EarthquakEarthquake Sciencee Science

(Seismology)(Seismology)

Announcements for next week’s labMagnetometer 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 - AlaskaM = 9.1 Up to 12 m vertical displacement

October 17, 1989 - Loma Prieta, CAM = 6.9 $6 B in property damage

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

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

August 17, 1999 - Izmit, TurkeyM = 7.4 15,700 casualties

December 26, 2004 - SumatraM = 9.15 250,000 deaths

• What are earthquakes? What are earthquakes? (… and what earthquakes (… and what earthquakes are not!)are not!)

• Earthquake Earthquake phenomenology (… fault, phenomenology (… fault, epicentre, seismic waves, epicentre, seismic waves, magnitude)magnitude)

4 mSan AndreasSan Andreas

FaultFault

Fence offset during the Great Earthquake (1906)

Earthquake: A sudden, violent dislocation in the subsurface caused by stress buildup on a 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.

Hypocentre (almost synonymous)

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

EpicentralDistance

Surface waves

Body waves

Seismic Waves 101

P waves

S waves

Love waves

Rayleigh waves

Primary (or compressional)

Secondary (or shear)

Fast

Slow

www.iris.edu

2005/09/26 01:55:35 85.4 km depth

Northern Peru, Magnitude 7.5

PS

SS

Rayleigh

ELFO

6.6 min

Estimating Epicentral Distance ()

P

S

t

For a distant earthquake:

~ 1000 t - 1500

t in minutes, in km

e.g., ~ 1000*6.6 - 1500 = 5100 km

Note: Just a crude estimate!

For a local earthquake:

~ 8 tt in seconds, in km

Earthquake epicentresare located by triangulation

www.quakechasers.ca

Requires t 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

MS = log10(A) + 1.656log10() + 1.818

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

~ (5100/111) ~ 46 degrees

MS = log10(A) + 1.656log10() + 1.818

= 7.4

Magnitude CalculationExample