•Section 6.1 Objectives1. Discuss the elastic rebound theory.2. Explain why earthquakes generally occur at plate boundaries.

Earthquakes & Plate Tectonics

•Earthquakes (EQs): vibrations of the Earth’s crust– Occur when rocks

under stress suddenly shift

•Elastic Rebound Theory: the theory geologists use to explain many EQs– Rocks on each side of fault are moving– If fault is locked, stress increases– At certain point rocks fracture & rebound

•Focus: the area along a fault where slippage 1st occurs.–Depths vary

•Epicenter: the point on the Earth’s surface directly above the focus.

•Aftershocks: series of smaller tremors.–Caused by released energy (seismic waves)•Increases stress in other rocks along the fault

•Focus depths vary–Shallow focus - w/in 70 km of surface•Tend to cause the most damage

•Focus depths vary– Intermediate focus - 70 to 300 km

•Focus depths vary–Deep focus (occur in subduction zone)•300 to 650 km

•Focus depths vary–About 90% of Continental EQs are shallow focus

3 Major EQ Zones•Links btwn EQs & Plate Tectonics

– Plate movements = stress buildup

•Pacific Ring of Fire: ring of tectonic/volcanic activity surrounding the pacific.

•Mid-ocean ridges: spreading motion = stress

•Eurasian-Melanesian mountain belt: Eurasian plate colliding w/African & Indian plates = stress

•Fault Zones: are groups of interconnected faults.– Form at plate

boundaries b/c of intense stress•e.g. San Andreas

fault

•Not all EQs occur at plate boundaries!– e.g. New Madrid, Missouri, 1812

•Ancient fault? (600 million yrs ago)