Earthquakes Explained: Crash Course Geology #14
CrashCourse
What Causes An Earthquake 0:00
An earthquake is any sudden shaking of the ground caused by waves of energy moving through the Earth. That energy can come from landslides, moving magma, or meteorite impacts, but most often it comes from a fault, an underground crack where blocks of rock slide against each other. Faults can stretch thousands of kilometers or be smaller than a fingernail, and many sit at the edges of tectonic plates, the massive slabs of crust that drift slowly over time. That slow movement builds stress in the rock, and when brittle rock can bend no further, one side of the fault slips and releases the pent-up energy as an earthquake.
How Seismologists Track The Waves 2:01
Seismologists study seismic waves using GPS, radar satellites, and sensitive machines called seismographs, with global data shared through a facility called SAGE. There are body waves, which start at the earthquake's hypocenter, including fast but usually mild P-waves and slower, more damaging S-waves that shake side to side. Surface waves travel along the ground and, despite being slowest, can be the most destructive because of their rolling motion. Intensity depends on distance from the epicenter and on rock age, which is why a 2011 Virginia quake was felt 600 miles away while a stronger California quake was felt only 250 miles away, since older East Coast rocks transmit waves farther. Magnitude, measured on the logarithmic moment magnitude scale, means each whole number jump represents ten times more energy.
Why Prediction Still Fails 6:31
The 2009 L'Aquila, Italy earthquake, a 6.2 magnitude event, killed over 300 people after officials wrongly reassured residents that earlier tremors were normal, leading to manslaughter convictions later overturned in 2014. True prediction, pinning down exact time, date, location, and severity, has never been achieved and isn't expected soon, according to the US Geological Survey. Instead, scientists forecast probability based on past earthquake patterns and sediment records, while places like Japan, Mexico, and California use early warning systems and stricter building codes to reduce harm.
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