What Causes Earthquakes

By Shawn Kyles

The earth is made up of several layers. The crust, which is the top layer, is up to 46 miles deep. It includes both land (continents) and oceans. Approximately 70% of the earth’s surface is covered by oceans. The average depth of the ocean is 2.5 miles. The crust contains iron, oxygen, silicon, magnesium, sulfur, nickel and small amounts of calcium, aluminum and several other elements.

The planet’s second layer is called the mantle. It is made of rocks with heavy concentrations of magnesium and iron. The earth’s crust, which is divided into plates, floats on the mantle. These plates (called tectonic plates) are always in motion. They are like a giant puzzle with moving pieces.

The earth’s third and fourth layers are the outer and inner core. The inner core is solid iron, and its outer layer is liquid.

Faults are the outer, rough edges of a plate. They can get stuck when a plate moves. When the edge of a plate unsticks, it results in an earthquake. The three types of faults are normal, reverse (thrust) and strike slip.

Reverse faults (thrust faults) cause the strongest earthquakes, above 8.0 or more. Strike-slip quakes can also be powerful – up to a magnitude 8. A normal fault generally produces quakes that are less than magnitude 7.

The hypocenter is where the quake starts below the surface. The epicenter is the location of the quake on the surface.

The “shake” you feel in an earthquake is the result of stored energy – the stress that has been building up over time. When the plates finally shift, the energy is released as seismic waves (waves of energy) that spread out like ripples in a pond. The waves make the ground shake.

Earthquakes usually occur in groups. They are related to each other location and time-wise. When a series of earthquakes occur in a similar location over a very short period of time, it is called an earthquake swarm.

In 2012, California’s Imperial Valley experienced a swarm of small to moderate quakes. There were dozens of quakes around magnitude 3.5 and a magnitude 5.3 quake. The largest quake was a magnitude 5.5. Although there were no injuries, windows were shattered and trailers were knocked off their foundation. Residential and government buildings suffered structural damage, requiring foundation repair, and various sidewalks and roads needed repairs.

Earthquakes occur frequently in California, with building damages totaling nearly $3 billion dollars per year. The Los Angeles-Long Beach-Santa Ana metropolitan area currently has the highest estimated annualized building loss and the highest annualized percent building loss in the state, followed by San Francisco-Oakland-Freemont, Riverside-San Bernardino-Ontario and San Jose-Sunnyvale-Santa Clara.

California homeowners can minimize the possibility of quake damage with regular building inspections by a foundation repair expert. Taking care of foundation cracks and similar types of problems will help maintain the home’s structural integrity and make it less likely to collapse during a quake. There are also many ways to strengthen a home so it can withstand quakes, like foundation bolting and earthquake retrofitting. For California homeowners, earthquake preparation is a must.

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