EarthquakesEarthquakesA Violent A Violent InterludeInterlude

Definition of earthquake

-vibration of the Earth's by shock waves generated by energy released from rocks rupturing under stress

-most (not all) earthquakes and ruptures occur along faults

-most faults and stress occurs along active plate tectonic boundaries

Fault Types

Dip-Slip [normal] Strike-Slip [right-lateral]

Elastic rebound theory

Crustal blocks move through time -“locked” segment along the fault: no slip

Rocks are strained: deformation of shape -the strain can be released catastrophically

Energy released causes earthquake Analogy: breaking rubber band

Other causes of earthquakes:

Landslides, rockslides, or slumping of rocks.

Movement of magma, gases, or rocks associated with volcanism

Morphology of earthquakes and related terms

Focus or Hypocenter -exact location of the earthquake disturbance-with few exceptions is located below the surface of the Earth

Epicenter -the position on the surface of the Earth directly above the focus

Slip: the distance of displacement along the fault

Foreshock(s)

-Earthquake(s) generated at or very near the focus of the main earthquake prior to the main shock

-smaller magnitude than the main shock and precede main quake by a short or substantial time interval

Aftershock(s)

-Earthquake(s) generated at or very near the focus of the main earthquake following the main shock

-smaller in magnitude than the main shock. Follow the main earthquake by a short or substantial time interval

Main shock

-is the main earthquake disturbance generated at the focus

Seismic waves

-waves which travel within the Earth

-P (primary) wave-moves in a straight line path with alternating compression and expansion

-S (secondary) wave-moves in a sinusoidal motion along its path of movement

Principles of P and S Waves

P-Wave Propagation

P wavesCompressional waves – similar to sound waves – Series of contractions and relaxations – Fastest, ~5 km/sec (depends on rock type) – Travel through solid, liquid and gas

S-Wave Propagation

S wavesShear waves – motion is right angles to direction of wave – Half speed of P waves – Travel only through solid

P-wave

S-wave

Surface Waves-move along the Earth’ssurface radiating out from epicenter– two types:

Rayleigh & Love

Seismographs-seismograph is the machine that detects seismic waves. -seismogram is the printed record.

Seismograms from multiple Seismographs around San Francisco Bay Area

Locating a focus/epicenter:

1. Need a seismogram from at least 3 different seismic stations.

2. Examine the seismograms and determine the elapsed time between the arrival of the first P-wave and the first S-wave.

3. Use a travel-time graph

-knowing the S- & P-wave travel-time, you can determine the distance to the focus/epicenter from the seismic station.

4. On a map, draw a circle around the seismic station.

5. Repeat for two other seismic stations. (Triangulation.)

Measuring EarthquakesSeveral different scales exist to measure earthquakes.

Modified Mercalli Scale

Measures earthquake intensity. -describes damage to structures.-ranges from I (felt by only a few) to XII (total destruction).

Intensity depends on: -Distance to epicenter -Building materials and design -Type of ground material - soil, rock, etc.

Modified Mercalli Intensity Scale

I. Not felt except by a very few under especially favorable conditions. II. Felt only by a few persons at rest, especially on upper floors of buildings. III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight. VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures. VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations. X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent. XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly. XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.

Richter Scale

-Measures earthquake magnitude.-determined by measuring the amplitude of the largest waves on the seismogram.

-logarithmic scale.-each number on the Richter Scale is ten times

greater in wave amplitude.

-each number on the Richter Scale involves an energy release about 32 times as great.

Moment magnitude scale-measures the amount of energy released

– Designed to differentiate large earthquakes– May be used to calculate energy of old

events by slip along fault– Moment = µ A D– µ = shear modulus A = LW = area

D = average displacement during rupture

Moment Magnitude forIndonesianQuake [2004]

Locations of Earthquakes

Most occur along tectonic plate boundaries: -around the Pacific Ocean (circum-Pacific belt is major site of large earthquakes) -Mediterranean Sea area through Iran and on to the Himalayas -Asia (Indonesia, Himalayan region) -Mid-ocean ridges

Some occur far from plate edges-intraplate earthquakes