Quaking, Shaking, Earth All about Earthquakes By: Mrs. Baker Edited by Mrs. Fowler 2015.

Quaking, Shaking, Earth

All about EarthquakesBy: Mrs. Baker

Edited by Mrs. Fowler 2015

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What is an earthquake?

• Simply put:– An earthquake is the shaking of the earth.


Many buildings in Charleston,

South Carolina, were damaged or destroyed by

the large earthquake that

occurred August 31,

1886.

Picture from the United States Geological Service www.usgs.gov


San Francisco, California, Earthquake April 18, 1906. East side of Howard Street near Seventeenth Street. All houses shifted toward the left. The tall house dropped from its south foundation wall and leaned against its neighbor. 1906.

Picture from USGS.GOV


San Fernando, California, Earthquake February 1971. Collapsed overpass connecting Foothill Boulevard and the Golden State Freeway. Feb 10, 1971.

Photo by R.E. Wallace, USGS.

www.usgs.gov


What causes an earthquake?

• Earthquakes are the Earth's natural means of releasing stress.

• Due to the constant motion of the Earth’ plates, this put stress on the edges of the plates.

• To relieve this stress, the rocks tend to bend, compress, or stretch.

An aerial view of the San Andreas fault in the Carrizo Plain, Central California. Picture from www.usgs.gov


Locatin' the Shakin'

• Focus: the place on the Earth’s crust where the pressure was released.

• Epicenter: the spot on the Earth’s surface directly above the focus.


Faults

• If the force is great enough, the rocks will break.

• An earthquake is the vibrations produced by the breaking of rock.

• Most earthquakes occur near plate boundaries.

The Hanshin expressway in Kobe, Japan collapsed due to an earthquake in 1995.Picture from http://www.ce.washington.edu/~liquefaction/html/quakes/kobe/kobe.html

• Faults are classified by how the rocks move.

• Up• Down• Sideways



Normal Fault

• Rock above the fault surface moves downward in relation to rock below the fault surface.

Normal Fault

• Now, consider this: if we hold the foot wall stationary, gravity will normally want to pull the hanging wall down, right? Faults that move the way you would expect gravity to move them normally are called normal faults! Not so hard, is it?


• See the block farthest to the right that is shaped kind of like a foot? That's the foot wall. Now look at the block on the other side of the fault. See how it's resting or hanging on top of the foot wall block? That's the hanging wall.

http://geomaps.wr.usgs.gov/parks/deform/gfaults.html


Reverse Fault

• Reverse faults result from compression forces that squeeze rock.

• If rock breaks from forces pushing from opposite directions, rock above a reverse fault surface is forced up and over the rock below the fault surface.

Reverse Fault

• Can you see the foot-shaped foot wall and the hanging wall resting or hanging above it? Think about this: if we hold the foot wall stationary, where would the hanging wall go if we reversed gravity?

• The hanging wall will slide upwards, right? When movement along a fault is the reverse of what you would expect with normal gravity we call them reverse faults!


http://geomaps.wr.usgs.gov/parks/deform/gfaults.html


Strike-slip Fault

• At a strike-slip fault, rocks on either side of the fault are moving past each other without much upward or downward movement.

• The San Andreas Fault is the boundary between two of Earth’s plates that are moving sideways past each other.


How does energy created by an earthquake move through the

Earth?• Seismic waves

are energy waves that travel outward from the source of the earthquake.


Surfin the Waves

• When earthquakes occur, three different types of seismic waves are produced.


P Waves• Primary waves (P-

waves) cause particles in rocks to move back and forth in the same direction that the wave is traveling.

• P-Waves are the fastest waves and are felt first, usually as a bang or a thump.


S Waves

• Secondary waves (S-waves) move through Earth by causing particles in rocks to move up and down.

• These waves are slower than P-Waves.


L Waves

• Surface waves cause most of the destruction resulting from earthquakes.

• Surface waves (L-Waves) or land waves move rock particles in a backward, rolling motion and a side-to-side, swaying motion.

Wave Summary Chart

Primary Secondary Surface

Movement Push/pull Up/down, side/side

Up/down.Side/side

Material can travel through

Solid, liquid and gas

Rock or solid only

None, travel at the surface

Speed Fastest Medium Slowest

Location Inside crust Inside crust Earth’s surface



How are seismic waves measured?

• Seismic waves from earthquakes are measured with an instrument known as a seismograph.

• Seismographs register the waves and record the time that each arrived.


Measuring Earthquake Magnitude

•Magnitude is a measure of the energy that is released during an earthquake.

Earthquake MeasurementEarthquake Measurement


Richter Scale

• The Richter magnitude scale is used to describe the strength of an earthquake and is based on the height of the lines on the seismogram.


Richter Scale

• For each increase of 1.0 on the Richter scale, the height of the line on a seismogram is ten times greater.

• However, about 32 times as much energy is released for every increase of 1.0 on the scale.


Earthquake Severity

• Richter Earthquake Magnitudes Effects• Less than 3.5 Generally not felt, but recorded. • 3.5-5.4 Often felt, but rarely causes damage. • Under 6.0 At most slight damage to well-designed

buildings. Can cause major damage to poorly constructed buildings over small regions.

• 6.1-6.9 Can be destructive in areas up to about 100 kilometers across where people live.

• 7.0-7.9 Major earthquake. Can cause serious damage over larger areas.

• 8 or greater Great earthquake. Can cause serious damage in areas several hundred kilometers across.

TsunamiTsunami

Tidal waves or Tsunamis result when a large section of the sea floor suddenly moves and therefore displaces a massive amount of water.

Quaking, Shaking, Earth All about Earthquakes By: Mrs. Baker Edited by Mrs. Fowler 2015.

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