Earthquake – vibration of Earth produced by the rapid release of energy

Focus – the point within Earth where the earthquake starts

Epicenter – location on the surface directly above the focus

Fractures in Earth where movement has occurred

Earthquakes are usually associated with faults in Earth’s crust and mantle

Elastic Rebound Hypothesis

When rocks are deformed, they first bend and then break, releasing stored energy

Most earthquakes are produced by the rapid release of elastic energy stored in rock that has been subjected to great forces

When the strength of the rock is exceeded, it suddenly breaks, causing the vibrations

Aftershocks – a small earthquake that follows the main earthquake

Usually much weaker than the main earthquake

Can sometimes destroy structures weakened by the main EQ

Foreshocks – small earthquake that often comes before a major earthquake

Can happen days or years before the major quake

Transform fault boundaries

Produce smaller, shallow-focus earthquakes that occur 0-40 miles deep

Converging plate boundaries

Can produce deep-focus earthquakes that occur 180 miles or more below surface

1. Surface waves – seismic waves that travel along Earth’s outer layer

a) Travel along the ground and cause the ground and anything resting upon it to move

b) Movement is like ocean waves that toss a ship

c) Up-and-down motion as well as side-to-side motion

d) Most destructive earthquake waves

2. Body waves – other waves that travel through Earth’s interior

a. P waves – push (compress) and pull (expand) rocks in the direction the waves travel

▪ Aka compression waves

▪ Travel through solids, liquids, and gases

▪ Have the greatest velocity of all earthquake waves

▪ Move through the entire earth-crust, mantle, and core like a slinky

b. S waves – shake particles at right angles to the direction that they travel

Aka transverse waves

Travel only through solids

Slower velocity than P waves

Move through the ground (Earth’s outer layer) like a rope

3. A seismogram shows all three types of seismic waves

a. First P wave, then first S wave, and then surface waves

▪ Hint: Alphabetical order

1. The greater the interval measured on a seismogram between the arrival of the first P wave and the first S wave, the greater the distance to the earthquake source

2. Earthquake direction

a. The precise location can be found when the distance is known from three or more different seismic stations

b. The point where the three circles intersect is the epicenter of the quake

3. Earthquake zones

a. Most earthquakes occur around the outer edge of the Pacific Ocean

1. Two different types of measurements to describe size of earthquake

a. Intensity – a measure of the amount of earthquake shaking at a given location based on the amount of damage

▪ Not quantitative measurement

b. Magnitudes – a measure of the size of seismic waves or the amount of energy released at the source of the earthquake

▪ Quantitative measurements – rely on calculations using seismograms

2. Richter Scale

a. Based on the amplitude of the largest seismic wave recorded on a seismogram

b. Most EQ measurements you hear on news reports use Richter scale

c. Scientists no longer use it

3. Moment Magnitude

a. Most widely used measurement for earthquakes because it is the only magnitude the estimates the energy released by earthquakes

b. Calculated factors

▪ Average amount of movement along the fault

▪ Area of surface break

▪ Strength of the broken rock

Damage to building and other structures depends on these factors:

Intensity and duration of the vibrations

Nature of the material on which the structure is built

Design of the structure

Landslides

The greatest damage to structures is from landslides, or the sinking of the ground triggered by the vibrations

Fire

Start when gas and electrical lines were cut

Water lines may also break so that fires can’t be stopped

Tsunamis Triggered by an earthquake

Occurs where a slab of the ocean floor is displaced vertically along a fault

Can also occur when the vibration of a quake sets an underwater landslide into motion

Although tsunamis travel very rapidly, there is sufficient time to evacuate all but the area closest to the epicenter ▪ Tsunami Warning Center in Honolulu