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Transcript
Earthquakes and Earth’s Interior 1. What are Earthquakes 2. Locate and Measure EQ 3. Use EQ to understand Earth’s structure 4. Plate Tectonics and EQs What is an Earthquake? n n Earthquake = ground shaking by waves Waves are caused by the passing of ENERGY 1 Definition of Earthquake: Motion of earth caused by transmission of energy (wave) released from breaking strained rocks (in lithosphere) EQ focus vs. epicenter Focus = location in Earth where breaking occurs Epicenter = projection of Focus onto surface 2 Earthquakes & Faults n n n EQs generated on faults Faults = brittle failure by stress The rupture/brittle failure of a fault is caused by stresses (tectonic or loading) in the lithosphere Earthquakes and Plate Tectonics: n n Thus, most Earthquakes are located at plate margins where tectonic stresses are greatest. Recall: EQs define lithospheric plate boundaries 3 The Elastic Rebound Theory n n n n n How faults create earthquakes Rocks deform or strain due to tectonic stresses (elastic deformation) Failure occurs (rupture strength exceeded) at one spot/point (focus) in rock when elastic deformation can no longer accommodate stress. Strained rocks around focus rebound, releasing energy in the form of waves (Earthquake!) Draw ELASTIC REBOUND (stress-strain diagram) and TYPES OF WAVES on board… 4 Elastic Rebound Theory Types of Seismic Waves P S P-wave animation S-wave Draw stress vs. strain, elastic moduli and Velocity equations. 5 Locating Earthquakes: Seismographs and difference in wave speeds First Seismometer (132 AD) invented by Chinese mathematician Chang Heng during Han Dynasty 6 Principle ideas behind locating earthquakes n n n P-wave velocity > S-wave velocity Thus the there is a lag time in the arrival of the P and S waves at a seismometer. Given the velocity of these waves through the Earth and the lag time, one can calculate a distance. Seismograph Animation Draw Lag Time 7 How many seismic stations are required to locate the focus of an earthquake? 8 Local or Richter Magnitude: ML = log10 (A/T) + f(d,D) Or Moment magnitude: Mw=2/3log10µAu – 6.0 n n n A = wave amplitude, T = wave period, d = distance, D = depth Note ML is function of depth and Distance to Focus - not desirable. Mw is ‘complex’ n A = area of fault, u = displacement along fault µ= shear modulus, Seismic Moment = µAu Reall just amount of slip * length of rupture * depth of rupture * strength of rock n n Mw better than Richter because Mw does not ‘saturate’ (give low values) at high magnitude – BETTER measurement of energy. Note these are log10 scales: n n one unit increase = 10x amplitude increase one unit increase = 32-time energy increase Illustrate difference between Richter and Moment Magnitudes 9