EarthquakesChapter 6.1

Earthquakes & Plate Tectonics

1. Earthquakes are vibrations of the earth’s crust.a. Earthquakes occur when rocks under stress suddenly shift

along a fault. b. The area along a fault where motion first occurs is called

the focus of an earthquake. i. When an earthquake occurs seismic waves radiate outward in all

directions from the focus.

Earthquakes & Plate Tectonics

c. The point on the earth’s surface directly above the focus is called the epicenter.

d. A fault zone is an area where there are groups of interconnected faults. i.For example, the San Andreas Fault zone is made up of many

faults and any movement could cause an earthquake within that zone.

Elastic Rebound Theory

2. The elastic rebound theory states that the rocks on each side of a fault are moving slowly.a. If the fault is locked, not moving, then the stress in the fault

increases, and when the stress in the rocks increases past a certain point the rocks fracture and spring back to their original shape, or rebound.

Elastic Rebound Theory

b. When the rocks fracture they release energy in the form of vibrations called seismic waves. i. This release of energy increases the stress of other rocks

along the fault and causes them to fracture and spring back.

ii. This spring back action is the cause of smaller tremors called aftershocks.

Earthquake Zones

3. Most earthquakes occur along or near the edges of the earth’s lithospheric plates. a.The stress is greatest and the rocks experience the most

amount of strain at these areas of the earth.

Earthquake Zones

b. The earth has three major earthquake zones.i.Pacific Ring of Fireii.Mid-ocean ridgesiii.Eurasian-Melanesian mountain belt