8.2 – Measuring Earthquakes – Part I

Do Now What is the difference between a seismograph and seismogram?

Do Now What is the difference between a seismograph and seismogram? Seismograph – instrument that records earthquake waves. Seismogram – The actual reading of earthquake waves (the data).

Key Word Amplify

Vocab Words Surface Wave P Wave S Wave Seismograph Seismogram

Measuring Earthquakes The study of earthquake waves, or seismology, dates back almost 2000 years. The first attempts to discover the direction of earthquakes were made by the Chinese.

Seismograph Instruments that record earthquake waves. When waves from an earthquake reach the instrument, the inertia of the weight keeps it stationary, while the Earth and the support vibrate. The weight stays almost motionless, it provides a reference point to measure the amount of movement that occurs as waves pass through the ground below. http://www.youtube.com/watch?v=Gbd1FcuLJLQ

Seismograph

Seismogram Modern seismographs amplify and electronically record ground motion, producing a trace, called a seismogram.

Earthquake Waves The energy from an earthquake spreads outward as waves in all directions from the focus. Seismograms show the two main types of seismic waves that are produced by an earthquake – surface waves and body waves.

Earthquake Waves Surface Waves (also called L waves) Seismic waves that travel along Earth’s outer layer. The motion is complex. Travel along the ground and cause the ground and anything resting upon it to move. Feels like ocean waves tossing a ship. Up-and-down and side-to-side. Large amplitudes and different movements, makes surface waves the most destructive earthquake waves.

Earthquake Waves Body Waves P Waves (http://www.youtube.com/watch?v=2rYjlVPU9U4) Push-pull waves They push (compress) and pull (expand) rocks in the direction the waves travel. Also known as compression waves. Temporarily change the volume of the material they pass through. S Waves (http://www.youtube.com/watch?v=en4HptC0mQ4) Shake the particles at right angles to their direction of travel. Transverse waves Temporarily change the shape of the material they pass through.

Earthquake Waves A seismogram shows all three types of seismic waves – surface waves, P waves and S waves. The waves arrive at different times because they travel at different speeds. Generally, in any solid material, P waves travel about 1.7 times faster than S waves. Surface waves travel the slowest at about 90% of the speed of S waves.

Earthquake Waves

Locating an Earthquake Earthquake Distance The epicenter is located using the difference in the arrival times between P and S wave recordings, which are related to distance. Earthquake Direction Travel-time graphs from three or more seismographs can be used to find the exact location of an earthquake epicenter. Earthquake Zones About 95 percent of the major earthquakes occur in a few narrow zones.

Locating an Earthquake

Earthquake Zones

Group Activity Fill out the worksheet on practicing to find the epicenter of an earthquake.

3.5 * 10^3 = 3500 km 1 * 10^3 = 1000 km