Transcript Earthquakes
Earthquakes Earthquakes • When plates move very slowly and over a long period of time • Eventually a sudden shaking of the crust occurs at a particular place: Earthquake Earthquakes • The majority of earthquakes occur at the boundaries of tectonic plates. • Individual earthquakes also occur where there is a fault. • A fault is a zone of weakness in the crust along which some movement of rock occurs. Three Types of Faults 1) Reverse – compression causes horizontal and vertical movement 2) Normal: Tension causes horizontal and vertical movement 3) Strike-Slip – shear causes horizontal and vertical movement Earthquake Terms • Focus: the area in the crust (below surface) where energy is released during an earthquake (where the earthquake begins) – Shallow: 0-70 km – Intermediate: 70-300 km – Deep: over 300 km • Epicenter: The point on the surface directly above the focus of an earthquake Process • As tectonic plates move, friction causes the rocks at plate boundaries to stretch or compress. • Like a stretched rubber band or a compressed spring, these rocks energy called the Elastic Rebound Theory. store • When the pressure builds up within the crust, the stored energy is suddenly released as the rocks slip along the fault, and an earthquake occurs. Process • More than ½ of all earthquakes occur along the edges of plates – Subduction zones – Mid-ocean ridge – Collision zones (continent-continent convergence) Seismology • The study of earthquakes • Scientists cannot predict the exact date and time of an earthquake • But… they can identify areas likely to have an earthquake in the next 10 years. Seismology • A seismograph measures earthquakes, and seismologists use seismic waves to study • The energy released by an earthquake travels away from the focus in waves • Magnitude: relative size of an earthquake depends on amount energy released How Seismographs Work the pendulum remains fixed as the ground moves beneath it http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM Seismology • Seismographs show the kinds of waves, their amplitude, and the timing of the waves • Foreshocks are small tremors that precede an earthquake • Aftershocks are small tremors that follow after an earthquake Scales • Magnitude is expressed on the Richter Scale – Study the waves to determine the magnitude and location of the earthquake – Uses number 1 & up – Each # indicates ten times stronger than the # below • Ex: A rating of 8 has a magnitude ten times as great as an earthquake with a magnitude of 7 – 7 or higher indicates a major earthquake ( 10-felt all over the earth) Scales • Modified Mercalli Scale: use to rate intensity – Roman numerals I to XII describes damage done by the quake – XII : total destruction – An earthquake with one magnitude, damage can vary depending on the location How are Earthquakes Measured? Mercalli Intensity Scale Click Link for Interactive Demo http://elearning.niu.edu/simulations/images/S_portfolio/Mercalli/Mercalli_Scale.swf Seismic Waves • The conversion of potential to kinetic energy results in seismic waves • Travel through the ground 20 times faster than the speed of sound • Seismic waves reach the ground at the epicenter Seismic Waves • Body waves – originate from the focus of the earthquake – P waves (primary waves) are compression waves that push and pull rock as they travel • Cause particles to move back & forth • Move through solids, liquids and gases • 1st waves to arrive Seismic Waves – S waves – (shear/ secondary waves) Body waves that have reached the surface • Cause particles to move side to side • Do not travel through liquids Seismic Waves • Surface Waves: Third & Fourth waves – Love waves & Rayleigh waves • Travel along the surface & more slowly • Cause the most damage • Love: no vertical movement of the surface • Rayleigh: both horizontal and vertical movement How to determine the distance to the epicenter • Need data from three seismographic stations including the spread of S and P waves and the time between the arrival of the s and p waves. • Forecasting – Based on calculating the probability – 2 factors • History of area • Rate at which strain builds up Locating Earthquakes http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM Locating Earthquakes http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM Locating Earthquakes http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM Earthquake Hazards • Factors that determine the severity of damage 1) Design of buildings - Wooden structure = less damage 2) Ground shaking - tall building – vibrations too rapid -short building – vibrations too slow - 5-15 stories collapsed Earthquake Hazards • 3) Soil Fracture – Sloping area = landslides – Liquefication = sand becomes liquidy due to vibrations – Soft materials amplify the motion – Hard/resistant materials lessen the motion Causes • sinking, falling over, buildings • Underground pipes & tanks rise to the surface Earthquake Hazards • 4) Tsunami: A huge wave generated by an underwater earthquake or landslide – Caused by vertical motion on the ocean floor • Open ocean – Height less than 1 m – Speed 500-800 km/h Earthquake Hazards • Shallow water – Height increases due to breakers/shallow water – Height may exceed 30 m Tsunami Warning System http://isu.indstate.edu/jspeer/Earth&Sky/EarthCh11.ppt