Earthquake Science (Seismology).

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Presentation transcript:

Earthquake Science (Seismology)

Announcements for next week’s lab Magnetometer survey, working in groups Please bring: small notebook pen graph paper umbrella (if raining) Be prepared to remove all metal (watches, belt buckles, jewelry, etc.)

March 28, 1964 - Alaska M = 9.1 Up to 12 m vertical displacement

October 17, 1989 - Loma Prieta, CA $6 B in property damage

January 17, 1994 - Northridge, CA Mw = 6.7 > $15 B in property damage

January 17, 1995 - Kobe, Japan Mw = 6.7 5000 dead, > $200 B in property damage

August 17, 1999 - Izmit, Turkey M = 7.4 15,700 casualties

December 26, 2004 - Sumatra M = 9.15 250,000 deaths

What are earthquakes? (… and what earthquakes are not!) Earthquake phenomenology (… fault, epicentre, seismic waves, magnitude)

Earthquake: A sudden, violent dislocation in the subsurface caused by stress buildup on a fault. Fence offset during the Great Earthquake (1906) 4 m San Andreas Fault

An earthquake is not: … a giant crack that opens up and swallows buildings whole …like an underground explosion … defined by a point in space or time … easily predictable

Fault: A surface across which two blocks can move relative to each other.

San Andreas Fault Trace, California

Focus: The location on a fault where earthquake rupture initiates. Focal depth

http://www.iris.iris.edu/sumatra/

Hypocentre: The calculated position of an earthquake focus. (almost synonymous)

Epicentre: That point on the Earth's surface directly above the hypocentre. Epicentral Distance

Seismic Waves 101 Body waves Fast Slow P waves Primary (or compressional) S waves Secondary (or shear) Surface waves Love waves Rayleigh waves

www.iris.edu

Northern Peru, Magnitude 7.5 SS P Rayleigh S ELFO 6.6 min 2005/09/26 01:55:35 85.4 km depth

Estimating Epicentral Distance (D) For a distant earthquake: D ~ 1000 Dt - 1500 Dt in minutes, D in km For a local earthquake: D ~ 8 Dt Dt in seconds, D in km Dt e.g., D ~ 1000*6.6 - 1500 = 5100 km Note: Just a crude estimate!

Earthquake epicentres are located by triangulation www.quakechasers.ca Requires Dt from at least 3 stations

Magnitude: A measure of the strength of an earthquake, as determined by seismographic observations. Different magnitude formulas are used. Examples: Richter magnitude Surface-wave magnitude

Small earthquakes happen often, but large earthquakes are infrequent... http://www.em.gov.bc.ca/Mining/Geolsurv/Surficial/quake/eq2.htm

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)

Magnitude Calculation 672 mm Magnitude Calculation Example D ~ (5100/111) ~ 46 degrees MS = log10(A) + 1.656log10(D) + 1.818 = 7.4