt intensities between three and five. See the related sidebar for more information on the Mercalli scale of intensity.

Intensity reports can be filed with the USGS office through their Web site, at pasadena.wr.usgs.gov/shake/, or at the tsunami center Web site, at wcatwc.gov. The reports are useful to help determine how earthquakes affect those in the vicinity of the activity, and the reports, like that of Bill and Vivian Swanson, are often useful years into the future.

Teaching a valuable lesson

Geologists have been flying over the Denali fault for the past week, assessing the aftermath of the 7.9 earthquake, and the effects of the hundreds of aftershocks that followed.

According to information released by USGS, scientists from their office and the Alaska Division of Geological and Geophysical Survey flew in a helicopter through valleys, over streams and along glaciers to record new features. They found a considerable amount of change.

According to information released from their office, large rock and snow slides involving boulders as big as houses were seen along valley slopes near the fault trace, with some slides traveling large distances. On the Black Rapids glacier, for example, rock slides crossed a valley a mile and a half wide, and continued up the slope on the opposite side.

Another interesting find resulting from the Nov. 3 earthquake has been the movement of earthquake activity. The Oct. 23 foreshock happened on the western portion of the rupture zone, west of the epicenter of the 7.9 main shock. Energy released from the main shock reportedly traveled eastward along the fault, and Whitmore said earthquake activity after the 7.9 quake has steadily moved east. Scientists will be able to use that information, along with measurements of the fault offsets, to determine how faults slip during earthquakes and extrapolate potential outcomes.

Ultimately, information gleaned from activity along the Denali fault can be applied to potential activity along the San Andreas fault, Till said.

"Among fault systems of this type, this is an unusually large earthquake," Till said. "For seismologists and geologists who study earthquakes, they want to study the effects right at the start of an earthquake. This earthquake was larger than most of the quakes that have occurred along the San Andreas system in the last 150 years. They're hoping to predict if an earthquake is of a certain size, what the likely outcome is."

Although scientists may not be able to determine when an earthquake is coming, each earthquake -- and especially those as large and significant as the Nov. 3 earthquake -- helps people around the world become better prepared for future earthquakes.

What to do during an earthquake

1. If you are indoors, duck or drop down to the floor. Take cover under a sturdy desk, table or other furniture. Hold on to it and be prepared to move with it. Hold the position until the ground stops shaking and it is safe to move. Stay clear of windows, fireplaces, woodstoves and heavy furniture or appliances that may fall over. Stay inside to avoid being injured by falling glass or building parts. If you are in a crowded area, take cover where you are. Stay calm and encourage others to do likewise.

2. If you are outside, get into the open, away from buildings and power lines.

3. If you are driving, stop if it is safe, but stay inside your car. Stay away from bridges, overpasses and tunnels. Move your car as far out of the normal traffic pattern as possible. If possible, avoid stopping under trees, light posts, power lines or signs.

4. If you are in a mountainous area, or near unstable slopes or cliffs, be alert for falling rock and other debris that could be loosened by the earthquake.

5. If you are at the beach, move quickly to higher ground or several hundred yards inland.

Quake aftermath -- what to do

1. Check for injuries. Do not move a seriously injured person unless they are in immediate danger of further injuries.

2. Perform a safety check. Check for the following hazards:

Fire or fire hazards.

Gas leaks. Shut off the main gas valve only if a leak is suspected or identified by the odor of natural gas. Wait for the gas company to turn it back on once the damage is repaired.

Damaged electrical wiring. Shut off power at the control box.

Downed or damaged utility lines. Stay away from downed lines even if power appears to be off.

Fallen objects in closets and cupboards. Displaced objects may fall when you open the door.

Downed or damaged chimneys. Approach chimneys with caution, as they may be weakened and could topple during an aftershock.

Check your telephone. Make sure each phone is on its receiver. Telephones that are off the hook tie up the telephone network unnecessarily.

3. Clean up. Clean up potentially harmful materials or medicines which may have spilled.

4. Be aware of potential tsunami hazards. If you live along the coast, be alert for news of tsunami warnings issued by the Alaska Tsunami Warning Center. If you experience a strong earthquake, there may not be time to issue a warning. Move to higher ground as soon as you are able, and stay there until the authorities issue an "all clear."

5. Expect aftershocks. Most of these are smaller than the main earthquake. Some may be large enough to do additional damage to weakened structures.

6. If the electricity is out, use flashlights or battery-powered lanterns. Do not use lighters, matches, candles, or lanterns until you are sure there are no gas leaks.

7. Use your telephone only in the event of life-threatening emergencies.

8. Turn on a battery-powered radio for information, damage reports, and for information on volunteering your assistance.

9. Keep streets clear for emergency vehicles. Cooperate with public safety officials.