EARTHQUAKES

earthquake: the shaking or vibrating of Earth caused by a

release of energy

elastic rebound: as two plates move past each other they get stuck and then slip; the plates suddenly move, release energy and then snap back into place

epicenter: point on the earth’s surface directly above the focus

focus: point at which rupture occurs along the fault

If you hear about an earthquake on the news,

you will always be told the location of the epicenter. If

you are told the depth of the earthquake, that is the

focus.

fault: a break in the

earth’s surface where

movement has

occurred

TYPES OF FAULTS

normal fault: blocks of rock move apart- found at divergent boundaries

reverse fault: blocks move toward one another

- found at convergent boundaries- also called thrust fault

strike slip: rocks move horizontally past one another

- found at transform boundaries

seismology: the study of earthquakes

seismograph: device that measures earthquakes

seismogram: earthquake graph made by a seismograph

Seismic Waves

As energy is released from an earthquake, it is distributed in waves

Body Waves: travel through the interior of the Earth

Surface Waves: travel along Earth’s surface

Types of Body Waves

P-wave (primary wave):- longitudinal wave- can travel through any material- 1st to arrive (6-7 km/s)- shortest period, smallest amplitude

S-wave (secondary wave)

- transverse wave- cannot travel through liquids (outer core)- 2nd to arrive (3.5 km/s)

S waves cannot travel through the Earth. P waves refract as they pass through the

liquid part of the core.

Surface Waves

- most damaging to structures- largest amplitude; slowest- last to arrive (2.5 km/s)

Types of Surface Waves

R- WavesRayleigh WavesMake the ground ripple up and

down

L-WavesLove WavesMake the ground ripple side to side

Determining the Location of the

Epicenter

epicenter

Shallow earthquakes tend to occur along transform

boundaries.

Deep earthquakes tend to occur along subduction zones.

* the deeper an earthquake, the less damage it causes at

the surface

How do we measure

earthquake magnitude?

Richter Scale

- measures amplitude of largest seismic wave 100 km from the epicenter

- wave height is ten times greater for each step that increases

The energy released is 33 times greater for each step that is increased

How many magnitude 6 earthquakes would you need to

release the same amount of energy in one magnitude 8

earthquake?

Richter Scale values

> 8 = total destruction (about 1/year)7-7.9 = bad (18/year)6-6.9 = moderate-serious (120/year)5-5.9 = felt by everyone (minor damage) (800/year)4-4.9 = not felt by everyone (6,000/year)< 3.9 very small (about 1 million per year)

Moment Magnitude Scale

-measures energy released-used by scientists- factors in amount of movement and rigidity of rocks

Comparison:

1964 Alaskan EQRichter- 8.4, Moment – 9.2

1906 San FranciscoRichter – 8.3, Moment – 7.9

Mercalli Intensity Scale

-measures damage done by the earthquake- scale of I – XII

I – felt by few people

V – felt by everyone (dishes broken)

VIII – slight damage to structure

X – wooden structures destroyed

XII – total destruction; see waves at surface

Earthquakes caused by HumansHoover Dam –

600 tremors over the 1st 10 years; some as high as magnitude 5

Rocky Mountains – In the 1960’s the US Army drilled a well

and injected liquid waste into the mountains

This lubricated the fault and earthquakes began

When the Army stopped injecting the waste, the earthquakes stopped too

Earthquake Prediction

long term years, not days or hours

short term Good Luck!

In order to study earthquake predictions, you need to predict an

earthquake.

(in order to set up equipment, etc.)

Recurrence Interval

- how often an earthquake of a specific magnitude occurs

Seismic Gap Method

-looking at gaps along faults where there is no

seismic activity

Earthquake Damage

1. Ground Displacementexamples: 1906 San Fran – 7 m of displacement 1964 Alaska – 12 m of displacement

(vertical)1989 Loma Prieta 2.2 m1994 Northridge 1 foot

Alaska, 1964

2. Landslides

1970 Peru – killed 70,000 people

3. Liquefaction

- sediments act as fluids

- Kobe, Japan

4. Tsunami- seismic sea waveex. Sumatra 2004

5.Fireex. Tokyo, San

Francisco

1906 – San Francisco

6. Valley Fever

- disease-sewage lines break- dust associated with fires causes bronchial problems

Most Devasting EQs

1500’s China 850,000 dead1923 Tokyo 143,000 dead1976 China 250,000 dead1985 Mexico 10,000 dead1995 Kobe, Japan 5,000 dead

$100 billion in damages

Northridge, CA 1994

Northridge 1994

Northridge 1994

Alaska, 1964

                                       

San Francisco Bay Bridge, 1989

Kobe Japan, 1995