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CHAPTER 12:EARTHQUAKES
Earthquakes
• An Earthquake is a sudden shaking of the crust
What causes earthquakes?
• Most earthquakes are associated with the breakage and displacement of rocks, often along a known fault
• Kinetic energy is released as seismic waves
• Seismic waves radiate through the Earth's interior resulting in "shaking“ at surface
More than 380 major cities lie on or near unstable regions of the Earth’s crust
(potential for casualties and infrastructure damage is high)
How do earthquake locations relate to plate boundaries? Identify some of the plates on this map.
There Are Several Types of Earthquake Hazards
• Ground shaking produces sediment liquefaction (instability of the ground), mass wasting (landslides), and damage to buildings and other structures
• Fires following an earthquake can be catastrophic and fire-fighting capabilities may be compromised
• Often water lines and emergency equipment are damaged and utility and communications systems are disrupted
• A tsunami may be generated
Tsunami Simulation Animation
Kashmir, 2005
Explain how an earthquake could lead to flooding.
Natural Resources Canada map of seismic shaking hazards
What is the relative
earthquake risk in your part of the country?
The Elastic Rebound Theory
• The elastic rebound theory explains the origin of earthquakes• Energy is stored in the form of elastically deformed rock. When the strain
exceeds the strength of the rock it breaks the energy is released.
Strain Release
• Eventually the rock is no longer able to accommodate the buildup of strain and it ruptures, either along a pre-existing fault or at a new location.
Is this a normal or a
reverse fault?
What are the definitions of “focus” and “epicenter”?
Clusters
• Earthquakes tend to occur in clusters, both in time and in space.
Intraplate Earthquakes
• Most seismicity occurs at plate boundaries,but intraplate earthquakes are also common
Identify some intraplate earthquakes on this map.
Divergent, convergent, and transform margins
• Divergent, convergent, and transform margins are the sites of frequent earthquake activity
Divergent margin Convergent margin
Intraplate locationTransform margin
Identify some divergent, convergent
and transform boundaries on this map where there
have been earthquakes.
Intraplate Earthquakes
• Poorly understood• Most have shallow foci• Tend to occur in soft, weak areas of plate interiors, caused by stresses
transmitted from plate boundaries,• Reactivation of old rift zones or faults (e.g., Logan Fault)• Hot Spots
– Yellowstone– Hawaii
Divergent Seismicity
Convergent Seismicity
• The zone of seismicity along the plane of the subducting plate is called the Wadati-Benioff Zone
Where can you see some
Wadati-Benioff zones on this
map?
Which plates are involved?
Transform Seismicity
What are some features that the San Andreas and North Anatolian Faults have in common?
Earthquakes produce four types of seismic waves
• Two types of Body Waves and two types of Surface Waves
Body waves (within the crust)
Surface waves
Liquids do not resist changes in shape - they absorb S WAVES rather than transmit them.
A Seismogram is produced by a Seismometer
Seismometers
• Seismometers are instruments that measure and help locate earthquakes• An earthquake near Vancouver is measured by a seismometer in Saskatoon
Seismometers in Iqaluit, Halifax and St. John’s are used to locate an earthquake in central Quebec
In fact, many seismometers are used to locate earthquakes.
Could an earthquake be
located with just two seismic
stations?
Expressing Earthquake Magnitude
• Earthquake magnitude is expressed as a whole number and a decimal fraction (e.g., 7.3)
Modified Mercalli (MM) intensity scale
• Intensity is expressed using the Modified Mercalli (MM) intensity scale
What is the difference between earthquake magnitude and intensity?
Magnitude of an earthquake using the Richter method
Comparison of energy released and damage done by different events
Seismology
• Seismology is the study of seismic waves in order to improve our understanding of the Earth’s interior
• Wave refraction is used to identify a discontinuity
P-wave and S-wave shadow zones
Why are S waves not transmitted
through the Earth’s core?
Seismic Data
• Seismic data confirm the existence of discontinuities in Earth’s interior
Mohorovicic Discontinuity
• ~ 8 km beneath oceanic basins• ~ 20 to ~70 km beneath continents
This map shows just a small fraction of all of the seismometers on Earth.
Seismic Tomography
• Seismic tomography uses seismic data from hundreds of seismic stations and hundreds of different earthquakes to provide a 3-dimensional seismic velocity image of the Earth’s interior.
Modeling with Tomographic Images
Why is the subducted Farallon Plate in the mantle beneath North America represented by a zone of higher seismic velocity than the surrounding mantle?
COPYRIGHT
Copyright © 2014 John Wiley & Sons Canada, Ltd. All rights reserved. Reproduction or translation of this work beyond that permitted by Access Copyright (The Canadian Copyright Licensing Agency) is unlawful. Requests for further information should be addressed to the Permissions Department, John Wiley & Sons Canada, Ltd. The purchaser may make back-up copies for his or her own use only and not for distribution or resale. The author and the publisher assume no responsibility for errors, omissions, or damages caused by the use of these programs or from the use of the information contained herein.