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In the aftermath of Haiti's devastating earthquake, nervous citizens can be forgiven for wondering where the next Big One will hit. Major quakes strike with alarming regularity: Earthquakes of magnitude 7 or greater occur approximately 18 times a year worldwide. They usually originate near faults where tectonic plates —tremendous fragments of the earth's crust—collide or push above or below each other. Geologists suspect that Haiti's destructive quake resulted from 250 years of seismic stress that has been building up between the North American and Caribbean tectonic plates. In fact, a group of U.S. geologists presented a study in the Dominican Republic (which shares the island of Hispaniola with Haiti) in 2008 saying that the region was at risk of an earthquake potentially even bigger than last week's magnitude 7.0 quake. Part of their presentation is particularly chilling in light of what would happen less than two years later: "This means that the level of built-up stress and energy in the earth could one day be released resulting in an earthquake measuring 7.2 or more on the Richter Scale. This would be an event of catastrophic proportions in a city [Port-au-Prince] with loose building codes, and an abundance of shanty-towns built in ravines and other undesirable locations." Earthquakes are still impossible to predict with precision; in the words of one of the geologists who predicted the Haiti quake, "It could have been the next day, it could have been 10 years, it could have been 100... This is not an exact science." But researchers have identified a handful of seismic zones around the globe that are storing up especial amounts of stress and are particularly hazardous. Browse through the gallery for a world tour of the planet's most seismically vulnerable regions. By Aline ReynoldsImage: USGS

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The San Andreas fault zone, which is about 800 miles long and at least 10 miles deep, slices through two-thirds of the length of California. The Pacific Plate, on the west side of the fault zone, has been grinding in a northwesterly direction past the North American Plate for millions years, at a rate of up to 2 inches per year. The 1906 earthquake, which measured an estimated 8.3 on the Richter scale, was one of the worst natural disasters in North American history, causing around 3,000 casualties. The major rupture occurred along the San Andreas fault off the San Francisco coast and stretched almost 300 miles from north to south. Fires raged for days, damaging much of the city's infrastructure. The San Andreas fault zone creeps in some places, especially in its northern section, and is fixed in others. In the area of Cholame, a community in San Luis Obispo County, underground energy is steadily building up. Smaller tremors have plagued the region since the recent magnitude 6 quakes in San Simeon and Parkfield, which, scientists speculate, might be a sign of a forthcoming large earthquake. Image: USGS / Google Earth

San Andreas Fault, California

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The Hayward fault, a crack in the earth's crust about 62 miles long in the San Francisco Bay area, is referred to by U.S. Geological Survey Scientists as a tectonic time bomb. It is considered the most dangerous urban fault zone in the country, experiencing quakes of magnitude 6.8 or greater about every 140 to 170 years. Having suffered from a magnitude 7 quake in 1868, the Bay Area is due for another large tremor in the very near future, geologists fear. If the 1868 earthquake were to occur today, the consultancy firm Risk Management Solutions estimates total economic losses to residential and commercial properties would likely exceed $165 billion. Image: USGS

Hayward Fault, California

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Padang, the capital of Indonesia's West Sumatra province, sits in the crosshairs of some of the world's most turbulent fault lines. Indonesia is situated near the convergence of the Eurasian, Australian and Pacific tectonic plates, which have pressed against each other for millennia, and lies near the long, underwater Sumatran fault line, which is approximately 130 miles off the west coast of Sumatra. These fault lines are part of the "ring of fire," an enormous horseshoe-shaped band of seismic stress surrounding the Pacific Ocean. The undersea earthquake of 2004 occurred along the fault where the Indo-Australian Plate is slipping beneath the Eurasian Plate; it triggered the horrific tsunami that killed approximately 150,000 people, and was estimated to have produced the energy of 23,000 Hiroshima-size atomic bombs. Yet that quake may have increased the danger elsewhere in the region by increasing pressure on some parts of the Sumatran fault. In a recent article published in the journal Nature Geoscience, scientists report forecast yet another large earthquake for Padang, the capital of west Sumatra, in the aftermath of the city's 2009 earthquake. The expected tremor of magnitude 8.5 or higher could rumble sometime in the next decade, and could trigger another tsunami. Image: USGS

Indonesia

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Pakistan's capital, Islamabad, is situated on five major fault lines and is near the convergence of the Arabian and Iranian micro-plates, making it highly prone to large earthquakes. But the problem doesn't stop at the capital's borders: Two-thirds of Pakistan rests on fault lines, making the country particularly susceptible to violent tremors, which can also affect nearby India and Nepal. A 2005 quake in Kashmir, Pakistan, with a magnitude of 7.6, left an estimated 4 million people homeless. It may also have begun a sequence of events in which future quakes will release seismic stress along different parts of the fault. One or more of the seismic gaps might be ready to burst, particularly in the Himalayan region, where underground pressure is accumulating. Image: USGS

Pakistan

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More than 90 percent of Iran blankets an active seismic zone, the Alpine-Himalayan belt. According to the earthquake research institute MCEER, the country has endured more than 130 earthquakes with magnitude 7.5 or higher in the past 70 years, with loss of life in the 20th century totaling approximately 125,000. Tehran, a city of 12 million people, might be the next target, having been last strongly rattled in 1830, when it was hit by a magnitude 7.2 quake. The Iranian government is even considering moving the nation's capital elsewhere for fear of the destruction that would occur if a large tremor hits Tehran. Image: USGS

Iran

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Japan sits on the crossroads of several continental and oceanic plates. The 1923 Great Kanto earthquake (approximately 8.2 magnitude) shook two densely populated cities, Tokyo and Yokohama, causing approximately 130,000 fatalities. More recently, the 1995 Kobe quake killed over 6,000 people in 1995. Some scientists fear that the Tokai/Suruga Bay region, located along the country's Pacific coast, southwest of Tokyo, is due for a tremor with a magnitude of 8 or higher, as the Philippine plate is inching underneath the Eurasian plate, forming a subduction zone. Tokai last ruptured in 1854, and before that in 1707. Is another mega-quake just around the corner? Image: USGS

Japan

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Earthquakes and volcanic eruptions are an ever-present threat in Mexico, which is located on another of the world's subduction zones. The country is situated above three of the earth's tectonic plates. Large tremors in the coming decades could result from the Cocos tectonic plate sliding beneath the North American plate, causing the two of them to grind against each other and generate a slip-fault like the San Andreas. The most recent disaster in Mexico occurred in 1985, when a massive 8-magnitude tremor to the north of Acapulco shook the streets of Mexico City and Acapulco and took at least 9,500 lives. Image: Wikimedia Commons / Woudloper

Mexico

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The Cascadia Subduction Zone, a 700-mile stretch of closely-knit landmass, is situated 50 miles off the coast of Oregon, Washington, and southern British Columbia. The zone is capable of producing magnitude 9 earthquakes that could cause widespread damage to Portland, Seattle, and Vancouver. The last megaquake took place in 1700, in which a 9-magnitude shock generated a tsunami that traversed the Pacific Ocean, damaging parts of the Japanese coast. A massive tremor of magnitude 8 or greater could occur any time now, as quakes of this size typically strike every few hundred years. Image: USGS

Cascadia Fault

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Scientists fear that the Xianshuihe, Kunlun, and Min Jiang faults, strike-slip cracks comparable to San Andreas, might cause turbulence in the years to come. The 1556 earthquake in Hausien, in the Shaanxi Province of China, which killed almost 1 million people, was one of the worst natural disasters in recorded history. Since 1920, approximately 15 strong tremors, most of which measured magnitude 7 or higher, have shaken the region. In 2008, a rupture underneath the Longmen Shan mountain range in Sichuan province caused the devastating 7.8-magnitude quake that killed around 70,000 and put additional stress on three other major fault lines in China (pictured above). Scientists estimate that there is a 57 to 71 percent chance that another large quake will strike China in the next decade. Image: Geophysical Research Letters

China

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Italy is one of the most earthquake-prone regions in Europe. To its south lies the boundary of the African and Eurasian tectonic plates that partition the earth's crust. In 1905 and 1908, Europe's strongest-ever quakes, both more than magnitude 7, rattled the Messina Strait, a narrow strip of water that divides Sicily and the mainland, killing approximately 200,000 people. Aftershocks continued into 1913. By evaluating radon emissions, researcher Giampaolo Giuliani claims to have forecast the 6.3 earthquake that left tens of thousands of people homeless in central Italy last April (seismic map pictured)—although his claim was met with considerable skepticism from other experts. If Giuliani continues his prediction research, maybe he can tell us if another quake of similar magnitude will indeed strike this century, as some scientists have suggested. Century-old buildings line the streets of cities like L'Aquila where the 2009 earthquake hit, making the large tremors all the more damaging. Image: USGS

Italy

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The New Madrid Seismic Zone, located in southeastern Missouri, was once the most active seismic area in the United States east of the Rocky Mountains. In 1811 and 1812, five quakes measuring magnitude 8 or higher reportedly shook the area with a force 10 times stronger than that of the famous 1906 San Francisco earthquake, caused by a breakage in the San Andreas. Since the New Madrid Zone has witnessed seismic events of this magnitude every 250 years or so, scientists have predicted that another will come in the next half-century. Recently, however, movement along the fault lines has slowed considerably, leading some geologists to speculate that the zone might be shutting down. Image: USGS

New Madrid Seismic Zone

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