Earthquakes Measuring Earthquakes Guide For Reading What are

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 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 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 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