An Earthquake is a rapid vibration or shaking of the Earth s crust created by a release in energy from sudden movement of a part of a plate along a fault.
Energy released radiates in all directions from its source, the focus. So the Focus is the point at which the first movement occurs during an earthquake. (Where the earthquake originates from).
The epicenter is the point on Earth s surface directly above the focus of an earthquake.
Energy propagates in the form of seismic waves. So seismic waves are the energy that is released from the earthquake.
A Fault is a fracture or break in rock (in the lithosphere) where movement is occurring or has occurred. Faults can be active or inactive, and can be associated with either current or old plate boundaries.
Label the focus, epicenter and fault for each pictures.
Where do Earthquakes occur?
Earthquakes Occur at Different Depths: Shallow depths along transform boundaries Shallow to Medium depths (up to 30km bgs) along divergent boundaries Deep depths (up to 700km bgs) along subduction boundaries
What causes an Earthquake??
So, how does energy released by slippage at a fault travel through the ground?
Energy from an Earthquake travels in seismic waves.
2 Main Types of Waves: Body Waves earthquake waves that travel from a earthquake through the Earth. Surface Waves earthquake waves that travel along the Earth s surface.
Body Waves P waves (Primary Waves) compression waves that squeeze and stretch rock materials as they move Can travel through any material - solid rock, magma, ocean water, air S waves (Secondary Waves) shear waves that cause the particles of rock material to move at right angles to the direction the waves are traveling. Can travel through solid material only (CANNOT travel through liquids or gas) S waves travel a little more than ½ the speed of a P wave (so, s-waves are slower than P waves)
P Wave (Compression Waves) move like a slinky S Wave (Shear Waves) seem more like a worm or snake
S-Wave Motion Side-to-Side Motion
Surface Waves earthquake waves that travel along Earth s surface Travel more slowly than P or S waves and can move: side to side (Love Wave) Elliptical pattern (Rayleigh Wave) Can cause considerable damage even far from the earthquake s epicenter
Surface Waves earthquake waves that travel along Earth s surface Two Types: Love & Rayleigh
How the seismic waves move through Earth s Interior: Depths of Earth s Interior are different densities, wave velocities corresponds to interior of Earth Velocity of waves increases when waves travel through dense material Velocity of waves decreases when waves travel through less dense material.
So think back to our layers of the Earth Solid = inner core, lithosphere, (upper parts of mantle), crust Liquid/melted = outer core, Asthenosphere, (lower parts of mantle)
How the seismic waves move through Earth s Interior: Boundary of core and mantle is at 2,900km below the surface Here P waves are slowed and S waves stop. So material below 2,900km must be liquid (because S waves CANNOT travel through liquids) The inner core and outer core boundary is at a depth of 5,200 km, here P waves increase in speed again (because inner core is solid like the lithosphere)
How the seismic waves move through Earth s Interior: Terms that describe layers of Earth in reference to earthquakes Moho boundary where dense mantle rock meet less dense crust rock. (32km under continents, 5-10km oceans) Transition Zone middle mantle at depth of 400-670km (separates upper mantle from denser lower mantle) Shadow Zone -the wide belt on other side of Earth opposite the focus where seismograph stations do not receive waves. (because waves are bent/refracted as they pass through layers/materials of different densities).
Transition Zone Terms that describe layers of Earth in reference to earthquakes Moho Shadow Zone (shaded area)
Label which is S wave & which P wave (Compression) is P wave. S wave (Shear)
Draw how S & P waves would travel through layers, also label the Moho, Transition Zone and Shadow Zone: 1 st Remember P waves travel through: S waves travel through: Solid ONLY Any material
Draw how S & P waves would travel through layers, also label the Moho, Transition Zone and Shadow Zone: Focus
Locating and Measuring Earthquakes
Geologists use a seismograph to detect and record waves of an earthquake. Richter Scale assigns a single number to quantify the energy contained in an earthquake. (It s the scale the measures the magnitude of the earthquake.
Seismograph
Seismograph: A device that records earthquake waves. Seismogram: The picture drawn by a seismograph.
Which type of wave would reach the seismograph first (because it travels faster than others)? P Wave (it travels faster and through all materials) Label which is the P wave and which is the S wave on the seismograph to below and to the right. P wave S wave
The Richter Scale measures the magnitude of an earthquake. It is a base-10 logarithmic scale based on the waves measured by a seismograph. Example: An earthquake that measures 5.0 on the Richter scale has a shaking amplitude 10 times larger.
To locate the epicenter of an earthquake scientist must know the distances from at least three (3) different stations
How far is the epicenter of an earthquake from a seismic station, if the difference between the arrival time of the P and the S wave is 5 minutes?
Draw the circles showing distances from each seismograph station to the epicenter of the earthquake to determine where the epicenter actually occurred. Epicenter of this earthquake is? (Reno 200 miles, Chicago 2,000 miles, Phoenix 700 miles) 200 600 1000 (miles)
Hazard and Damage Caused by Earthquakes: Most buildings can withstand fairly violent up-anddown shaking Few buildings can survive side-to side shaking (so many collapse) Liquefication is when loose soil temporarily takes on some of the properties of liquid building on top of this loose soil that undergoes liquification or settling may cause collapse in buildings
Hazard and Damage Caused by Earthquakes: A large Earthquake can be followed by a series of smaller earthquakes called aftershocks. Aftershocks can still cause damage and be fairly strong and their frequency diminishes quickly over time. When large earthquakes occur they may rupture (break) gas lines, which could cause fires. Underwater earthquakes and landslides can sometimes cause huge ocean waves called tsunamis. Indonesia, Sri Lanka, Thailand (Indian Ocean) killed over 200,000 people on December 26, 2004.
Preventing Earthquake Damage What can we do?? Implement and enforce good engineering practices and building codes so that buildings can withstand a certain amount of shaking (especially in earthquake prone areas) Try to predict when, where, and at what magnitude an earthquake will occur. (By using data from past, and plotting earthquake foci along a fault to see where the fault has not moved, thus being where stress is building).