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

EarthquakEarthquake Sciencee Science

(Seismology)(Seismology)

• Quick review of last lecture Quick review of last lecture (fault, epicentre, seismic (fault, epicentre, seismic waves, magnitude)waves, magnitude)

• Earthquake Intensity Earthquake Intensity

• Elastic-rebound theoryElastic-rebound theory

• Fault classification Fault classification

• Earthquakes and plate Earthquakes and plate boundariesboundaries

• Benioff-Wadati zoneBenioff-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

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

A =672 m (measured)

~ 1000*6.6 - 1500 = 5100 km

~ 5100/111 ~ 46 degrees

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

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

1855

1460

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