Virtual Class Presentation Earthquakes Virtual Class Presentation

Stress Types Compression decreases the volume of a material. Tension pulls a material apart. Shear is stress that causes a material to twist. The deformation of material in response to stress is strain.

Rock Deformation Elastic deformation is caused when a material is compressed, bent or stretched. Deformation of the rock disappears when stress is reduced to zero. Plastic deformation is beyond elastic limit; rocks are permanently deformed

Most materials exhibit both elastic and plastic behavior but on different degrees. Glass and dry wood will fail before much plastic deformation occurs. Putty and rubber can undergo a great deal of deformation before failure occurs.

Temp and Pressure Deformation impacted by temp and pressure. As pressure increases rocks require greater stress to reach elastic limit. As temperature increases, solid rock melts, reducing stress.

Faults Fracture or system of fractures along which Earth moves. Crustal rocks fail when stresses exceed the strength of the rock. Movement along a fault results in earthquakes.

Fault Types Reverse fault - compression causes horizontal and vertical movement. Seen at convergent plate boundaries. Normal fault - Tension causes horizontal and vertical movement. Strike and slip faults - Shear causes horizontal movement. Ex. San Andreas in Ca.

Seismic Waves - vibrations of the ground during an earthquake. Primary waves (P-waves) - squeeze and push rocks in the direction along which the waves are traveling. Secondary waves (S-waves) - cause rocks to move at right angles in relation to the direction of the wave. Slower than p-waves.

Both P-waves and S-waves can pass through Earth’s interior. Surface waves are the slowest type of wave and travel only along Earth’s surface. Cause ground to move sideways and up and down. Cause most destruction.

Generation of Seismic Waves Point in the Earth where waves originate is the focus. Focus is usually several kilometers below surface. Point on Earth’s surface directly above focus is the epicenter.

Measuring Earthquakes Seismometer is a sensitive instrument that can detect seismic waves even when far from the epicenter. Vary in design but all include a frame that is anchored to the ground and a mass that is suspended from a spring or wire. Mass and pen stay at rest due to inertia while ground beneath shakes.

Seismogram

Travel Time Curves Show how long it takes for P-waves and S- waves to reach seismic stations.

Modeling Earth using Seismic Waves P-waves get refracted by Earth’s core. S-waves will not travel through liquid. By recording travel time curves and path of each wave, seismologist learn about structure of Earth.

Richter Scale Numerical rating system that measures the energy of the largest seismic wave. Magnitude - wave energy; each number higher is 32 times more energy. Amplitude - height of the largest seismic wave in an earthquake event; each number higher is 10 times more amplitude.

Richter Scale Problems How much more energy is an earthquake of magnitude 9 than magnitude 7? How much more amplitude does an earthquake have that measures 5 than 2?

Locating an Earthquake Epicenter Difference in time recorded at seismic station between p and s-waves was 6.4 minutes. Locate on travel- time curve separation of 6.4 minutes between p and s-waves. The seismic station must be 4000 km away from epicenter.

Locating an Earthquake Epicenter Draw a circle around the seismic station with the radius being the calculated distance to the epicenter. The epicenter can be any direction from the seismic station. Adding data from a second seismic station, the two circles will overlap at two points, one is the epicenter. Adding data from a third seismic station results in one overlap of the rings, at the epicenter.