Seismic Waves P waves primary waves compress and

Seismic Waves S waves (secondary waves) • travel at half the speed of P

Seismic Waves Surface waves • move most slowly • produce severe ground movements; can

Measuring Earthquakes • Seismograph: instrument that records and measures the seismic waves.

Measuring Earthquakes Modified Mercalli Scale • Rates the amount of shaking • Rated by

Measuring Earthquakes Richter Scale • Finds the magnitude; a number assigned to an earthquake

Measuring Earthquakes Moment Magnitude Scale • Rates the total energy an earthquake releases. •

Comparing Magnitudes • A 1 -point increase on a scale represents an increase of

Locating an Earthquake • You need seismogram info from 3 places: – Find the

Chapter 4: Earthquakes 4. 3 Monitoring Earthquakes

Earthquake Risk • Largely depends on how close a given location is to a

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Seismic Waves P waves (primary waves) • compress and expand the ground • first waves/fastest moving • travel through solid, liquid, and gas

Seismic Waves S waves (secondary waves) • travel at half the speed of P waves and arrive second • travel through solids only • strong enough to shake structures

Seismic Waves

Seismic Waves Surface waves • move most slowly • produce severe ground movements; can cause ground to roll or buildings to shake back and forth.

Measuring Earthquakes • Seismograph: instrument that records and measures the seismic waves.

Measuring Earthquakes

Measuring Earthquakes Modified Mercalli Scale • Rates the amount of shaking • Rated by observations; useful in areas where there aren’t instruments available.

Measuring Earthquakes

Measuring Earthquakes Richter Scale • Finds the magnitude; a number assigned to an earthquake based on size. • Depend on the size of the waves recorded by a seismograph.

Earthquake Magnitude • Bill Nye!

Measuring Earthquakes Moment Magnitude Scale • Rates the total energy an earthquake releases. • Geologists use data from seismographs and other sources.

Measuring Earthquakes

Comparing Magnitudes • A 1 -point increase on a scale represents an increase of 32 -times more energy. • The higher an earthquake is rated, the more energy it has and the more damage it can do.

Locating an Earthquake • You need seismogram info from 3 places: – Find the difference of arrival times between the P and the S waves. – The difference is used to find the distance to the epicenter – the longer it takes, the farther away the epicenter. – A circle is drawn around each point (location). The point where they all cross is the epicenter.

Locating an Earthquake

Chapter 4: Earthquakes 4. 3 Monitoring Earthquakes

Earthquake Risk • Largely depends on how close a given location is to a plate boundary. • In N. America, there are plate boundaries in California, Washington, and Alaska. • Around the world, the Ring of Fire contains boundaries where several volcanoes and earthquakes occur.