earthquakes bill menke september 16, 2005. summary what is an earthquake? why do earthquakes occur?...
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Earthquakes
Bill Menke
September 16, 2005
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Summary
• 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?
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What is an earthquake ?
An earthquake is the shaking of the ground that is caused by sudden slip on a geological fault.
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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.
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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)
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Example: San Andreas fault
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From the air, the fault really does look more-or-less line a line
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Step 1
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Step 2
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Step 3
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It really happens …
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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)
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The Type of Plate Boundary Matters
Lithosphere – brittle part of earth – earthquakes happen here
Asthenosphere – viscous part of earth – no earthquakes
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Convergent Plate Boundary
Convergent BoundaryPlates move towards one anotherOceanic Lithosphere: Subduction Zone (shown here)Continents: Collisional mountain belt (e.g. Himalayas)Extremely Large Earthquakes
(since lithosphere old, cold and thick)
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Divergent Plate Boundary
Divergent BoundaryPlates move away from one anotherOceanic Lithosphere: Mid-Ocean Ridge (shown here)Continents: Rift Zone (e.g. East African Rift Valley)Smallish Earthquakes
(since lithosphere young, hot and thin)
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Strike-Slip Plate Boundary
Strike Slip BoundaryPlates slide past one anotherOceanic Lithosphere: fracture zone (shown here)Continents: (e.g. San Andreas Faults)Intermediate Size Earthquakes
(since lithosphere intermediate in age, temperature and thickness)
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What about the US ?
West Coast – Plate Boundary East Coast – No plate Boundary
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Oregon and Washington:Convergent Plate Boundary,the Cascadia Subduction Zone
(but note divergent plateboundary offshore)
CaliforniaStrike Slip boundarythe San Andreas Fault
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In the US, Where Have the Big Damaging Earthquakes Been ?
Oops – patternNot quite whatwe expected!
California OKBut why:
None in Cascadia
Some east ofMississippi!
Why?
Hey! What aboutAlaska, Hawaii andPuerto Rico?
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Quantifying Earthquake Size
Size, a tricky buisness …
What is a big person?a tall person, with height in metersa heavy person, with weight in kilogramsa rich person, with fortune in dollarsan influential person, with influence in
% of population impacted
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Richter: an earthquake is bigwhen the ground shakes a lot
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Earthquake Magnitude
An earthquake’s size is defined to beMagnitude 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 kmIts Magnitude 5 if it causes 36 microns at 100 kmAnd so forthNote that an increase of 1 magnitude unit corresponds to a factor of
ten increase in ground shaking … the scale is logarithmic
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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.
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Seismic Moment
Seismic Moment = Fault area times fault slip times rock rigidity
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Faulting with high momentTend to causeEarthquakes with high
magnitude
With a factor of 30 increasein moment causinga one-unit inceasein magnitude (theline in the graph)
But the relationship is notexact. Muchvariability isobserved (the dotsin the graph)
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Getting Back to Plate Boundaries …
“Maximum” size of earthquakeDivergent
mid-ocean ridge: magnitude 5continental rift: magnitude 7
Strike Slipoceanic fracture zone: magnitude 7continental fracture zone: magnitude 8
Convergentcollisional mountain belt: magnitude 8.5subduction zone: magnitude 9
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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.
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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.
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How frequently do earthquakes occur?
There are many more small earthquake than large ones:
Magnitude range
number
8.0-9.9 1
7.0-7.9 14
6.0-6.9 127
5.0-5.9 1199
4.0-4.9 8143
World Earthquakes in 2001
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1341 earthquakes with magnitudegreater 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.
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My Motto
There’s always the next earthquake …
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The rate of earthquakes is fairly constant with time …
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Gutenberg-Richter Statistics
This graph for 2001 data shown in a previous slide
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?
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Why do Earthquake Cause Damage ?
“Earthquakes don’t killpeople …
… buildings kill people”
Prof. Chris ScholzColumbia University
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Types of Earthquake Hazard
Ground Shakingbuilding and other structures collapse
Landslidesshaking causes collapse of hills
Tsunamisshaking causes ocean-crossing wavescoastal areas experience very rapid flooding
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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.
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1994 Northridge Earthquake
maximum shaking exceeded 66% g (red) over a wide
area
but note that acceleration
decreases rapidly with distance
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Landslide induced by 1994 Northridge Earthquake blocks Highway
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Before and afterAerial photos of damageCause by tsunami fromDec. 26, 2004 Sumatra-Andaman IslandEarthquake.