Earthquakes and faults

Deep under the ground, rocks are always being pushed.

Sometimes they get stuck. Then, all at once, they slip. The ground shakes. That is an earthquake.

Most of the time the ground feels perfectly still. But deep underground, rocks are being slowly pushed and pulled by Earth's moving plates. The British Geological Survey (BGS) explains that an earthquake happens when rocks suddenly move along a fault, a crack in the Earth where blocks of rock can slide past each other.

As the rocks slip, they send out waves of energy through the Earth, spreading outward like the ripples when you drop a stone into a pond. When those waves reach the surface, the ground shakes. Most earthquakes happen near the edges of tectonic plates, but they can happen in other places too.

BGS defines earthquakes as the result of sudden movement along faults within the Earth, releasing stored elastic strain energy as seismic waves that make the ground surface shake. The movement is generally a response to long-term deformation and the build-up of stress, often from the slow motion of tectonic plates.

The point on the fault where rupture begins is the hypocenter, or focus; the point on the surface directly above it is the epicenter. Most earthquakes are associated with plate boundaries, and plate tectonics gives a simplified explanation of their global distribution. Earthquakes also occur within plates, though BGS notes that plate interiors do not usually have large ones, and the USGS lists examples such as New Madrid, Missouri (1811 to 1812) and Charleston, South Carolina (1886), with many years usually passing between such shocks.

A fault is a long crack in the rock.

The rocks on each side push on each other. They bend a little, like a ruler. Then they slip and spring back. Snap!

Imagine bending a plastic ruler. It bends and bends, storing up energy, until it suddenly snaps back. Rocks along a fault do something similar. The plates keep moving, but the rocks on each side of the fault are locked together by friction. They slowly bend and store energy. When the push gets too strong, the fault suddenly slips and the rocks spring back, in new places.

There are three main kinds of fault. In a normal fault, the rock above the fault slides down. In a reverse fault, it is pushed up. In a strike-slip fault, the blocks slide sideways past each other, like the San Andreas Fault in California.

Elastic rebound theory, proposed by geologist Henry Fielding Reid after the 1906 San Francisco earthquake, explains the cycle: stress builds in the rocks on either side of a fault, deforming them gradually, until it exceeds the friction holding them; sudden slip releases the accumulated stress, and the rocks rebound to their original shape but offset across the fault. The rupture relieves some, but generally not all, of the stress (BGS).

Faults are classified by their dip and the direction of slip. In a normal fault, the block above the fault moves down relative to the block below; in a reverse (thrust) fault, it moves up; in a strike-slip fault, motion is horizontal, right-lateral or left-lateral; oblique-slip faults combine both. Each plate-boundary type is dominated by one fault type: normal faults at divergent boundaries, reverse at convergent and strike-slip at transform boundaries.

An earthquake sends out waves of shaking.

The fastest wave arrives first. Then a slower one. Then the big rolling waves at the surface.

An earthquake sends out different kinds of waves, and they travel at different speeds. P-waves are the fastest, so they arrive first. They push and pull the rock, like a slinky squeezed and stretched. S-waves come next. They shake the rock from side to side, and they cannot travel through liquid.

Last come surface waves, which travel along the ground's surface. BGS says their shaking is often much bigger than the other waves, and it is the kind of shaking that damages buildings. Scientists record all of these waves with instruments called seismometers.

Body waves travel through the Earth. Compressional P (primary) waves move at about 5 to 7 km/s in typical crust and over 8 km/s in the mantle and core, and so arrive first. Transverse S (shear or secondary) waves travel at about 3 to 4 km/s in the crust and cannot pass through fluids, so they are absent in the liquid outer core, water and magma (BGS).

At a free surface the two combine into surface waves: Rayleigh waves, with a rolling motion like water waves, and Love waves, with horizontal side-to-side motion, both at about 2.0 to 4.5 km/s. Their ground motions are often much larger than those of body waves, and they produce the kind of shaking that causes buildings to fail. Seismometers work on inertia: a suspended mass tends to stay still while the ground moves, and three sensors record up-down, east-west and north-south motion.

Scientists use machines that feel the shaking.

They can tell where an earthquake happened. They can tell how big it was. Each step up the scale is a lot bigger.

Because P-waves are faster than S-waves, the farther away an earthquake is, the longer the gap between them. By measuring that gap at three or more seismometers, scientists can work out exactly where the earthquake started.

To say how big an earthquake was, scientists use magnitude. Each earthquake has one magnitude. The scale goes up in big jumps: the U.S. Geological Survey (USGS) explains that each step up, from 5 to 6, for example, means the waves on the seismometer are 10 times larger, and the earthquake released about 32 times more energy. The first magnitude scale was made by Charles Richter in 1935.

P- and S-waves leave the source together but separate with distance, so the S-minus-P time at a station gives its distance from the earthquake; distances from three stations fix the epicenter (BGS). Size is described in two ways. Intensity is a qualitative measure of shaking from its effects on people, objects and buildings, and it usually decreases with distance. Magnitude is a single value for the earthquake, a measure of the energy released, independent of distance.

Magnitude scales are logarithmic: each whole-number increase means a tenfold increase in measured amplitude and about 32 times more energy (USGS). Charles Richter developed the first scale in 1935 for southern California. Today moment magnitude (Mw), based on the fault's rupture area, slip and rock strength, is the standard because Richter's scale underestimates large events; reported magnitudes can change as more data are analyzed (BGS).

Engineers build houses and towers to be strong.

They tie the walls and floors together. Tall buildings can be designed to survive strong shaking. Strong buildings help keep people safe.

BGS explains that how much damage an earthquake does depends not only on its size, but also on the ground and on how buildings are made. Soft ground, such as river floodplains or filled-in land, shakes more than hard rock.

Engineers design buildings to stay standing. Walls and floors must be tied together properly. Wooden and steel frames that are braced correctly do much better than heavy bricks held by weak mortar. Tall buildings are made with stronger, heavier lower floors, and some sit on special foundations that reduce how much shaking reaches the building.

Damage reflects magnitude, local geology and construction (BGS). Soft sediments, as in floodplains, reclaimed land or former landfill, amplify shaking, and saturated ground can even liquefy and behave like quicksand. Poorly tied slab walls and floors, rubble masonry and weak mortar perform badly; correctly braced wooden and steel frames perform much better, and lightweight walls and roofs are safer than heavy ones.

Resonance matters too: a building shakes most when the frequency of the seismic waves is close to its own natural frequency, which depends on its height, so low-frequency shaking can strongly affect tall buildings while leaving low ones nearby less affected. Designers therefore make lower floors stronger and heavier than upper ones, avoid large unsupported spaces, add reinforcement such as steel cables, and may place a building on a foundation that reduces the shaking transmitted to it.

If the ground shakes, you can stay safe.

Drop down onto your hands and knees. Cover your head and neck. Hold on until the shaking stops.

The U.S. Geological Survey (USGS) says no scientist has ever predicted a major earthquake. Scientists can only work out the chance of one happening in an area over a number of years. So people in earthquake areas practice what to do. The U.S. government's Ready.gov site teaches three steps:

1. Drop where you are onto your hands and knees. 2. Cover your head and neck with one arm and hand. If a sturdy table or desk is nearby, crawl under it. If not, get next to an inside wall, away from windows. 3. Hold on until the shaking stops.

If you are inside, stay inside and do not run outside. If you are outside, move to an open area away from buildings, trees and power lines.

Ready.gov's guidance is Drop, Cover and Hold On: drop to hands and knees (or lock wheels for wheelchair users); cover the head and neck with one arm, sheltering under a sturdy table or desk if one is near, otherwise beside an interior wall away from windows; and hold on to the shelter, ready to move with it, until the shaking stops. If indoors, stay there rather than running outside, and avoid doorways; if in bed, turn face down and cover the head and neck with a pillow; if outdoors, move to an open area away from buildings, trees, streetlights and power lines.

The USGS states that neither it nor any other scientists have ever predicted a major earthquake, which would mean giving the date and time, location and magnitude, and that it does not expect to know how in the foreseeable future; it can only calculate the probability of a significant earthquake in an area within a certain number of years. The What If? lab on predicting earthquakes explores what might change if prediction were possible.