3. What Are Earthquakes?
Earthquakes are sudden motions of the Earth's crust due to waves caused by
faulting or volcanic activity
Two types of seismic waves:
"P waves"
"S waves"
Graphic: (top) Earthquake damage following the Good Friday Earthquake, Alaska,
March 1964. Photo courtesy of NOAA, (bottom) Garrison 4th Ed. Fig. 3.7, pg 65,
5th Ed. Fig. 3.4, pg 58.
4. Seismic Waves: "P" and "S"
"P" (primary) - compression
"S" (secondary) - shear
These and other seismic waves can cause the ground to move more than 10 meters
during an earthquake
Graphic: Garrison, 4th Ed. Fig. 3.5, pg 62, 5th Ed. Fig. 3.2, pg 55.
5. How Are Earthquakes Measured?
Seismographs measure displacement of the ground at a particular location
A world-wide seismograph network measures:
- intensity (energy released)
- location (epicenter)
Graphic: (top) Mass and spring seismograph, graphic courtesy of USGS National
Earthquake Information Center, (bottom) P and S wave signatures recorded by a
seismograph, courtesy of USGS.
6. Ranking Earthquakes by Ground Motion
Richter scale: reports ground displacement on a continuous exponential scale
Richter
Energy (expressed
Example
magnitude
in tons TNT)
2.0
1
Quarry blast
4.0
1000
Small nuclear explosion
4.5
5000
Typical tornado
7.0
32,000,000
Large nuclear explosion
8.0
1,000,000,000
San Francisco 1906
10.0
1,000,000,000,000
San Andreas type fault circling Earth
12.0
160,000,000,000,000
Earth split in half through center
7. Determining an Earthquake's Location
The delay between the arrival of P and S waves indicates distance to the
epicenter
Near epicenter: S waves received soon after P waves
Far from epicenter: S waves received much later than P waves
Graphic: Courtesy of IRIS One-Pager Educational Resource No. 6.
8. Triangulation
If several monitoring sites measure the distance to the epicenter, the location
of the epicenter can be determined by triangulation
Graphic: Courtesy of IRIS One-Pager Educational Resource No. 6.
9. Recent Earthquakes
see http://www.iris.edu/seismon/ for updates
10. What do P and S Waves Reveal?
P waves travel through solid and liquid parts of the Earth
S waves travel only through solid parts of the Earth
Measuring P and S waves indicates which parts of the Earth are solid and which
are liquid
Graphic: P and S wave propagation, courtesy of USGS.
11. Seismic Waves in a Homogeneous Earth
If the Earth were completely solid and of uniform density...
P and S waves would travel through the Earth along straight lines. Their speed
would not change as they traveled.
P and S waves from a particular earthquake would be detected everywhere on Earth
Graphic: Garrison, 4th Ed. Fig. 3.6a, pg 63, 5th Ed Fig. 3.3a, pg 56.
(animation)
12. Seismic Waves in a Density-Stratified Earth
If the Earth were completely solid, but more dense in the center...
P and S waves would bend and change speed as they traveled through the Earth
P and S waves from a particular earthquake would be detected everywhere on Earth
Graphic: Garrison, 4th Ed. Fig. 3.6b, pg 63, 5th Ed. Fig. 3.3b, pg 56.
(animation)
13. "S" Waves in a Layered Earth
S waves:
- only travel through solids
- cannot travel through the liquid outer core
- cannot reach the inner core
The Earth's core absorbs S waves and produces a large shadow zone where S waves
are not detected at the surface
Graphic: Garrison, 4th Ed see Fig. 3.6c, pg 63, 5th Ed. Fig. 3.3c, pg 56.
(animation)
14. "P" Waves in a Layered Earth
P waves:
- travel through solids and liquids
- bend as they travel through the Earth
Bending creates a small shadow zone where P waves are not detected at the
surface
Graphic: Fig. 17.8, Physical Geology, 1999. McGraw-Hill. See Garrison, 4th Ed.
Fig. 3.6d, pg 63 (Note that the P wave exiting at 120 degrees can only be
detected by very sensitive instruments. In general, P waves are not detected
between 103 and ~143 degrees) or 5th Ed. Fig. 3.3d, pg 56. (animation)
15. Seismic Waves - Summary
I. Two types of seismic waves:
- P waves (compressional waves) - pass through solids and liquids
- S waves (shear waves) - only pass through solids
II. Density stratification bends P and S waves as they travel creating shadow
zones where waves are not detected
III. A global network of seismographs
- detects of the arrival times of P and S waves
- determines the locations of shadow zones
- reveals internal structure of the Earth
16. Density - The Concept
Density = relative heaviness of a substance (mass per unit volume)
Examples: water = 1 g/cm3
granite rocks = 2.7 g/cm3
Heavier materials have higher densities
Wave speeds change as they move through materials of different density
Graphic: Deploying an anchor for oceanographic instruments off Antarctica. M.Van
Woert, photographer, courtesy of NOAA.
17. Density Stratification
Layering of materials by density (more dense materials below less dense
materials)
Example 1: Icebergs float because ice is less dense than water
Example 2: Rocks sink because they are more dense than water
Graphics: (top) Icebergs near Antarctica, M.Van Woert, photographer. (bottom)
Artificial reefs created from concrete pipes, J.P.McVey, photographer. Both
courtesy of NOAA.
18. The Layered Earth
Analysis of seismic waves shows that the Earth is layered
Layers can be classified by:
- chemical properties
- physical properties
Graphic: Garrison, 4th Ed. Fig. 3.2, pg 60, 5th Ed. Fig. 3.1, pg 54.
19. Chemical Properties of Earth's Layers
Continental crust
- mostly granite (2.7g/cm3)
Oceanic crust
- mostly basalt (2.9 g/cm3)
Mantle
- silicon, oxygen, iron, magnesium (~4.5 g/cm3)'
Core
- mostly iron (~13 g/cm3)
Graphic: Garrison, 4th Ed. Fig. 3.2, pg 60, 5th Ed. Fig. 3.5 pg 59.
20. Physical Properties of Earth's Layers
Lithosphere
- crust and upper mantle
- cool and rigid
Asthenosphere
- hot, partially melted, slowly flowing part of upper mantle
Lower mantle
- hot, but not melted
- flows very, very slowly
Core
- liquid outer core
- solid inner core
Graphic: Garrison, 4th Ed. Fig. 3.2, pg 60, 5th Ed. Fig. 3.5 pg 59.
21. Can Earthquakes Help Explain the Dynamic Earth?
Earthquakes are not randomly distributed
Observations like this paved the way for a revolution in geology ... the theory
of plate tectonics
Graphic: Garrison, 4th Ed. Fig. 3.11, pg 69, 5th Ed. Fig. 3.11, pg 64.
22. Preview of Next Lecture
Seafloor Magnetism, Seafloor Spreading: Developing the Theory of Plate Tectonics
Reading:
4th Ed. Ch 3 Sec 5, 12-17, 24-25, 29-30, Ch 4 Sec 5, 12-13, 16
5th Ed. Ch 3 Sec 11, 12, 14-18, 25-26, 29-30, Ch 4 Sec 6, 12-13, 15
Graphic: Topography of land and sea.