Chapter 6: Earthquakes

Transcription

1 Section 1 (Forces in Earth s Crust) Chapter 6: Earthquakes 8 th Grade Stress a that acts on rock to change its shape or volume Under limited stress, rock layers can bend and stretch, but return to their original shape when the stress is gone. If the stress is too great, rock layers will bend and stretch to a limit, then break suddenly. Types of Stress: (caused by plates moving slowly over time) 1. tension stress that stretches, or pulls rock layers apart so that it becomes thinner in the middle. (can create valleys) 2. compression stress that squeezes, or pushes rock layers together until they fold or break. (can create mountains) 3. shearing stress that causes rocks to slide past each other Fault a break or crack in Earth s lithosphere along which the rocks move When enough stress builds up in rock, the rock breaks, creating a fault. The rock on both sides of the fault can move up, down, or sideways. 1. normal fault hanging wall Types of Faults: type of fault where the hanging wall slides downward in relation to the caused by tension in the crust. (where plates diverge, or pull apart) the block of rock that forms the upper half of a fault the block of rock that forms the lower half of a fault 1

2 normal faults 2. reverse fault type of fault where the hanging wall slides upward in relation to the. caused by compression in the crust (where plates converge, or come together) reverse faults hanging walls hanging walls Tension stress compression stress 3. strike-slip fault type of fault where rocks on either side move past each other sideways with little up or down motion caused by shearing stress formed at transform boundaries San Andreas fault in California is a strike-slip fault. strike-slip fault View from the sky. Anticline Syncline anticlines synclines Creates folded mountains. an upward fold in rock layers formed by compression in Earth s crust (No break in the rock layers has occurred yet.) a downward fold in rock layers formed by compression Section 2 (Earthquakes and Seismic Waves) Plateau forms when rock layers break and are pushed up. plateau compression stress focus epicenter the area beneath Earth s surface where rock that is under pressure breaks and causes an earthquake seismic waves the point on the surface directly above the focus vibrations that travel through Earth from an earthquake travel out in all directions from the focus (like ripples in a pond) 2

3 fault develops epicenter focus Earth s surface Seismic waves travel through Earth like ripples in a pond. Types of Seismic Waves: 1. P waves (primary waves) seismic waves that compress and expand the ground like an accordion P waves start at the focus. These waves travel fastest and are the first to reach the epicenter. 2. S waves (secondary waves) seismic waves that move the ground from side to side or up and down. S waves also start at the focus. travel slower than P waves and reach the epicenter 2 nd. 3. surface waves seismic waves that form when P and S waves reach the epicenter start at the epicenter. move the slowest. move up and down and with a rolling motion produce the most ground movement and the most damage S and P waves surface waves form from the epicenter when P and S waves reach the surface Earth s surface Focus where the rock first breaks Mercalli scale Measuring Earthquakes: rates earthquakes (1 through 12) according to their intensity and how much damage they cause at a particular place. The same earthquake can have a different Mercalli rating, depending on what location you look at. Richter scale magnitude rates an earthquake s magnitude (1 through 10) based on the strength and size of its seismic waves as measured by a seismograph. measurement of an earthquake s strength, or size Each step on the magnitude scale represents about 32 times more energy produced than the step below it. seismograph instrument that measures and records ground movements caused by seismic waves 3

4 water water 8/18/2014 Example: (Richter Scale) Richter scale 5 Richter scale 6 32 times the energy as in 5 (energy of scale 5 times 32) Richter scale 7 32 times the energy as in 6 (or 1024 times the energy in 5) (32 32) Moment magnitude scale rates earthquakes by estimating the total energy released by an earthquake. This rating takes into account strength and size of seismic waves, and the amount of damage caused. Richter scale 8 32 times the energy as in 7 (or 32,768 times the energy in 5) ( ) Section 3 (Monitoring Earthquakes) Seismogram the record of an earthquake s seismic waves produced by a seismograph. level tilted ground Calm tiltmeter measures very slight raising of the ground that may take place before an earthquake creep meter measures very slight horizontal movement of the ground with a wire stretched across a fault. (one side of the wire is anchored to the ground the other side is free to move) instrument housing fault (view from overhead) wire anchored stake laser-ranging device The laser beam is sent to a reflector and timed going out and back (If this time changes, the distance has changed, indicating the Earth surface moved.) laser beam uses laser beams to detect very slight horizontal movement fault (view from overhead) reflector If either distance changes, the ground has moved. 4

5 GPS satellites Friction (global positioning system) to monitor horizontal and vertical movements. GPS measures the precise longitude, latitude, and elevation of markers set up on the opposite sides of a fault. the that opposes (slows down) the motion of one surface as it moves across another surface Friction and Earthquakes: Earthquake intensity depends on the amount of friction between both sides of a fault. Where friction is low (2 smooth rock surfaces), the rocks on both sides slide without sticking. (stress or pressure does not build up very mild earthquakes) Where friction is moderate the sides jam together and jerk free from time to time (small earthquakes) Where friction is high both sides lock together and do not move for a long time, until enough stress is built up to violently jar them free (strong earthquakes) 5