Earthquakes Chapter 3 Earthquake Facts • It is estimated that there are 500,000 detectable earthquakes in the world each year. 100,000 of those can be felt, and 100 of them cause damage. • The largest recorded earthquake in the United States was a magnitude 9.2 that struck Prince William Sound, Alaska on Good Friday, March 28, 1964 UTC. • From 1975-1995 there were only four states that did not have any earthquakes. They were: Florida, Iowa, North Dakota, and Wisconsin. http://earthquake.usgs.gov/learning/facts.php Important Earthquakes • 1755 - Lisbon, Portugal o Killed 70,000, Raised Waves in Lakes all over Europe o First Scientifically Studied Earthquake • 1886 - Charleston, South Carolina o Felt All over East Coast, Killed Several Hundred. o First Widely-known U.S. Earthquake • 1906 - San Francisco o Killed 500 (later studies, possibly 2,500) o First Revealed Importance of Faults • 1923 - Tokyo o Killed 140,000 • 1976 - Tangshan, China. Hit an Urban Area of Ten Million People o Killed 240,000 http://earthquake.usgs.gov/learning/facts.php What is an Earthquake? • The vibration of the earth produced by the rapid release of energy • Caused by large fractures in the Earth called faults • Faults are where 2 plates intersect • Ex: San Andreas Fault Where do earthquake occur? • Most earthquakes occur at the boundaries where the plates meet. • Spreading Zones (Divergent) • Most spreading zones are found in ocean. • Usually have earthquakes at shallow depths (within 30 kilometers of the surface). • Transform faults • Where plates slide past one another. • Earthquakes at transform faults tend to occur at shallow depths and form fairly straight linear patterns. • Subduction zones (Convergent: Cont/Ocn) • Where one plate overrides, or subducts, another, pushing it downward into the mantle where it melts. • Subduction zones are characterized by deep-ocean trenches, shallow to deep earthquakes, and mountain ranges containing active volcanoes. Earthquake Hazards • Indirect • Tsunamis • Caused by underwater earthquakes • Travel about 400 M.p.h. • Wave heights up to 100 feet as approach the shore • Landslides • Fire • Direct • Liquefaction • Breaking things • Shaking • Collapse Tsunami Fire Landslide What causes earthquakes Elastic Rebound Theory • Here we have a landscape with a road, a fence, and a line of trees crossing a fault. As the crust moves, the rocks adjacent to the fault are deformed out of shape (in reality the deformation is spread across many kilometers - if it were this obvious, earthquake prediction would be easy). Eventually the rocks are so stretched out of shape that they cannot bear the stress any longer. The fault slips, and the stage is set for the next cycle of strain buildup and release. Think of a rubber band snapping back into place • • Faults • A fault is a fracture in the Earth's crust along which two blocks of the crust have slipped with respect to each other. Faults are divided into three main groups, depending on how they move. 1. Normal 2. Reverse 3. Transform Normal Fault • Commonly called an Extensional fault • the block above the fault moves down relative to the block below the fault. • This fault motion is caused by tensional forces and results in extension. Normal Fault Reverse Fault • Commonly called a Compressional Fault • the block above the fault moves up relative to the block below the fault. • This fault motion is caused by compressional forces and results in shortening. Reverse Fault Transform Fault • the movement of blocks along a fault is horizontal. Rocks slide across one another. • The fault motion of a transform fault is caused by shearing forces. • Ex: San Andreas fault Transform Fault Parts of an earthquake • Focus • This is the actual location where fault movement begins. • Almost every earthquake has its focus located below the earth's surface. • Epicenter • This is the point on the land surface directly above the focus and is the location normally reported in the news or shown on maps. • Note that an epicenter need not be located on a fault line. Seismograph • A seismograph is nothing more than a glorified pendulum with the base anchored to bedrock. The base moves as the pendulum remains motionless (inertia). • There is a pen attached to the pendulum, and a rotating drum at the base records motion. • Readings need to be taken from at least 3 stations to determine the origin of an earthquake. Waves There are three types of seismic waves 1. P 2. S 3. Surface P Waves • • • • • • Primary (P) waves (or "longitudinal waves") The 1st wave Travel through fluids, and solids. They are compression waves. The motion of the material’s particles that transmit the energy move parallel to the direction of the wave. P waves are the fastest seismic waves. They travel at roughly 6.0 km/s in the crust (more than seven times the speed of sound). S Waves • • • • • Secondary (S) waves The 2nd Wave Travel through solids only. They are transverse waves. The motion of the material’s particles that transmit the energy move perpendicular to the direction of the wave. (Slinky time!) You shake it from side to side but the wave travels forward and perpendicular to the direction of shaking. S waves do not travel as fast as P waves and have a velocity of about 3.5 km/s in the crust. • Surface Waves • • • • • • • Surface waves The last wave Travel through solids only, at the surface. The motion of the wave is both up and down, and side to side. (Rolling) Typically, it is the surface waves that do the most damage. The velocity of surface waves varies with their wavelength (crest to crest), but always travel slower than P and S waves Greatest amplitude (height) of all the waves. Waves Review (in motion) Measuring Earthquakes • Magnitude • Is a quantitative measurement of the energy released • It is exponential; an increase of one magnitude means thirty times as much energy. • Richter scale (there is no upper or lower limit). • Generally an earthquake with a magnitude of 3.5 or less is not felt. • Intensity • Is a measure of the actual effects at a certain location • Generally decreases with distance from epicenter • Modified Mercalli Scale (I to XII) • I: not felt except rarely by a few • V: Felt by nearly everyone; many awakened. some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop. • XII: Damage total. Lines of sight and level are distorted. Objects thrown into the air. When Will Cali Fall into the Ocean? • The ocean is not a great hole into which California can fall, but it is itself land at a somewhat lower elevation with water above it. • It’s absolutely impossible that California will be swept out to sea. Instead, southwestern California is moving ever so slowly (2 inches per year) towards Alaska as it slides past central and eastern California. 15 million years (and many earthquakes) from now, Los Angeles and San Francisco will be next-door neighbors. Right on dude!
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