Measuring

Measuring

Inside Earth Chapter 2.2Pages 64-70

Earthquakes

Fault• A break in Earth’s lithosphere (crust) where

plates slide or move past each other.

Focus• Focus: the starting point of an earthquake,

usually underground• Depth of the focus is related to the type of

boundary.– Transform: Shallow <300m– Divergent: 300m– Subduction: Deep 700m

Epicenter• The point directly above the focus on the

surface of the Earth is called the Epicenter.

Seismic Waves• Vibrations caused by the energy of the

earthquake that travel through the Earth’s interior and across the surface away from the focus P-Waves S-Waves Surface Waves

Primary Waves• First Wave to Arrive– Squeeze and stretch rock, (Accordion-like)• Compression / tension stress

–Passes through…• Solid rock• Magma• Ocean water• Air• Everything!

Secondary Waves• Arrive second, after P-waves– Vibrate from side to side, up and down– Shake the ground back and forth– Can not pass through liquids or gasses• Indicates that Earth has a liquid outer core.

Surface Waves• Travel along the Earth’s surface• Produced by P and S waves that reach the

surface– Produce severe ground movement–Most damaging waves.

Seismograph

• An instrument used to detect and record the seismic waves produced by earthquakes.

Seismograph• Consists of a heavy

weight attached to a frame by a wire.

• A pen is attached to a heavy weight.

• The pen stays in place, the drum moves during an earthquake.

Seismogram• The record sheet the pen records on

as the drum rotates.

Seismograph• More than 10,000 StationsUse Data to…• Determine an epicenter– Need 3 Stations

• Earthquake Magnitude

Locating an Epicenter• Need 3 Seismograph stations to determine

their distance from the epicenter

• Determine distance by:D = S-wave arrival time - P-wave arrival time.

Locating an Epicenter• Each station draws a circle.– Where all three intersect is the epicenter.

• The measure of the amount of energy released in an earthquake

– Three types• Mercalli• Richter Scale• Moment Magnitude.

Magnitude

• 12 steps describe how earthquakes effect people, buildings, and the surface

• Rates earthquakes according to damage dealt to people and structures

• Not a precise measurement.

Mercalli Scale

• Developed in the 1930’s by Charles Richter• Rates the size of the seismic waves.• Measures the intensity of ground movements– Provides accurate measurements for small, nearby

earthquakes, but does not work well for large or distant.

Richter Scale

Richter Scale

Richter Scale

• Indicates the total energy released during earthquake’s more accurately

• Scale used by scientists• Can be used for any size earthquake, near or

far.

Moment Magnitude

• Combines the following to determine the moment magnitude:1. The type of seismic waves produced and their

strength2. How much movement occurred on the fault3. Strength of the rocks that broke• 5.0 Little damage• Above 5.0 major damage

Moment Magnitude

Moment Magnitude

E. Epicenter

A. Fault

B. Seismic Waves

C. Earth’s Crust

D. Focus

The point under the surface directly where the Earthquake originates is…

The point on the surface above where the Earthquake originates.

E. Epicenter

A. Fault

B. Seismic Waves

C. Earth’s Crust

D. Focus

The wave that arrives first is the…

E. Tsunami Wave

A. Secondary Wave

B. Seismic Waves

C. Sound Wave

D. Primary Wave

The scale that measures the total energy released is the…

E. Magnitude

A. Richter Scale

B. Mecallie Scale

C. Bathroom Scale

D. Moment Magnitude

How many seismograph stations are needed to locate an earthquake?

E. 4

A. 5

B. 3

C. 1

D. 6

What to Work On

• Read textbook pages 64-69• Answer the Section Review questions on

page 69 (#’s 1-4)

DUE: Friday, October 21st

Locating an Epicenter Lab• Purpose: Geologists who study earthquakes

are called seismologists. If you were a seismologist, you would receive data from all across the country. Within minutes after an earthquake, seismographs located in Denver, Houston, and Miami would record the times of arrival of the P waves and S waves. You would use this data to zero in on the exact location of the earthquake’s epicenter.

Locating an Epicenter Lab

City Denver, CO

Houston, TX

Miami, FL

Difference in P & S

Wave Arrival Times

2min 10sec 4min 5min 40sec

• Seismograph Data:



200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000 4200 4400 4600 4800 5000 (km)

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