Ch. 6.2 Recording Ch. 6.2 Recording EarthquakesEarthquakes

SeismographSeismograph—an instrument used for —an instrument used for detecting and recording seismic waves.detecting and recording seismic waves.

Detected waves are converted into Detected waves are converted into electrical signals, and recorded onto tape, electrical signals, and recorded onto tape, or directly loaded into a computer for or directly loaded into a computer for analysis.analysis.

Types of Seismic WavesTypes of Seismic Waves

Three main types:Three main types: Primary waves (P waves)Primary waves (P waves)—fastest, so first —fastest, so first

to be recorded. Can travel through solids to be recorded. Can travel through solids andand liquids. They are compression waves liquids. They are compression waves that move particles together and apart that move particles together and apart along the direction of the wave’s travel.along the direction of the wave’s travel.

Secondary waves (S waves)Secondary waves (S waves)—2—2ndnd waves waves to be recorded on a seismograph. Only to be recorded on a seismograph. Only travel through solids. They can’t penetrate travel through solids. They can’t penetrate the liquid outer core. They are shear the liquid outer core. They are shear waves; they move particles at right angles waves; they move particles at right angles to the direction of the wave’s travel.to the direction of the wave’s travel.

The energy of P and S waves is converted The energy of P and S waves is converted to to Surface WavesSurface Waves when they reach the when they reach the earth’s surface. They are the slowest earth’s surface. They are the slowest waves, with movement similar to rolling waves, with movement similar to rolling ocean waves. They cause the greatest ocean waves. They cause the greatest amount of damage.amount of damage.

Locating an EarthquakeLocating an Earthquake

Since P waves travel about 1.7 times Since P waves travel about 1.7 times faster than S waves, by analyzing the faster than S waves, by analyzing the difference in arrival times of the waves at a difference in arrival times of the waves at a seismograph station, the distance to the seismograph station, the distance to the earthquake can be determined.earthquake can be determined.

Info from at least Info from at least threethree stations is needed stations is needed to determine the location of the epicenter.to determine the location of the epicenter.

Earthquake MeasurementEarthquake Measurement

The energy released by an earthquake The energy released by an earthquake (magnitude) is measured using the (magnitude) is measured using the RichterRichter scale. Going up the scale, 1 to 10, each scale. Going up the scale, 1 to 10, each whole number represents 10 times greater whole number represents 10 times greater energy.energy.

Major earthquakes have magnitude 7 or Major earthquakes have magnitude 7 or above.above.

The The Mercalli Mercalli scale uses roman numerals, I scale uses roman numerals, I to XII, and expresses the earthquake’s to XII, and expresses the earthquake’s intensityintensity. Uses descriptive phrases to . Uses descriptive phrases to describe effects.describe effects.

When calculating energy differences in the When calculating energy differences in the Richter scale, remember that each whole Richter scale, remember that each whole number differs from the adjacent whole number differs from the adjacent whole number by a factor of 10.number by a factor of 10.

A magnitude 3 has 10 times more energy A magnitude 3 has 10 times more energy than a magnitude 2.than a magnitude 2.

A magnitude 4 has 100 times more energy A magnitude 4 has 100 times more energy than a magnitude 2 (10 x 10).than a magnitude 2 (10 x 10).

A magnitude 5 has 1000 times more A magnitude 5 has 1000 times more energy than a magnitude 2 (10 x 10 x 10).energy than a magnitude 2 (10 x 10 x 10).

Calculate the energy difference between a Calculate the energy difference between a magnitude 3 earthquake and a magnitude magnitude 3 earthquake and a magnitude 7 earthquake (Richter Scale).7 earthquake (Richter Scale).

7 – 3 = 47 – 3 = 4 10 x 10 x 10 x 1010 x 10 x 10 x 10 10,00010,000