Seismic Waves Surface Waves Seismic Waves are shock waves given off by earthquakes. There are 2 types: 1. Body Waves originate from the focus (F) travel through the earth. They’re used to locate epicenters (E.)

2 Types of Body Waves Surface Waves 1)Primary (P) Waves – compression waves travel fastest through solids first waves to arrive 2Secondary (S) Waves shear waves cannot travel through liquids second to arrive only half as fast as P waves

2 Types of Body Waves 1)P Waves – Compressional (or Longitudinal) Waves 2)S Waves – Shear Waves

Seismic Waves Surface Waves Seismic Waves are shock waves given off by earthquakes. There are 2 types: 2.Surface Waves (also called L (Long) Waves) Start forming when body waves reach the surface at the epicenter, They arrive much later than body waves.

S Waves do not travel through liquids.

S Waves do not travel through the outer core.

The outer core must be liquid.

P waves slow down in the outer core and then speed up in the inner core.

Bending of P Waves Creates Two Small “Shadow Zones”

P Wave Travel Paths P Wave DirectionP Wave Wave Fronts

Conclusion: The inner core must be solid.

Two Shadow Zones Note: S Waves are totally blocked by the outer core, so they have a huge shadow zone.

Seismic Waves and Earth Layers Seismic Waves travel faster through cold, dense rock and slow down traveling through warm, less dense rock.

Seismic Waves and Earth Layers There is a sharp rise in wave speed between the crust and mantle and again between the mantle and the outer core.

Seismic Waves and Earth Layers These sudden changes in wave speed are due to sharp changes in the type and density of rock which are called discontinuities.

Mohorovicic Discontinuity (Moho) Andrija Mohorovicic Sharp boundary between the crust and upper mantle

When you get a CT scan (Computerized Tomography Scan), the machine records your body image in a series of “slices”…

Computerized Tomography …which the computer puts together into a detailed 3-D image of your body’s soft tissue and internal organs.

Seismic Tomography Geophysicists use a similar technique with hundreds of seismic stations worldwide to make a 3 – D seismic tomographic image:

Layers of the Earth Basic seismology told us that Earth has four main layers.

4 Earth Layers* Seismic Tomography allows us to see smaller details in Earth’s structure that we didn’t know about before.

4 Earth Layers* - Plus One There are 4 main layers… *plus another: The D” layer is a thin layer in the lower 100 km of the lower mantle. The Core/Mantle boundary is very rugged and mountainous.

Anti-Continents and Anti-Oceans

2 More Boundary Layers (Discontinuities) below the Moho 1) 2) 3)

Ultra-Low Velocity Zone (ULVZ) A soft “squishy “ region found along the D” Core/Mantle Boundary is called the Ultra- Low Velocity Zone (ULVZ).

Core-Mantle Boundary

D” Layer – Bottom 100 km of Mantle

Mantle “Blobs” Hot Blobs are less dense and rise towards the surface. Cold Blobs are more dense and sink lower in the mantle.

Tomogram and Topographic Map - Mantle Blobs Bring up QuickTime

Tomograms of Mid and Deep Mantle Red – hot light rock Blue – cold heavy rock

Mantle Blobs and Gravity Anomalies

Tomographic Map of Japan and China

Cross-Section across Japan and China Blue - heavy rock: Sinking ocean crust “flattens out” at 500 km and then eventually continues to sink into the lower mantle.

Tomogram of Pacific Ocean Crust and Mantle Brown = warm, light rock Blue = cold, heavy rock

World Tomogram Blue blobs show heavy sinking slabs of crust. Red blobs show warm, rising blobs of rock.

“Convection Plumes” Hot, less dense plumes rise up, while cold, denser plumes sink in a circular motion called convection.

Surface Waves 1)Love Waves - Horizontal shear 2)Rayleigh Waves – Vertical shear 3)The combined effect of horizontal and vertical shear is to wrench buildings and highways apart.