Earthquakes PDF

Introduction  Earthquakes are caused by a rapid release of energy.  Energy moves outward as an expanding sphere of waves.  This waveform energy can be measured around the globe.  Earthquakes are common on this planet.  They occur every day.  More than a milli...

Introduction  Earthquakes are caused by a rapid release of energy.  Energy moves outward as an expanding sphere of waves.  This waveform energy can be measured around the globe.  Earthquakes are common on this planet.  They occur every day.  More than a million detectable earthquakes per year.  Most earthquakes result from tectonic plate motion. Are there moon-quakes or mars-quakes? Box 8.1b  What Causes Earthquakes?  Seismicity (earthquake activity) occurs due to:  Sudden slip along a new or existing fault.  Movement of magma in a volcano.  Giant landslides.  Nuclear detonations.  Fault slip is the most common cause. Box 8.1a  Components of Earthquakes  Hypocenter/ Focus—the place where fault slip occurs.  Usually occurs on a fault surface.  Earthquake waves expand outward from the hypocenter.  Epicenter—land surface right above the hypocenter.  Maps often portray the location of epicenters. Fig. 8.3  Faults in the Crust  Faults are found in many places in the crust.  Active faults—ongoing stresses produce motion.  Inactive faults—motion occurred in the geologic past.  A fault trace shows the fault intersecting the ground.  Displacement at the land surface creates a fault scarp.  Not all faults reach the surface. Blind faults are not visible. Fig. 8.4b  Generating Earthquake Energy  Tectonic forces add stress (push, pull, or shear) to rock.  The rock bends slightly without breaking (elastic).  Continued stress causes cracks to develop and grow.  Eventually, cracking progresses to the point of failure (faults = brittle deformation).  Stored elastic energy is released at once, creating a fault. Fig. 8.6  Generating Earthquake Energy  Rocks slide past one another along a fault.  Fault motion cannot occur forever.  Fault motion is stopped by friction.  Friction is the force that resists sliding on a surface.  Friction is due to irregularities (bumps) along the fault. What kind of fault (normal, reverse, or strike-slip) would this be? Fig. 9.9a  Generating Earthquake Energy  Rocks slide past one another along a fault.  Fault motion cannot occur forever.  Fault motion is stopped by friction.  Friction is the force that resists sliding on a surface.  Friction is due to irregularities (bumps) along the fault. What kind of fault (normal, reverse, or strike-slip) would this be? Fig. 9.9a  Generating Earthquake Energy  Slip on a preexisting fault causes earthquakes.  Faults are weaker than surrounding crust.  Over time, stress builds up leading to slip along the fault.  This behavior is termed stick-slip behavior. Stick—friction prevents motion. Slip—friction is briefly overwhelmed by motion. Fig. 9.9c  Generating Earthquake Energy  A major earthquake may be preceded by foreshocks.  Smaller tremors indicating crack development in rock.  Aftershocks usually follow a large earthquake.  May occur for weeks or years afterward. Aftershock map of the August 23, 2011, 5.8M Earthquake near Mineral, Virginia http://earthquake.usgs.gov/earthquakes/seqs/events/se082311a/  Amount of Slip on Faults  How much does a fault slip during an earthquake?  Larger earthquakes have larger areas of slip. Displacement is greatest near the hypocenter. Displacement diminishes with distance.  Fault slip is cumulative.  Faults can offset rocks by hundreds of kilometers over geologic time. Geology at a Glance  Seismic Waves  Release of energy from fault slip = waves of energy emanating from focus  Body waves—pass through Earth’s interior.  P-waves (primary or compressional waves).  S-waves (secondary or shear waves).  Surface waves—travel along Earth’s exterior.  L-waves (Love waves – slowest and most destructive)  R-waves (Rayleigh waves) Usgs.gov  Seismic Waves  Body waves—pass through Earth’s interior.  P-waves (primary or compressional waves). Waves travel by compressing and expanding material. Material moves back and forth parallel to wave direction. P-waves are the fastest. They travel through solids, liquids, and gases. http://www.wwnorton.com/college/geo/animations/seismic_wave_motion.htm Fig. 8.7a  Seismic Waves  Body waves—pass through Earth’s interior.  S-waves (secondary or shear waves). Waves travel by moving material back and forth. Material moves perpendicular to wave travel direction. S-waves are slower than P-waves. They travel only through solids, never liquids or gases. Fig. 8.7b  Seismic Waves  Surface waves—travel along Earth’s exterior.  R-waves (Rayleigh waves) P-waves that intersect the land surface. Cause the ground to ripple up and down like water. Fig. 8.7c  Seismic Waves  Surface waves—travel along Earth’s exterior.  Surface waves are the slowest and most destructive.  L-waves (Love waves) S-waves that intersect the land surface. Move the ground back and forth like a writhing snake. Fig. 8.7c  How Do We Measure Earthquakes?  Seismometer—instrument that records ground motion.  A weighted pen on a spring traces movement of the frame. Vertical motion—records up-and-down movement. Horizontal motion—records back-and-forth motion. Fig. 8.8a, b  How Do We Measure Earthquakes? https://www.youtube.com/watch?v=fUlRuiSKzgs Fig. 8.9a http://www.wwnorton.com/college/geo/animations/how_seismograph_works.htm  How Do We Measure Earthquakes?  Measure wave arrivals and the magnitude of motion.  The first wave causes the frame to sink (pen goes up).  The next vibration causes the opposite motion.  The recording captures waves racing through a region. Fig. 8.9a http://www.wwnorton.com/college/geo/animations/how_seismograph_works.htm In groups, predict which waves 1, 2, and 3 represent in the seismograph below: 3 3 1 2  How Do We Measure Earthquakes?  Seismogram—data recording earthquake wave behavior.  Earthquake waves arrive at a station in a specific order. P-waves first S-waves second Surface waves last  Arrival times determine the distance to the epicenter. Fig. 8.9b  Finding the Epicenter  P-waves always arrive first; then S-waves.  P-wave and S-wave arrivals are separated in time.  Separation grows with distance from the epicenter.  The time delay is used to establish this distance. Fig. 8.10a  Finding the Epicenter  Data from three or more stations pinpoints the epicenter.  The distance radius from each station is drawn on a map.  Circles around three or more stations will intersect at a point.  The point of intersection is the epicenter. Fig. 8.10c  Earthquake Size  Earthquake size is described by two measurements.  The severity of damage (intensity) – Mercalli Scale  The amount of ground motion (magnitude)  Earthquake Size  Earthquake size is described by two measurements.  The severity of damage (intensity) – Mercalli Scale  The amount of ground motion (magnitude)  Mercalli Intensity Scale—amount of shaking damage.  Roman numerals assigned to different levels of damage. I = low XII = high  Damage occurs in zones.  Damage diminishes in intensity with distance. Fig. 8.11  Earthquake Size  Magnitude—a uniform measurement of size.  The maximum amplitude of ground motion.  Several magnitude scales are used. Richter Scale—useful near the epicenter. Moment Magnitude Scale—most accurate measure.  Magnitude scales are logarithmic.  M6.0 is 10 times a M5.0.  M6.0 is 100 times a M4.0  M8.0 is ______ times a M4.0. Fig. 8.12  Earthquake Size  Magnitude—a uniform measurement of size.  The maximum amplitude of ground motion.  Several magnitude scales are used. Richter Scale—useful near the epicenter. Moment Magnitude Scale—most accurate measure.  Magnitude scales are logarithmic.  M6.0 is 10 times a M5.0.  M6.0 is 100 times a M4.0  M8.0 is 10,000 times a M4.0. Fig. 8.12  Measuring Earthquake Size  Energy released can be calculated.  M6.0—the energy of the Hiroshima atomic bomb. Fig. 8.13  Earthquake Occurrence  Earthquakes are linked to plate tectonic boundaries.  In groups, draw where earthquakes are likely to occur on the cross sections below. 1 2 1 2 Fig. 8.14  Earthquake Occurrence  Earthquakes are linked to plate tectonic boundaries.  Shallow—divergent and transform boundaries.  Intermediate and deep—convergent boundaries. 1 2 Fig. 8.14  Earthquakes at Plate Boundaries  Divergent plate boundary—mid-ocean ridges.  Develop two kinds of faulting. Normal faults at the spreading ridge axis. Strike-slip faults along transforms.  Shallow: