VIEWS: 1 PAGES: 40 POSTED ON: 9/13/2012
Earthquakes and Plate Tectonics What is an earthquake? An earthquake is the vibration of Earth produced by the rapid release of energy Energy radiates in all directions from its source, the focus Energy moves like waves Seismographs record the event Anatomy of Earthquakes Causes of earthquakes Sudden release of accumulated strain energy Creation of new faults by rupturing rocks Shifting of rocks at preexisting faults Elastic Rebound Theory Rocks on sides of fault are deformed by tectonic forces Rocks bend and store elastic energy Frictional resistance holding the rocks together is overcome by tectonic forces Slip starts at the weakest point (the focus) Earthquakes occur as the deformed rock “springs back” to its original shape (elastic rebound) The motion moves neighboring rocks (((( ((( )))) ))) The 3 Basic Types of Plate Movement (a review…) Divergent Boundary Convergent Boundary Transform Fault Boundary Horizontal Movement Along Strike-Slip Fault San Andreas: An active earthquake zone San Andreas is the most studied fault system in the world Displacement occurs along discrete segments 100 to 200 kilometers long Most segments slip every 100-200 years producing large earthquakes Some portions exhibit slow, gradual displacement known as fault creep Fence offset by the 1906 San Francisco earthquake Earthquake focus and epicenter Seismology Seismometers - instruments that record seismic waves Records the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape A seismograph designed to record vertical ground motion The heavy mass doesn’t move much The drum moves Lateral Movement Detector In reality, copper wire coils move around magnets, generating current which is recorded. Two Types of Surface Waves P and S waves Demo: P and S waves Smaller amplitude than surface (L) waves, but faster, P arrives first, then S, then L Earthquakes are located by finding the difference in velocities of P and S waves Note how much bigger the surface waves are Graph to find distance to epicenter Locating the epicenter of an earthquake Three seismographs needed to locate an epicenter Each station determines the time interval between the arrival of the first P wave and the first S wave at their location A travel-time graph then determines each station’s distance to the epicenter Locating Earthquake Epicenter Locating the epicenter of an earthquake A circle with radius equal to distance to the epicenter is drawn around each station The point where all three circles intersect is the earthquake epicenter Epicenter located using three seismographs Measuring the size of earthquakes Two measurements describe the size of an earthquake Intensity – a measure of earthquake shaking at a given location based on amount of damage Magnitude – estimates the amount of energy released by the earthquake Intensity scales Modified Mercalli Intensity Scale was developed using California buildings as its standard Drawback is that destruction may not be true measure of earthquakes actual severity Earthquake destruction Amount of structural damage depends on Intensity and duration of vibrations Nature of the material upon which the structure rests (hard rock good, soft bad) Design of the structure Magnitude scales Richter magnitude - concept introduced by Charles Richter in 1935 Richter scale –Based on amplitude of largest seismic wave recorded –LOG10 SCALE Each unit of Richter magnitude corresponds to 10X increase in wave amplitude and 32X increase in Energy Magnitude scales Moment magnitude was developed because Richter magnitude does not closely estimate the size of very large earthquakes –Derived from the amount of displacement that occurs along a fault and the area of the fault that slips Investigating Earth’s Interior Seismology helps us understand Earth’s Interior Structure. We use: Speed changes in different materials due changes rigidity, density, elasticity Reflections from layers with different properties Attenuation of Shear Waves in fluids Direction changes (Refraction) Three Major Components of Earth 35 Seismic-wave velocities are faster in the upper mantle Mohorovičić discontinuity Velocity increases w depth, waves bend back to surface. Waves that travel via mantle arrive sooner at far destinations Wave Velocities Upper Mantle Fast Asthenosphere Slow Lower Mantle Fast Mineralogy of Earth’s Layers http://pubs.usgs.gov/gip/interior/ The P-Wave Shadow Zone http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/p_lehmann.html P-waves through the liquid outer core bend, leaving a low intensity shadow zone 103 to 143 degrees away from the source, here shown as the north pole HOWEVER, P-waves traveling straight through the center continue, and because speeds in the solid inner core are faster, they arrive sooner than expected if the core was all liquid. Inge Lehmann Behavior of waves through center reveal Earth’s Interior The S-Wave Shadow Zone http://en.wikipedia.org/wiki/Richard_Dixon_Oldham Since Shear (S) waves cannot travel through liquids, the liquid outer core casts a larger shadow for S waves covering everything past 103 degrees away from the source.
Pages to are hidden for
"Earthquakes - Chapter 10"Please download to view full document