chapter 12: earthquakes. earthquakes an earthquake is a sudden shaking of the crust

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?

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