Dynamic Earth
Class 8
2 February 2006
Any Questions?
Note:
Exam #1
Date shown on hardcopy syllabus as
February 14th, but said February 15th.
The correct date is February 14th
(On-line version is correct)
What Plate Tectonics
Theory explains

Age distribution of oceanic crust
(and why the oceans are MUCH
younger than the continents)

Distribution of earthquakes and
volcanoes
Seismology

Study of the propagation of
mechanical energy released by
earthquakes.

When energy is released in this
fashion, waves of motion (like the
effect of a pebble tossed into a pond)
are set up in the Earth.
Earthquakes

earthquake: movement of rock
bodies past other

fault: locus of the earthquake
movement

faults come at all scales, mm to
separation of lithospheric plates
(e.g., San Andreas).
Earthquake terms
focus: site of initial rupture
Focus = point
where an
earthquake
originates
Earthquake terms
focus: site of initial rupture
epicenter: point on surface above
the focus
Seismic Waves Radiate from the
Focus of an Earthquake
Earthquakes
Most damage
from ground
shaking
Eric Marti/AP Photo
Chile, May 22, 1960




Magnitude 9.6
earthquake, the
largest earthquake
ever recorded
The city of Valdivia
suffered
catastrophic
damage
Severe shaking
lasted for over 15
minutes
Coastal areas
subsided
Chile, May 22, 1960


The earth ruptured
for 450 miles along
the Chile coast
The area of rupture
was the size of
California
Elastic Rebound Theory
Elastic Rebound Theory
Elastic Rebound Theory
Elastic Rebound Theory
1906 San Francisco Earthquake
Fig. 18.2
1906 San Francisco Earthquake
Fault Offset
(~2.5m)
Seismic waves

Waves are started because of initial
tension or compression in the rock.

Instruments used to measure these
waves are called seismographs.
Seismographs Record Vertical or
Horizontal Ground Motion
Vertical
Horizontal
Modern
Seismograph
Kinematics
Seismograph Record and Pathway
of Three Types of Seismic Waves
Two kinds of waves from earthquakes

P waves (compressional) 6–8 km/s.
Parallel to direction of movement (slinky),
also called primary waves. Similar to
sound waves.

S waves (shear) 4–5 km/s. Perpendicular
to direction of movement (rope); also called
secondary waves. Result from the shear
strength of materials. Do not pass through
liquids.
Comparison
of
P-wave and
S-wave
Motion
Two Types of Surface Waves
Time Lag Between S and P waves
with Distance from Epicenter
Seismic Travel-time Curve
Locating the Epicenter
Locating an epicenter

The difference between the arrival
times of the P and S waves at a
recording station is a function of the
distance from the epicenter.

Therefore, you need three stations to
determine the location of an epicenter.
Measuring the force of earthquakes
1. Surface displacement

1964 Alaska earthquake displaced some
parts of the seafloor by ~ 50 ft.

1906 San Francisco earthquake moved the
ground ~8.5 ft.
2. Size of area displaced
Alaska — 70,000 sq. miles
Measuring the force of earthquakes
3. Duration of shaking
Up to tens of seconds
4. Intensity scales
Based on damage and human perception
5. Magnitude scales
Based on amount of energy released
Modified Mercalli Intensity Scale
I
II
III–IV
V–VI
VII
VIII
IX
X
XI
XII
Not felt
Felt only by persons at rest
Felt by persons indoors only
Felt by all; some damage to plaster, chimneys
People run outdoors, damage to poorly built structures
Well-built structures slightly damaged; poorly built structures
suffer major damage
Buildings shifted off foundations
Some well-built structures destroyed
Few masonry structures remain standing; bridges destroyed
Damage total; waves seen on ground; objects thrown into air
Richter scale

Richter scale: amount of energy received 100 km
from epicenter

Largest quake ever recorded = 9.6 (rocks not
strong enough for more).

Earthquakes less than M = 2 are not felt by people.

Scale is logarithmic:
Increase 1 unit = 10 times greater shaking
Increase 1 unit = 30 times greater energy
Maximum Amplitude of
Ground Shaking Determines
Richter Magnitude
Richter Magnitude Versus Energy
Earthquake prediction
Long term—imprecise (can be
done)
Short term—precise (very difficult)
We can't stop earthquakes, so we
have to be prepared for them.
New Housing
Built Along the
1906 Trace of
the San
Andreas Fault
R.E. Wallace, USGS
Seismic
Hazard
Map
Distribution of earthquakes

Not random

Focused in linear zones
World Seismicity, 1963–2000
Earthquake distribution defines
plate boundaries
Core
Mantle
Yoke
Crust
Core
?
The Earth
has a
number of
layers, just
like an egg
or an apple
Earth’s
layers
Lithosphere
Asthenosphere
Plates
 Rigid
Lithosphere with definite
boundaries
 Can
have both oceanic and
continental crust or just one
kind.
Note that
crust under
continents
is thicker
(~45 km)
than under
oceans
(~8 km).
The Earth’s Major Plates
Types of plate boundaries
Divergent:
mid-ocean ridges
 Convergent: collision zones
volcanic arcs


Strike-slip:
San Andreas Fault
(California)
Anatolian Fault
(Turkey)
Divergent
(Spreading)
Convergent
(Subduction
Zone)
Transform
Three Types of Plate Boundaries
Earthquakes Associated with Divergent and
Transform Margins
Strike-slip Faults
Subduction Zones
Earthquakes in
subduction
zones
Convection within the Earth
Lithosphere is created at spreading
centers and destroyed at Trenches
(Subduction Zones)
Subduction zones
Ocean-continent
convergence
Ocean-ocean
convergence
Continent-continent
collision
Growth of Continents



Addition of volcanic arc material to
continent (e.g. Andes).
Collisions: Continents
Collisions: Exotic terranes
Exotic terranes: approaching arc
or microcontinent
Collision
Accreted Terrane
The West Coast of North America
Exotic
Terranes:
plastered to
the
continent at
subduction
zones
Any Questions?
Tuesday

Finish reading Chapter 3 in the text
Thursday

Homework # 3 due