Arkansas Center for Earthquake Education and Technology Transfer

Earthquake Frequently Asked Questions
Faults - Earthquakes - Probability - Seismographs


Faults

A fault is a thin zone of crushed rock between two blocks of rock, and can be any length, from centimeters to thousands of kilometers.

It is a fracture in the crust of the earth along which rocks on one side have moved relative to those on the other side. Most faults are the result of repeated displacements over a long period of time.

Normal, dip-slip fault - The fault plane of a normal fault dips away from the uplifted crustal block. Faulting occurs in response to extension.

Reverse, dip-slip fault - The fault plane of a reverse fault dips beneath the uplifted crustal block. Faulting occurs in response to compression.

Strike-slip fault - Crustal blocks slide past each other. The slip may be left lateral or right lateral. This example shows a left-lateral, strike-slip fault.

A thrust fault is a reverse fault with a gently incline, or low angle of dip. If the slip on a fault is partly strike-slip and partly dip-slip, the fault is called an oblique-slip fault.

EQs occur on faults. When an EQ 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.

Strike-slip - occurs on an approximately vertical fault plane as the rock on one side of the fault slide horizontally past the other.

Dip-slip - The fault is at an angle to the surface of the earth and the movement of the rock is up or down.

(1) if the EQ left surface evidence, such as surface ruptures or fault scarps (cliffs made by EQs);

(2) if a large EQ has broken the fault since we began instrumental recordings in 1932; and

(3) if the faults produces small EQs that we can record with the denser seimographic network established in the 1970s.

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

Fault rupture almost always follows preexisting faults which are zones of weakness. Rupture may occur suddenly during an EQ or slowly in the form of fault creep. Sudden displacements are more damaging to structures because they are accompanied by shaking.

Fault creep is the slow movement of faults in the earth's crust.


Earthquakes

An EQ is caused by a sudden slip on a fault. Stresses in the earth's outer layer push the sides of the fault together. Stress builds up and the rocks slips suddenly, releasing energy in waves that travel through the rock to cause the shaking that we feel during an EQ.

An EQ occurs when plates grind and scrape against each other. In Arkansas, the North American plate attempted to split apart (rift) millions of years ago. Over time, sediments filled the rift. Today, the rift is gradually closing.

No. An EQ can occur virtually anywhere at any time. They cannot be prevented, however, damage, destruction and loss of life can be significantly reduced but because Mother Nature provides us with regular reminders of the dangers, we can prepare and reduce loss of life, injuries and propery damage if everyone sufficiently prepared themselves, their homes, work places and communities for a major EQ.

The hypocenter is the point where the earthquake rupture begins, usually deep down on the fault.

The epicenter is the point on the surface directly above the hypocenter.

There is nothing unusual about what's happening, it's perfectly natural. Many parts of the U.S. are known to be highly seismically active. Sometimes there is a short-term increase in seismicity in that region, in an area where we commonly see quakes, just usually not so many at once. These types of increases are not usually alarming or cause for concern. It is a high level of activity that has since slowed and the norm is just an occasional earthquake.


Probability

No. the Arkansas Earthquake Center does not predict earthquakes. Nor do we expect to do so any time in the foreseeable future.

No. Probabilities are estimated from the rate of aftershocks and these are sometimes confused with the prediction of a particular event.

Seismologists Arch Johnston and Susan Nava calculated in 1985 that a moderate magnitude New Madrid zone earthquake has a 100-year recurrence interval. It has been nearly 100 years since the 1895 earthquake, and no large quake has occurred. Johnston and Nava's study would indicate the statistical probability is high that another large earthquake could occur in the region.


Seismographs

Seismographs are instruments used to record the motion of the ground during an EQ. They are installed in the ground throughout the world and operate as seismographic networks; 1st developed ~1890.

Seismograms are the records (paper copy) produced by seismographs used to calculate the location and magnitude of an EQ. Shows how the ground moves with the passage of time. SEISMO

The HORIZONTAL axis = time (measured in seconds); VERTICAL axis= ground displacement (usually measured in millimeters) When there is NO EQ reading = straight line except for small wiggles caused by local disturbance or "noise."

A seismometer ia an internal part of the seismograph which may be a pendulum or a mass mounted on a spring.

There are several ways:

(1) a pen drawing an ink line on paper revolving on a drum;

(2) a light beam making a trace on a moving phographic film or

(3) electromagnetic system generating a current that is recorded electronically on tape

A modern digital seismic station recording a wide range of seismic motions.

Adapted from USGS Earthquake FAQ, 1996 April 12


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Arkansas Earthquake Center
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Last Updated: July 18, 2012

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