GEOLOGY LECTURE OUTLINE –

Earthquakes and Faulting – (Ch 11)

I. Lecture Content

Introduction – Earthquakes in Our Own Backyard

Fundamentals of Earthquakes and Faults – Definitions

Earthquakes and Plate Tectonics

Elastic Rebound Theory

Fundamentals of Seismic Waves

Locating Earthquake Epicenters

Measuring the Size of Earthquakes

Earthquakes as Natural Hazards

Earthquake Preparation

Earthquake Prediction

Prologue - Living with Earthquakes in So-Cal

II. Living with Earthquakes in Southern California

A. Southern California is one Huge Active Fault Zone

· Earthquakes occur along numerous active faults

· Map of California's Active Faults

--- Web page: http://quake.wr.usgs.gov/recenteqs/latest.htm

· The San Andreas Fault Zone – a Transform Plate Boundary

ü Mojave Desert Faults

ü San Andreas Fault

ü San Jacinto Fault

ü Ellsinore Fault

ü Newport-Inglewood Fault

ü Rose Canyon Fault

ü Continental Borderland Faults

ü Regional “Blind Thrust” Faults

B. Location, Frequency, and Size of Recent Quakes

Mojave Desert Faults – Landers -

San Andreas Fault – 1908 SF;

Tejon Pass;

Imperial Valley;

Loma Prieta;

San Jacinto Fault

Ellsinore Fault

Newport-Inglewood Fault

Rose Canyon Fault

Continental Borderland Faults

Regional “Blind Thrust” Faults; Sylmar; Northridge

C. Personal Experiences of Earthquakes

D. Understanding Earthquake Hazards

III. Fundamentals of Earthquakes and Faults

A. Earthquake defined:

· The sudden release of energy, usually along a fault,

which produces “seismic” vibrations within Earth.

1. Earthquakes vary in several ways.

Size/Intensity

Duration

Nature of ground motion

2. The record of an earthquake (a seismogram) is made by a seismic wave-measuring device (a seismograph).

(see Fig. 9.5)

3. The point where an earthquake’s energy is released is known as the focus. (see Fig. 9.6)

4. The point on the Earth’s surface directly over the earthquake’s focus is called the epicenter. (see Fig. 9.6)

5. Monitoring, measuring, and predicting earthquakes is the job of the seismologist.

B. Fault defined: A plane of weakness, within the upper crust, where displacement occurs, usually in the form of earthquake.

1. The nature of fault displacement (rupture) varies from

minute creep (mm’s) to massive offsets (10’s of m’s).

2. Faults, like volcanoes, are classified as either, active

(frequent rupture) or inactive/extinct (no ruptures).

3. Faults are also classified by the geometry of the rupture: Normal, reverse, strike-slip, and oblique.

C. Seismology defined:

· The scientific study of earthquakes (and earthquake-related processes and mechanisms).

1. Includes the monitoring and measuring of both seismic waves (earthquakes), and fault motions (rupture).

2. Most seismologists live in Earthquake Country.

IV. Earthquakes, Fault Zones and Plate Tectonics

A. Global Distribution of Earthquakes

1. Vast majority of quakes occur along plate boundaries.

2. 80% of all earthquakes occur in the circum-Pacific belt.

3. 15% of quakes within the Mediterranean-Asiatic belt.

4. Remaining 5% occur along ocean spreading ridges and

within plate interiors.

5. About 150,000 earthquakes (strong enough to be felt)

are recorded by seismographs worldwide each year.

Another 900,000 tiny (micro) earthquakes also occur.

B. Concentration of Earthquakes at Plate Boundaries

1. Most major quakes occur in four tectonic regions

· Subduction zones - Along dipping "Benioff zones"

ü Downgoing slab surface is a giant thrust fault

· Continental Collision zones - Major thrust/fold belts

· Transform boundaries - Giant strike-slip fault systems

· Oceanic spreading ridges - Rift Valleys

2. Average earthquake focal depth varies for each region

· Subduction zones - shallow to deep focus

· Continental collision zones - shallow to intermediate

· Transform and Divergent boundaries - shallow focus

C. Interactive Maps of Recent Earthquakes on the Web

1. Recent Quakes in California

http://www.scecdc.scec.org/recenteqs/

2. Recent Quakes in The United States

http://earthquake.usgs.gov/activity/index.html

3. Recent Quakes from around The World

http://neic.usgs.gov/neis/current/world.html

4. Shake Maps of the U.S.

http://earthquake.usgs.gov/shakemap/

D. Interactive Maps of Historic Earthquakes on the Web

1. Historical Quakes of Southern California

http://www.scecdc.scec.org/clickmap.html

2. Historical Quakes of the World

http://earthquake.usgs.gov/activity/past.html

3. World Quakes Greater than 7.0

http://neic.usgs.gov/neis/eqlists/MAG7/mag7.html

V. The Elastic Rebound Theory – The “Best” Explanation

A. Elastic Rebound Theory Explained:

· Fault (rocks) ruptures when pressure (binding stress) accumulates in the rocks on either side of the fault to a level that exceeds the rocks’ strength.

· Prior to rupture, the rocks slowly deform (bend) as they build up stored energy.

· Stored energy from the accumulated stress is released as the ruptured rock snaps back to their original position.

B. Illustration of the ERT - (see Figure 9.3)

VI. Fundamentals of Seismic Waves

A. Three Forms of Energy Released from Earthquakes

1. Heat Energy - caused by friction in the fault zone

2. Body Waves - Travel through the solid Earth in a manner

somewhat like sound waves. There are two types of

body waves; they are named P- and S-waves.

3. Surface Waves - Travel only along the ground surface of

the Earth in a manner similar to water waves. There

are two types of surface waves; they are named R-

and L- waves.

4. Each type of seismic wave has a unique behavior in

the way it moves (propagates) through the ground.

B. Body Waves - Two Types: P-waves and S-waves

1. P-waves - short for primary waves (see Figs. 9.12 & 9.14)

· The fastest seismic wave

· Can travel through solids, liquids, and gases

· Compressional, or push-pull

· Are similar to sound waves

2. S-waves - short for secondary waves

· Slower than P-waves - (see Figs. 9.12 & 9.14)

· Can only travel through solids

· Shear waves; material moves side-to-side

3. Velocity of body waves varies with type of material.

· Faster through denser, more elastic rocks

· Slower through less dense, more plastic rocks

· Wave velocity increases with depth, as a rule

· P-wave velocities are always faster than S-waves

for any given material

· S-waves cannot travel through liquids or gases

· P-waves arrive at seismic stations before S-waves

4. Illustration of Body Waves (see Figs. 9.12)

C. Surface Waves - Two Types: R-waves and L-waves

1. R-waves - short for Raleigh waves

· Slower of than the L-waves

· Wave motion along surface similar to a rolling

ocean wave = back-and-forth, up-and-down

· The sensation of a R-wave is like that on a boat

2. L-waves - short for Love waves

· Faster than the Raleigh waves

· Similar in form to a S-wave

· Wave motion along surface similar to a slithering

snake = undulating, side-to-side

· Most destructive of the seismic waves

3. Surface waves are most destructive in loose/wet material

4. Illustration of Surface Waves (see Fig. 9.13)

VII. Earthquake Epicenters - Finding its Location

A. Locating an Earthquake's Epicenter

· An earthquake's epicenter is located using seismic data

gathered from several seismic stations

· The fact that P-waves travel faster the S-waves means

that with increasing distance from the epicenter the

difference in arrival time between the P- and S-waves

will progressively increase. (see Fig. 9.14)

· Since average speeds for P- and S-waves are well known

for any specific distance from their source (focus), the

difference in arrival time between the P- and S-waves

can be used to calculate the distance to the epicenter.

· Seismologists have assembled time-distance graphs for

all P-S time intervals for calculating the distance to the

epicenter for each seismic station. (see Fig. 9.15)

B. Calculating an Earthquake's Epicenter

1. Measure the arrival time for both, the very first P-wave

arrival and the very first S-wave arrival.

2. Calculate the P-S time interval.

3. Using the time-distance graph, a circle with a radius

equal to the distance from the epicenter is drawn

around each seismic station. All potential epicenters

are located on the circle. (see Fig. 9.16)

4. It takes data from three differently located seismic

stations to calculate the actual earthquake epicenter.

That means you have to do steps 1 through 3 for at

least three stations (see Fig. 9.16)

VIII. Earthquake Size - Measuring its Intensity and Magnitude

A. Measuring an Earthquake's Size (Strength)

· Size (strength) of an earthquake is measured two ways.

1. Intensity - based on kinds of damage done

2. Magnitude - based on amount of energy released

B. Measuring an earthquake's intensity is done by

qualitatively assessing the quake experience and

damage aftermath.

1. Physical damage to buildings and structures

2. Physical sensations felt by people and animals

3. Behaviors of people and animals

Ø The most widely used intensity scale is the

Modified Mercalli Intensity Scale (see Table 9.2)

Ø Modified Mercalli Intensity Maps (Fig. 9.17 & 9.18)

C. Measuring an earthquake's magnitude is done by

calculating the total amount of energy released by the

quake. This is a quantitative measurement.

1. Magnitude is based on measuring the amplitude of

of the largest seismic wave as recorded by a

seismograph.

1. The most accurate method for determining the magnitude,

In terms of energy released, is the moment magnitude.

2. The most widely used magnitude scale is the

Richter Magnitude Scale (see Fig. 9.19).

3. Richter's Magnitude Scale is open-ended and starts

at one (1). Each jump in whole number represents a

30-fold increase in energy released.

4. Each jump in whole number represents a 10-fold

increase in wave amplitude.

5. Calculating the magnitude on the Richter Scale is done

by taking the measured maximum amplitude of the

largest seismic wave and charting it against the

measured P-S time interval (represented in the P-S

time-distance graph). (see Fig. 9.19)

6. The line drawn between the two scales interests the

centrally-located magnitude scale at the appropriate value.

7. The largest recorded earthquake on Earth has been 8.6 M

D. Create and Analyze Your Own Virtual Earthquake

http://www.sciencecourseware.com/eec/Earthquake/

1. This website has an interactive program that allows you

to choose a region (several possible locations) where you

can create a virtual earthquake.

2. You are then given three seismograms from three

different seismic stations in the region, which you will

use to calculate both, the epicenter and magnitude of

the virtual earthquake.

IX. Earthquake Hazards

A. Ground Shaking

1. The type of ground (subsurface) beneath a location is a

critical factor on how much ground shaking occurs,

for a given distance from the epicenter.

2. Structural failure and collapse

· Homes and Buildings

· Bridges and Dams

3. Ground failure

· Hillside failure (mass wasting)

· Liquefaction

4. Falling/Flying objects

5. Getting knocked off your feet or thrown against something

B. Fire

1. Broken gas mains

2. Damaged electrical wire

3. Wooden structures

C. Tsunami

1. Razing and flooding of coastal communities

2. Drowning

D. Volcanic Eruption

1. Earthquake-triggering of an active volcano

2. Volcanic eruption hazards

X. Major Earthquake Preparation and Survival - 3 Steps

A. Always be prepared for an earthquake

· Know how to make your home earthquake-ready

· Have an earthquake survival kit at home.

B. Know what to do when an earthquake occurs

· While at home

· While traveling

C. Know what to do after an earthquake occurs

· While at home

· While traveling

D. Quake Preparation and Survival Info on the Web

http://www.preparedness.info/lafd/disprep/earthquaketips.htm

XI. Earthquake Prediction

A. Compiling Historical Earthquake Data of Active Faults

1. Statistical analyses of earthquakes recorded by

seismographs for specific faults and fault systems

2. Paleo-rupture histories of active faults done by trenching

across known fault surfaces.

· Use stratigraphic principles to decipher a sediment pile

that is laced with buried rupture surfaces

3. The above combined information can be used to predict

a fault's frequency of rupture, including how big.

B. Monitoring Active Fault Zones

1. Real-time monitoring on/around active faults

using a variety of instruments

· Seismographs

· GPS/Laser sights

· Water well sensors

· Electrical sensors

C. Potential 6.9 Magnitude Quake on the Rose Canyon Fault

1. An earthquake planned 6.9 M scenario created for the RCF

using a computer-generated simulation

· Perceived shaking

· Potential damage

· Peak acceleration

· Peak velocity

· Instrumental intensity

2. Earthquake scenario map of Rose Canyon Fault

http://www.trinet.org/shake/Rose_Canyon6.9_se/intensity.html

XII. Earthquake Resources on the Internet

A. General Information and Activities

1. http://scign.jpl.nasa.gov/learn/eq.htm

2. http://www.gns.cri.nz/earthact/earthquakes/index.html

B. Historical Earthquake Archives

1. http://earthquake.usgs.gov/activity/past.html

XIII. Earthquake and Fault Vocabulary

Internet Image Glossary: http://earthquake.usgs.gov/image_glossary/

Aftershock

Body wave

Compressional force

Dip-slip

Earthquake

Elastic rebound theory

Epicenter

Fault

Fault rupture

Focus