Lecture 21 more earthquakes

Chapter 10: Earthquakes (Part 2)


Announcements
- Papago Park Fieldtrip
Sign up at website.
See maps and directions.

- Quiz postponed until Monday
Volcanoes (Chap. 9)
Earthquakes (Chap. 10)
Earth’s Interior (Inter. C)

- Midterm 2
Rescheduled for a week from
Friday, April 11.
Review outline will be posted on
the web this coming Monday.

-


Most earthquakes occur along
the margins of tectonic plates

Fig. 10.22
W. W. Norton


Today’s Lecture

- Student presentation
“Taiwan Earthquake- 9-21-99”Vilasinee Kongsomboonvech

- Introduction to earthquakes
Where do Earthquakes occur?
Plate tectonic context

- Types of earthquake waves
Body waves
Surface waves
(Video simulations)


Earthquake “belts”
95% of energy from earthquakes comes from thin
zones marking the edges of tectonic plates


Major Earthquake “belts”
Some earthquakes are quite deep (up to 700 km or more)
Deep quakes are found at subduction zones and are
confined to downgoing slabs of lithosphere.

Active Volcanoes


Gaps in earthquake activity define
areas where stress is accumulating.

Fig. 10.36c

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Seismic gaps:
Mark likely places for future large earthquakes!


n)

Fig. 10.04
EXTENSIONAL STRESS

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shear)


Earthquakes & Plate Tectonics
Normal Faults (Extensional Stress)
-At divergent plate boundaries, plates move away from each
other at constant rates.
-Examples:
* Red Sea
* Mid-Atlantic
Ridge.
- See video
for opening of
the Red Sea.


n)

Fig. 10.04

COMPRESSIONAL STRESS

W. W. Norton

shear)


Earthquake “belts” & faulting styles

At convergent plate boundaries, oceannic
plates come together forming subduction
zones. Example: Peru-Chile Trench.


Deep Earthquakes in Subduction Zones


Earthquake “belts” & faulting styles

Where converging plates are both
made of continental crust, they collide, forming
mountain belts. Example: Himalayan Mts.


n)

Fig. 10.04

Lateral (Shear) Stress
W. W. Norton

shear)


Earthquakes & Plate Tectonics
Transform Faults (Lateral Shear Stress)
-At transform boundaries, plates slip pass each
other at constant rates.
-Example: San
Andreas Fault
-At the present
rate (~ 1 cm/yr)
San Francisco
and LA will be
neighbors in
~10 million yrs!



Fig. 10.00
U.S. Geological Survey


Summarizing…

Fig. 10.24
W. W. Norton

Major Tectonic Environments of Earthquakes


EARTHQUAKES GENERATE
TWO TYPES OF INTERNAL
OR “BODY” WAVES
 P-waves
- Travel faster.
- Move through both solids and liquids.

 S-waves: Shear waves
- Travel slower.
- Only travel through solids.
 Information carried by these waves is used to:
-Locate the focus of the earthquake.
-Calculate the earthquake magnitude.
- “See” into the Earth’s interior.


P-waves:
- Compression
waves.

Fig. 10.13ab

- Travel fastest.
- Move through
both solids
and liquids.

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S-waves:
-Shear waves.
-Travel slower.
- Only travel
through
solids.

W. W. Norton


Types of Earthquake Waves

“Body waves”

travel in the Earth’s interior

P-waves (“P” for primary)

Expansion/compression:
push/pull motion
S-waves (“S” for secondary)

Shear:
side-to-side motion
“Surface Waves” travel at the Earth’s surface

Surface Waves
- Cause the
most damage.
- Two types.
* Rayleigh waves:
Forward,
rolling motion.
* Love waves:
Side-to-side
(lateral) motion
- Both are shear
waves.


Love Waves

Fig. 10.13ef
W. W. Norton


In summary: Types of seismic waves
Motion produced by the
different wave types

P-wave

S-wave

Surface-wave