Discussion
Have you ever felt an earthquake?

• 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?

An earthquake is the shaking of the ground that is caused by sudden slip on a geological fault.

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.

From the air, the fault really does look more-or-less like a line

What factor promote big earthquakes?

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 Boundary
Biggest Earthquakes collisional mountain belt: “up to” magnitude 8.5
subduction zone: “up to” magnitude 9.5

Divergent midocean ridge: “up to” magnitude 5 continental rift: “up to” magnitude 7

Strike-Slip Plate Boundary intermediate oceanic fracture zone: “up to” magnitude 7 continental fracture zone: “up to” magnitude 8

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?

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

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

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:
World Earthquakes in 2001
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

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 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.

There’s always the next earthquake …

Why do Earthquake Cause
Damage ?
“Earthquakes don’t kill people …
… buildings kill people”
Prof. Chris Scholz
Columbia University

A building that ‘pancaked’ during an earthquake

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

What are “risk factors” for each of these:
Building and other structures collapse
Landslides
Tsunamis

Quantified by ground acceleration units: meters per second squared or percent of gravity (g=9.8 m/s 2 )
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 Sumatra-
Andaman Island
Earthquake.

Earthquake Predictibility,
Forcasting and Early Warning

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?

Why might long-term predictions be possible ?
1. Most earthquakes are on plate boundaries
2. Plate motions are very constant over long periods of time
3. 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

Long-term Predictability
Most earthquakes are on plate boundaries

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 Predictability
Faults segmentation: characteristic large earthquakes

Segmentation in Japan

Long-term Predictability
Earthquake occurs when loading exceeds strength
Loading rate correlates
With plate-tectonic motions
Maximum load
Eq time, years
Eq
Now: where
Are we in the
Loading cycle ?

Results of this kind of analysis
Is a prediction of likelihood of a large earthquake on each segment of each fault
Assuming: long-term loading rates determined by GPS and/or geological studies and closeness to failure based on when last large event occurred

Why might short-term or intermediateterm predictions be possible ?
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 rate of moderate earthquakes prior to the big one – intermediate term

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 term heightened emergency preparedness curtain endangered activities evacuate people
Intermediate term redirect preparedness funds re-site future construction

Early Warning
- or every second counts -

Strong ground motion sensors
10 km fault city
50 km
50 km distant
At 2 km/s shear wave velocity
Is 25 seconds
Minus 10 seconds to
Detect strong motion at a
Few stations near fault
Is …

15 seconds

But say the damaging effects extend to 100 km …
50 km
< 15 second warning
> 15 second warning
100 km
… There may be a lot of people & structure in the >15 second warning region area

For this to have any hope of working you must plaster the earth with sensors 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 available that both the announcement of impending strong shaking and the response must be fully automated

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 occurred can help in formulating an emergency response
Strong ground motions after the 1995 Kobe, Japan earthquake