Guide to Earthquakes
Earthquakes are tremors of the ground we often take for granted as solid. As there is often few places of refuge and earthquakes are so unpredictable they can be terrorizing. Some more powerful earthquakes such as the San Francisco earthquake of 1906 which killed more than 700 people can cause widespread damage.
Most earthquakes occur along the edges of tectonic plates or where they meet. Tectonic plates are very large pieces of rock below the earth’s crust. The edges of these plates can be brittle and are known as fault lines. The plates move and shift over time and these lines or faults absorb the motion of the plate action. An earthquake happens when these plates move suddenly under the earth's surface.
Small tremors, in fact millions, are happening constantly throughout the world as the plates often move though so slight and slow the motion is barely measured or undetectable. This is called creep or aseismic. When the plates stick to one another stress accumulates. When the stress reaches a tipping point one or both of the plates shift enough to cause the earth’s crust to shake or a seismic vibration. This is when an earthquake happens. Seismic energy moves on the crust like waves. The two types of seismic waves are called body waves and surface waves. A body wave moves three dimensionally in a centrifugal pattern in all directions from the epicenter. Surface waves move slower and only along the surface of the earth’s crust.
There are known faults throughout the world and scientists monitor and study the faults and movements of the plates. Many earthquake belts run along coastal areas. The epicenter of an earthquake is the spot where the earthquake is the strongest. The seismic waves get weaker the farther they travel. The epicenter can be far below the ground and have been measured as deep as 435 miles. There are different types of faults such as a strike-slip fault. In this case, plates slide past each other moving in opposite directions. The other major type of faults are dip-slip faults. In these instances the plates are either pushing together or pulling apart, as one plate or the other slides either up or down a sloped plane.
Seismologists are the scientists who study earthquakes. Seismologists use a Richter scale to determine the amount of energy caused by an earthquake. The Richter scale measures earthquake on a scale from 0 to 9. Scientists collect data from earthquakes in an effort to one day accurately predict when and where they will take place. The Richter magnitude scale was invented by seismologist Charles Richter. The scale calculates magnitude based on the strength of the vibrations and distance from the earthquake’s epicenter as measured by an instrument called a seismograph. The Richter scale is logarithmic. Each magnitude on the scale is ten times stronger than the previous. A 7.0 earthquake tremor is ten times more powerful than a 6.0 tremor. Energy release is much greater. An earthquake magnitude of 6.0 sends out 32 times the seismic energy as an earthquake magnitude of 5.0.
The Richter scale is limited and only accurate for earthquakes as far away as about 300 miles or 500 kilometers from the instrument. There are other instruments that measure earthquakes and waves in different ways. One system is moment magnitude, which uses the actual area of fault ruptured for a more constant measure of the earthquake’s size.
How Earthquakes Cause Damage
When an earthquake happens the first waves are called body waves or P or primary waves. These actually push ahead causing the air to move sometimes creating sound. Then the secondary or S waves arrive. These waves are often felt as a sudden jolt. The next waves are the surface waves which can cause the motion that makes the ground appear to roll. These waves can cause mass destruction making buildings, structures, and bridges fall.
When a powerful earthquake occurs, surface waves can move faster than the rate of gravity and throw heavy objects into the air. After the waves there are often series of aftershocks that can even last for days.
Earthquakes can also cause further destruction by initiating landslides, mudslides, and avalanches. An earthquake in 1692 moved the seaside town of Port Royal 50 feet below the ocean’s surface. Shifting plates can also cause ground to rise.
Ground liquefaction is another hazard of strong earthquakes. Soil or sand is violently shaken until it separates and becomes fluid and can swallow anything on the surface including buildings.
As much a seismologists and the general population would like, at this time there is no accurate method for predicting earthquakes. One basic idea behind quake prediction is that faults send out subtle but detectable warnings. Seismologists have studied tremors, weather, earthquake patterns, fault lines, water tables, mineral deposits, electrical currents, and even animal behavior in an effort to find any warning signals that would allow them to predict earthquakes.
While some methods have produced results they turned out not to be reliably consistent. China successfully predicted a quake in 1975 by studying foreshocks but was then caught off guard by the Tangshan earthquake in 1976 which killed an estimated 240,000 people.
Seismologists in the US predicted an earthquake would strike in Parkfield, California around 1988 based on the timing of previous area quakes with magnitudes around 6.0 occurring every 22 years for over a century. The spent a great amount of money to monitor the region. As of today, 2010, nothing has happened.
These and many other inconsistent forecasts of earthquakes have eroded faith in seismologists attempts to foretell the future and left people speculative about any human ability to ever make predictions. Even so seismologists around the world are still working to develop a method to be able to predict specific earthquakes. Preventing the possible loss of life and preparing for the destruction is a worthwhile and admirable goal. If earthquakes could be accurately forecast even years in advance specific regions could implement building methods that prevent or minimize loss and people could prepare.
Incredibly Useful Earthquakes Resources
National Earthquake Information Center US
All about earthquakes from PBS.
US Government information on earthquakes.
Learning fun about earthquakes with graphics.
Fault line seismic science.
Introduction to Elastic Rebound Animation
Virtual museum: 1906 San Francisco earth quake.
Collection of earthquake photographs.
Earthquakes In History
Understanding earthquakes tutorial and quiz.
The Earthquake Engineering Online Archive
A Brief History of Seismology to 1910
Earthquakes, Volcanoes, and More
Department of Earth & Atmospheric Sciences with volumes of earthquake information.
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