Earthquakes

• An earthquake is a shaking of the Earth’s crust caused by a release of energy.

• A fault is a break in the lithosphere along which movement has occurred.

• The focus of an earthquake is the point at which movement first occurs.

• The epicenter of an earthquake is the point on the surface of the Earth directly above the focus.

Earthquake animation

Earthquake animation

April 18th 1906 San Francisco Earthquake, Van Ness Street

Days after the earthquake residents watch the city burn.

Downtown Anchorage Alaska after the 1964 earthquake.

Tsunami from the 1964 earthquake destroyed the waterfront in Kodiak Alaska.

A 1976 earthquake near Guatemala City shattered this bridge in Agua Caliente, cutting off the city’s main supply route to the Atlantic. The 7.5-magnitude quake killed more than 23,000 people

An earthquake's destructive power varies depending on the composition of the ground in an area and the design and placement of manmade structures.

The Tsunami caused by the 1964 Alaskan earthquake was responsible for 122 deaths.

The Tsunami moved at 400 mph reaching the Hawaiian Islands, Oregon, and California.

Ten people died in Crescent City California.

Downtown Crescent City, March 1964

1952 Bakersfield Earthquake

Magnitude 7.5 along the White Wolf Fault

Beale clock tower after the 1952 earthquake.

Locations of Earthquakes

• Earthquakes occur predominantly at plate boundaries where neighboring plates move past each other.

• Understanding the theory of plate tectonics helps geoscientists understand and predict earthquakes.

USGS Earthquake Location Website

Causes of Earthquakes

• Magma rising towards the surface of a volcano. Yellowstone and the Long Valley Caldera (Mammoth)

• Rock bursts/collapses in deep mines. • The majority of earthquakes are caused by two

faces of rock slipping past each other along an approximately flat internal surface, called a fault.

What Governs the Timing of Earthquakes?

• There is currently no agreed-upon method to predict the location, size, and probable time of an impending earthquake on a particular fault.

• It is possible to forecast the likelihood of earthquakes of a given size in a fault region over time periods of decades to centuries.

Look what happens above Look what happens above and below the ground.and below the ground.

Types of Faults• Strike-slip faults are vertical (or

nearly vertical) fractures where the blocks have mostly moved horizontally.

• Dip-slip faults are inclined fractures where the blocks have mostly shifted vertically.

• If the rock mass above an inclined fault moves down, the fault is termed normal, whereas if the rock above the fault moves up, the fault is termed reverse. A thrust fault is a reverse fault with a dip of 45° or less.

Fault Animation

Earthquakes lift and turn Earthquakes lift and turn mountains.mountains.

Types of Stress

• Compression stress is the stress that squeezes something. Thrust Fault

• Tensional stress is the stress that tends to pull something apart. Normal Fault

• Shear stress is the stress component parallel to a given surface. Strike Slip Fault

Measuring an Earthquake

• The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions and measured with a seismometer.

Type of Seismic Waves

• P- Wave is a seismic body wave that shakes the ground back and forth in the same direction and the opposite direction as the direction the wave is moving.

• S-Wave, or shear wave, is a seismic body wave that shakes the ground back and forth perpendicular (right angle) to the direction the wave is moving.

• Surface Wave (2) a. Love, b. Raleigh- travel near the earth's surface.

Wave Animation

The Earthquake sets off a The Earthquake sets off a chain reaction.chain reaction.

Surface waves are responsible for most of the damage associated with earthquakes, because they cause the most intense vibrations.

•S waves cannot travel through the liquid core

•P waves are refracted by the liquid core

Earthquake Scales

Richter Scale is calculated from the amplitude of the largest seismic wave.

The scale is logarithmic which means that each unit increases the level of ground shaking by a magnitude of ten.

Each unit increase release an amount of energy equal to 32. Therefore a magnitude 7 releases 32 times the energy of a magnitude 6 and shakes 10 times as much.

Intensity Scale Description Effects

less than 2 Micro not felt

2-3 Very Minor not felt but recorded

3-4 Minor often felt, no damage

4-5 Light shaking observed

5-6 Moderate Some damage

6-7 Strong damaging over a 100 mile area

7-8 Major serious damage over wider area

greater than 8 Great Serious damage over several hundred miles

Richter Scale

World's Biggest Earthquakes

California Earthquakes

The Mercalli scale measure the amount of damage an earthquake causes.

IntensityScale

Classification Effect description

I Instrumental Detected only by seismographs

II Feeble Noticed by sensitive people

III Slight Similar to a passing lorry

IV Moderate Loose objects are rocked

V Rather strong Felt generally

VI Strong Trees sway, loose objects fall

VII Very strong Walls crack

VIII Destructive Chimneys fall; masonry cracks

IX Ruinous Houses collapse where ground starts to crack

X Disastrous Ground badly cracked; buildings destroyed

XI Very disastrous Bridges and most buildings destroyed; landslides

XII Catastrophic Ground moves in waves; total destruction