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Earthquake Case Study
Discussion
Have you ever felt an earthquake?
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?
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.
Example: San Andreas fault
From the air, the fault really does look more-or-less like a line
Step 1
Step 2
Step 3
Discussion
What factor promote big earthquakes?
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)
Convergent Plate BoundaryBiggest Earthquakes
collisional mountain belt: “up to” magnitude 8.5subduction zone: “up to” magnitude 9.5
Divergent Plate BoundarySmallish Earthquakes
Divergentmid-ocean ridge: “up to” magnitude 5continental rift: “up to” magnitude 7
oceanic fracture zone: “up to” magnitude 7continental fracture zone: “up to” magnitude 8
Strike-Slip Plate Boundaryintermediate
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?
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
Richter: an earthquake is bigwhen the ground shakes a lot
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
Discussion
Should we give up on the Richter Scale, and switch to something non-logarithmic ?
For example, something that directly measures fault size?
e.g. fault area fault slip
Discussion
There are three subduction zones near the United States
What are they?
Which one is the riskiest?
• Aleutian Subduction Zone, in western Alaska. Magnitude 9.2 earthquake in 1964.
• Puerto Rico Subduction Zone. Magnitude 8.1 in 1946 near the Dominican Republic.
• 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?
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
Discussion
If there are only 15 earthquakes per year in the world with magnitude 7
How fequent are they given region?
What are the implications in terms of education?
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.
My Motto
There’s always the next earthquake …
Why do Earthquake Cause Damage ?
“Earthquakes don’t killpeople …
… buildings kill people”
Prof. Chris ScholzColumbia University
A building that ‘pancaked’ during an earthquake
School collapses all near fault
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
Discussion
What are “risk factors” for each of these:
Building and other structures collapse
Landslides
Tsunamis
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 afterAerial photos of damageCause by tsunami fromDec. 26, 2004 Sumatra-Andaman IslandEarthquake.
Earthquake Predictibility,Forcasting
and Early Warning
Summary
Are long-term predictions of earthquakes possible?
Are short short-term (or intermediate term) predictions of earthquakes possible?
Can specific earthquakes be forecast?
Are a few seconds or minutes of Early Warning useful.
Is rapid assessment useful?
1. Most earthquakes are on plate boundaries2. Plate motions are very constant over long periods
of time3. Faults at plate-boundaries are long term features
4. Long-term fault slip rate of faults are fairly constant
5. Segments of faults seem to rupture time and time again in similar earthquakes
6. Earthquake occurs when loading exceeds strength
Why might long-term predictions be possible ?
Long-term PredictabilityMost earthquakes are on plate boundaries
Almost no new faults
Faults grow slowly
A big earthquake on a fault tends to increase the length of the fault
The bigger the fault, the bigger the earthquake that can occur
Strategy: map the faults to determine where earthquakes will occur
(but look for evidence of recent motion, make sure it’s a recently active fault)
Problem: deeply buried faults, such as blind thrusts(especially if they have few small earthquakes)
(example fault that caused 1994 Northridge Earthquake)(But now we know it’s there!)
Long-term PredictabilityFaults segmentation: characteristic large earthquakes
Segmentationin Japan
Long-term PredictabilityEarthquake occurs when loading exceeds strength
time, years
load
ing
Maximum load
Loading rate correlatesWith plate-tectonic motions
Eq Eq Now: whereAre we in theLoading cycle ?
Results of this kind of analysisIs a prediction of likelihood ofa large earthquake on eachsegment of each fault
Assuming:
long-term loading ratesdetermined by GPS and/orgeological studies
and
closeness to failurebased on when last largeevent occurred
Detectable changes in fault behavior as it approaches failure
Examples:
Foreshocks – small earthquakes that occur before the big one – short term
Seismicity rate changes – increase in rateof moderate earthquakes prior to the big one – intermediate term
Why might short-term or intermediate-term predictions be possible ?
Foreshock little one before the Big One
• In California, foreshocks occur less than 5 days before about half of the large earthquakes. For these reasons, the California Office of Emergency Services issues an advisory of an increased likelihood of a major earthquake within the next 5 days following moderate-sized earthquakes.
Discussion
What can you do with a prediction of an earthquake ?
Especially if it has low skill
Short termheightened emergency preparednesscurtain endangered activitiesevacuate people
Intermediate termredirect preparedness fundsre-site future construction
Early Warning
- or every second counts -
How long do you have ?
Strong ground motionsensors
city
50 km distantAt 2 km/s shear wave velocityIs 25 secondsMinus 10 seconds toDetect strong motion at aFew stations near faultIs …
10 km
fault
50 km
15 seconds
50 km
100 km
But say the damaging effects extend to 100 km …
… There may be a lot of people & structure in the >15 second warning region area
> 15 second warning
< 15 secondwarning
For this to have any hope of working
you must plaster the earth withsensors capable of detecting
strong ground motion and immediately sending that
information to a processing and distribution center
Seismic Intensity Stations in Japan
So little time is availablethat both the
announcement of impending strong shaking
and the responsemust be fully automated
Discussion
How much are you willing to trust automation?
And to do what?
What can you do in 15 seconds ?
Shut down delicate or dangerous equipment
Have people dive for shelter (?)
Just knowing where the strong shaking occurredcan help in formulating an emergency response
Strong ground motions after the 1995 Kobe, Japan earthquake