EARTHQUAKES CHAPTER 8

Earthquake – a vibration in the earth caused by a rapid release of energy, usually a slippage along a fracture in the Earth’s crust Focus – the point within the earth where the earthquake starts The energy released from an earthquake spreads out in all directions in the form of waves Epicenter – the location on the surface directly above the focus

Elastic Rebound Theory – Forces within the Earth slowly deform the crustal rocks on both sides of the fault. When rocks are deformed, rocks first bend storing elastic potential energy. The resistance caused by the internal friction that holds the rocks together is overcome and then the rocks slip at the weakest point 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 of an earthquakes.

Aftershocks – smaller much weaker earthquakes that occur for several days after the main earthquake Foreshocks – small earthquakes that often come before a major earthquake The San Andreas fault system is the most studied fault system in the world. Each segment behaves slightly different than the other segments.

Seismographs – instruments used to measure earthquake waves Seismogram – recorded trace of a vibration

Earthquake waves – there are two main types of seismic waves, surface waves and body waves (P waves and S waves) Surface waves – seismic waves that travel along the ground, move up and down or side-to-side. Te most destructive of all earthquake waves. P waves – push-pull waves, they compress and expand rocks in the direction the waves travel. This type of wave is called compressional or longitudinal waves. Travel through solids liquids and gases. S waves – shakes the rock at right angles to the direction of travel. S waves are transverse waves. Gases and liquids will not transmit S waves because they do not rebound elastically.

P waves travel faster than S waves. Surface waves travel the slowest. Locating an Earthquake – earthquakes are located by the differences in velocities of P and S waves from three seismic stations Earthquake distance can be determined by the difference in travel time. See chart on figure 8 page 225. How far is the epicenter if the difference in travel time is 3 seconds? Earthquake direction – a circle is drawn around each seismic station. The epicenter is where the three circles overlap.

Pacific ring of Fire – most earthquakes occur around the outer edge of the Pacific Ocean

Richter Scale – measures the magnitude of earthquakes on a scale from 1 to 10. Moment magnitude – the amount of displacement along the fault, estimates the energy released

Earthquake destruction – the damage to buildings and other structures from earthquake waves depends on the vibrations, the nature of the material on which the structure is built, and the design of the structure Liquefaction – stable soil is turned into a liquid that is unable to support buildings and other structures

Tsunamis – tidal waves triggered by an earthquake where a slab of ocean floor is displaced vertically along the fault. Tsunamis are also generated by vibrations from underwater landslides.

Landslides – violent shaking of an earthquake can cause the soil and rock on slopes to slide downhill or collapse Fire – gas and electric lines are cut starting fires

Predicting Earthquakes – accurate short- and long-term predictions when earthquakes will occur are not accurate.

Earth’s Structure Moho boundary – the boundary that separates the crust from the underlying mantle. His is where the velocity of seismic waves increases abruptly. Seismic P waves are refracted or bent as they travel through the earth. The refraction is explained by the different composition of the crust, mantle, and core. This deflection creates a shadow zone. Seismic S waves are stopped at the outer core, therefore it must be liquid

Crust Thin, rocky outer layer Divided into oceanic and continental Ocean crust is 7 km thick, density of 3.0 g/cm3, younger 180 million years or less Continental crust, 8-75 km thick, density of 2.7 g/cm3, some over 4 billion years Made of lighter, granitic rocks

Mantle 82% of the Earth’s volume Density 3.4 g/cm3 Basaltic composition Core Iron-nickel alloy Extreme pressure and high temperature Density 13 g/cm3

Lithosphere – crust and uppermost mantle, cool rigid shell Asthenophere – rocks close to melting temperature and are easily deformed Lower mantle – rocks are very hot and capable of plastic flow Outer core – liquid layer, the flow of metallic iron generates the Earth’s magnetic field Inner core – the material is compressed into solid iron-nickel rock

Earthquake Destruction