Earthquakes

Measuring Earthquakes

Guide For Reading

• What are the different kinds of seismic waves?

• How does the energy of an earthquake travel through Earth?

• What are the scales used to measure the strength of an Earthquake?

There are about ______ earthquakes per day, worldwide.

• 8,000

Most earthquakes begin below the earth’s surface in the _______ within ______ kilometers from Earth’s surface.

• Lithosphere

• 100

Focus

• The point beneath Earth’s surface where rock breaks under stress and causes an earthquake

Epicenter

• The point on Earth’s surface directly above an earthquake’s focus

Seismic Waves

Seismic Waves

• A vibration that travels through Earth carrying the energy released during an earthquake

Figure 11: At what point do seismic waves first reach the surface?

• The epicenter

What determines how much the ground shakes during an earthquake?

• How close a location is to the epicenter

• The types of rock and soil surrounding the epicenter determines how much the ground shakes

Guide For Reading: What are the different kinds of seismic waves?

• There are three types of seismic waves:

• Primary Waves (P waves)• Secondary Waves (S waves)• Surface Waves

• An earthquake sends out two types of waves P waves and S waves

• When the waves reach Earth’s surface at the epicenter, surface waves develop.

Primary Waves

Primary Waves (P Waves)

• A type of seismic wave that compresses and expands the ground

• The first wave to arrive at an earthquake

Describe how P waves move?

• P waves compress and expand like an accordion

• When P waves arrive they vibrate the particles of the crust forward and back along the path of the wave

Secondary Waves

Secondary Waves (S Waves)

• A type of seismic wave that moves the ground up and down or side to side

How are P waves different from S Waves?

• P waves compress and expand the ground

• These waves cause buildings to contract and expand

• P waves travel through solids and liquids

• S waves vibrate from side to side • These waves vibrate from side to side and thrust

the ground up and down, or back and forth• S waves can not move through liquids

Surface Waves

Surface Waves

• A type of seismic wave that forms when P waves and S waves reach Earth’s surface

Why do you think surface waves produce more severe ground movements than P waves and S waves?

• Surface waves consist of loose soil, sand, gravel, mud, small rocks, not solid rock

• These more loose substances are more likely shift and slide

Guide For Reading: How does the energy of an earthquake travel through Earth?

• Seismic waves carry the energy of an earthquake from the focus, through Earth’s interior, to the epicenter, and across the surface

Detecting Seismic Waves

Seismograph

• A device that records ground movements caused by seismic waves as they move through Earth

Describe how a mechanical seismograph records ground movement.

• A heavy weight attaches to a frame by spring or wire

• A pen connected to the weight rests its point on a rotating drum

• During an earthquake the seismic waves cause the drum to shake while the pen stays in place

• The pen records lines on the paper around the drum

Measuring Earthquakes

There are at least ______ different measures for rating earthquakes.

• 20

Magnitude

• The measurement of an earthquake’s strength based on seismic waves and movement along faults

The Mercalli Scale

Mercalli Scale

• A scale that rates earthquakes according to their intensity and how much damage they cause

• Developed in the early twentieth century

An earthquake’s ______ is the strength of ground movement in a given place.

• Intensity

The Mecalli scale has ______ steps and describes how an earthquake affects ______, ______, and the ______.

• 12

• People

• Buildings

• Land surface

Figure 14: How would you rate the damage to the Foligno city hall on the Mercalli scale?

• The damage would probably rate VII - VIII

The Richter Scale

Richter Scale

• A scale that rates seismic waves as measured by a particular type of mechanical seismograph

How does the Richter scale measure an earthquake?

• The Richter scale measures seismic waves using a seismograph

• Over time, mechanical seismographs were replaced by electronic seismographs

What is a strength of the Richter scale when measuring earthquakes? What is a weakness?

• Strength: Provides accurate measurements for small, nearby earthquakes

• Weakness: Does not measure larger and more distant earthquakes well

How are the Mercalli scale and the Richter scale similar? How are they different?

• Both measure the strength of an earthquake

• The Mercalli scale measures the strength in terms of extent people notice the earthquake and the amount of damage caused

• The Richter scale measures the size of seismic waves

The Moment Magnitude Scale

Moment Magnitude Scale

• A scale that rates earthquakes by estimating the total energy released by an earthquake

Why is the moment magnitude scale used today by geologists to measure earthquakes?

• The moment magnitude scale determines the total energy released by an earthquake

• This scale uses a electronic seismograph that can measure earthquakes that are big or small, and near or far

• Geologist examine movement along the fault and the strength of broken rock

• These two measurements give a more accurate measurement of an earthquake

On which scale would an earthquake’s strength vary from one place to another? Explain.

• The Mercalli scale because the amount of shaking that people would feel and the damage to objects would be greater in a place closer to the earthquake’s epicenter

Checkpoint:What are three scales for measuring earthquakes? Explain what each one measures.

• Mercalli Scale• Measures earthquakes on how much damage

they cause

• Richter Scale• Measures seismic waves using a seismograph• Measures small and nearby earthquakes

• Moment Magnitude Scale• Looks at the total energy released• Measures large and distant earthquakes• Helps scientists predict how much fault

movement their was

Locating the Epicenter

How do scientists calculate how far a location is from the epicenter of an earthquake?

• Scientists calculate the difference between arrival times of the P waves and S waves

• The further away an earthquake is, the greater the time between the arrival of the P waves and the S waves

Figure 17: Use the map scale to determine the distances from Savannah and Houston to the epicenter. Which one is closer?

• Houston • 800 Km

• Savannah• 900 km