Earthquakes, Faults, Plate Tectonics, Earth Structure

Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. NOT ALL earthquakes result in surface rupture.

Earthquakes occur in the crust or upper mantle, which ranges from the earth's surface to about 700 kilometers deep (about 500 miles).

An earthquake is caused by a sudden slip on a fault. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel.

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

Sometimes earthquakes leave surface evidence, such as surface ruptures or fault scarps (cliffs made by earthquakes).

Sometimes a large earthquake has broken the fault since instrumental recordings began in 1932.

Sometimes the fault produces small earthquakes that can be recorded with the denser seismographic network established in the 1970s.

Earthquakes occur on faults - strike-slip earthquakes occur on strike-slip faults, normal earthquakes occur on normal faults, and thrust earthquakes occur on thrust or reverse faults. When an earthquake occurs on one of these faults, the rock on one side of the fault slips with respect to the other. The fault surface can be vertical, horizontal, or at some angle to the surface of the earth. The slip direction can also be at any angle.

A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement may occur rapidly, in the form of an earthquake - or may occur slowly, in the form of creep. Faults may range in length from a few millimeters to thousands of kilometers. Most faults produce repeated displacements over geologic time. 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.

Plate tectonics is the continual slow movement of the tectonic plates, the outermost part of the earth. This motion is what causes earthquakes and volcanoes and has created most of the spectacular scenery around the world.

For further information, see:
This Dynamic Earth: The Story of Plate Tectonics

Five billion years ago the Earth was formed by a massive conglomeration of space materials. The heat energy released by this event melted the entire planet, and it is still cooling off today. Denser materials like iron (Fe) sank into the core of the Earth, while lighter silicates (Si), other oxygen (O) compounds, and water rose near the surface.

Earthquakes induced by human activity have been documented in a few locations in the United States, Japan, and Canada. The cause was injection of fluids into deep wells for waste disposal and secondary recovery of oil, and the use of reservoirs for water supplies. Most of these earthquakes were minor. The largest and most widely known resulted from fluid injection at the Rocky Mountain Arsenal near Denver, Colorado. In 1967, an earthquake of magnitude 5.5 followed a series of smaller earthquakes. Injection had been discontinued at the