# Earthquake Terminology And Science

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## Earthquake Terminology And Science

Tom Jordan (Director, Southern California Earthquake Center (SCEC)) gives expert video advice on: What is a geologic fault?; What is the "Richter Scale" of an earthquake?; What is the "Moment Magnitude Scale" of an earthquake? and more...

### What is a geologic fault?

A geologic fault is a zone within the earth's crust where there has been displacement of one side relative to the other. We see geologic faults as off-sets in geologic formations.

### What is the "Richter Scale" of an earthquake?

The Richter Scale was invented by Charles Richter in 1935, in Southern California, and was designed to measure the size of earthquakes, the size of fault ruptures that cause earthquakes specifically. Charles Richter did that by looking at the size of the waves that the earthquakes produced. The scale is based on measurements of the size of those waves.

### What is the "Moment Magnitude Scale" of an earthquake?

The "Moment Magnitude Scale" measures the size of the earthquakes, much like the Richter Scale does. However, the Moment Magnitude Scale uses different types of measurements that give us a more precise reading on the size of the fault that ruptured to produce the earthquake.

### What is the "epicenter" and "hypocenter" of an earthquake?

The epicenter of an earthquake is the point on the surface of the ground directly above where the earthquake begins. The hypocenter of an earthquake is the point below the earth's surface where the rupture begins. In the 1964 San Francisco earthquake, for example, the hypocenter was located about 1 kilometers below the surface, just west of the San Francisco peninsula.

### What is an earthquake "fault"?

An earthquake fault is a zone within the earth's crust where the rocks have been weakened by previous earthquakes, and where we expect earthquake slip to occur.

### What are the different types of earthquake faults?

There are various types of earthquake faults, usually recognized by the orientation of the fault and the slip on the fault. For example, we have "Strike-Slip" faults, where the orientation is a vertical plane across which motion occurs horizontally. We have "Normal" earthquake faults, where the fault plane is dipping at an angle to the surface and the motion is the upper block, downward. We have "Reverse" faults, where the plane is dipping and the motion is the upper block, upward. A "Thrust" earthquake fault is a low-angle Reverse fault, so that corresponds to a fault plane which is dipping at shallow angles to the surface, and along which the motion of the upper block is upward.

### What are tectonic plates?

The earth's surface is broken into a number of tectonic plates. These are regions that comprise the earth's crust and the upper part of its mantel, about 1 kilometers thick, that can extend laterally for large distances. For example, the Pacific Plate occupies most of the Pacific Basin and has dimensions as large as 1, kilometers across. We have, in addition to large plates such as the Pacific Plate, the North American Plate, the Eurasian Plate, we have a number of smaller plates, which can also be significant sources of tectonic activity.

### What is the USGS?

The USGS is an acronym that stands for the United States Geological Survey. The USGS is the government agency that has the responsibility for mapping geologic features in the United States and for understanding geologic hazards, such as earthquakes, landslides and volcanoes.

### What is SCEC?

SCEC stands for the Southern California Earthquake Center, which is a large collaboration among scientists, in universities and in the government, to understand earthquakes in Southern California and communicate that understanding to the public. The SCEC works so that we might understand earthquakes, throughout the country and across the world, much better.