At 12 minutes past five on a spring morning in 1906, San Francisco was still asleep. Then, without warning, the ground began to roll. For about a minute, the whole city shook. Buildings fell, gas lines broke, and fires burned for three days.
0:16What happened beneath San Francisco that morning happens all over the planet. Somewhere on Earth, the ground shakes hard enough for people to feel it about every five minutes. Most of those earthquakes are small. A few change history.
0:30In this lecture, we'll start from the very beginning, with four questions. What makes the ground shake? Why do earthquakes strike where they do? How do we measure them? And what decides how much harm they do?
0:48Stuck, Then Snap
To understand that morning, start with the ground itself. Earth's outer shell isn't one solid piece. It's broken into giant plates that fit together like a cracked eggshell. The plates are always moving, but slowly, just a few centimeters a year. That's about as fast as your fingernails grow.
1:06Where two plates meet, their edges don't slide smoothly. They catch on each other and lock, while the plates keep moving. The rock along the edge slowly bends, like a ruler you push at one end. Year after year, it stores up energy.
1:20Eventually, the rock can't hold any more. It breaks along a crack called a fault, and the two sides lurch past each other. All that stored energy rushes out as waves through the ground. That sudden slip is an earthquake.
1:35That's what happened in 1906. The San Andreas fault slipped along nearly 300 miles, about 480 kilometers. Roads and fences that crossed it were torn apart. In places, the two sides moved as much as 20 feet, about 6 meters.
1:53A geologist named Harry Fielding Reid compared land surveys made before and after the earthquake. They showed that the ground had been slowly bending for decades, and then snapped back, like a spring let go. He called it elastic rebound. It's still how scientists explain earthquakes.
2:13Where They Strike
Put every earthquake on a map, and a pattern jumps out. They line up along the edges of the plates. The busiest edge circles the Pacific Ocean: the Ring of Fire, which has a lecture of its own. Plates meet in three ways, and each one shakes the ground in its own way.
2:29Where plates pull apart, as they do in Iceland, the crust stretches and cracks, and blocks of rock drop down. These earthquakes are usually shallow and fairly small.
2:40Where plates grind past each other sideways, the ground lurches side to side. Most of these faults lie under the sea. But on land, like the San Andreas, they can be very dangerous, because they run close to cities.
2:54Where plates push together, one usually slides beneath the other, at what's called a subduction zone. The two can lock together over a vast area. That's where the giants happen. Every earthquake of magnitude 9 or more ever measured struck a subduction zone.
3:11Where two continents push together, neither one sinks. Instead, the crust crumples and rises into mountains, like the Himalaya. Some of the deadliest earthquakes on land have struck along this belt, from Pakistan to China.
3:25A few earthquakes strike far from any plate edge. In the winter of 1811 and 1812, a series of powerful earthquakes shook the middle of the United States, around New Madrid, Missouri. They're rare, but they show that plate edges aren't the whole story.
3:44Measuring the Shaking
Every earthquake starts at one spot underground, where the fault first breaks, often less than 20 kilometers down. The break then races along the fault. The spot on the surface right above the start is the epicenter: the dot on an earthquake map. Waves spread out in every direction, like ripples in a pond.
4:04Instruments called seismometers record those waves, and they always arrive in the same order. First come P waves, which push and pull the rock. They're the fastest, and usually gentle. Next, S waves shake the rock up and down or side to side. Last come surface waves, rolling along the ground. The S waves and surface waves bring the strongest shaking.
4:27To compare earthquakes, scientists give each one a magnitude: a single number for how much energy it released. Each step up the scale means about 32 times more energy. So a magnitude 7 isn't a little bigger than a five. It releases about 1,000 times more energy.
4:45You may have heard of the Richter scale, the first magnitude scale, created in 1935. Scientists now use a newer scale, called moment magnitude, that works even for the very largest earthquakes.
4:59Magnitude describes the earthquake. Intensity describes the shaking: how hard the ground moved at one particular place. One earthquake has one magnitude, but many levels of shaking, strongest near the fault and weaker farther away.
5:15Intensity depends on more than distance. Of two earthquakes the same size, the shallower one shakes the surface harder. Soft ground, like loose sand or filled land, can make the shaking far worse. That's what happened in San Francisco's Marina District in 1989, the story told in our lecture on the World Series earthquake.
5:40Buildings, Not Earthquakes
Engineers have a saying: earthquakes don't kill people, buildings do. In January 2010, a magnitude 7 earthquake struck just outside Haiti's capital, Port-au-Prince. Many buildings were poorly built concrete and brick, with too little steel, and they collapsed. More than 100,000 people died.
5:59About six weeks later, a far stronger earthquake struck Chile. At magnitude 8.8, it released about 500 times as much energy. The shaking was strong, but Chile had strict building codes, learned from many deadly earthquakes, and most buildings stood. About 500 people died.
6:19Warning systems can help too. Because P waves arrive first, sensors near a fault can catch them and send an alert before the strong shaking arrives. Japan and the U.S. West Coast both have systems like this. The warning may last only seconds, but that's enough to slow trains, open fire station doors, and let people take cover. Right next to the fault, there may be no warning at all.
6:45Be ready before it starts: fasten tall, heavy furniture to the wall. When the shaking starts, drop to your hands and knees. Cover your head and neck, under a sturdy table if one is close. Hold on until the shaking stops. Don't run outside, and skip the doorway: in a modern home, it's no stronger than anywhere else.
7:05Strong shaking can bring other dangers: landslides on steep hills, fires from broken gas lines, and near the coast, a tsunami. If you're near the coast and the shaking is strong, or lasts more than about 20 seconds, head to high ground or inland right away. Don't wait for an official warning. The earthquake itself may be your only warning.
7:27After a big earthquake come aftershocks: smaller quakes nearby that can go on for weeks, even years. No one has ever reliably predicted the time, place and size of a major earthquake. Instead, scientists forecast the odds. California's official forecast calls a strong earthquake somewhere in the state all but certain within 30 years.
7:53Three Things
So, three things to remember. First, an earthquake is a sudden slip on a fault, releasing strain that built up slowly as the plates moved. Second, most earthquakes strike where plates meet, and the largest where one plate slides beneath another. And third, magnitude measures the earthquake, intensity measures the shaking, and buildings decide how much harm it does.
8:16The ground beneath you is moving a few centimeters every year. Earthquakes are the moments when it catches up. In Earthquakes two oh one, we'll go deeper: inside the fault, down through the planet's core, and into the science of forecasting.
