Big quakes can be devastating and deadly but thousands of earthquakes take place around the world every single day. The vast majority of all earthquakes are so weak that no one notices them-except for scientists who monitor sensitive seismic equipment at more than 4,000 scattered stations.
Earthquakes originate underground in the crust and upper mantle at depths of up to 500 miles (800 kilometers). Most big quakes happen at the faults where large tectonic plates of rock meet and move past one another in a slow, gradual process. When stresses cause sudden shifts of these plates they move violently from side to side or up and down, sending shock waves through the crust which we experience as earthquakes.
Scientists with seismic equipment monitor about half a million earthquakes a year and very few of them are even strong enough to be noticed by people on the surface. But larger earthquakes take a heavy toll. About 10,000 people lose their lives to earthquakes each year.
In terms of size the biggest earthquake of the modern era was a magnitude 9.5 temblor which rocked Chile on May 22, 1960. That quake killed some 1,655 people and left two million homeless. It also sparked tsunamis which brought subsequent death and destruction to the shores of Hawaii, Japan, and the Philippines.
Truly epic disasters, the so-called “great quakes” which level cities or spawn giant tsunamis, happen about once every five years on average.
The January 2010 Haitian quake occurred at the boundary of the Caribbean and North American plates. It left more than 220,000 people dead and 1.3 million displaced. Official estimates listed nearly 300,000 homes as destroyed or damaged.
Scientists cannot predict where and when earthquakes will occur, but they can use seismic data to estimate the likelihood of future events. The San Francisco Bay area has a 67 percent chance of experiencing a major earthquake sometime in the next 30 years. California is crossed by the San Andreas Fault system, where the Pacific and North American plates are slowly moving past one another horizontally. California is in no danger of dropping into the Pacific, but Los Angeles and San Francisco will one day sit next to one another.
The deadliest quake on record also struck in China. The epicenter of the 1556 event was about 50 miles (80 kilometers) northeast of Xi’an. Contemporary accounts tell of widespread destruction, city walls and homes in collapse, and fissures gushing canals of groundwater. Loss of life was appalling and is estimated to have been at least 830,000 people.
Earthquake frequency appears to be relatively consistent through the years. Yet earthquake danger to human is increasing because populations in earthquake-prone areas, like urban California and Japan, continues to rise and put more people at risk.
Where Do Most Earthquakes Occur?
Some 80 percent of all the planet's earthquakes occur along the rim of the Pacific Ocean, called the "Ring of Fire" because of the preponderance of volcanic activity there as well. Most earthquakes occur at fault zones, where tectonic plates—giant rock slabs that make up the Earth's upper layer—collide or slide against each other. These impacts are usually gradual and unnoticeable on the surface; however, immense stress can build up between plates. When this stress is released quickly, it sends massive vibrations, called seismic waves, often hundreds of miles through the rock and up to the surface. Other quakes can occur far from faults zones when plates are stretched or squeezed.
Earthquakes are unpredictable and can strike with enough force to bring buildings down. Find out what causes earthquakes, why they're so deadly, and what's being done to help buildings sustain their hits.
Earthquake Magnitude Ratings and Their Impacts
Scientists assign a magnitude rating to earthquakes based on the strength and duration of their seismic waves. A quake measuring 3 to 5 is considered minor or light; 5 to 7 is moderate to strong; 7 to 8 is major; and 8 or more is great.