What Causes Earthquakes?

Local Shakes

The Herd of Elephants Theory

Have you ever felt the ground shake as a herd of elephants stampeded by? Has your mom ever told you that you sound (and feel!) like a herd of elephants because you are making the whole house shake? Is that an earthquake? Maybe it's just a "housequake". Have you ever felt your house shake when a truck drives by? Well, that is a very local earthquake. In all these cases, the earth shakes in response to a local shock. These shakes would show up on a seismograph. In fact, they do show up on seismographs and scientists have to know how to tell the difference between a big truck going by outside (or a herd of elephants stampeding down the hall) and a large earthquake halfway around the world.

The Nuclear Explosion Theory

On a larger scale an explosion can cause the earth to shake for a considerable distance. Scientists use seismographs to monitor nuclear tests. People in Las Vegas could feel the shaking caused by underground nuclear tests in the desert miles away. The government analyzes the shock waves (earthquakes) produced by nuclear explosions to study the effects of nuclear tests and to monitor tests elsewhere in the world.

The Extraterrestrial, or Meteor, Theory

Every day tiny meteors hit the earth, as we move through space. The vast majority of them burn up in the atmosphere, leaving no more trace than a shooting star across the sky. Once in a while, a meteorite will reach the surface of the earth. Very rarely a great meteorite will hit, causing the ground to shake and creating a large crater. The Meteor Crater in Arizona is an excellent example of this type of crater. Imagine how the ground shook for miles around when it was formed!

The moon is full of meteor craters that we can see because they have not eroded away. The earth also has been struck many times over its history. Erosion by wind and rain wear down the craters so we can't see most of them anymore. Scientists studying the earth have found traces of many meteor impacts around the world. Each impact creates an earthquake.


Earthquakes are one of the indicators of increased volcanic activity leading up to an eruption. As magma forces its way up into a volcano, it pushes aside the rocks in its way, causing bulges in the ground and a flurry of earthquakes. Scientists studying volcanoes watch for an increase in earthquakes to tell them that an eruption may be on the way. Using this and other measures of volcanic activity, they have been able to warn residents to evacuate before eruptions. Although they still cannot predict eruptions with absolute certainty, they are learning more all the time about what to look for to make better predictions. Much of this knowledge comes from studying volcanic earthquakes.

Plate Tectonics

Most Earthquakes are caused by Plate Tectonics. The earth's crust consists of a number of sections or plates that float on the molten rock of the mantle. These plates move on convection currents caused by heat rising from the center of the earth. The hot magma rises and spreads out on the surface, creating new crust. The crust spreads out forming a new plate until it meets another plate. One of the plates will be pushed down into the interior of the earth and reabsorbed into the mantle. Plates can also be compressed to push up mountains when they collide or move sideways along transform faults.

The plates are the Earth's crust that float on the molten rock in the center of the Earth ("isostasy"). Most of the inside of the Earth is so hot that the rock melts. Just as a pot of hot chocolate on the stove will bubble as it is heated; the molten rock, or magma, very slowly bubbles up in great currents under the surface of the Earth. The crust that floats on the magma moves with it, like the skin that might form on the hot chocolate. The Plates are just pieces of the crust. The part that makes it hard to understand is that it all moves so slowly. Even though the magma is very hot it is also very thick and under tremendous pressure in the middle of the Earth. So it moves only a few centimeters a year. Over millions of years, that adds up to a lot of movement. The three different types of earthquakes are 1) Transform , 2) Compressional and 3) Extensional.

1.) Strike-slip (Transform or "Sliding") Faults

In these, the fault plane is vertical or near vertical, and the motion of one block is in a horizontal direction parallel to the plane. The most famous example is that of the main San Andreas Fault. In the 1906 Earthquake, the west (Pacific) side moved north relative to east (California) side. There is little or no vertical movement on the fault, although the surface may show local slumping and slippage.

These faults typically occur when a plate is moving sideways relative to an adjoining one (eg. Pacific and North American Plates). They also occur near mid-ocean ridges (eg. central Atlantic), and combined with other fault zones (Kobe 1995 occurred on a transform fault).

2.) Compressional ("Thrust") Faults

These occur where forces are pushing two blocks of rock together. The fault plane will be dipping, with the upper ("Hanging Wall") block overriding the lower ("Foot Wall") block. These faults typically occur in mountain ranges where the thrusting of blocks causes mountain uplift (eg. Himalayas and Alps), or in subduction zones (eg. Japan). Due to the compression, the crust tends to thicken, hence it is no coincidence that most areas of compressional faulting are of relatively high terrain.

3.) Extensional, or "Normal" Faults

These occur where rocks are being stretched. Again, there is a dipping fault, but the upper ("Hanging Wall") block moves down. These are often likened to dominos leaning against one another. This effect can be seen in Nevada, where the faults form the steep sides of the mountain ridges.

Due to the stretching, the crust tends to thin, so areas experiencing extensional faulting tend to be low-lying. About 100 to 200 million years ago, this occurred in the North Sea. This provided an ideal environment for oil formation, and the area is still below sea level.

Extensional faults are, also, very common along mid-ocean ridges. For example, the Daly City Earthquake, because of the San Andreas fault, was, predominately, a transform fault. We would expect a strike-slip mechanism, like that of the 1906 San Francisco event and most other San Andrean earthquakes. Initial work immediately after the 1957 earthquake showed that this was unlikely. Unfortunately, the earthquake was limited in size, so records were limited. It was decided that the most likely mechanism was that of an extensional event. At first sight, it might be but there were a number of pieces of evidence that told us that we should expect some extensional faulting near Daly City.

Block Sliding