EarthquakesChapter 3 Earthquakes and Their Damages: Shaking Ground, Collapsing BuildingsChapter 4 Earthquake Prediction and Tectonic EnvironmentEarthquakes 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 TheoryTypes of FaultsTypes of Tectonic StressTensional StressDip-Slip Faults - Normal FaultsNormal 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.Dip-Slip Faults - Reverse FaultsReverse 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 15oSubduction BoundariesAt 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 = 9.51964 Alaska = 9.2Collision BoundariesAt 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 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 SystemIntraplate EarthquakesEarthquake Seismic WavesBody waves travel through the interior (body) of the earth as they leave the focus. They include P-waves and S-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 motionRayleigh Waves - Rolling or elliptical motion.Earthquake MeasurementSeismographs 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 P-waves arrive, then S-waves, then surface waves.Locating the Epicenter Via SeismogramsEarthquake 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 Magnitude. Because of the limitations of all three magnitude scales, a new scale, known as moment magnitude, or MW, was developed. MW is a measure of the seismic moment, or total energy expended during an earthquake. MW depends on the rock strength, area of rock broken, and amount of offset across the fault.Modified Mercalli scaleBased on people’s reported perceptions of shaking (subjective), and the type and extent of damage produced (objective).Ranges from I (not felt by people) to XII (catastrophic destruction)Not all fault movements result in violent earthquakes. Some faults move slowly and fairly continuously, a movement called fault creep.Earthquake Hazardsearthquakes don’t kill people, buildings do”.Earthquake Damage Susceptibility Depends onMagnitude of the earthquake - the higher the magnitude, the more intense the shaking, the longer the duration of shaking, and the greater the displacement.Distance from the epicenter – Seismic waves attenuate (amplitude diminishes) with distance.Surface Geology – Surface Faulting and ground rupture, and soil amplification.Integrity of Structures and Type of Construction - Building codes; Building material: concrete/masonry vs. wood/steel. Structural Integrity.Integrity of Utilities.Population Density, building density, time of day, etc. - the more people and buildings, the greater the potential for structural damage and death.Earthquake Magnitude and Ground AccelerationGround acceleration is the rate of increase in velocity, or the strength of the shaking. During an earthquake, the ground accelerates from being stationary to a maximum velocity before slowing and reversing its movement.Acceleration is normally designated as some proportion of the acceleration due to gravity (g). 1.0 g is the acceleration felt by a freely falling body.Earthquake Magnitude and Shaking Time An increase in magnitude significantly increases the time of shaking, and the potential damageMagnitude and Fault DisplacementThe amount of displacement during fault movement and the length of surface rupture are generally proportional to the magnitude of the accompanying earthquake.Distance from the Epicenter Seismic waves attenuate with distance.Surface Faulting and Ground Rupture - Land Uplift and SubsidenceGround rupture and surface faulting occur due to horizontal or vertical displacement of faults that break the surface. Areas right next to the fault can experience direct damage from the ground
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