Understanding Earthquakes

Earthquakes are defined as "shaking and vibration at the surface of the earth resulting from underground movement." Although volcanoes, explosions, and underground collaptions can cause earthquakes, the most destructive (the ones we're most familiar with) are caused by the movement of the earth's plates. According to the United States Geology Survey, there are more than 3 million earthquakes each year. Fortunately, most of these quakes are very small and many of the larger ones occur in uninhabited parts of the earth.

When a major earthquake does occur near inhabited areas, it's destruction is widespread and deadly. Over the past one hundred years, more than 1.5 million deaths were the result of earthquake activity. This is because the earth's shaking usually doesn't hurt people. Its the destruction that occurs around them, such as man-made structures collapsing, avalanches, landslides, and tsunamis.

As mentioned earlier, most large earthquakes are the result of the earth's tectonic plates moving over the asthenosphere. The plates have a range of motion varying from moving apart, pushing together, or sliding against each other. The resulting energy released from the earthquake is expressed with the Richter scale.

Description	Richter magnitudes	Earthquake Effects	Frequency of Occurrence
Micro	Less than 2.0	Micro earthquakes, not felt.	About 8,000 per day
Very minor	2.0-2.9	Generally not felt, but recorded.	About 1,000 per day
Minor	3.0-3.9	Often felt, but rarely causes damage.	49,000 per year (est.)
Light	4.0-4.9	Noticeable shaking of indoor items, rattling noises. Significant damage unlikely.	6,200 per year (est.)
Moderate	5.0-5.9	Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well-designed buildings.	800 per year
Strong	6.0-6.9	Can be destructive in areas up to about 100 miles across in populated areas.	120 per year
Major	7.0-7.9	Can cause serious damage over larger areas.	18 per year
Great	8.0-8.9	Can cause serious damage in areas several hundred miles across.	1 per year
Rare great	9.0 or greater	Devastating in areas several thousand miles across.	1 per 20 years

Surviving an Earthquake

Be Prepared

• Have an earthquake survival kit on hand.
• All family members should know how to turn off gas, water, and electricity.
• Plan family emergency procedures, and make plans for reuniting your family.
• Know emergency telephone numbers (doctor, hospital, police, 911, etc)
• Anchor heavy objects to walls (bookcases, wall units, mirrors, cabinets, etc.)
• Never place heavy objects over a bed. Keep heavy objects lower than the height of the shortest member of your family.

When an earthquake occurs

• If inside, stand in a doorway or crouch under a desk or table well away from windows or glass dividers.
• If you are outside, stand away from buildings, trees, telephones and electrical lines.
• If on the road, stay clear of underpasses and overpasses. Stop in safe area and stay in your vehicle.

After an earthquake occurs

• Check for injuries and provide first aid if needed.
• Perform safety-check for gas, water, sewage breaks. Check for downed power lines and short circuits. Turn off appropriate utilities.
• Check for building damage and potential problems during aftershocks.
• Clean up any dangerous spills or request appropriate assistance.
• Wear protective shoes.
• Turn on your radio and listen for instructions from public safety agencies.
• Use telephone for emergencies, only.

All photos courtesy of USGS