CHAPTER 8.3

DESTRUCTION FROM EARTHQUAKES PGS. 229 - 232

SEISMIC VIBRATIONS

• The damage to buildings and other structures from earthquake waves depends on several things: intensity and duration of the vibrations, the nature of the material on which the structure is built, and the design of the structure.

BUILDING DESIGN

• A steel frame will withstand earthquake vibrations while a poorly designed building could possibly be greatly damaged. Stone or brick buildings do not withstand an earthquake as well as other materials.

LIQUEFACTION

• When we have sediments that are saturated with water earthquakes can cause liquefaction. This is where stable soil turns into liquid that cannot support a building. This may cause structures to collapse.

TSUNAMIS

• • • A tsunami is caused by seismic sea waves.

A tsunami is caused by an earthquake where a slab of the ocean floor is displaced vertically along a fault. It can also occur when the vibration of a quake sets an underwater landslide into motion. While a tsunami is in the ocean it moves very quickly but may not be noticeable because it is not very high. But when it hits the shore, the wave will slow down and cause the water to pile up on itself making a very high and large wave.

TSUNAMI WARNING SYSTEMS

• • Usually seismic monitoring stations will also check the level of the ocean. If there is thought of a tsunami then a warning is issued. It will be issued an hour before the tsunami is to hit – this is enough time for coastal towns to evacuate. Not all countries have a warning system – in 2004 Indonesia was hit with a tsunami but was not prepared because they had no warning system. http://www.youtube.com/watch?v=-5fKvN-RwKs

OTHER DANGERS

• • Earthquakes can also cause landslides, ground subsidence, and fires. Ground subsidence is the sinking of the ground triggered by the vibrations. In 1906 in San Francisco a major earthquake caused many fires throughout the city.

PREDICTING EARTHQUAKES

Short-range predictions want to provide early warning of the location and magnitude of a large earthquake. There are things that may predict a large earthquake. They can measure uplift, subsidence, and strain in the rocks near a fault. This is still not a very successful way to predict an earthquake.

• • • Long-range forecasts – this is trying to predict earthquakes that will happen within 30 to 100 years. This is based on the idea that earthquakes are repetitive, meaning they will occur in the same areas over and over. Seismic gap is an area along a fault where there has not been any earthquake activity for a long time. Long-range forecasts are a little more accurate than short-range.