Earthquakes
Bill Menke
September 16, 2005
Summary
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What is an earthquake?
Why do earthquakes occur?
How is size quantified?
Where do earthquakes occur?
How frequently do earthquakes occur?
How do earthquakes cause damage?
What is an earthquake ?
An earthquake is the shaking of the ground
that is caused by sudden slip on a
geological fault.
Why do Earthquakes Occur ?
Forces in the earth slowly build up to where they
exceed the factors impeding fault motion,
causing sudden slip on the fault.
Both friction and unbroken rocks are factors
impeding fault motion.
Sudden slip causes earthquakes. Slow, steady
slip (=creep), which can sometimes occur on
faults, does not.
Analogy: breaking a twig
Step 1: Slowly bend twig. Energy, provided
by your muscles, is stored in twig,
because it is elastic.
Step 2: Keep bending (=straining).
Eventually the force (=stress) is great
enough to cause cracking (=faulting).
Step 3. The twig cracks. The stored elastic
energy is used up by 1) tearing molecules
apart (90%) and 2) making the cracking
sound (vibrations, the earthquake)
Example: San Andreas fault
From the air, the fault really does
look more-or-less line a line
Step 1
Step 2
Step 3
It really happens …
Forces are highest on Plate
Boundaries
If one plate in moving in one direction …
And the other plate in moving in another
direction …
Then the boundary between the two plates
will be experiencing lots of force (=stress)
The Type of Plate Boundary Matters
Lithosphere – brittle part of earth – earthquakes happen here
Asthenosphere – viscous part of earth – no earthquakes
Convergent Plate Boundary
Convergent Boundary
Plates move towards one another
Oceanic Lithosphere: Subduction Zone (shown here)
Continents: Collisional mountain belt (e.g. Himalayas)
Extremely Large Earthquakes
(since lithosphere old, cold and thick)
Divergent Plate Boundary
Divergent Boundary
Plates move away from one another
Oceanic Lithosphere: Mid-Ocean Ridge (shown here)
Continents: Rift Zone (e.g. East African Rift Valley)
Smallish Earthquakes
(since lithosphere young, hot and thin)
Strike-Slip Plate Boundary
Strike Slip Boundary
Plates slide past one another
Oceanic Lithosphere: fracture zone (shown here)
Continents: (e.g. San Andreas Faults)
Intermediate Size Earthquakes
(since lithosphere intermediate in age,
temperature and thickness)
What about the US ?
West Coast – Plate Boundary
East Coast – No plate Boundary
Oregon and Washington:
Convergent Plate Boundary,
the Cascadia Subduction Zone
(but note divergent plate
boundary offshore)
California
Strike Slip boundary
the San Andreas Fault
In the US, Where Have the Big
Damaging Earthquakes Been ?
Oops – pattern
Not quite what
we expected!
California OK
But why:
None in Cascadia
Some east of
Mississippi!
Why?
Hey! What about
Alaska, Hawaii and
Puerto Rico?
Quantifying Earthquake Size
Size, a tricky buisness …
What is a big person?
a tall person, with height in meters
a heavy person, with weight in kilograms
a rich person, with fortune in dollars
an influential person, with influence in
% of population impacted
Richter: an earthquake is big
when the ground shakes a lot
Earthquake Magnitude
An earthquake’s size is defined to be
Magnitude 3 on the Richter Scale
if it causes 0.36 microns of ground shaking
at points 100 km distant from the fault
Its Magnitude 4 if it causes 3.6 microns at 100 km
Its Magnitude 5 if it causes 36 microns at 100 km
And so forth
Note that an increase of 1 magnitude unit corresponds to a factor of
ten increase in ground shaking … the scale is logarithmic
So magnitude quantifies the amount of shaking caused by
a fault, without saying anything particular about the
physical size of the fault or the amount of slip on it …
But what about quantifying the size of the faulting ?
(Note the phrase “size of the faulting”, which is not the
same as the “size of the fault”, since it must account both
for the “size of the fault” (that is, its length and width),
and the amount of slip that occurred.
Sumatra-Andaman Island Earthquake of Dec 26, 2004. The
faulting was 15 meters of slip on a fault 1000 km long
and 200 km wide.
Seismic Moment
Seismic Moment = Fault area times fault slip times rock rigidity
Faulting with high moment
Tend to cause
Earthquakes with high
magnitude
With a factor of 30 increase
in moment causing
a one-unit incease
in magnitude (the
line in the graph)
But the relationship is not
exact. Much
variability is
observed (the dots
in the graph)
Getting Back to Plate Boundaries …
“Maximum” size of earthquake
Divergent
mid-ocean ridge: magnitude 5
continental rift: magnitude 7
Strike Slip
oceanic fracture zone: magnitude 7
continental fracture zone: magnitude 8
Convergent
collisional mountain belt: magnitude 8.5
subduction zone: magnitude 9
Getting back to the US
Subduction zones are BIG sources of hazard
!
• There are three subduction zones near the
United States
• Aleutian Subduction Zone, in western
Alaska. Magnitude 9.2 earthquake in
1964.
• Puerto Rico Subduction Zone. Magnitude
8.1 in 1946 near the Domincan Republic.
Cascadia
• The Cascadia Subduction zone (western
Oregon and Washington) is capable of a
magnitude 9 earthquake (although none
have occurred there since the European
settlement of that area in the early 1800’s)
• But on January 26, 1700 a large tsunami
hit Japan. It was probably from a
magnitude 9 earthquake on Cascadia.
How frequently do earthquakes
occur?
World Earthquakes in 2001
There are many more small
earthquake than large
ones:
Magnitude
range
8.0-9.9
7.0-7.9
6.0-6.9
5.0-5.9
4.0-4.9
number
1
14
127
1199
8143
1341 earthquakes with magnitude
greater than or equal to 5.0 in 2001 !
I’ve picked the lower limit of magnitude 5
because earthquakes that are smaller
rarely cause significant damage.
Fortunately, most of these earthquakes
occurred beneath the sea floor or in
sparsely inhabited regions. Nevertheless,
23534 people died.
My Motto
There’s always the next earthquake …
The rate of earthquakes is fairly
constant with time …
Gutenberg-Richter Statistics
The cumulative number of
earthquakes per unit time (a year
in this case) with a magnitude
greater than or equal to a given
value is highly predictable.
Can the red line be extrapolated
to predict the rate of occurrence of
the very largest earthquakes?
Does a magnitude 9 really occur
once every ten years, as the
dotted red line would predict?
This graph for 2001 data shown
in a previous slide
Why do Earthquake Cause
Damage ?
“Earthquakes don’t kill
people …
… buildings kill people”
Prof. Chris Scholz
Columbia University
Types of Earthquake Hazard
Ground Shaking
building and other structures collapse
Landslides
shaking causes collapse of hills
Tsunamis
shaking causes ocean-crossing waves
coastal areas experience very rapid flooding
Ground Shaking
Quantified by ground acceleration
units: meters per second squared
or
percent of gravity (g=9.8 m/s2)
An ground shaking of 10% g is big enough to do
significant damage, especially if it includes
horizontal motions.
1994
Northridge
Earthquake
maximum shaking
exceeded 66% g
(red) over a wide
area
but note that
acceleration
decreases rapidly
with distance
Landslide induced by 1994 Northridge
Earthquake blocks Highway
Before and after
Aerial photos of damage
Cause by tsunami from
Dec. 26, 2004 SumatraAndaman Island
Earthquake.