CHAPTER 8.2 MEASURING EARTHQUAKESCHAPTER 8.2 MEASURING EARTHQUAKES

The study of SEISMOLOGY (earthquake waves) dates back 2000 years.

The study of SEISMOLOGY (earthquake waves) dates back 2000 years.

1. HSW: Earthquakes: The Science of Earthquake Prediction2. HSW: Earthquakes: The Science of Earthquakes

Earthquakes WavesEarthquakes Waves

Surface WavesSurface Waves

= seismic waves that travel along earth’s surface.

Shakes anything that they hit.

Pg 224 Figure 7E & 7F

Shaking trees video

Body WavesBody Waves

= seismic waves that travel through Earth’s interior.

P waves = push-pull waves(compression waves)

Alternates compressing and expanding material.

Pg 224 Figure 7A & 7B

Compare P and S Waveshttp://www.classzone.com/books/earth_science/terc/content/visualizations/es1002/es1002page01.cfm?chapter_no=visualization

P Waves

Body WavesBody Waves

= seismic waves that travel through Earth’s interior.

S waves = up-down waves

Shakes particles at right angles to the direction they are traveling.

Pg 224 Figure 7C & 7D

S Waves

Surface Waves – Ocean typehttp://earthquake.usgs.gov/regional/nca/1906/simulations/movies/sf1906livermore.mov

Seismogram = shows all three types of seismic waves – surface waves, P waves, and S waves.Seismogram = shows all three types of seismic waves – surface waves, P waves, and S waves.

P waves travel fastest. S waves travel 1.7x slower than P waves.Surface waves travel slowest (90% of the speed of S waves.

Pg 223 Figure 6

How a Seismograph works.

http://piers.wikispaces.com/How+a+seismogram+works+animation

P wave S wave

Surface waves

1 m

in1 min

Locating Earthquake Distance

1 min

1 m

in

3800 miles from station to epicenter

1000 2000 3000 4000 5000

Draw a circle correlating to the number of miles determined using the travel-time graph. Repeat with two more seismograms and drawing two more circles. Where the circles intersect is the epicenter!

3800

mi

EPICENTER

Same techniques apply when using a map!

Now try your hand at the Exploration Lab on page 240.

500 mi

Earthquake zones

95% of major earthquakes occur in a few narrow zones.

Ring of Fire around pacific rim.

A simple pattern of high intensity close to the San Andreas Fault, dropping off with distance. But why is there a disconnected island of high intensity in central California? The band of low (IV) intensity parallel to the coast coincides with the Coast Ranges. Soils here are very shallow - usually less than a meter to bedrock. Observers here felt mostly a sharp jolt.In contrast, the high intensity in central California coincides with the Central Valley, where young and unconsolidated sediments are kilometers deep. Unconsolidated material shakes like jelly in an earthquake.Note how intensity VI follows the shoreline of San Francisco Bay, where there are also thick unconsolidated sediments.

Two types of measurements to describe the size of an earthquake.

Intensity = amount of shaking

Google.com search “earthquakes” to see current earthquakes and access maps.

Magnitude = quantitative measurement rely on calculations.

Used by reporters.

Chile 1960 moment magnitude mapChile 1960 moment magnitude mapMoment MagnitudeMoment Magnitude

More precise.Used by scientists.Derived from the amount of displacement that occurs along a fault zone.

More precise.Used by scientists.Derived from the amount of displacement that occurs along a fault zone.

Derived from• Average movement along the fault.• Area of surface break.• Strength of broken rock.

Most widely used measurement for earthquakes.Estimates energy released.

Most widely used measurement for earthquakes.Estimates energy released.

Pg 227 Table 1

1886 Charleston SC

1923 Tokyo1960 Chile

1964 Alaska1970 Peru

1971 San Fernando, CA 1985 Mexico City

1988 Armenia1988 Armenia1989 Loma Prieta, CA1990 Iran 1993 Latur, India1994 Northridge, CA 1995 Kobe, Japan1999 Izmit, Turkey 1999 Chi Chi, Taiwan 2001 El Salvador2001 Bhuj, India