2Why Earthquakes Happen? Rocks along faults pressed tightly together.Stress occurs but friction prevents them from moving – fault is said to be locked.Rocks under stress suddenly shift along a faultFault – break in body of rock where one block slides relative to anotherWhen rocks along a fault move, a sudden release of energy occurs causing movements on the ground...
4FOCUS Area along a fault where the slippage first occurs Focus is beneath the earth’s surfaceThe epicenter is directly aboveThe depth of the focus varies – 90% of continental earthquakes have shallow focusShallow foci – within 70km from surfaceDeep foci – subduction zones farther from plate boundaries than shallow quakes
6FOCUS DEPTH OF EARTHQUAKES IN WASHINGTON 1977-1996
7EPICENTERA point on the earth’s surface directly above the focus
8Seismic Waves travel outward in all directions from focus What happens when rocks in motion release energy in the form of vibrations?Seismic Wavestravel outward in all directions from focus
9Seismic Waves Body waves – travel through body of a medium Two types of waves produced from earthquakes :Body waves – travel through body of a mediumSurface waves – travel along surface of body rather than through middle
10Surface WavesForm from motion along shallow fault or by energy transfer from P and S waves reaching earth’s surface.Slowest moving seismic wavesCause greatest damage at surfaceTwo types : Love and Rayleigh
11Love and Rayleigh Waves Love waves – rock moves side-to-side and perpendicular to wave directionRayleigh waves – move ground in an elliptical rolling motion
12Earthquakes and Plate Tectonics Most earthquakes are located at or near tectonic plate boundaries
13Convergent Oceanic Environments Plates move toward one another and collideSubduction occurs as denser plate sinks under overriding plate - earthquakes occurOceanic-oceanic or oceanic-continental
14Divergent Oceanic Environments Plates moving away from each other and mid-ocean ridgeSpreading motion causes earthquakes
15Continental Environments Two continental plates converge, diverge, or move horizontally to one anotherRocks build up stressMountains form and earthquakes occur
16MAJOR EARTHQUAKE ZONES Pacific Ring of FireMid-ocean ridgesEurasian-Melanesian Mountain Belt
21MagnitudeThe measure of the strength of an earthquake, determined by measuring the amount of ground motion.
22Richter ScaleUsed for most of the 20th century. Each number represents a factor of 30 (a 5 earthquake has 30 times the energy of a 4). We now prefer to use the……
23Moment MagnitudeBased on the size of the area of fault movement, the average distance that the fault blocks move, and the rigidity of the rocks in the fault zone. More accurate for large earthquakes.
24IntensityBased on an earthquake’s effects. The Mercalli scale (p. 304) uses Roman numerals I-XII. Depends on magnitude, distance from the epicenter, local geology, earthquake duration, and human infrastructure.Charleston, SC
25I & II Not felt except by a very few. Felt by a few persons on the upper floors of buildings.
34A rapid drop or rise in the ocean floor Caused by an earthquake whose epicenter is on or below the ocean floor that causing:A rapid drop or rise in the ocean floorORAn underwater landslide that displaces a large volume of water
35Much Safer To Be Out In The Open Most injuries result from the collapse of buildings, falling objects, and flying glass.
36Other Dangers Include Landslides Fires Explosions (broken electric and gas lines)Floods (from collapsing dams)
37Ground Type Is Important Loose soil and rock can vibrate like Jell-O.
38KOBE SEISMOGRAPH Top - Station on solid ground Right - Station on water saturated, soft ground
46BEFORE - Be prepared DURING - Stay calm AFTER - Stay cautious EARTHQUAKE SAFETY People who live near active faults should be ready to follow a few simple earthquake safety rules to help prevent death, injury, and property damageBEFORE - Be preparedDURING - Stay calmAFTER - Stay cautious
47Before an EarthquakeBe prepared. Keep an adequate supply of food, water, batteries, flashlights and a radio.Prepare an earthquake plan and discuss it with your family.Learn how to turn off the gas, water, and electricity in your home.
48During an Earthquake Protect yourself by moving to a safe position. Best way is the “Triangle of Life” (next slide--)Indoors – get under a table or if not possible in a inside door frame away from heavy furnitureOutdoors – get away from tall buildings, tunnels, power lines or bridges out in the open
49Triangle of Life during a Quake Newest research shows your first best position to get into during a quake is next to and below the level of a heavy object – table or furnitureCover your head in bracing position (like shown on a plane)Any falling debris will hit table or furniture and arc over you and you will be protected