Chapter 19 - Earthquakes

EARTHQUAKES CAN BE VERY POWERFUL. FORTUNATELY ONLY

AROUND 75 MAJOR EARTHQUAKES HAPPEN EACH YEAR. THEY ARE CAUSED

WHEN ROCKS FRACTURE OR BREAK.

1. What causes an earthquake?

A. Rocks break or fracture because of stress.

When the stress overcomes the strength of the rock, the rock

breaks.

Rocks act like a wooden stick being bent. The rocks bend until they reach a breaking point, and then they move, or break, when they cannot take the stress any longer.

1. What causes an earthquake?

Rocks break or fracture because of stress. When the stress overcomes the strength of

the rock, the rock breaks.

There are 3 types of stress:

B. Tensional stress = forces that pull rocks apart

1. What causes an earthquake?

C. Compressional stress = forces that push rocks together

D. Shear stress: forces that twist the shape of a rock

Strain: Rocks becoming deformed because of stress.

2. What is a fault?

A. The place (a crack) where the rock fractures and moves.

B. This movement releases the energy feel as an earthquake.

TYPES OF FAULTS

Faults are cracks in the Earth’s crust, along which movement has taken place. There are 3 major types (shown on the

next slides).

C. Normal faults - Caused by TENSION - The crust becomes lengthened.

D. Reverse faults - Caused by COMPRESSION- The crust becomes shortened.

E. Strike-slip fault

- Caused when rocks move past one another horizontally.

- Caused by SHEAR stress

- Example is the San Andreas Fault.

3. Earthquake waves

A. Energy is released through vibrations of Earth

B. Energy is released in waves, called seismic waves, and travel outward in ALL directions.

C. P Waves- compression waves

(squeezed and pull rock in same direction) - fastest - move through solids and liquids

D. S Waves- “slinky waves,” rock moves up and down- slower- move only through solids

E. Surface Waves- - on the surface of the Earth only

- move up, down and sideways- cause the most damage

- slowest

F. Focus - The point INSIDE the Earth where the earthquake begins.

G. Epicenter - The point on Earth’s SURFACE where the earthquake begins. - It is directly above the focus.

So…why do earthquakes occur??

TO RELEASE BUILT UP STRESS IN THE ROCKS!!