Introduction

Earthquakes are one of the most feared natural disasters because scientists have found no way to predict them.

What is an Earthquake?

Let's first take a look at the earth. The earth is made up of 4 main layers-the inner core, the outer core, the mantle and the crust. The crust is where we live, on the surface of the earth. Below the crust lies the mantle, which is made up of solids, liquids and gases.

The lithosphere is made up of the crust and the upper most layer of the mantle, and is divided up into 12 major plates. As the plates of the lithosphere shift, weak spots, or "faults" develop. When this shifting has built up over long periods of time, the crust of the earth weakens and an earthquake occurs. The place where the crust is weakened is called a "fault".

Sometimes the movement of plates is slow, with great pressure accumulating over time. Other times, plates become locked together and when pressure has reached a certain point, the plates are released and an earthquake happens. If this earthquake happens in a populated area, the effects on mankind can be devastating.

Earthquake Terms

Faults
As described above, a fault is a weak spot, or fracture (break) in the earth's crust. There are three main types of faults. A normal fault occurs as a result of pulling tension that develops as one plate slips away from another plate (Diagram 2, "B"). A thrust (reverse) fault occurs as a result of the compression of plates pushing towards each other (Diagram 2, "C"). A strike-slip (lateral) fault occurs as a result of two adjacent places sliding next to each other in opposite directions. (Diagram 2, "A").

Focus and Focal Depth
The focal depth of an earthquake is the depth from the Earth's surface to the region where an earthquake's energy originates (the focus). Earthquakes with focal depths from the surface to about 70 kilometers (43.5 miles) are classified as shallow. Earthquakes with focal depths from 70 to 300 kilometers (43.5 to 186 miles) are classified as intermediate. The focus of deep earthquakes may reach depths of more than 700 kilometers (435 miles). The focuses of most earthquakes are concentrated in the crust and upper mantle. The depth to the center of the Earth's core is about 6,370 kilometers (3,960 miles), so event the deepest earthquakes originate in relatively shallow parts of the Earth's interior.

Epicenter
The epicenter of an earthquake is the point on the Earth's surface directly above the focus. The location of an earthquake is commonly described by the geographic position of its epicenter and by its focal depth.

Tsunamis
Earthquakes beneath the ocean floor sometimes generate immense sea waves or tsunamis (Japan's dread "huge wave"). These waves travel across the ocean at speeds as great as 960 kilometers per hour (597 miles per hour) and may be 15 meters (49 feet) high or higher by the time they reach the shore.

Seismology
The vibrations produced by earthquakes are detected, recorded, and measured by instruments call seismographs. The zig-zag line made by a seismograph, called a "seismogram," reflects the changing intensity of the vibrations by responding to the motion of the ground surface beneath the instrument. From the data expressed in seismograms, scientists can determine the time, the epicenter, the focal depth, and the type of faulting of an earthquake and can estimate how much energy was released.

Vibrations
The two general types of vibrations produced by earthquakes are surface waves, which travel along the Earth's surface, and body waves, which travel through the Earth. Surface waves usually have the strongest vibrations and probably cause most of the damage done by earthquakes.

Richter Scale

The severity of an earthquake can be expressed in several ways. The magnitude of an earthquake, usually expressed by the Richter Scale, is a measure of the amplitude of the seismic waves. The moment magnitude of an earthquake is a measure of the amount of energy released - an amount that can be estimated from seismograph readings. The intensity, as expressed by the Modified Mercalli Scale, is a subjective measure that describes how strong a shock was felt at a particular location.

The Richter Scale, named after Dr. Charles F. Richter of the California Institute of Technology, is the best known scale for measuring the magnitude of earthquakes. The scale is logarithmic so that a recording of 7, for example, indicates a disturbance with ground motion 10 times as large as a recording of 6. A quake of magnitude 2 is the smallest quake normally felt by people. Earthquakes with a Richter value of 6 or more are commonly considered major; great earthquakes have magnitude of 8 or more on the Richter scale.

Earthquakes in History
Information provided courtesy of the USGS

The earliest earthquake for which we have descriptive information occurred in China in 1177 B.C. The Chinese earthquake catalog describes several dozen large earthquakes in China during the next few thousand years.

Earthquakes in Europe are mentioned as early as 580 B.C., but the earliest for which we have some descriptive information occurred in the mid-16th century.

The earliest known earthquakes in the Americas were in Mexico in the late 14th century and in Peru in 1471, but descriptions of the effects were not well documented.

By the 17th century, descriptions of the effects of earthquakes were being published around the world - although these accounts were often exaggerated or distorted.

The most widely felt earthquakes in the recorded history of North America were a series that occurred in 1811-1812 near New Madrid, Missouri. A great earthquake, whose magnitude is estimated to be about 8, occurred on the morning of December 16, 1811. Another great earthquake occurred on January 23, 1812, and a third, the strongest yet, on February 7, 1812. Aftershocks were nearly continuous between these great earthquakes and continued for months afterwards. These earthquakes were felt by people as far away as Boston and Denver. Because the most intense effects were in a sparsely populated region, the destruction of human life and property was slight.

The San Francisco earthquakes of 1906 was one of the most destructive in the recorded history of North America - the earthquake and the fire that followed killed nearly 700 people and left the city in ruins.

Locating Earthquakes

Given a single seismic station, the seismogram records will yield a measurement of the S-P time, and thus the distance between the station and the event. Multiply the seconds of S-P time by 8 km/s for the kilometers of distance. Drawing a circle on a map around the station's location, with a radius equal to the distance, shows all possible locations for the event. With the S-P time from a second station, the circle around that station will narrow the possible locations down to two points. It is only with a third station's S-P time that you can draw a third circle that should identify which of the two previous possible points is the real one:

This example uses stations in Boston, Edinborough, and Manaus. With the distances shown, all three circles can intersect only at a single point on the Mid-Atlantic Ridge spreading center

Planning for an Earthquake

1. Make sure each member of your family knows what to do no matter where they are when EQs occur:
   • Establish a mtg place where you can all reunite afterward
   • Find out about EQ plans developed by children's school or day care
   • Remember transportation may be disrupted, keep some emergency supplies--food, liquids, and comfortable shoes, for example--at work
     
2. KNOW where you gas, electric and water main shutoffs are and how to turn them off if there is a leak or electrical short. Make sure older members of the family can shut off utilities
   
3. LOCATE your nearest fire and police stations and emergency medical facility
   
4. TALK to your neighbors--how could they help you, or you them after an EQ
   
5. TAKE Red Cross First Aid and CPR Training Course.

1. If you are INDOORS--STAY THERE! (Get under a desk or table and hang on to it, or move into a hallway or get against an inside wall. STAY CLEAR of windows, fireplaces, and heavy furniture or appliances. GET OUT of the kitchen, which is a dangerous place (things can fall on you). DON'T run downstairs or rush outside while the bldg is shaking or while there is danger of falling and hurting yourself or being hit by falling glass or debris.
   
2. If you are OUTSIDE-- get into the OPEN, away from bldgs, power lines, chimneys, and anything else that might fall on you.
   
3. If you are DRIVING--stop, but carefully. Move your car as far out of traffic as possible. DO NOT stop on or under a bridge or overpass or under trees, light posts, power lines, or signs. STAY INSIDE your car until the shaking stops. When you RESUME driving watch for breaks in the pavement, fallen rocks, and bumps in the road at bridge approaches.
   
4. If you are in a MOUNTAINOUS AREA--watch out for falling rock, landslides, trees, and other debris that could be loosened by quakes.

Tsunamis: Ocean waves that move at aircraft speeds

Tsunamis are often incorrectly called tidal waves, yet they have nothing to do with tides. Commonly triggered by undersea earthquakes, tsunamis are enormous ocean waves that travel hundreds of miles at speeds near 500 mph - as fast as commercial jets. Only two or three tsunamis are spawned each year. Seemingly small at sea, the waves gain extraordinary size when they plow ashore, at times towering more than 100 feet high.