Chapter 6 Earthquakes Ch. 6.1 Earthquakes and Plate Tectonics Earthquakesvibrations of the earths crust. Earthquakes normally occur along faults, or fractures in the crust where the rock moves. A fault is normally locked, or pressed together tightly, until stress overcomes the pressure holding it together, and the rocks suddenly grind past each other. The vibrations that result are felt and recorded as earthquakes. The vibrations are released as energy known as seismic waves. The movement of rock may cause pressure on other rock, which then breaks and releases more energy (aftershocks). Focusthe point along a fault where slippage of rock (the rock breaks) first occurs. Usually underground. Epicenterthe point on the earths surface directly above the focus. Seismic waves radiate outward in all directions from the focus. 90% of earthquakes have a shallow focus, within 70 km of earths surface. 70 km to 300 kmintermediate focus. 300 km to 650 kmdeep focus, in subduction zones. The most damaging earthquakes usually have a shallow focus. stop Because they occur in subduction zones, intermediate and deep focus earthquakes usually occur further inland than shallow focus ones. Elastic Rebound Theory Stress on rocks will slightly bend rock, but too much stress will cause the rock to break, and it will spring back to its original shape. Major Earthquake Zones Most earthquakes occur at lithospheric plate boundaries. The earth has 3 major earthquake zones. 1. The Pacific Ring of Fire. 2. Along mid-ocean ridges. 3. The Eurasian-Melanesian mountain belt. Major Earthquake Zones Pacific Ring of Fire Fault zonesgroup of interconnected faults at plate boundaries. Example: San Andreas fault, which extends almost the length of California. A major earthquake occurred in the central United States in 1812, far from any known plate boundary. Evidence reveals an ancient but now inactive plate boundary in the central U.S. Instead of breaking apart, the North American continent is being compressed by the actively spreading Atlantic Ocean.