Earthquakes begin as parts of the complicated process of the Earth's heat flow.
To begin with, the vast majority of the heat in Earth's interior—up to 90 percent—is fueled by the decaying of radioactive isotopes like Potassium 40, Uranium 238, 235, and Thorium 232 contained within the mantle. The amount of heat caused by this radiation is almost the same as the total heat measured leaving the Earth.
Both the measurements and simulations show that the hottest part of Earth's interior is the iron core. Part of the heat down there is actually left over from the fiery formation of Earth; part is from latent heat released by the freezing of liquid iron in the outer core onto the solid inner core, and part is (possibly) from the slow decay of naturally radioactive elements like uranium and potassium mixed in the core. The core heats the bottom of the rocky mantle. The hottest rock near the bottom of the mantle becomes slightly less dense than the somewhat cooler rock above it, so buoyancy forces try to push the hottest rocks upward. Although the rock in the mantle is solid, the pressures and heat are so great that the rock can deform slowly, like hot wax. So the hot rock creeps upward through the cooler rock. As the hot rock rises, cooler rock flows downward to take its place next to the core, where it is heated and becomes buoyant enough to rise again later. The rising hot rock comes in contact with cold rocks near the surface of Earth where it gives off its heat, cools, and sinks again. Most of the rock in the mantle moves in this broad cyclic flow, indicated by the arrows in the figure. This zone, where rock is soft enough to flow, is called the asthenosphere.
When material in the center of the Earth changes from a liquid to a solid, heat is released. The solidified material also expands, which increases the pressure, thereby increasing the temperature. "The inner core is becoming larger by about a centimeter every thousand years," Marone says.
(This means of heat transport--the cyclical movement of hot and cold material--is called convection. You can see an example of this in your kitchen by heating a pan of water to what is called a "rolling boil": hot water from the bottom of the pan rises up the sides, flows to the center, and sinks to the bottom again.)
Occasionally, however, masses of hotter-than-normal rock rise independently of the broad flow, like bubbles through a flowing stream. These masses of very hot rock form rising columns with rounded tops, called plumes.
Rock near the surface of Earth is so cold and at such low pressures that it cannot flow like mantle rock. So how does heat get through this rigid layer lithosphere, to the surface? One way is by conduction which describes heat flow in an iron pan held over a fire. The part of the pan over the flame gets hot first, followed by the handle, which is not over the flame. The heat in the handle came from the pan, but there was no movement of hot material from one part of the pan to another as in convection. (The metal in the pan and handle certainly didn't flow!) The heat, which is vibrations of atoms in the solid pan, moves as a result of fast moving (hot) atoms bouncing off slow moving (cool) atoms, causing the slow atoms to move faster (heat up). So at the top of the asthenosphere, the hot rock flows along the bottom of the lithosphere, transferring its heat to the cold rocks by conduction. The heat then flows through to the surface, again by conduction.
A second way of getting heat through the lithosphere is more exciting: melt some of the mantle rock and let it flow through cracks in the lithosphere to the surface! Sound familiar? Places where liquid rock (lava) flows onto Earth's surface are usually called volcanoes!
How does all this relate to the motion of the plates on Earth's surface? The movement of heat by convection in the asthenosphere causes the rock of the mantle to slowly move in huge streams. The solid (but brittle) rock of the lithosphere is resting directly on top of the solid (but soft) rock of the asthenosphere. As the rock of the asthenosphere moves in different directions, it carries parts of the lithosphere along with it. The lithospheric rock can't stretch, so it breaks into pieces--forming the plates. Interestingly, once the plates form, they begin to act somewhat independently of the convection flow because their cold edges tend to sink into the mantle. The detailed relation between of the motions of the plates and the underlying convective motions is still being studied.
This whole group of observations and ideas describing the motions of the plates and their associated geologic features is called plate tectonics. The word plate, of course, refers to the pieces of rigid lithosphere that comprise Earth's surface. Tectonics is derived from the Greek word for builder and is used in geology to describe structures like folds, faults, and mountains. Since one of the important results of plate collisions is rock fracture and mountain building, the use of this word should also be clear. Plate tectonics is the current "paradigm," or unifying philosophy, for understanding most of the geologic features on the surface of Earth. The development of plate tectonics in the 1960s and 1970s represented an enormous leap forward in understanding how Earth works.
The Earth’s outermost layer is fragmented into about 15 major slabs called tectonic plates. These slabs form the lithosphere, which is comprised of the crust (continental and oceanic) and the upper part of the mantle. Tectonic plates move very slowly relative to each other, typically a few centimetres per year, but this still causes a huge amount of deformation at the plate boundaries, which in turn results in earthquakes.
Observations show that most earthquakes are associated with tectonic plate boundaries and the theory of plate tectonics can be used to provide a simplified explanation of the global distribution of earthquakes, while some of the characteristics of earthquakes can be explained by using a simple elastic rebound theory.
Plate tectonic map of the world showing direction of movement. BGS ©UKRI.
The Ring of Fire is where 81% of earthquakes in the world occur.
Over 81 per cent of large earthquakes occur around the edges of the Pacific Ocean, an area known as the ‘Ring of Fire’; this where the Pacific plate is being subducted beneath the surrounding plates. The Ring of Fire is the most seismically and volcanically active zone in the world.
Below the tectonic plates lies the Earth’s asthenosphere. The asthenosphere behaves like a fluid over very long time scales. There are a number of competing theories that attempt to explain what drives the movement of tectonic plates. Three of the forces that have been proposed as the main drivers of tectonic plate movement are:
- mantle convection currents: warm mantle currents drive and carry plates of lithosphere along a like a conveyor belt
- ridge push (buoyant upwelling mantle at mid-ocean ridges): newly formed plates at oceanic ridges are warm, so they have a higher elevation at the oceanic ridge than the colder, more dense plate material further away; gravity causes the higher plate at the ridge to push away the lithosphere that lies further from the ridge
- slab pull: older, colder plates sink at subduction zones because, as they cool, they become more dense than the underlying mantle and the cooler, sinking plate pulls the rest of the warmer plate along behind it
has shown that the major driving force for most plate movement is slab pull, because the plates with more of their edges being subducted are the faster-moving ones. However, ridge push is also presented in recent research to be a force that drives the movement of plates.
Mantle convection currents, ridge push and slab pull are three of the forces that have been proposed as the main drivers of plate movement (based on What drives the plates?
). BGS © UKRI. All rights reserved.
There are three types of plated boundary:
divergent: plates moving apart
- convergent: plates coming together
- transform: plates moving past each other
Boundaries between tectonic plates are made up of a system of faults. Each type of boundary is associated with one of three basic types of fault, called normal, reverse and strike-slip faults.
Elastic rebound theory was originally proposed after the great San Francisco earthquake in 1906 by the geologist Henry Fielding Reid, to explain the deformation caused by earthquakes.
Before an earthquake, the buildup of stress in the rocks on either side of a fault results in gradual deformation. Eventually, this deformation exceeds the frictional force holding the rocks together and sudden slip occurs along the fault. This releases the accumulated stress and the rocks on either side of the fault return to their original shape (elastic rebound) but are offset on either side of the fault.
Over time stresses in the Earth build up (often caused by the slow movements of tectonic plates). At some point the stresses become so great that the Earth breaks. An earthquake rupture occurs and relieves some of the stresses (but generally not all). BGS ©UKRI.
There are three basic types of fault: normal, reverse and strike-slip. Certain types of fault are characteristic of the different plate boundaries, although often more than one type of fault occurs there. This can help us understand the relative movement of the plates and the type of deformation.
are earthquakes that precede larger earthquakes in the same location. An earthquake cannot be identified as a foreshock until after a larger earthquake in the same area occurs.
are smaller earthquakes that occur in the same general area during the days to years following a larger event or "mainshock."
They occur within 1-2 fault lengths away and during the period of time before the background seismicity
level has resumed. As a general rule, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the mainshock. The frequency of these aftershocks decreases with time. Historically, deep earthquakes (>30 km) are much less likely to be followed by aftershocks than shallow earthquakes. (modified from Univ. of Washington)
In California there are two plates - the Pacific Plate and the North American Plate. The Pacific Plate consists of most of the Pacific Ocean floor and the California Coast line. The North American Plate comprises most the North American Continent and parts of the Atlantic Ocean floor. The primary boundary between these two plates is the San Andreas Fault. The San Andreas Fault is more than 650 miles long and extends to depths of at least 10 miles. Many other smaller faults like the Hayward (Northern California) and the San Jacinto (Southern California) branch from and join the San Andreas Fault Zone.
The Pacific Plate grinds northwestward past the North American Plate at a rate of about two inches per year. Parts of the San Andreas Fault system adapt to this movement by constant "creep"
resulting in many tiny shocks and a few moderate earth tremors. In other areas where creep is NOT constant, strain can build up for hundreds of years, producing enormous earthquakes when it finally releases and the resulting earthquake may destroy a city.
During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between. Faults are classified using the angle of the fault with respect to the surface (known as the dip) and the direction of slip along the fault.
Faults that move along the direction of the dip plane are called dip-slip faults while strike-slip faults are classified as either right-lateral or left-lateral. Faults which show both dip-slip and strike-slip motion are known as oblique-slip faults.
Boundaries between tectonic plates are made up from a system of faults. Each type of boundary is associated with one of three basic types of fault, called normal, reverse and strike-slip faults. BGS ©UKRI. All rights reserved.
If we look at the pattern of where earthquakes occur around the world, it is clear that most of the activity is concentrated in a number of distinct earthquake belts; for instance the edge of the Pacific Ocean, or in the middle of the Atlantic Ocean.
Let's turn to the Richter Scale, which measures the strength of an earthquake and runs from 1 - 10 (1 being the least in magnitude and 10 being the greatest), but it is logarithmic. This means that for each 1 point in increase on the scale we get 10 times more ground shaking. Let's look at an example. Say we have a magnitude 1 earthquake on the Richter scale, which is the lowest magnitude earthquake. Compare that with a magnitude 2 earthquake, which is only one step higher (remember, the scale runs from 1 - 10), and you now have 10 times more ground shaking than with the magnitude 1 quake.
Take another step up the scale, so from magnitude 1 to magnitude 3, and this is 10 times more than that first step, so we now have 100 times more ground shaking with just two steps up the scale. That's a lot of seismic activity!
Now see how this relates to the total energy released during an earthquake. Through measurements of seismic activity, scientists know that the energy released by an earthquake, which is what causes all that shaking and moving in the first place, increases 32 times for each step up the Richter scale. Take our example from before where we have a magnitude 1 and a magnitude 2 quake. The magnitude 2 quake will have 32 times more energy than the magnitude 1 earthquake.
Now let's compare our magnitude 1 and magnitude 3 quakes. The magnitude 3 quake is two steps up the Richter scale, so we have 32 times more energy for the first step and then 32 times more energy than that up the second step. 32 times 32 gives us about 1,000 times the energy with just two steps up the scale! This is even more amazing when you compare this with the difference in ground shaking. A magnitude 3 earthquake has 100 times more ground shaking than a magnitude 1 quake but about 1,000 times the energy. That's a big difference!
The Richter scale was originally designed to measure medium-sized earthquakes, those between magnitude 3 and 7, and within a distance of about 400 miles. The moment magnitude scale was created in 1979 to deal with these issues, but it builds on the Richter scale because it was already so accurate for small- to medium-sized quakes. The moment magnitude scale is the currently accepted scale used to measure medium- to large-sized earthquakes.
The moment magnitude scale (MMS) was created in 1979 as a means of measuring medium to large earthquakes because of problems and inability to give reliable results (when applied to earthquakes of magnitudes of 7and above) using the Richter Scale, which was developed by Charles Richter and Beno Gutenberg in the 1930’s.
In 1979, because the Richter Scale had problems in measuring medium-sized to large quakes; two Cal Tech seismologists; Thomas C. Hanks* and Hiroo Kanamori*, came up with the moment magnitude scale, successor to the Richter Scale (or local magnitude). Their goal was to quantify medium-sized earthquakes between 3.0 and 7.0 in Southern California.
The moment magnitude scale enables seismologists to compare the energy released by different earthquakes on the basis of the area of the geological fault that ruptured in the quake. The MMS symbol is M with W written as superscript which means “mechanical work.” The “W” indicates the work accomplished; the magnitude is based on the moment of the earthquake, which is equal to the rigidity of the earth multiplied by the average amount of skip on the fault and size of the area slipped. The seismic moment M and O written superscript is a measure of the total amount of energy that is transformed during an earthquake. It is now the most common measure for medium to large earthquake magnitudes but it does break down for smaller earthquakes. The US Geological Survey does not use this scale for earthquakes with a magnitude of less than 3.5, which is the majority of earthquakes.
The moment magnitude scale assigns a single number to quantify the size of an earthquake. The magnitude is based on the moment of earthquake which is equal to the rigidity of the earth multiplied by the average amount of slip on the fault and size of the area that slipped. The formulae is different than the Richter scale; however, the moment magnitude scale still has a continuum of magnitude values defined by the older one. The moment magnitude scale is used to estimate magnitudes for all modern and large earthquakes by the US Geological Survey. It is logarithmic like the Richter.
Seismologists often no longer follow Richter’s original methodology because it was not designed to use data from earthquakes recorded at epicenter distances greater than about 600 km. According to USGS Geologiocal Survey, the moment magnitude is preferred for all earthquakes listed in their catalog. The least complicated and probably most accurate terminology is to use the term “magnitude” and to use the symbol “M” (without any subscripts or superscripts).
For very large earthquakes, the moment magnitude gives the most reliable estimate of earthquake size. The recent earthquake that hit Haiti measured 7.0 on the moment magnitude scale, according to the U.S. Geological Survey, thought to have killed between 100,000 and 200,000 people. Eight days later, an aftershock was also measured by the moment magnitude scale as 6.0.
* Their original 1979 paper: A Moment Magnitude Scale,
THOMAS C. HANKS
U.S. Geological Survey, Menlo Park, California 94025
Seismological Laboratory, California Institute of Technology, Pasadena, California 9II25
Earthquakes can potentially happen anywhere, but there are areas around the world that are more likely to experience an earthquake. These earthquake zones are areas in which tectonic plates meet and move more often. When I lived and worked in the Los Angeles area I experienced many earthquakes and I was often awakened by them, realized I had just experienced a low intensity earthquake, and just rolled back over in bed and went back to sleep - such was their number and low intensity for those years.