Do you live on a fault?

If you took two books, put them side-by-side, and then pushed one of the books while keeping the other still, you would be imitating the process that makes earthquakes. The surface that the two books slipped along is called a fault. In the Earth's crust, huge blocks of rock can move past each other along faults. When these blocks suddenly slip, an earthquake is produced. Faults are common in the Earth's crust, but only some will produce earthquakes. These are called "active faults."

Particularly severe damage is likely where structures are built directly on top of active faults. The 1989 Loma Prieta earthquake in California is an example of where a moderate-size earthquake on a fault near a populated area caused a lot of damage. The location of active faults can often be determined before a major earthquake. View a map of some of the active faults in Alaska. Buildings that lie on top of, or very close to, these features should be considered especially dangerous.

Many earthquake-producing faults in Alaska lie far beneath the Earth's surface. In fact, most residents of southern coastal Alaska live above a huge fault called the "Aleutian Megathrust." The 1964 earthquake, and most major earthquakes in southern Alaska are related to movement along this fault. Ground shaking caused by earthquakes on these deep faults is a more widespread hazard than actually straddling a fault. Nevertheless, a small earthquake nearby can be as destructive as a big earthquake farther away.

Soil Liquefaction

When loosely packed and very wet sand is shaken during an earthquake, it may flow like liquid. This is called liquefaction. Anyone who has walked on a beach may have seen a small-scale version of this process. Stamp your foot in the sand near the water's edge and suddenly the area of your footprint vibrates like shaky gelatine.

Earthquake-caused liquefaction is often accompanied by cracks in the ground surface and small eruptions of sand and water called sandblows. When a soil liquefies, it is unable to support the weight of any ground or structures above it. Bridges and buildings may settle and tilt even though they may have been designed to withstand strong ground shaking. If the liquefied area is on a slope, massive landslides may result.

The Bootlegger Cove Formation is the name of a soil that underlies much of Anchorage. Liquefaction of a part of the Bootlegger Cove Formation caused much of the destruction in the Anchorage area during the 1964 earthquake. Formations or soils that liquefy are not limited to the Anchorage area, but are present in many low-lying parts of Alaska where there is a high water table.