Earthquake Science (Seismology). Quick review of last lecture (fault, epicentre, seismic waves, magnitude) Quick review of last lecture (fault, epicentre,

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Presentation transcript:

Earthquake Science (Seismology)

Quick review of last lecture (fault, epicentre, seismic waves, magnitude) Quick review of last lecture (fault, epicentre, seismic waves, magnitude) Earthquake Intensity Earthquake Intensity Elastic-rebound theory Elastic-rebound theory Fault classification Fault classification Earthquakes and plate boundaries Earthquakes and plate boundaries Benioff-Wadati zone Benioff-Wadati zone

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. Earthquake Terminology Magnitude: A measure of the strength of an earthquake, as determined by seismographic observations.

Surface waves: travel around the surface of Earth. Body waves: travel through Earth. Seismic Waves 101 P waves S waves Love waves Rayleigh waves Primary (or compressional) Secondary (or shear) Fast Slow

M S = log 10 (A) log 10 (  ) Surface-wave magnitude formula (most common) A = amplitude in microns (  m)  = epicentral distance in degrees (divide by 111 to go from km to degrees) 672  m A =  672  m (measured)  ~ 1000* = 5100 km  ~ 5100/111 ~ 46 degrees M S = log10(A) log10(  ) = 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

Recently, intensity measurements have been carried out using the web.

Internet Intensity Map A map of earthquake intensity is also called an isoseismal map. Parkfield, CA, 28 Sept., 2004 M = 6 Produced using web response forms.

Elastic Rebound Theory Fault Original position Fence Stress Accumulates Earthquake: stress released Rebound to original shape

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

Elastic Rebound Theory: The Earthquake Cycle For an active fault, earthquakes will repeat periodically. 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 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