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Download Earthquake intensity, elastic rebound theory and plate boundaries.
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Earthquake Science (Seismology) • Quick review of last lecture (fault, epicentre, seismic waves, magnitude) • Earthquake Intensity • Elastic-rebound theory • Fault classification • Earthquakes and plate boundaries • Benioff-Wadati zone Earthquake Terminology Fault: A surface across which two blocks can move relative to each other. Focus: The location on a fault where earthquake rupture initiates. Hypocentre: The calculated position of an earthquake focus. Epicentre: That point on the Earth's surface directly above the hypocentre. Magnitude: A measure of the strength of an earthquake, as determined by seismographic observations. Seismic Waves 101 Fast P waves Primary (or compressional) S waves Secondary (or shear) Love waves Slow Rayleigh waves Body waves: travel through Earth. Surface waves: travel around the surface of Earth. Surface-wave magnitude formula (most common) MS = log10(A) + 1.656log10(D) + 1.818 A = amplitude in microns (mm) D = epicentral distance in degrees (divide by 111 to go from km to degrees) 672 mm A = 672 mm (measured) D ~ 1000*6.6 - 1500 = 5100 km ... D ~ 5100/111 ~ 46 degrees MS = log10(A) + 1.656log10(D) + 1.818 = 7.4 Intensity • Intensity is a measure of the effects of an earthquake on the natural and built environment. • While the magnitude of an earthquake is constant, the intensity changes with epicentral distance. • In N.A., the Modified Mercalli intensity scale is used (I-XII) Earthquake Intensity Map A map of earthquake intensity is also called an isoseismal map. New Madrid Earthquake sequence of 1811-1812 http://www.abag.ca.gov/bayarea/eqmaps/doc/mmi.html http://www.abag.ca.gov/bayarea/eqmaps/doc/mmi.html http://www.abag.ca.gov/bayarea/eqmaps/doc/mmi.html http://www.abag.ca.gov/bayarea/eqmaps/doc/mmi.html http://www.abag.ca.gov/bayarea/eqmaps/doc/mmi.html Recently, intensity measurements have been carried out using the web. Internet Intensity Map Produced using web response forms. A map of earthquake intensity is also called an isoseismal map. Parkfield, CA, 28 Sept., 2004 M = 6 Elastic Rebound Theory Fault Fence Original position Stress Accumulates Earthquake: stress released Rebound to original shape Elastic Rebound Theory 1. Stress on a fault slowly accumulates 2. During an earthquake, stress on the fault is released 3. After an earthquake, stress begins to re-accumulate Elastic Rebound Theory: The Earthquake Cycle For an active fault, earthquakes will repeat periodically. 1460 1855 Turakirae Head, New Zealand Earthquakes and Plate Boundaries More than 90% of earthquakes occur near plate boundaries … in fact the earthquake distribution can be used to outline tectonic plates ... Earthquakes and Plate Boundaries Although there are exceptions … rare earthquakes do occur in stable continental regions New Madrid Earthquake sequence of 1811-1812 Seismic Intensity Map Three basic categories of faults Normal fault: upper block drops down Thrust (reverse) fault: upper block pushes up Strike-slip fault: blocks slide past each other Normal fault • Occurs where plates are pulling apart under tension • Divergent plate boundary • Example: Iceland Strike-slip fault • Occurs where plates are sliding past each other • Transform plate boundary • Example: San Andreas Fault, California Thrust (reverse) faults • Occurs where plates are pushing together under compression • Convergent plate boundary • Example: Japan Benioff-Wadati Zone • Dipping zone of earthquakes • Occurs at a convergent plate boundary
