Devastating earthquakes seem to be more frequently in the news. In early 2010 alone, the strong earthquakes in Haiti and Chile have everyone’s attention. Strong quakes continued in April 2010 with a 6.9 magnitude in Qinghai, China; a 7.8 in Indonesia; and a 7.0 in Baja California, sourced from the United States Geological Survey (USGS) database . Many still remember the overwhelming 2008 quake in China that killed more than 80,000 people. The December 26th, 2004 tsunami in the Indian Ocean caused by a 9.0 earthquake in Indonesia killed more than 150,000 people. (USGS website. Earthquake Hazards Program. "Significant Earthquake and Headline News" accessed March 1st, 2010)
Are There More Earthquakes Now?
Are we are experiencing more earthquakes? Have they been increasing in recent years? No, says the United State Geological Survey (USGS) in Common Myths about Earthquakes. Earthquakes with a magnitude 7.0 or greater have remained fairly constant in numbers throughout the last century.
However, Dave Santek (CIMSS/SSEC) at the University of Wisconsin-Madison points out that there have been stronger earthquakes recently. There were no quakes of 8.5 or stronger in the 1970's, 80's or 90's, but already 4 in the 2000's. Quakes 8.0 and greater also register more since 2000, with 6 in the 1970s, 4 in the 1980s and 6 in the 1990s, yet already 13 in the 2000s.
According to the USGS, it may seem there are more earthquakes today because of more and better communication, and better technology. But also, says Steve Dutch at the University of Wisconsin, growing population bases in at-risk areas cause more damage and bring more media attention. There is no question that scientific and technological advances lead to better monitoring and study - and more awareness.
In 1931 there were about 350 seismograph monitoring stations in the world. Today, says USGS, there are more than 4,000 stations, with more rapid and comprehensive data reporting by satellite and computer.
What Causes Earthquakes?
Movement of Earth’s tectonic plates causes quakes. This movement causes stress between the rock masses and the subsequent tremors or waves of energy. The word tectonic comes from the Greek “to build” and refers to the earth’s surface being made up or built of plates. There are about a dozen tectonic plates around the earth, also called lithospheric plates. The lithosphere is the crust (thin and rigid) and the upper mantle (hot, dense semi-rock) of the earth.
These tectonic plates vary in size, and in the speed and direction they move. Major plates include the North American Plate, the Pacific Plate, the Antarctic Plate, and the South American Plate. Several plates are smaller but considered important, such as the Nazca (off the Chilean coast), Caribbean, and Philippine Plates.
Tectonic Plate Movement
There are three general categories of plate movement related to earthquakes, as explained in the USGS online booklet, "This Dynamic Earth: The Story of Plate Tectonics" (Robert I. Tilling and and W. Jacqueline Kious, version 1.14):
- Divergent – plates pull apart, new crust is created. This is usually evidenced by seafloor spreading or a rift valley.
- Convergent – one plate is pushed into another and under it, with crust destroyed. One plate is usually ‘subducted’ (forced down) into the mantle of the earth during convergence. Subduction zones can create very strong earthquakes, as evidenced in Chile.
- Transforming – plates slide past each other
In addition, there are three combinations of Plate interactions:
- Oceanic – Oceanic. This occurs often in the Pacific Rim.
- Oceanic – Continent. This occurred in Chile, where the Nazca Oceanic Plate is pushing up against and under the South American continent. Typically an Oceanic Plate is subducted and ‘loses the battle’ with a Continental Plate.
- Continent – Continent. The Himalayas were formed in this kind of interaction, as a prime example.
Faults can result from any combination of oceanic or continental plate movement. As an example, the San Andreas fault was formed as the North American Plate slid against the Pacific Plate.
Richter Scale
Earthquakes are measured by the Richter scale. The Richter Scale, developed in 1935 by American seismologist Charles Richter, measures tremblors on a scale of 1 to 9, with 9 being the most powerful. The magnitude is determined from a logarithm recorded by seismographs, and is represented in whole numbers and decimal fractions. Because of the logarithmic basis, each whole number increase represents a tenfold increase in power, says the USGS.
Quakes with a magnitude of 4.5 are strong enough to cause damage to buildings, while a 7 is considered quite severe. The strongest earthquake ever measured was the Great Chilean earthquake of May 22, 1960, which measured 9.5 and set off a tsunami that devastated Hilo, Hawaii. However, the Richter Scale is not used to define damage caused. Keep in mind that many strong earthquakes may occur in unpopulated areas or below the ocean floor.
Earthquakes Cannot be Predicted Precisely
Scientists have a developed a good understanding of the plate movements and how they relate to earthquakes, the USGS says, but predicting earthquakes precisely is not possible. Scientists have plotted the “hot spots”. They know in general how the plates are moving and what areas are vulnerable. They track daily seismic activity. But predicting exactly when an earthquake may happen isn’t possible, at least not yet.
The U.S. Geological Survey, a bureau within the Department of Interior, is the nation’s largest water, earth and biological science agency.
Kathy Heshelow - Kathy Heshelow is a freelance writer who has published two non-fiction books (a third is in progress). She has published articles in ...
