earthquakes 11 earthquakes occurred worldwide by 9am yesterday (local time)

Earthquakes

11 Earthquakes Occurred Worldwide by 9AM Yesterday (local time)

Why Study Earthquakes?• January 2010 – Haiti – Magnitude 7.0

>250,000 deaths, ~3 million need emergency aid

• May 2008 – China – Magnitude 7.887,652 deaths, estimated ~86 billion dollars loss.

• October 2005 – Pakistan – Magnitude 7.6Estimated 86,000 deaths, 4 million left homeless.

• December 2004 – Sumatra – Magnitude 9.1227,898 deaths, 1.7 million left homeless.

• 187 earthquakes have occurred which killed 1,000 or more since 1900.

http://www.youtube.com/watch?v=4Y-62Ti5_6s

http://www.youtube.com/watch?v=4Y-62Ti5_6s

Earth is Composed of Multiple Layers from Core to CrustCrust and Mantle Lithosphere = Locked Together as Rigid Plate

Locations of earthquakes during past 5 years clearly mark the outlines of the Earth’s tectonic plates.

Convergent Plate Margins: Large Earthquakes to ~700 kmDivergent Plate Margins: Small Shallow EarthquakesTransform Plate Margins: Large Shallow Earthquakes

Ductile and Brittle Strain Diagram

As stress is applied 3 different responses:

Elastic strain – reversible, rock can return to original shape.

Plastic strain – rock flows, strain rate increases faster as stress goes up, irreversible.

Brittle strain – rock breaks suddenly as stress increases, irreversible.

What is the connection between earthquakes and rock deformation?

Rocks in the field (as in lab simulations) absorb stress elastically up to their yield strength and then release it when they break. This occurs along faults. Thus, earthquakes provide an image of the strike of faults (left) and the dip of faults (right).

Subduction Zones Are “Imaged” By Earthquakes

Plate Boundaries Imaged by Earthquakes

Can You Tell Which Way the

Subduction Zones Dip?

Earthquakes and Related Terminology

Focus (Hypocenter)Exact location of the earthquake origin - with few exceptions 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.

Seismic Waves

Shock 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.

Surface wave – moves along Earth’s surface.

Illustration of Difference Between P and S Waves

P Waves• Compressional Waves – similar to sound waves.• A series of contractions and relaxations of the rock.• Thus, motion is along direction of wave propogation.• Fastest, ~5 km/sec, but in detail depends on rock type.• May travel through solid, liquid and gas.

S-Wave Propagation

S Waves• Shear Waves – motion is at right angles to direction of wave propogation.• Move at only about half the speed of P waves.• Travel only through solids.

Surface WavesMove along the Earth’ssurface radiating out from epicenter.

There are two types: Rayleigh & Love

Surface WavesMove along the Earth’ssurface radiating out from epicenter. There are two types: Rayleigh & Love


Motion along faults is often expressed at Earth’s surface. A highway (left) is displaced by a right-lateral strike-slip fault. A fault scarp (right) is exposed on the footwall of a normal dip-slip fault.


• Earthquakes are most common in the upper crust.

• This is expected because the upper crust is cold and brittle.

• Deep earthquakes tend to be restricted to narrow zones such as subduction zones, where cold rock is carried to great depths.


Fault motion without earthquakes: some faults are soweak that little elastic energy is stored from stress. Instead the fault “creeps” almost constantly by small increments of strain.

Elastic rebound along the Hayward fault in California.

Elastic rebound is a “bend-bend, break, snap-back” process (rubber band analogy).

However, it is not possible to know how much stress a given interval of a fault can take before it breaks.


• Foreshocks, mainshock, aftershocksForeshocks, mainshock, aftershocks• ForeshocksForeshocks are produced by small displacement fault are produced by small displacement fault

motions that occur prior to the main shock.motions that occur prior to the main shock.• The The mainshockmainshock produced by a single large slip on the fault produced by a single large slip on the fault

and a sudden large release of energy.and a sudden large release of energy.• AftershocksAftershocks are small displacements that occur after the are small displacements that occur after the

mainshock. There may be tens to hundreds, and although mainshock. There may be tens to hundreds, and although usually weak, they often hamper relief efforts.usually weak, they often hamper relief efforts.

• Foreshocks are not useful indicators in predicting Foreshocks are not useful indicators in predicting earthquakes as there is no way to tell a foreshock to a earthquakes as there is no way to tell a foreshock to a major earthquake from a lone, small earthquake.major earthquake from a lone, small earthquake.


Aftershocks along a fault system in Alaska following two major earthquakes. Aftershocks may stay in the mainshock region, or may transfer to adjoining faults as stresses change and adjust.

Earthquakes - Summary

• Earthquakes result from motion along faults. Earthquakes are caused by brittle failure of rock and thus usually occur in the upper crust where temperature and pressure are relatively low (exception: subduction zones).

• Not all motion on faults produces earthquakes - rocks can creep along faults if the fault/rock is too weak to store up the energy of prolonged stress.

• Elastic rebound theory explains deformation before and during earthquakes as brittle failure following accumulation of elastic strain. Bending rocks on either side of a locked fault store energy that releases as an earthquake when it exceeds the rocks yield strength.

• Foreshocks and aftershocks are smaller earthquakes that occur before and after the main earthquake.

How do we detect and measure earthquakes?

• Intensity – a qualitative measure of the force of an earthquake - observed effects such as damage to structures and secondary effects like landslides.

• Mercalli Scale – made by Giuseppe Mercalli (1902) and runs from a low of I to a high value of XII• Current revision is called the Modified Mercalli scale. • Intensities are commonly plotted on maps with intensity

contours.• Can be used to determine intensities of historic earthquakes

through newspapers, diaries, etc.


Examples of Mercalli iso-intensity (contours of equal intensity) maps. The New Madrid earthquakes of 1811–1812 are among the largest earthquake events ever.


• Magnitude – a quantitative measure of earthquake size based on the energy released during the earthquake.• Note that this is not a direct measure like temperature, but is

calculated from measurements of seismic waves after the earthquake occurs.

• Richter Scale – the first magnitude scale (1935) developed by Charles Richter.• Measured by surface wave amplitude on a seismometer

and the time interval between P-S waves.• Scale is logarithmic, each numeric value is 10x the force of

the one before.• E.g. magnitude 5 is 10x stronger than magnitude 4.

Calculation of Richter Scale Magnitude


Comparison of specific earthquakes energy released to that of other events such as volcanic eruptions, bombs, etc…

How are earthquakes destructive?How are earthquakes destructive?

• Ground shakingGround shaking• Surface waves radiating outward from the epicenter Surface waves radiating outward from the epicenter

cause up-down and side-to-side ground motion. cause up-down and side-to-side ground motion. This causes damage to buildings, bridges, dams, This causes damage to buildings, bridges, dams, and other structures.and other structures.

• Disrupts supplies of water, gas, and power, and Disrupts supplies of water, gas, and power, and damages roads. Resulting fires and the inability to damages roads. Resulting fires and the inability to quickly get to them often produces more damage quickly get to them often produces more damage than the ground shaking alone.than the ground shaking alone.

• Ground shaking often triggers landslides. Ground shaking often triggers landslides.


Ground shaking Landslides

How are earthquakes destructive?How are earthquakes destructive?Damage from ground shaking

Peru - 1970Peru - 1970An earthquake caused a rock and snow debris avalanche which buried the towns of Yungay and Ranrahirca.

The landslide started as a moving mass of glacial ice and rock about 3,000 feet wide and one mile long.

The landslide traveled 11 miles to the villages, reaching speeds of more that 100 miles an hour.

By the time it reached Yungay and Ranrahirca, the debris is estimated at 80 million cubic yards of water, ice, mud, and rocks.

The death toll from the landslide alone was 18,000 (including fatalities from the earthquake a total of 66,000 people died).

http://landslides.usgs.gov/learning/photos/images/international/peru_earthquake_mt._huascaran_1970/huascaranperu.jpg


The ground material greatly affects susceptibility of an area to damage.

Saturated sediments undergo liquefaction (becoming like fluid) during the shaking event).


Generation of a seismic sea wave, or tsunami. (Note that “tidal wave” is not a correct term.)

A M9.1 earthquake near Sumatra in 2004 caused a huge tsunami.~225,000 people in 11 countries around the Indian Ocean killed.

A 1200 km section of the subduction zone moved over a period of a few minutes. Uplifted the seafloor as much as 3-4 meters, displacing billions of gallons of seawater.Released stored elastic energy equivalent to ~ 23,000 Hiroshima atomic bombs.

http://upload.wikimedia.org/wikipedia/commons/8/89/2004_Indian_Ocean_earthquake_-_affected_countries.png


Tsunamis may travel great distances and still cause major damage when they come onshore.Sumatra and Sri Lanka are over 1,000 miles apart.

M9 Earthquake

Sri Lanka

The tsunami was up to 10 meters (33 feet) high in some areas.

http://upload.wikimedia.org/wikipedia/commons/2/2d/2004-tsunami.jpg

How are earthquakes destructive?How are earthquakes destructive?Tsunami in Sri Lanka from the Dec. 2004 Sumatra earthquake.


• Earthquakes cause destruction by:• Ground shaking causing buildings, bridges, etc, to

collapse.• Destruction of power, water and gas supplies and

disruption of roads. Fires are common and hard to deal with.

• Landslides are triggered by earthquakes, some can be extremely destructive.

• Tsunamis are caused by earthquakes and may travel thousands of miles before coming onshore.

Detecting and Locating Earthquakes – How?

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

Different Types of Seismic Waves Travel at Different Speeds

Detecting and Locating Earthquakes – How?

First – We need a seismogram from at least 3 seismographs at different locations.

Second – We determine the elapsed time between the arrival of the first P-wave and the first S-wave at each seismic station.

Third – If we know the S- and P-wave travel time characteristics, we can determine the distance to the focus/epicenter from the seismograph.

Fourth – We draw circles around the seismic stations, the radius is the distance calculated previously.

Fifth – The location of the epicenter is where these circles all overlap. This is called triangulation.

Shock Wave Propogation Through A Homogeneous Planet

How Do We Image The Interior Of The Earth?

Shock Wave Propogation Through An Inhomogeneous PlanetDensity Increases Smoothly Towards Center


Refraction Occurs Where Abrupt Changes In Density Occur



• Changes in seismic wave velocities indicate density increases with depth.• P-wave shadow zone indicates presence of iron core (refraction).• S-wave shadow zone indicates that outer core is liquid (waves blocked).

Reflection Also Occurs Where Abrupt Changes In Density Occur


Density and Physical State Control Velocity Of Seismic Waves


Experimental petrologyprovides laboratorymeasurements of seismicvelocities and how thesechange with temperatureand pressure.



Seismic Velocities Are Measured In The Laboratory And Are Well-Known.

Measured seismicwave velocities in the Earth match laboratoryvelocities for knownminerals.

Composition of Earth’sinterior can be inferredfrom seismic data.


Knowledge of Earth’s Interior – Seismic Data and SamplesXenoliths (foreign rock) brought up by volcanic eruptions.

From the crust and the upper ~200 km of the mantle.

Mantle Peridotite Deep Crust

Layers of the Earth – Composition and Thickness

• Upper Continental Crust – Felsic to Intermediate - Granite to Diorite• Lower Continental Crust – More Mafic – Diorite to Gabbro• Oceanic Crust – Mafic - Basalt and Gabbro• Mantle – Ultramafic – Peridotite• Core – Iron/Nickel Alloy (outer liquid, inner solid)

earthquakes 11 earthquakes occurred worldwide by 9am yesterday (local time)

Documents

large earthquakes

earthquakes11 earthquakes

kmdivergent plate margins

sound waves

seismic waves shock

earths surface

speed of p waves

rock deformation