Earthquakes

Tangshan, China 1976 Lisbon 1755

Earthquake is a sudden slip (rupture) on a fault and the resulting ground shaking caused by the radiated seismic energy generated by the slip (rupture)

What is an Earthquake?

EarthquakesAlternate definition:A natural vibration of the ground caused

when the stress built up between tectonic plates is suddenly released.

There are three types of stress:Compression – caused by two plates being

push together.Tension – Pulls plates apart.Shear – causes plates to twist.

Types of faultsThere are three types of Faults

1. Normal fault caused by tension.

2. Reverse fault caused by compression.

3. Strike-slip fault caused by shearing.

Three Types of Seismic Waves1. Primary waves (p-waves)

Longitudinal waves – these vibrate by the compression of material in the direction of the wave’s motion. These can travel through the core.

2. Secondary waves (s-waves) Transverse waves (Like in water) – these vibrate

by moving the particles perpendicular to the direction of wave’s motion. These cannot travel through the core.

3. Surface waves (Love and Rayleigh waves) Vibrate in all directions.

Body Waves• P waves are compressional (or

longitudinal) body waves in which rock vibrates back and forth parallel to the direction of wave propagation– Fast (4 to 7 kilometers per

second) wave that is the first or primary wave to arrive at a recording station following an earthquake

– Can pass through solids and fluids (liquids or gases) Fig 10.7

Body Waves

• S waves are shearing (or transverse) body waves in which rock vibrates back and forth perpendicular to the direction of wave propagation– Slower (2 to 5 kilometers

per second) wave that is the secondary wave to arrive at a recording station following an earthquake

– Can pass only through solids

Fig 10.7

Surface Waves• Slowest type of seismic

waves set off by earthquakes

• Love waves involve only side-to-side motion of the ground surface– Can’t travel through fluids

• Rayleigh waves behave like ocean waves, and cause the ground to move in an elliptical path opposite the direction of wave motion– Extremely destructive to buildings

Fig 10.7

The Earth’s InteriorEarthquakes (P and S waves)Were instrumental in the Study of the Earth’s Core

S-waves can only propagate Through the Mantle,

P-waves can propagate Through the core.

This shows a densityDifference inside the Earth

Figs 10.23 & 10.24 & 10.26

Measuring EarthquakesRichter Scale

Based on size of the largest seismic waves generated by the Earthquake.

Each level increase represents an increase of energy by a factor of 32 and a wave size of 10 times.

Anatomy…Worst damage is not necessarily nearest to focus: it depends on ground type, time, magnitude, terrain, nature of structures…

Fig 10.2

Measuring EarthquakesMoment of Magnitude Scale

Based on the magnitude of seismic waves as well as the size of the fault rupture, amount of movement, and rock characteristics.

More accurate than Richter scale.

Modified Mercalli ScaleMeasures the amount of damage caused by the

Earthquake.

Strongest Earthquakes of 20101. Chile (Feb 27th) Mag. 8.82. Indonesia (April 6th) Mag. 7.73. Vanuatu (May 27th) Mag. 7.74. Indonesia (May 9th) Mag. 7.25. Mexcali Valley, California (April4th) Mag. 7.26. Solomon Islands (Jan 3rd) Mag. 7.17. Haiti (Jan 12th) Mag. 7.08. Ryukyu, Japan (Feb 27th) Mag. 7.0

There have been 7460 earthquakes so far this year. Most are below a magnitude of 4.9.

Chile7th Strongest earthquake in

History130 after shocks over the next

week.The focus ground at epicenter

moved 3.0 up and 10 m west. 500 times more powerful than

HaitiCalled a megathrust quake

because it is in a subduction zone

Earthquake Size and Strength…• Modified Mercalli Intensity Map

– 1994 Northridge, CA earthquake, Richter magnitude 6.7

Los Angeles area…

Collapsed Cypress Freeway

Copyright © Houghton Mifflin Company. All rights reserved.

Source: Richards/Sipa Press

Japan (Tohoku) Earthquake and Tsunami

CBC News Reportshttp://

www.cbc.ca/news/world/story/2011/03/11/japan-quake-tsunami.html

The Guardianhttp://

www.guardian.co.uk/world/video/2011/mar/14/japan-tsunami-amateur-footage-video

TsunamiThe 9.0 magnitude

earthquake and subsequent 40 m tsunami in Japan in March 2011, took nearly 20,000 lives and threatened many more with concerns over radiation from the damaged Fukushima Nuclear Power Plant.

The Fukushima evacuation zone continues to displace thousands of residents of the region.

Generation of a Tsunami from EQ

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Location of the EarthquakeFocus

The location in the Earth’s crust where the earthquake occurs.

It can occur anywhere between the surface and a depth of 700 km. The closer to the surface it occurs, the more powerful the earthquake is.

Epicenter The location on the Earth’s surface above the

focus.

Finding the EpicenterEpicenters are found using triangulation from

three or more seismograph stations.

P-waves travel a lot faster than s-waves. The difference between the arrival time of each type of wave tells you the distance to the epicenter.

The speed of each wave is a constant factor.

Steps for finding the Epicenter1. Determine the difference in time between

the arrival of the s & p waves.

Steps for finding the Epicenter

1. Use the travel-time graph at which the p-curve and s- curve have the same separation.

2. Record this distance.

Steps for finding the Epicenter1. Repeat the steps #1-3 for two other

stations

Steps for finding the Epicenter1. Use a compass to draw a circle around

each station with the radius in step #3. 2. The epicenter is located where all three

circles intersect.

Locating Earthquakes

• P- and S-waves start out from the focus of an earthquake at same time

• P-wave gets farther and farther ahead of the S-wave with distance and time from the earthquake

• Travel-time curve can be used to determine the distance to the focus based on the time gap between first P- and S-wave arrivals– Plotting distances from 3

stations on a map, as circles with radii equaling the distance from the quake, will show the location of the epicenter

TRIANGULATION OF DATA

Fig 10.10

Earthquake Location Activity

Class Activity:http://

earthguide.ucsd.edu/eoc/teachers/t_tectonics/swf_earthquake_triangulation/p_activity_eqtriangulation.html

Class lab Activityhttp

://www.sciencecourseware.org/eec/earthquake/

HomeworkP. 499 #1-4p. 504 #2, 4P. 510 #1-4