What causes an earthquake? Giant snakes, turtles, catfish, and spiders?
What causes an earthquake? The movement of Earth s plates creates enormous forces that squeeze or pull the rock in the crust as if it were a candy bar. Rock becomes brittle and breakable or soft and bendable. Draw a candy bar!
What causes an earthquake? Plate Movement Stress Stored Energy Earthquake Faults Rock Breaks
What is? Stress is a force that adds energy to a rock. The energy is stored in the rock until the rock changes shape or breaks. This break creates a fault. Stress Stored Energy Changed Rock
What is? = Fault A fault is a break in the rock where rocks slip past each other. Most occur along plate boundaries. EX:
The San Andreas Fault
Are there different types of faults?
Earthquakes Every day, worldwide, there are several thousand earthquakes. Are there earthquakes in Arizona?
Are there earthquakes in Arizona? http://earthquake.usgs.gov/regional/states/arizona/seismicity.php Where do earthquakes begin?
Where do? Most earthquakes begin in the crust within about 100 km of the Earth s surface. Focus: area beneath the Earth s surface where rock breaks Epicenter: point on the Earth s surface directly above the focus
How do geologists locate the epicenter? Geologists use seismic waves to locate an earthquake s epicenter.
What are? Earthquakes produce vibrations called waves.
Seismic waves carry energy from an earthquake away from the focus, through Earth s interior, and across the surface.
What are the three types? Primary waves Secondary waves How do these move?
Primary Waves 1. P (primary): a) arrive first b) compress and expand the ground c) travel through solid and liquid d) velocity through granite is about 6 km/sec.
Secondary Waves 2. S (Secondary): a) arrive second b) vibrate from side to side and up and down c) cannot travel through liquid (liquids and gases have no resistance to change of shape, so S-waves can't travel through them.) d) velocity through granite is about 3.6 km/sec.
Surface Waves When P and S waves reach the surface some of them become surface waves. Surface waves aren't generated at the focus. They are generated by P and S waves hitting the earth's surface. They travel about 0.9 km/sec.
Surface waves move more slowly than P waves and S waves, but they produce the most severe ground movements.
Earthquakes cause tsunamis. A tsunami spreads out from an earthquake's epicenter and speeds across the ocean. http://www.jayepurplewolf.com/sites/tsunami/tsunami.html
How are earthquakes measured? 1. Mercalli scale: rate earthquakes according to the amount of damage at a given place. Focus p. 173
How are earthquakes measured? 2. Richter Scale: rating of earthquake magnitude or the size of the seismic waves. 10x10 each level!
Mercalli and Richter Scale
What does the damage look like for people? http://library.thinkquest.org/03oct/01428/earthquake1en.html
San Francisco 1906
What is an aftershock? Aftershocks are earthquakes on a smaller scale after the main quake. They happen because the newly moved rock have to be settled. Aftershocks are dangerous because they are usually unpredictable, can be of a large magnitude, and catch people off guard if they are salvaging items from a damaged building.
Monitoring Earthquakes Earthquakes are dangerous so people want to monitor (watch closely) them. During the late 1800s,scientists developed seismographs that were much more sensitive and accurate than any earlier devices. What is a seismograph?
What? A seismograph measures seismic waves or the shaking.
The Modern Seismograph Seismic waves cause the seismograph s drum to vibrate. But the suspended weight with the pen attached moves very little. Therefore, the pen stays in place and records the drum s vibrations.
Instruments That Monitor Faults In trying to predict earthquakes, geologists have developed instruments to measure changes in elevation, tilting of the land surface, and ground movements along faults.
How do scientist use this data? Scientists use this data to monitor faults and determine the earthquake risk for an area. They also try to predict earthquakes by measuring the stress along a fault, but huge stress does not always equal an earthquake!
Earthquakes and Volcanoes
What is the difference between faults and plate boundaries? A fault is simply a plane along which two masses of rock move relative to one another. A plate boundary is a fault in which the opposite sides are different plates. All plate boundaries involve faults, but not all faults are at plate boundaries.