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USA's Largest Earthquake

Prince William Sound, Alaska 1964 Magnitude 9.2
This great earthquake and ensuing tsunami took 125 lives (tsunami 110, earthquake 15), and caused about $311 million in property loss. Earthquake effects were heavy in many towns, including Anchorage, Chitina, Glennallen, Homer, Hope, Kasilof, Kenai, Kodiak, Moose Pass, Portage, Seldovia, Seward, Sterling, Valdez, Wasilla, and Whittier.

Anchorage, about 120 kilometers northwest of the epicenter, sustained the most severe damage to property. About 30 blocks of dwellings and commercial buildings were damaged or destroyed in the downtown area. The J.C. Penny Company building was damaged beyond repair; the Four Seasons apartment building, a new six-story structure, collapsed; and many other multistory buildings were damaged heavily. The schools in Anchorage were almost devastated. The Government Hill Grade School, sitting astride a huge landslide, was almost a total loss. Anchorage High School and Denali Grade School were damaged severely. Duration of the shock was estimated at 3 minutes.

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Earthquake Physics

First, quakes themselves are caused by abrupt shifts of rocks separated by a geological fault. It's these motions that generate the vibrations that we feel, and if they are strong enough, this results in property damage.
An earthquake usually begins with a roar, a sudden noise, and is then followed by those vibrations and the swaying sensation. Although vertical movement can occur, it's the horizontal movements that really generate the damage.

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Soil Factors

Local ground conditions play a large role in the characteristics of earthquake motion. In areas noted for "soft" soil, the degree of quake motion is much greater. The quake forces are amplified on water-saturated soils. A quake tends to turn this type of soil from a solid into a liquid -- quicksand.
You can have your local building authority or even soil engineers check this. If it turns out that your building is situated on such soil, the foundations should and can be reinforced.

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Nuclear Tests

Both earthquakes and nuclear tests can rapidly release a large amount of energy. The energy source for small yield (typically less than 50 kilotons) thermonuclear devices is the splitting of heavy radioactive isotopes. This process produces about 20 million times the energy of each reacting atom in a chemical explosive. The energy source for an earthquake is tectonic strain accumulated by the relative motion of Earth's tectonic plates which is driven by mantle heat flow in the presence of the earth's gravitational field. In a nuclear test, all of the energy is suddenly (within milliseconds) released in the form of heat from a relatively small volume surrounding the thermo- nuclear device. The tremendous heat causes rapid expansion of a spherical cavity, which in turn generates seismic waves. The heat gradually conducts away from the cavity into the surrounding rock. However, rock is a poor conductor of heat so it can take many years for the thermal signature of the thermonuclear explosion to subside and the increase in the surface temperature above the explosion is insignificant.

Nuclear tests are also very shallow sources with the depth of burial generally less than a few hundred meters (the depth of burial is typically proportional to the cube root of the expected yield). The estimated yields of the larger Indian and Pakistani tests are approximately 2-40 kilotons. In a large earthquake, the elastic strain energy stored in the Earth's crust is released, within a few seconds to a few tens of seconds, by rupture along a fault and the strain energy is released from a relatively large volume of rock surrounding the fault rupture. For example, the recent (5/30/98 at 06:22:28 UT) magnitude 6.5 earthquake in Afghanistan (37.4 N, 70.0 E), had a source duration of about 5 seconds and an estimated source volume of order 4000 cubic kilometers. This earthquake also had a focal depth of 18 km. The energy release is equivalent to a 2000 kiloton nuclear explosion.

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Largest Quakes in the World

Location Date UTC Magnitude Coordinates
1. Chile 1960 05 22 9.5 Mw 38.2 S 72.6 W
2. Alaska 1964 03 28 9.2 Mw 61.1 N 147.5 W
3. Russia 1952 11 04 9.0 Mw 52.75 N 159.5 E
4. Ecuador 1906 01 31 8.8 Mw 1.0 N 81.5 W
5. Alaska 1957 03 09 8.8 Mw 51.3 N 175.8 W
6. Kuril Islands 1958 11 06 8.7 Mw 44.4 N 148.6 E
7. Alaska 1965 02 04 8.7 Mw 51.3 N 178.6 E
courtesy of USGS
8. India 1950 08 15 8.6 Mw 28.5 N 96.5 E
9. Chile 1922 11 11 8.5 Mw 28.5 S 71.0 W
10. Indonesia 1938 02 01 8.5 Mw 5.25 S 130.5 E

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Predicting Quakes

Can earthquakes be predicted? Unfortunately, not yet. Scientists or the USGS have never predicted a major quake. The focus of current research is more on improving structures and preparedness rather than on prediction. What we can do is "estimate probability" -- in some opinions "guestimate probability" would be a more accurate description.

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Nuclear Explosions

On January 19, 1968, a thermonuclear test, codenamed Faultless, took place in the Central Nevada Supplemental Test Area. The codename turned out to be a poor choice of words because a fresh fault rupture some 1200 meters long was produced. Seismographic records showed that the seismic waves produced by the fault movement were much less energetic than those produced directly by the nuclear explosion.

Analysis of local seismic recordings (within a couple of miles) of nuclear tests at the Nevada Test Site shows that some tectonic stress is released simultaneously with the explosion. Analysis of the seismic wavefield generated by the blast shows the source can be characterized as 70-80 percent dilational (explosive-like) and 20-30 percent deviatoric (earthquake-like). The rock in the vicinity of the thermonuclear device is shattered by the passage of the explosions shock wave. This releases the elastic strain energy that was stored in the rock and adds an earthquake-like component to the seismic wavefield. The possibility of large Nevada Test Site nuclear explosions triggering damaging earthquakes in California was publicly raised in 1969. As a test of this possibility, rate of earthquake occurrence in northern California (magnitude 3.5 and larger) and the known times of the six largest thermonuclear tests (1965-1969) were plotted and it was obvious that no peaks in the seismicity occur at the times of the explosions. This is in agreement with theoretical calculations that transient strain from underground thermonuclear explosions is not sufficiently large to trigger fault rupture at distances beyond a few tens of kilometers from the shot point.
The largest underground thermonuclear tests conducted by the US were detonated in Amchitka at the western end of the Aleutian Islands and the largest of these was the 5 megaton codename Cannikin test which occurred on November 6, 1971. Cannikin had a body wave magnitude of 6.9 and it did not trigger any earthquakes in the seismically active Aleutian Islands. Suggested reading: "Nuclear Explosions and Earthquake, the Parted Veil", by Bruce A. Bolt, W. H. Freeman and Co., San Francisco, 1976.
UC Berkley

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What is an Earthquake

An earthquake is the motion produced when stress within the earth builds up over a long period of time until it actually exceeds the strength of the rock, which then fails by breaking along a fault.

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Building Models

There are two ways of constructing earthquake models. One is commonly used by scientists - a computer generated 3D model. These normally require expensive software and a good grasp of computer lingo. The other is a live scale model. The easiest ones to construct are built from springs and ball bearings. A flat piece of plywood is anchored to 4 springs in each corner. The springs are then anchored to a fixed platform. From here, structures, buildings, and mountains composed of pebbles are placed on the plywood.

For example, a group of university students built a similar model, but included 3 walls. They attached pictures to the walls, a computer and desk, kitchen table and chairs, a bed, and a ceiling light. 4 people stood behind each corner of the room and pushed in an X manner. The harder they pushed the greater the simulated magnitude was. They simulated a 7.5 magnitude to test new anchor brackets used to secure household objects.

This is just one of the examples used for quake models. Located in the links section of this site will be a link to the USGS where there is a detailed article on another way of constructing quake models.

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