Forces Within Earth • Most earthquakes are the result of movement of the Earth’s crust produced by plate tectonics • At the boundary between two plates, stress often builds up • Stress is the total force acting on crustal rocks per unit area • When stress > strength of rocks, earthquake is felt
3 Types: • Compression: SQUEEZING, tries to shorten a material • Tension: PULLING APART, tries to lengthen a material • Shear: SLIDING, stress that causes a material to twist
Deformation • The deformation of a material in response to stress is called strain • Elastic Deformation: when stress is low, it is caused when a material is compressed, bent or stretched; when the stress returns to zero, the original rock shape is maintained • Plastic Deformation: more stress than elastic deformation, this type of strain produces permanent deformation
Faults • A fracture or system of fractures along which Earth moves • Reverse faults form as a result compression. • This causes rock on one side of a reverse fault to be pushed up relative to the other side. • Normal fault forms from the pulling apart of rock and stretches the crust. • Strike-slip faults are caused by horizontal shear. The movement at a strike-slip fault is mainly horizontal and in opposite directions, similar to the way cars move in opposite directions on either side of a freeway.
Earthquake Waves • The vibrations of the ground produced during an earthquake are called seismic waves. • Every earthquake generates three types of seismic waves: primary waves, secondary waves, and surface waves. • Focus: the point IN earth where earthquake waves originate • Epicenter: the point ON Earth’s surface directly above the focus
Earthquake Waves 3 Types of Seismic Waves: 1) Primary Waves: (P-waves, or Pressure waves) similar to pushing a slinky in the direction in which it is stretched 2) Secondary Waves: (S-waves) similar to holding a jump rope in one hand and raising and lowering it 3) Surface Waves (Love and Rayleigh): the most destructive, cause the ground to move sideways and up and down like an ocean wave Because they happen INSIDE the earth, P-waves and S-waves are also called body waves
Determining what’s inside the earth • Why does light change angles when you look at a fish tank from various sides? • Light waves change as they pass through different mediums (media?)
Earth’s internal structure In the same way, seismic waves change speed and direction when at the boundaries between different materials.
Determining Earth’s Interior • Because the speed of seismic waves depends on temperature and composition, it is possible to use seismic waves to determine the composition of the earth. • When an earthquake happens, seismometers all over the earth receive the resulting waves at different times.
Earth’s internal structure • As P-waves and S-waves initially travel through the mantle, they follow fairly direct paths. • When P-waves strike the core, they are refracted, which means they bend.
Earth’s internal structure • the back-and-forth motion of S-waves does not travel through liquid. • However the longitudinal motion of P-waves DOES go through liquids
Earth’s internal structure • The data collected for the paths and travel times of the waves inside Earth led to the current understanding that Earth has an outer core that is liquid and an inner core that is solid.
Earth’s Composition • Seismic waves change their speed as they encounter boundaries between zones of different materials. • In general, the speed of seismic waves decreases as temperature increases.