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  • 1. Earthquakes and Earths InteriorEarthquakes are vibrations of the earth caused by the rupture and sudden movement of rocks that havebeen strained (deformed) beyond their elastic limit. Earthquakes occur along faults. Faults are breaks in the lithosphere where regions of rock movepast each other. Most major faults occur along tectonic plate boundaries. The focus is the point on the fault where the rupture begins. The epicenter is the point on the earths surface directly above the focus. When the fault ruptures, waves of energy spread out in all directions.Elastic rebound theory states that the waves of energy from an earthquake result from the sudden releaseof stored up strain energy in rock as it deforms. Much like a rubber band stretched past its breaking point,the rock on either side of a fault snaps suddenly to a new position, releasing energy in the process.Types of FaultsThe majority of earthquakes (90%) are caused by rocks rupturing in response to tectonic stresses at activeplate margins.Faults can be divided depending on the direction of relative displacement.There are 2 main categories. Dip-slip faults Strike-slip faultsRelative displacement is largely a function of the type of tectonic stress the rock is under. Tensional Stress Compressional Stress Shear StressTypes of Tectonic StressDip-Slip Faults - Normal FaultsNormal faults result from tensional stresses along divergent boundaries.The hanging wall block moves down relative to the footwall block.Low Richter magnitudes due to the tendency of rocks to break easily under tensional stress.Shallow focus (less than 20 km) because the lithosphere is relatively thin along diverging plate boundaries.Dip-Slip Faults - Reverse FaultsReverse faults are faults that result from horizontal compressional stresses where the hanging wall blockhas moved up relative to the footwall block.Reverse faulting occurs along convergent boundaries.There are two types of converging plate boundaries.1. Subduction boundaries where oceanic lithosphere is pushed beneath either oceanic or continental lithosphere.2. Collision boundaries where two plates with continental lithosphere collide.Subduction BoundariesAt subduction boundaries there is a continuum of stress along the subducting plate. Shallow focusearthquakes can be generated near the trench, but focal depths can reach down to 700 km as earthquakesare 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 = 9.51964 Alaska = 9.2

2. Collision Boundaries At collision boundaries two plates of continental lithosphere collide resulting in fold-thrust 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 AsiaStrike-Slip Faults - Transform FaultsStrike-slip faults where the relative motion on the fault has taken place along a horizontal direction due toshear stresses acting on the lithosphere.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.Earthquake Seismic WavesBody waves travel through the interior (body) of the earth as they leave the focus. They include P-wavesand S-waves. P - wavesPrimary wavesPush-pull waves S wavesSecondary wavesShear wavesSurface waves travel parallel to the earths surface. They are the slowest and most damaging. They includeLove and Rayleigh Waves. Love Waves - complex, horizontal motion Rayleigh Waves - Rolling or elliptical motion.Seismographs are instruments that detect and record ground shaking produced by earthquake waves. Due totheir different speeds, the different waves arrive at the seismograph at different times: first P-wavesarrive, then S-waves, then surface waves.Seismogram - the record of an earthquake as recorded by a seismograph. It is a plot of vibrations versustime.Remember: P-waves are faster than S-waves. Therefore the time gap between their arrival at aseismograph increases precisely with distance from the quake.Therefore, lag time is proportional to distance traveled.We can use the lag time between the P-waves and S- waves to calculate the distance to an earthquake! If wedo this for a minimum of three different seismic stations, we can precisely locate the epicenter. In thefigure, each circle has a radius equal to the distance to the earthquake from three separate seismic stations.The three circles intersect at only one point -- the epicenter!Earthquake MeasurementRichter Magnitude scale- ML; based on the highest amplitude wave measured on a seismogram, corrected for distance from theseismograph to the epicenter- ranges from 1.0 (smallest) to infinity, but 9.0 is typically the highest possible value for an earthquake. 3. - 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.Modified Mercalli scale- based on peoples reported perceptions of shaking, and the type and extent of damage produced- ranges from I (not felt by people) to XII (catastrophic destruction)Fault CreepNot all fault movements result in violent earthquakes. Some faults move slowly and fairly continuously, amovement called fault creep.Fault creep never killed anyone, but it can cause damage to roads or other structures.Earthquake Hazards and MitigationNow that you are familiar with some important concepts related to earthquakes and their measurement, weshall now consider the specific types of hazards generated by earthquakes, and the specific steps people cantake to mitigate (reduce) those hazards.The hazards we will review are:ground shakingliquifaction uplift or subsidence of land fire tsunamisGround ShakingAn old saying among geologists is that earthquakes dont kill people, buildings do. The vast majority ofdeaths in earthquakes occur when ground shaking from earthquake waves (particularly S-waves and surfacewaves) causes buildings or other structures collapse, killing the people inside.Most damage and collapse of structures like buildings, bridges, and roads occurs due to sideways movementof the ground from earthquake waves. This process is called horizontal ground acceleration, or base shear. Base shear causes the building to deform from a rectangle into a parallelogram and causes buildings constructed on so-called cripples to fall sideways, causing damage The most deadly type of failure from base shear is story-shift, in which the sideways acceleration causes floors to shift and collapse onto one another -- a situation called pancaking. Few or no occupants survive such collapses. In addition to buildings, highway overpasses, bridges, and multi-decked freeways also suffer major damage from base shear. Collapse of freeways is most commonly caused by failure of the concrete supporting columnsLiquifactionLiquifaction occurs in water-saturated soils and rock. The shaking of earthquake waves causes the soil orrock to turn into a weak, fluid-like mass. Structures built on areas that liquify may simply fall overLand Uplift and Subsidence 4. Areas right next to the fault can experience direct damage from the ground shifting upward (called uplift)or downward (called subsidence).FiresFires commonly break out during quakes due to ruptured gas lines or downed electrical lines. In some urbanquakes, fires have caused more damage than the ground shaking itself.Tsunamis Tsunamis are waves generated by physical disturbances of the ocean. Shifting of the sea floor duringan earthquake is the most common cause. Undersea volcanic eruptions, landslides, or even meteoriteimpacts can also cause tsunamis. When part of the sea floor drops the water drops with it. Almost immediately, water from thesurrounding are rushes in to fill the depression, form a flat (~1m), high speed (up to 700km/hr),spread out wave with a wavelength measuring 10 to 100 km. In deep water tsunamis waves are nearlyundetectable. But as the leading waves of a tsunami approach a shoreline, friction with the sea floorslows the waves down (, This compresses the wave and the distance between successivecrests decreases as the wave height increases. The waves surge onto shore typically as a rapidlyrising flood of water with great destructive power. Most destructive tsunamis occur in Pacific Ocean. This is clearly related to plate tectonics: theborders of the Pacific Ocean are dominated by active subduction zones that produce frequentviolent earthquakes (as well as undersea volcanic eruptions and landslides).Earthquake Prediction / ForecastingMillions of dollars, and great research effort, has gone toward finding a reliable system for predictingearthquakes in the short term (several hours to days before the event). The assumption of this research hasbeen that large earthquakes produce precursors -- some type of signal before they happen.Ground deformation: Measurements taken in the vicinity of active faults sometimes show that prior to anearthquake the ground is uplifted or tilts due to the strain building on the fault.Foreshocks: Small earthquakes that precede a large quake by a few seconds to a few weeks. The patternand intensity of foreshocks usually increase in magnitude and may cluster or migrate down a fault to theplace where the main shock will eventually occur.Abnormal Animal Behavior.Alas, no reliable short-term precursors have been found. Therefore research today focuses on longer-termwarnings or forecasts. In this approach, geologists attempt to identify regions where large earthquakes arelikely to occur within the next several years or decades. While this does not provide short-term warnings, itis useful for long-range planning for building codes and emergency response services.Statistical Methods use the history of