Earthquake...

EARTHQUAKE !

By Deepak

Topics What is an earthquake?

What makes the earth quake?

Measuring Earthquakes.

Can we predict earthquakes?

Should you worry about earthquakes?

How can you minimize your risk?

TSUNAMI !

What is an Earthquake? An earthquake is a shaking of the ground

caused by the sudden breaking and movement of large sections (tectonic plates) of the earth's rocky outermost crust. The edges of the tectonic plates are marked by faults (or fractures). Most earthquakes occur along the fault lines when the plates slide past each other or collide against each other.

Tectonic Activity Volcanic Activity Meteoroids Trucks

What makes the earth quake ?

Tectonic Earthquakes

where do earthquakes happen?

Earthquake map

Tectonics and Earthquakes Along what boundaries do DEEP

earthquakes occur? Along what boundaries do ONLY

small earthquakes occur? Along what boundaries does

strike-slip motion occur?

FOCAL MECHANISMS

The DIRECTION the ground moves in at seismometers tells us the direction of motion AT THE FOCUS.

seismometer

UP

FOCUS

FOCAL SPHERE

At the location on thelower half of the focalsphere where the wavecomes out, color thespot BLACK if the first-motion is OUT of thesphere, or white if thefirst motion is in. Withreadings from manyseismometers, the shapeof the “focal mecha-nism” is defined.

Map views of bottom half of focal spheres

What does the focal mechanism for a strike-slip earthquake look like?

What does the focal mechanism for a strike-slip earthquake look like?

FOCAL MECHANISMS AND TECTONICS

SpreadingCednter

TransformFault

TransformFault

SubductionZone

What is the relative motion? How many plates are shown?

SpreadingCednter

TransformFault

TransformFault

SubductionZone

If earthquakes occur along these plate boundaries, what will the focalMechanisms look like?

SpreadingCednter

TransformFault

TransformFault

SubductionZone

SHOW TECTONIC MAP

MEASURING EARTHQUAKES

Seismometers Locating earthquakes Discovering the interior

of the earth

Chinese Seismoscope - first earthquake sensor

Seismograms from many stations for one earthquake

Earthquake Location if P and S-wave data are available

What if you only have P-wave arrivals at three

seismometers… Can you locate the quake?

Station: Arrival Time A 10:22:04 B 10:22:04 C 10:23:34

x

x

x

A

B

C

The wave arrives much later at C and thus thequake location must be much farther from C than itis from A or B. The source must lie along a curvewhere all the points on the curve are locations thatare 90 sec LATER at C than at A. This curve is aHYPERBOLA. Similarly for C and B:

EPICENTER

Interior of the Earth

Almost everything we know about the deep interior of the earth comes from the study of

seismic waves

Seismic velocities and density changes in the earth

Predicting Earthquakes

types of prediction

long-term

intermediate term

short term

earthquake model

science, politics, and the law

How can you tell a VALID prediction from a scam?

• Three factors are needed:- HOW LARGE will the quake be? - WHERE will it occur- WHEN will it happen

•Predicting very small earthquakes is easy - they occur all the time.

•Predicting LARGE quakes may be impossible.

LONG-TERM Predictions (years to decades) are based on tectonic indicators - fault activity, distance to active faults, seismic gaps etc

INTERMEDIATE-TERM: (Year to days) based on increased activity in an area

SHORT-TERM: (Day to hours) very difficult, but very valuable

Predictions should include STATISTICS, such as:

“There is an 80% probability that a magnitude 7.5 or larger earthquake will occur in the San Francisco area within the next two weeks.”

WHAT HAPPENS IF YOUR PREDICTED QUAKE DOESN’T HAPPEN?

Law suits.

WHAT HAPPENS IF IT’S BIGGER OR SMALLER OR OCCURS EARLIER OR LATER OR IN A DIFFERENT LOCATION?

Law suits.

We need to know how earthquakes are generated

at faults. Tectonic plate motions Elastic Rebound Properties of materials -

elasticity,strength Strength of rocks vary with

pressure, temperature and composition

Elastic Rebound Theory

QuickTime™ and aGIF decompressor

are needed to see this picture.

A Simple Earthquake Model:

What if the STRAIN across the fault continues to grow until a fixed maximum value is reached, at which time the earthquake occurs? AND -

What if the STRAIN always drops to the same minimum value?

With a fixed maximum and minimum strain, earthquakes will all have the same size and will occur at predictable times.

ti me

maximum strain

minimum strain

What if only the Maximum Strain is fixed?

.

time

bend

ing

maximum bending

In this case, the TIME of the next quake will be predictable, but the SIZE will not be.

Sm

a ll q

u ake

LA

RG

E q

u ake

.

minimum bending

time

bend

ing

What if only the Minimum Strain is fixed?

In this case the SIZE of the next quake can be predicted - if it happens tomorrow, but WHEN it occurs is not constrained.

Let’s Try an experiment to see if either of these assumptions hold with a SIMPLE earthquake model…

crankspring

weight

fault

Turning the crank represents the passage of time, and the spring stretches (representing strain across the fault) until the weight moves - representing an earthquake.

What information do we need?crank

springweight

fault

A

B

01020

We need to measure the length “A” to get our measure of how much time has passed, andWe need to measure the length “B” to see how large the earthquakes are. “B” only changes during an earthquake.A-B is a measure of strain, and A is a measure of time, so, from these two measurements, we can make plots of strain vs. time and check our hypotheses.

If we plot “A” along the x axis and “B” along the y axis we get plots like:

A (time)

B (

tota

l motion

on

fault

)

large earthquake

small earthquake

If we plot “A” along the x axis and “A-B” along the y axis we get plots like:

A (time)

A-B

(st

retc

h of

sprin

g)

large earthquake

small earthquake

What did we learn?

Did our hypothesis fit the data? Could we predict the time and size

of the earthquakes? What about predicting earthquakes

along a REAL fault?

Should you worry about earthquakes?

Learn the earthquake history in

your area

Are you likely to be hurt?

Are you likely to loose property?

Damage is caused by things falling on you from above

Adobe is NOT a good building material in earthquake-prone areas.

LOMA PRIETA, I 80

Sometimes things drop out from under you…

Alaska, 1964: Often the GROUND fails, not the building. Build on HARD ROCK!

KOBE

San FranciscoArea Earthquakes

The California 1906 quake

history

the earthquake

the fire

could it happen again?

Before the earthquake and fire

Buildings built on soft fill often collapsed.

Much damage in 1989 earthquake occurred in soft fillWhere the 1906 trash had been dumped into the bay.

Seismogram wiggle size Vs. earthquake magnitude

How can you minimize your risk? avoid earthquake-prone regions

LEARN the earthquake history and fault locations in

your area

don’t live below something that can fall on you, like a

steep hill

don’t live below a dam

Don’t build on soft ground

store emergency supplies

TSUNAMI ! How can you minimize your risk? What are the differences between

surfing waves and tsunami? What would you do if you were on

the beach and the siren blows? What would you do if you were on

the beach and the water starts to slowly recede?

Why did the Dec 26 2005 tsunami waves appear to be many short period waves?

LandOcean

100 m30 m/s

50 m22 m/s

30 m17 m/s

10 m10 m/sv=Ćgh

h=v=

Because the wave speed is decreasing as water depth decreases, the wave piles up on itself as the depthshallows. This may create a series of ŅstepsÓwhere the speed decreases sharply on the shallow side. Thewave itself has a period of tens of minutes, but each individual step may be only tens of seconds apart.Notice that the water depth continues to increase as each step approaches, unlike a surfing wave, wherethe water depth is constant between waves.

LandOcean

Normal Surf Wave

The water level after a normal surf wave passes returns to the same level as beforethe wave. The time between waves varies from a few seconds to 20 seconds.

A tsunami is a long wave with the time between trough to peak of up to 20-30minutes, or a time from normal sea level to maximum of 10 to 15 minutes.

Calm water

Thank you