Seismic waves grade 10 unit 1 module 1

Grade 10

1. MODULE 2: The Earth’s Interior

2. SEISMIC WAVES Energy released during an earthquake that radiates in all directions from the focus. Recorded in Seismographs.

3. TWO MAIN TYPES OF SEISMIC WAVES Surface Waves – can only travel through the surface of the earth. They arrive after the main P and S waves. Two types of surface waves: a. Love waves

4. Named after A.E.H. Love, a British mathematician who worked out the mathematical model for this kind of wave in 1911. It is faster than Rayleigh wave. Moves the ground side to side horizontal motion. Cause most damage to structures.

5. b. Rayleigh Wave Named after John William Strutt , Lord Rayleigh who mathematically predicted existence of this wave in 1885. Rolls along the ground. Moves the ground either up and down or side to side. Most shaking felt during earthquake is due to Rayleigh wave.

6. 2. Body Waves – can travel through the earth’s inner layers. Used by Scientists to study the earth’s interior. Of higher frequency than the surface waves. Two types of Body waves: a. P-waves (primary waves) –also called compressional waves.

7. Pulse energy that travels through earth. It can travel through solids, liquids and gases. Faster than S-waves. Travel by particles vibrating parallel to the direction the wave travel, forcing the ground to move backward and forward as they are compressed and expanded.

8. b. S-wave (secondary waves or Shear waves). A pulse energy that travels slower than a P-wave through earth and solids. Cannot travel through liquids. Move as a shear or transverse waves and force the ground to sway side to side in rolling motion that shakes ground back and forth perpendicular to the direction of the waves.

9. Scientists gained information about the earth’s internal structure by studying how seismic waves travel through the earth. - By measuring the time it takes for both types of waves to reach seismic wave detecting stations from the epicenter of an earthquake.

10. Andrija Mohorovicic – Yugoslavian Seismologist, 1909, found that velocity of seismic waves changes and increases at a distance of about 50km below earth’s surface – there is a difference in density between crust and mantle. Mohorovicic ( Moho ) discontinuity- Boundary between crust and mantle.

11. During earthquake, Seismic waves bend due to change in density of the medium, as the depth increases, density also increases. Beno Gutenberg – German Seismologist, the existence of a shadow zone could only explain that the earth contained a core composed of material different from the mantle. Gutenberg discontinuity - mantle-core boundary.

12. S-waves do not travel all throughout the earth’s body – outer core is liquid. Inge Lehmann – Danish Seismologist,1936, predicted the innermost layer of the earth and discovered a new region of seismic reflection within a core. A core within a core must be solid with a different density than the surrounding materials.

13. Three layers of the earth: There is a varied thickness of the different layers of the earth. Size of inner core was accurately calculated through Nuclear underground test in Nevada. lAYER Thickness in Km. Crust 40 Mantle 2900 Outer core 2200 Inner core 1278