Danger, Damage, and Hazards

Some of the most tragic natural disasters of the world have been caused by earthquakes. In the Chinese earthquake of 1956, over 800,000 people were killed. After the Loma Prieta earthquake in Northern California in 1989 there was over $6 billion in property damage.

Each year there are on average about 100 earthquakes of magnitude 6 or greater, and 20 earthquakes of magnitude 7 or greater. This comes to about 1 potentially damaging earthquake every 3 days, and 1 severe earthquake every 3 weeks.

The Main Earthquake Hazards

Ground shaking results directly from an earthquake. It is the most common danger and can cause many other secondary effects which are listed below.

Falling Objects: Objects can fall from shelves, ceilings, building, etc. and easily injure humans or other animals.

Fires: Fires are caused by disruption of electric, gas, or oil lines which can rupture from the ground shaking.

Avalanches:Due to the ground shaking, avalanches cause snow to behave like a liquid and slide down a mountain. Avalanches can bury mountain side homes and destroy forests.

Land and Mud Slides: Similar to the avalanche the ground shaking breaks apart soil and causes it to cascade down the mountain side destroying everything that gets in its way. Land slides involve dry soil, and mud slides are caused by a water and soil mixture.

Floods: Floods can occur when the ground shaking disrupts a dam. When the dam breaks the water escapes, flooding the surrounding area.

Soil Liquefaction: This is the process in which soil and sand are turned from a solid to a liquid state. When wet sand is subjected to repititious motions the pressure of the water between the sand grains increases untill the pore pressure is equal to that of the external pressure of the soil. It is now in a dense liquid state instead of a solid state. The flow of the soil liquefaction causes large buildings and other structures to tilt.

Ground Lurching: A general term for all seismic motion, including ground acceleration.

Settlement: The gradual drawdown movement of strata due to compression.

Structural Collapse: Structures such as buildings and houses can collapse for a number of different reasons. The most common is ground shaking.

Tsunamis: Tsunamis, or harbor waves, develop when the epicenter of an earthquake is in the middle of the ocean. The earthquake causes long amplitude waves, which are barely noticable in the deep sea. As these waves travel closer to shore, the wave height and velocity increases. When the waves finally reach the shoreline and crash down, their height can exceed 20 meters. Tsumanis cause disasterous effects to the shoreline towns.

Photos courtesy of US Department of Commerce NOAA Geophysical Data Center