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