The State of Oregon, which is at-risk for earthquakes, has released a preparedness brochure. Dr. Althea Rizzo, the state’s Geologic Hazards Program Coordinator, has included this image in the brochure (together with this description): “Earthquakes occur when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves that make the ground shake.” Click on the image for a much-better view.
Earthquakes happen all the time, although we only hear about the massive ones. The USGS (United States Geological Survey) creates hourly maps which locate "felt quakes" in California and Nevada plus the entire world. It’s surprising to realize how many earthquakes measured at least 5.5 on the Richter Scale during the past five years.
The USGS also encourages people to report earthquake tremors (or "tremblors" as they are typically called). Such events can predict volcanic eruptions - just like tremors in 79 AD (had they been understood) would have predicted the eruption of Mt. Vesuvius which destroyed Pompeii and Herculaneum.
Why do earthquakes happen? The process (this is a very helpful animation from the BBC) has everything to do with how the earth is constructed. Tectonic plates, for example, sometimes “grind” against each other. Those movements can dislocate segments of the Earth’s crust and is one cause of earthquakes:
An earthquake is the vibration, sometimes violent, of the Earth's surface that follows a release of energy in the Earth's crust. This energy can be generated by a sudden dislocation of segments of the crust, by a volcanic eruption, or event by manmade explosions. Most destructive quakes, however, are caused by dislocations of the crust. The crust may first bend and then, when the stress exceeds the strength of the rocks, break and "snap" to a new position. In the process of breaking, vibrations called "seismic waves" are generated. These waves travel outward from the source of the earthquake along the surface and through the Earth at varying speeds depending on the material through which they move. Some of the vibrations are of high enough frequency to be audible, while others are of very low frequency. These vibrations cause the entire planet to quiver or ring like a bell or tuning fork. (USGS, "How Earthquakes Happen.")
Earthquakes typically occur along a “fault line” which is, in essence, a fracture of the Earth’s crust. In the United States, the San Andreas Fault (located in Central California’s Carrizo Plain) is the most famous. It has been the source of California’s worst earthquakes including the Loma Prieta quake which disrupted the World Series in 1989.
Haiti, which is located near the boundary region separating the Caribbean and North American Tectonic Plates, has two major fault systems. The southernmost fault is called the Enriquillo-Plaintain Garden. According to the USGS, that fault was involved in a massively damaging earthquake which occurred on the afternoon of January 12, 2010.
The epicenter of that 7.0 quake - 18.457°N, 72.533°W - was only 15 miles (25 km) west/southwest of Port-au-Prince, the country's heavily populated capital city (and seat of a beautiful National Palace). Its depth - at 5 miles (8 km) - was relatively shallow, adding to the potential for extraordinary damage.
Within seconds of the quake - which was captured live by security cameras at the U.S. Embassy in Port-au-Prince - seismic waves traveled across America and were recorded at the University of Portland (Oregon). Despite some water elevation, no tsunami (this is a BBC animation) occurred because of the quake's characteristics (strike-slip as opposed to subduction).
Aftershocks - compounding the horror of the quake and the widely spread, ongoing suffering of Haitians in the capital and elsewhere - could continue indefinitely (according to the USGS).
Hampered by damaged or destroyed infrastructure, rescuers were unable to quickly render assistance to shell-shocked survivors who did their best to care for others, and themselves, before aid arrived. At least 1.5 million people were instantly rendered homeless, and more than 100,000 (WARNING - GRUESOME PICTURE LINKED NEXT) died. (Click here for resources to help children understand events in Haiti.)
The quake reduced landmark buildings in Port-au-Prince - including the Justice Palace, a cathedral, the Montana Hotel - to unrecognizable ruin. As rescuers searched for people in collapsed buildings, homeless families either fled or erected tent cities on a golf course, in a stadium, near the airport and in front of the Presidential Palace.
Another stupendous earthquake, causing massive damage to a Caribbean island, happened in seventeenth-century Jamaica. At the time, its leading city - Port Royal - was an important seaport (and a base of operations for pirates).
Port Royal is located on the relatively small Caribbean Tectonic Plate. The size of the plate, however, was no measure for the damage which occurred on June 7, 1692. And ... after the earth moved that late spring day ... Port Royal ceased to be the center of activity for pirates of the Caribbean.