VII. Earthquakes

A. Introduction

B. Source of seismic energy

C. Propagation of seismic energy

D. Recording earthquakes

E. Magnitude scales

Building design could not withstand accelerations

$Millions of damage

Thousands of people killed

San Francisco, 1906

Geology in the News

Two die in 6.5 magnitude Earthquake near San Lois Obispo California

Earthquake triggers mudslides

http://news.nationalgeographic.com/news/2003

California, 2003

San Andreas Fault

Earthquake Bam, Iran

A Magnitude 6.5 Earthquake hits a stone- and mud-house city of 100,000 in Iran December 26, 2003 30,000 Dead 30,000 Refugees

US sends aid and releases sanctions

Relations improved

Photos from AP

Geological Hazards Related to Earthquakes

Landslides

Volcanoes

Mudslides Tsunamis

B: Sources of Seismic Energy

Elastic Rebound Buildup of elastic energy

during elastic strain Sudden release due to

slippage along a fault or brittle rupture

Anatomy of an Earthquake

Fig. 8.35

Focus: Source of energy Epicenter: Location directly above focus at the surface

(ground motion is greatest) Fault Trace: Shows intersection of fault and the surface

of the land Fault Scarp: Indicates vertical motion of fault

Propagation of Seismic Energy

Body Waves travel through the earths interior (crust, mantle, core) P wave: Compression

and expansion of rock S Wave: Shearing motion

of particles

Fig. 8.42

Propagation of a P-Wave

Propagation of Seismic Energy

Body Waves travel through the earth’s interior (crust, mantle, core) P wave: Compression

and expansion of rock S Wave: Shearing

motion of particles Surface Waves

Propagation of an S-Wave

Propagation of Surface Waves

Surface Waves travel along the earth’s surface Love Wave: Lateral

movement of the surface Rayleigh Wave: Rolling

movement of the surface (similar to an ocean wave)

Propagation of a Surface Wave

Propagation of a Surface Wave

Recording Earth Motion Seismograph:

An instrument that measures the horizontal or vertical motion of Earth’s surface

Seismograms: The plot of the motion

Measuring Velocity of Seismic waves

Because the P wave travels faster the the S wave

The S-P interval increases with distance

Time of EarthquakeTime of Earthquake

Reading a Seismogram Ground motion vs. Time

Each tick mark is 1 minute P-S Time interval indicates distance to epicenter

First P waveArrival

First S waveArrival First Surface wave

P-S Interval

See Fig. 8.44

Time-Distance Relationships

Use P-S interval to determine distance to focus

See Fig. 8.45

Distance from focus (Kilometers)

Seismic Waves and Velocities P-waves

= ( + ¾): Bulk modulus

: Mod. of rigidity : Density

S-waves

= /

Body Waves and Surface Waves

Locating the Epicenter

Triangulation using 3 seismograph stations

Depth can be determined with four or more stations

Finding the Depth of Earthquakes

Using 4 or more seismograph stations

Seismicity of the Pacific Rim 1975-1995

Depth(km)

03370

300

150

500

800

Shallow quakes at mid ocean ridges (<33km) and

Oceanic trenches Deep quakes over

the subduction zone (>70 km)

Earthquakes, Plate Interior

New MadridFault Zone Faults activated by

crustal warping Bowling Green

Fault Largest Earthquake

on the N. American Continent

New Madrid Earthquake, 1811

Modified Mercalli Intensity Scale Subjective

observations of Damage and Ground motion

Is not a quantitative measure

Earthquake Intensity Scales

Modified Mercalli Scale Subjective observations of

Damage and Ground motion

Is not a quantitative measure Richter Scale

Indicates Ground Motion Amplitude Logarithmic (e.g., 6 is ten times stronger than 5) Does not directly indicate energy or destruction

Earthquakes around the World

Assessing Risk“Major Quake Likely to

Strike San Francisco Bay Region Between 2003 and 2032”

• Assessing Risks• Avoiding Risks• Preventing Damage• Predicting Impact

Geologic Hazards

Seismic Risk Analysis

Quake history (statistics) Locations of active faults Competency of surficial

materials (soil and rock) Ocean basin source

Tsunamis

Solomon Islands Earth Quake and Tsunami 4-1-07

Profiling Earth’s Interior

Velocities of seismic waves vs depth Fig. 19.19 & 20

Imaging Earth’s Interior

P and S waves are refracted (bent) within the earth

S waves do not travel through fluids

Fig. 9.21