Earthquakes • An earthquake is shaking or movement of the Earth. • They are caused by plate tectonics.
Faults • Earthquakes occur on faults. Faults are fractures along blocks of rock • Faults allow the blocks to move relative to one another
Types of Faults • The direction and angle of the slip determines the type of fault. • The type of fault determines the type of earthquake. • During an earthquake, the rock on one side of a fault moves in relation to the other side. This is called slip. • Most faults produce repeated displacement of blocks over geologic time
Strike-slip faults are nearly vertical fractures where the blocks have moved mostly horizontally. strike slip animation Strike-Slip Faults
Strike-slip faults occur in regions with transform boundaries, such as , the San Andreas Fault. Strike-Slip Fault Example
Bigger Faults=Bigger Earthquakes
More time=Bigger Earthquake
Normal Faults • Normal faults are faults where one block has moved downward. • These faults are seen in areas where there are divergent boundaries • Normal Fault Animation
Normal Fault Examples Valley-Fairview Peaks, Nevada earthquake December 16, 1954
Reverse Fault • Reverse faults are faults where one block moves upwards • Thrust faults are reverse faults movement of 45 degrees or less • Reverse Fault Animation
Reverse faults can be found in areas with convergent boundaries. There needs to be subduction zones for reverse faults to occur. Reverse Fault Example
Magnitude • The magnitude of an earthquake is a measure of the amount of energy released by the earthquake.
Amount of Shaking • The amount of shaking is controlled by: • Magnitude • More energy released • Distance • Shaking decays with distance • Local soils • amplify the shaking
Seismic Waves • When an earthquake occurs, it releases energy in the form of seismic waves. • The different types of energy waves shake the ground in different ways and also travel through the earth at different velocities.
There are two categories of waves: Body waves travel through the Earth’s interior Surface waves travel along or near the earth’s surface. Types of Waves
Body Waves • A P wave, or compressional 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.
P Waves • The fastest wave, and therefore the first to arrive at a given location, is called the P wave. The P wave, or compressional wave, alternately compresses and expands material in the same direction it is traveling
Body Waves • An S wave, or shear wave, is a seismic body wave that shakes the ground back and forth perpendicular to the direction the wave is moving.
S Waves • The S wave is slower than the P wave and arrives next, shaking the ground up and down and back and forth perpendicular to the direction it is traveling.
There are two categories of waves: Body waves travel through the Earth’s interior Surface waves travel along or near the earth’s surface. Types of Waves
Surface Waves: Surface waves follow the P and S waves. • A Rayleigh wave is a seismic surface wave causing the ground to shake in an elliptical motion, with no transverse, or perpendicular, motion.
Surface Waves • A Love wave is a surface wave having a horizontal motion that is transverse (or perpendicular) to the direction the wave is traveling.
Magnitude
An estimate of the energy of an earthquake is called the Seismic Moment (Mo). Typically given in units of Newton-meters (Nm) What factors contribute to the amount of energy released by an earthquake? S = Surface Area of Fault (m2) (Fault length x Fault width) D = Displacement or slip (m) µ = Rigidity of rock along the fault (N/m2) Mo = S*D* µ
