How Earthquakes work Investigating Earthquakes--San Francisco

oEarthquakes oEarthquakes: Vibrations (seismic waves) within Earth materials are produced by the rapid release of energy Earth’s crust is in constant motion because of tectonic forces Earth’s crust can store elastic energy When forces exceed the elastic limits and structural strength of the rocks, the rocks will break and/or move producing vibrations that travel outward in all directions What is an Earthquake?

focus oThe actual place underground where the rocks break producing vibrations is called the focus epicenter oThe place on the surface directly above the focus is called the epicenter Earthquakes

Tension Force Tension Force: stretching or pulling force normal Makes a normal fault What types of forces What types of forces are created?

Compression Force Compression Force: pushingtogether force pushing something together reverse Makes a reverse fault What types of forces What types of forces are created?

Shear Force Shear Force: a system of forces that operates against a body from different sides strike-slip Makes a strike-slip fault What types of forces What types of forces are created?

released Movement along faults: occurs when the energy exceeds the friction holding the sides of the fault together and is suddenly released. Movement of magma (volcanic) Volcanic eruptions What causes Earthquakes?

Originate at the focus and travel outward in all directions Foreshocks Foreshocks: small earthquakes that come before a major earthquake Aftershocks Aftershocks: Are adjustments in the crust after in earthquake weeksmonths oSmaller than main earthquake, but can cause as much or more damage. They can continue for weeks to months. Not every earthquake produces aftershocks Seismic Waves

P waves P waves (primary waves) Compressional wave  Particles move back and forth in the same direction as the wave  Travels the fastest  Can pass through solids and liquids (gases also)  Does not cause damage 3 Types of seismic Waves

S wave S wave (secondary wave, shear wave)  Particles move at right angles to the direction of the wave  Travels slower than P waves  Can pass through solids only  Does not cause damage Types of Waves

L wave L wave (long wave, surface wave, ground wave)  Particles move in elliptical orbit  Originates on the surface after the P and S waves go straight up from the focus and reach the surface  The L wave causes the damage and will be the strongest at the epicenter  Travels the slowest Types of Waves

How do we Measure Earthquakes? Earthquake waves are recorded by a seismograph and the recording of waves on paper is called seismogram

How do we Measure Earthquakes? Intensity Intensity – a measure of the effects on an earthquake at a particular location Magnitude Magnitude: a measure of the strength or amount of energy released during an earthquake

Richter Scale Richter Scale: Measures the amplitude of earthquake waves on seismograms  Scale from 1-10  Each number is 10 times the amplitude of the number below How do we Measure Earthquakes?

Locating the epicenter Locating the epicenter P wave and the S wave  Lag time between the arrival of the P wave and the S wave to the seismograph station is converted to a distance  A circle with a radius that equals the distance is drawn around the station.  Two stations can narrow down the location to two places where the two circles intersect Locating the focus Locating the focus: the lag-time of the L wave will determine the depth of the focus Measuring Earthquakes

Earthquake Dangers oMost injuries and deaths are caused by falling objects and most property damage results from fires that start Tsunami Tsunami: seismic sea wave sometimes generated when an earthquake originates on the ocean floor

Tsunami—December 2004

Earthquake Dangers Seiche Seiche: rhythmic sloshing of small bodies of water A seiche is the sloshing of a closed body of water from earthquake shaking. Swimming pools often have seiches during earthquakes.

Liquefaction Liquefaction: unconsolidated materials that are water saturated may turn to a fluid causing some underground objects such as storage tanks to float to the surface Earthquake Dangers Ground fissures caused by liquefaction near the mouth of the Pajaro River in California during the 1989 Loma Prieta earthquake. When the surface of the ground oscillates, wet, sandy, and muddy soils can flow like a liquid. This is liquefaction. You can liquefy wet sand at the beach by pumping it up and down with your feet. Photo courtesy of the Loma Prieta Collection, Earthquake Engineering Research Center, UC Berkeley.

Landslides Landslides Earthquake Dangers

Earthquake Safety Protect yourself from falling objects (GET UNDER SOMETHING) or stand in a hallway or doorway (watch out for a swinging door) Do not try to go outside during the earthquake After the earthquake and before the aftershocks, go outside Do not return to the building until it has been inspected