Earthquakes Chapter 3 Earthquakes and Their Damages Shaking Ground Collapsing Buildings Chapter 4 Earthquake Prediction and Tectonic Environment Earthquakes are vibrations of the earth caused by the rupture and sudden movement of rocks that have been strained deformed beyond their elastic limit The forces that cause deformation and the buildup of strain energy in the rock are referred to as stresses Earthquakes occur along faults Faults are fractures in the lithosphere where regions of rock move past each other displaced The focus is the point on the fault where rupture occurs and the location from which seismic waves are released The epicenter is the point on the earth s surface directly above the focus When the fault ruptures waves of energy called seismic waves spread out in all directions Elastic Rebound Theory Types of Faults The majority of earthquakes 90 are caused by rocks rupturing in response to tectonic stresses at active plate margins Types of Tectonic Stress Tensional Stress Compressional Stress Shear Stress Faults can be divided depending on the direction of relative displacement There are 2 main categories Dip slip faults where the displacement is vertical Strike slip faults where the displacement is horizontal Relative displacement is largely a function of the type of tectonic stress the rock is under Dip Slip Faults Normal Faults Normal faults result from tensional stresses along divergent boundaries The hanging wall block moves down relative to the footwall block Earthquakes generated tend to have low Richter magnitudes due to the tendency of rocks to break easily under tensional stress The earthquakes tend to be shallow focus less than 20 km because the lithosphere is relatively thin along diverging plate boundaries Examples all mid ocean ridges Continental Rift Valleys such as the basin and range province of the Western U S and the East African Rift Valley Dip Slip Faults Reverse Faults Reverse faults result from compressional stresses along convergent boundaries The hanging wall block has moved up relative to the footwall block A Thrust Fault is a special case of a reverse fault where the dip of the fault is less than 15o There are two types of converging plate boundaries Subduction Boundaries At subduction boundaries cold oceanic lithosphere is pushed down into the mantle producing a continuum of stress along the subducting plate Shallow focus earthquakes can be generated near the trench but focal depths can reach down to 700 km as earthquakes are generated along the subducting plate Rocks are strong under compression and can store large amounts of strain energy before they rupture Therefore these earthquakes can be very powerful 1960 Southern Chili 9 5 1964 Alaska 9 2 Collision Boundaries At collision boundaries two plates of continental lithosphere collide resulting in fold thrust Page 1 of 6 mountain belts Earthquakes occur due to the thrust faulting and range in depth from shallow to about 200 km Example The Himalayas from the collision of India with Asia Strike Slip Faults Transform Faults Strike slip faults result from shear stresses acting on the lithosphere along transform boundaries Horizontal motion can be right lateral or left lateral Earthquakes along these boundaries tend to be shallow focus with depths usually less than about 100 km Richter magnitudes can be large Example The San Andreas Fault System Intraplate Earthquakes These are earthquakes that occur in the stable portions of continents that are not near plate boundaries Many of them occur as a result of re activation of ancient faults although the causes of some intraplate earthquakes are not well understood Earthquake Seismic Waves Body waves travel through the interior body of the earth as they leave the focus They include Pwaves and S waves P waves Primary waves Push pull waves S waves Secondary waves Shear waves Surface waves travel parallel to the earth s surface They are the slowest involve the greatest ground motion and are therefore most damaging They include Love and Rayleigh Waves Love Waves complex horizontal motion Rayleigh Waves Rolling or elliptical motion Earthquake Measurement Seismographs are sensitive instruments that detect and record ground shaking produced by earthquake waves Due to their different speeds the different waves arrive at the seismograph at different times first Pwaves arrive then S waves then surface waves Seismogram the record of an earthquake as recorded by a seismograph It is a plot of vibrations versus time Earthquake Seismic Waves Wavelength distance between two equal points on a wave Amplitude the amount of positive or negative wave motion Period the number of seconds between successive peaks of a wave P seconds 1 wave P and S waves have shorter periods 0 1 sec wave to 1 sec wave than surface waves 1 3 sec wave Frequency the number of waves passing a point of reference per second F waves 1 second P and S waves have higher frequencies 1 wave sec to 10 waves sec than surface waves less than 1 wave sec Locating the Epicenter Via Seismograms o P waves are faster than S waves and the time gap between their arrival at a seismograph increases precisely with distance from the focus Basically the lag time between the arrival of your first recorded P wave and first recorded S wave is proportional to distance traveled Page 2 of 6 o We can use the lag time between the P waves and S waves to calculate the distance to an earthquake If we do this for a minimum of three different seismic stations we can precisely locate the epicenter In the figure each circle has a radius equal to the distance to the earthquake from three separate seismic stations All three circles intersect at only one point the epicenter Earthquake Measurement Richter Magnitude Scale ML based on the highest amplitude wave measured on a seismogram corrected for distance from the seismograph to the epicenter ranges from 1 0 smallest to infinity but 9 0 is typically the highest possible value for an earthquake logarithmic scale each whole unit on the Richter scale represents a ten fold increase in wave amplitude ground shaking and an thirty fold increase in the energy released The Local Magnitude Scale developed by Richter was strictly valid only for certain frequency and distance ranges Therefore new magnitude scales were developed all calibrated to Richter s original method These include body wave magnitude MB and surface wave magnitude MS Each is valid for a particular frequency range Moment
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