Earthquakes are natural ground motions caused as the Earth releases energy. The science of earthquakes is seismology, "study of shaking" in scientific Greek.
Earthquake energy comes from the stresses of plate tectonics. As plates move, the rocks on their edges deform and take up strain until the weakest point, a fault, ruptures and releases the strain.
Earthquake Types and Motions
Earthquake events come in three basic types, matching the three basic types of fault. The fault motion during earthquakes is called slip or coseismic slip.
- Strike-slip events involve sideways motion—that is, the slip is in the direction of the fault's strike, the line it makes on the ground surface. They may be right-lateral (dextral) or left-lateral (sinistral), which you tell by seeing which way the land moves on the other side of the fault.
- Normal events involve downward movement on a sloping fault as the fault's two sides move apart. They signify extension or stretching of the Earth's crust.
- Reverse or thrust events involve upward movement, instead, as the fault's two sides move together. Reverse motion is steeper than a 45 degree slope, and thrust motion is shallower than 45 degrees. They signify compression of the crust.
Earthquakes can have oblique slip that combines these motions.
Earthquakes don't always break the ground surface. When they do, their slip creates an offset. Horizontal offset is called heave and vertical offset is called throw. The actual path of fault motion over time, including its velocity and acceleration, is called fling. Slip that occurs after a quake is called postseismic slip. Finally, slow slip that occurs without an earthquake is called creep.
The underground point where the earthquake rupture begins is the focus or hypocenter. The epicenter of an earthquake is the point on the ground directly above the focus.
Earthquakes rupture a large zone of a fault around the focus. This rupture zone may be lopsided or symmetrical. Rupture may spread outward evenly from a central point (radially), or from one end of the rupture zone to the other (laterally), or in irregular jumps. These differences partly control the effects that an earthquake has at the surface.
The size of the rupture zonethat is, the area of fault surface that rupturesis what determines the magnitude of a earthquake. Seismologists map rupture zones by mapping the extent of aftershocks.
Seismic Waves and Data
Seismic energy spreads from the focus in three different forms:
- Compression waves, exactly like sound waves (P waves)
- Shear waves, like waves in a shaken jumprope (S waves)
- Surface waves resembling water waves (Rayleigh waves) or sideways shear waves (Love waves)
P and S waves are body waves that travel deep in the Earth before rising to the surface. P waves always arrive first and do little or no damage. S waves travel about half as fast and may cause damage. Surface waves are slower still and cause the majority of damage. To judge the rough distance to a quake, time the gap between the P-wave "thump" and the S-wave "jiggle" and multiply the number of seconds by 5 (for miles) or 8 (for kilometers).
Seismographs are instruments that make seismograms, or recordings of seismic waves. Strong-motion seismograms are made with rugged seismographs in buildings and other structures. Strong-motion data can be plugged into engineering models, to test a structure before it is built. Earthquake magnitudes are determined from body waves recorded by sensitive seismographs. Seismic data is our best tool for probing the deep structure of the Earth.
Seismic intensity measures how bad an earthquake is, that is, how severe shaking is at a given place. The 12-point Mercalli scale is an intensity scale. Intensity is important for engineers and planners.
Seismic magnitude measures how big an earthquake is, that is, how much energy is released in seismic waves. Local or Richter magnitude ML is based on measurements of how much the ground moves, and moment magnitude Mo is a more sophisticated calculation based on body waves. Magnitudes are used by seismologists and the news media.
The focal mechanism "beachball" diagram sums up the slip motion and the fault's orientation.
Earthquakes cannot be predicted, but they have some patterns. Sometimes foreshocks precede quakes, though they look just like ordinary quakes. But every large event has a cluster of smaller aftershocks, which follow well-known statistics and can be forecasted.
Plate tectonics successfully explains where earthquakes are likely to occur. Given good geologic mapping and a long history of observations, quakes can be forecasted in a general sense, and hazard maps can be made showing what degree of shaking a given place can expect over the average life of a building.
Seismologists are making and testing theories of earthquake prediction. Experimental forecasts are beginning to show modest but significant success at pointing out impending seismicity over periods of months. These scientific triumphs are many years from practical use.
Large quakes make surface waves that may trigger smaller quakes great distances away. They also change stresses nearby and affect future quakes.
Earthquakes cause two major effects, shaking and slip. Surface offset in the largest quakes can reach more than 10 meters. Slip that occurs underwater can create tsunamis.
Earthquakes cause damage in several ways:
- Ground offset can cut lifelines that cross faults: tunnels, highways, railroads, powerlines and water mains.
- Shaking is the greatest threat. Modern buildings can handle it well through earthquake engineering, but older structures are prone to damage.
- Liquefaction occurs when shaking turns solid ground into mud.
- Aftershocks can finish off structures damaged by the main shock.
- Subsidence can disrupt lifelines and harbors; invasion by the sea can destroy forests and croplands.
Earthquake Preparation and Mitigation
Earthquakes cannot be predicted, but they can be foreseen. Preparedness saves misery; earthquake insurance and conducting earthquake drills are examples. Mitigation saves lives; strengthening buildings is an example. Both can be done by households, companies, neighborhoods, cities and regions. These things require a sustained commitment of funding and human effort, but that can be hard when large earthquakes may not occur for decades or even centuries in the future.
Support for Science
The history of earthquake science follows notable earthquakes. Support for research surges after major quakes and is strong while memories are fresh, but gradually dwindles until the next Big One. Citizens should ensure steady support for research and related activities like geologic mapping, long-term monitoring programs and strong academic departments. Other good earthquake policies include retrofitting bonds, strong building codes and zoning ordinances, school curricula and personal awareness.