Earthquakes

Earthquakes Earth Science Fall 2013Earthquakes & Plate Tectonics Earthquake- vibration of the earths crust Usually occur when rocks under stress suddenly shift along a fault Elastic Rebound Theory Theory that rocks that are strained past a certain point will fracture and spring back to their original shape. Energy is released by the rocks along the fault in the form of vibrations called seismic waves. The release of energy usually increases the stress in other rocks along the fault causing aftershocks.Aftershocks usually follow a major earthquake and are a series of tremors Elastic Rebound Theory Focus- area along a fault where slippage first occurs; under the surface of the earth Epicenter- point on the earths surface directly above the focus When an earthquake occurs seismic waves radiate outward in all directions from the focus Elastic Rebound Theory Earthquake depth statisticsShallow focus quakes occur within 70km of the earths surface 90% of continental earthquakes are shallow focus Usually cause the most damage Intermediate focus quakes occur at a depth of 70km 300 km Deep focus quakes occur in subduction zones at a depth of between 300km 650kmUsually occur farther inland from the subduction zone than shallow and intermediate focus earthquakes Elastic Rebound Theory Video Elastic Rebound Theory Earthquakes 101Major Earthquake Zones Most earthquakes occur on or near the edges of the earths lithospheric platesSome plate boundaries contain groups of interconnected faults called fault zones Ex: San Andreas Fault

3 Major Earthquake Zones Pacific Ring of Fire Includes the west coasts of North & South America, east coast of Asia, and the western Pacific islands of the Philippines, Indonesia, New Guinea, and New ZealandSubduction is occuring Mid-ocean ridgesOceanic crust is pulling away from both sides of the ridge Eurasian-Melanesian mountain beltCollision of plates Recording Earthquakes Seismograph- instrument used to detect and record seismic waves Consists of 3 separate sensing devices:One records vertical motionOne records horizontal motion in an east-west directionOne records horizontal motion in a north-south direction Types of Seismic Waves The major types of seismic waves:S wavesP wavesSurface waves Three Types of Seismic WavesP waves (primary waves)Move the fastest First to be recorded by a seismographCan travel through solids & liquids The more rigid the material the faster they travelCompression waves (cause rock particles to move together and apart along the direction of the waves) Three Types of Seismic Waves S waves (secondary waves)Second waves recorded by the seismographOnly travel through solid material Cannot be detected on the side of the earth opposite the earthquakes epicenter. Why? Shear waves ( they cause rock particles to move at right angles to the direction in which the waves are traveling) Three Types of Seismic Waves Surface wavesWhen P waves and S waves reach the earths surface their energy is converted in surface waves. Slowest-moving waves Last to be recorded by a seismographMove similar to that of ocean waves Cause the earths surface to rise & fall Particularly destructive when traveling through loose earth Locating an Earthquake To find the epicenter of an earthquake scientists analyze the difference between the arrival time P waves and S wavesP waves travel 1.7 times faster than S waves So is S waves arrive soon after P waves the earthquake originated really close and vice versa To determine how far an earthquake is from a given seismographScientists plot the difference between the arrival times of the two waves Need data from at least three seismograph stations to do this Earthquake Measurement Magnitude- amount of ground motion Seismologists report the magnitude of an earthquake by using a magnitude scale like the Richter scale or the moment of magnitude scale Scientists prefer the moment of magnitude scale because it is related more to cause of the earthquake, where the Richter scale is more related to the effect of the earthquake Earthquake Measurement Major earthquakeMagnitude of 7 or above Moderate earthquakeMagnitude between 6 and 7Minor earthquake Magnitude between 2.5 and 6 MicroquakeMagnitude less than 2.5Usually not felt by people Earthquake Measurement Mercalli Scale Expresses the intensity of an earthquake or the amount of damage it causes Expressed as Roman numerals from I to XII and a description Ex: rating of II (low intensity) Felt only by a few persons at rest, especially on the upper floors of buildings. Delicately suspended objects may swing. Ex: rating of XII Total Destruction Earthquake Damage A moderate earthquake that continues for a long time often causes more damage than an earthquake of high magnitude that only lasts for a short time. Earthquake Damage Destruction of Buildings & PropertyType of ground beneath a building can affect the way a building responds to seismic waves. Loose soil & rock is much more likely to be damaged during an earthquake than one built on solid ground Earthquake Damage TsunamisGiant ocean wave caused by a major earthquake with an epicenter on the ocean floorMost are caused by 2 events related to under sea earthquakesFaulting Underwater landslides Pacific Tsunami Warning Center (PTWC)Network of seismograph stations around and in the Pacific Ocean that alerts scientists to the location and magnitude of earthquakes Earthquake Safety Safety RulesBefore an earthquake occurs, be prepared.During an earthquake, stay calm.After an earthquake, be cautious Earthquake Warnings & Predictions Identified faults in populated areas Along some faults there are zones of immobile rock called seismic gaps A seismic gap is an area where the fault is locked and unable to move Scientists believe that seismic gaps are likely the location of future earthquakes Earthquake Warnings & Predictions Sometimes there is slight tilting of the ground shortly before an earthquake Some earthquakes are preceded by a decrease in the speed of local P waves Can last for several days or several years Longer the decrease lasts the stronger the earthquake will be Japan TsunamiEarthquake & Tsunami Relationship