The answer to what causes an earthquake is found in the geography of the earth. The ground you are standing on may be solid, but what lies beneath is not. The earth is in a constant state of change. Earthquakes are evidence of this and are not a rare occurrence. According to the United States Geological Survey, “The USGS estimates that several million earthquakes occur in the world each year.”
Layers of the Earth
To understand what causes an earthquake, you first need to understand the different layers of the earth. The earth is made up of layers that are in constant motion. The innermost layer is the inner core, followed by the outer core, then the mantle, and finally the crust where we all live. The inner core, the absolute center of the earth, is under so much pressure that it is solid rather than a liquid. The outer core is liquid nickel and iron, while the mantle, which is the thickest layer at about 1800 miles, has the consistency of a thick liquid. The crust is only 3 to 5 miles thick when measured from the sea floor.
The mantle, that hot liquid rock, creates its own currents known as convection currents. The currents are caused by the temperature variations within the mantle. The closer the mantle is to the core, the hotter it is. This hot mantle rises, pushing the cooler mantle and causing a current under the crust of the earth.
How do geologists know what the layers of the earth look like? From drilling holes deep into the earth, study of the variations in gravity, modeling experiments, analyzing rocks, and from earthquakes themselves, geologists have been able to understand what lies beneath our feet.
Tectonic Plates – Like Living on Floating Icebergs
Not only does the earth’s crust sit on a bed of fluid rock in constant motion, but its surface is also broken up like puzzle pieces called tectonic plates. And these puzzle pieces do not fit perfectly or securely together.
The crust of the Earth sits on the upper mantle. The crust and upper mantle are referred to as the lithosphere, a region that is broken up into several different plates. These plates do not connect. Instead, because the plates are floating on the mantle, they are constantly bumping and pushing into each other. Where the plates meet is called a fault. At tCredit: USGShe fault, the plates may be pushing against each other (a convergent boundary), slipping past each other in opposite directions (a transform boundary), or pulling away from each other (a divergent boundary).
Most of the continental U.S. sits on the North American Plate. Pressing against the Oregon and Washington coastline is the small Juan de Fuca Plate. This plate is actually plunging under the North American Plate. Pushing against the Juan de Fuca Plate is the large Pacific Plate. Part of California sits on the Pacific Plate and part of California sits on the North American Plate. Where the two plates meet is the San Andreas Fault, which runs through many California cities including San Francisco, Hollister, and San Bernardino. The North American Plate is moving southeast, while the Pacific Plate is moving northwest. According to PlateTectonics.Credit: Linda Tannercom, two thirds of California along the coast moves 5 centimeters a year towards the northwest. With hundreds of earthquakes in California each week, Californians know to be prepared for the next big quake and don’t generally consider any quake less than a 5.0 to be newsworthy.
Causes of an Earthquake
Most plate movement is not felt. But with all of the pushing and pulling and slipping of the tectonic plates, energy builds up and must be released at some point. This tectonic plates movement and release of energy is how earthquakes happen. The earthquake a sudden large movement of the plates that can be felt at the surface. The epicenter is the focal point of the earthquake. What most people feel are the seismic waves created by the earthquake. Like ripples on water when you throw a rock in a lake, the earthquake can have far-reaching effects with its seismic waves.
The Ring of Fire
The Pacific Ring of Fire gets its name from the frequent earthquake and volcanic activity. About 90 percent of all earthquakes occur along the ring of fire reports the USGS. This Pacific fire ring makes an upside down horseshoe shape as it runs along the coasts of South America, North America, Alaska, through Japan and the Indonesian Islands. This roughly follows the borders of the large Pacific Plate and some smaller plates, such as the Nazca Plate that meets the South American Plate and the Philippine Plate.
Other Earthquake Zones
Where tectonic plates meet are not the only earthquake zones, as residents of the central states know. Faults do develop in weakened areas within tectonic plates as well. In the U.S., the New Madrid Fault system runs through parts of Illinois, Missouri, Arkansas, Kentucky, and Tennessee. From 1811 to 1812, this fault system produced several major earthquakes that rival those of California’s largest known earthquakes. Three of those earthquakes measured approximately 7.5 to 7.7, were felt 900 miles away in Washington DC, and were said by witnesses to have caused the Mississippi to flow backwards. Two hundred quakes a year are produced by the New Madrid Fault reports the USGS.
Predicting and Preparing for an Earthquake
Geologists have a good understanding of what causes an earthquake. The next steCredit: Linda Tannerp is to understand the signs that a large earthquake is about to occur. Geologists are researching techniques that may one day serve as methods for predicting the chance of an earthquake along a fault in a given year. Much of that research is being conducted along the San Andreas Fault in Parkfield, CA. However, even with such predictions, preparation for an earthquake needs to occur years in advance. In earthquake prone locations, buildings must be designed to withstand the shaking and rolling and families need to prepare and maintain disaster preparedness kits with enough food, water, medical supplies, and housing supplies to meet the family’s needs for at least three days. Even if predicting an earthquake becomes a reality, the damage it may cause will be largely unpredictable.