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Unit 1.4 Earthquakes Where do earthquakes Occur? • Earthquakes can occur near the Earth’s surface or far below the surface. • Most earthquakes occur at plate boundaries, but some happen at faults located in the middle of tectonic plates. Earthquake Locations Around the World Vocabulary to Know • Stress: Force that acts on rock to change its shape or volume • Fault: When enough stress builds up and rock breaks • Tension: Stress force that pulls on Earth’s crust causing rock to stretch in the middle • Compression: Stress force that squeezes rock until it folds or breaks • Epicenter: The point on the Earth’s Surface above an earthquake’s starting point • Focus: The point inside the Earth where the earthquake begins What Causes Earthquakes? • Earthquakes are caused by movement along faults. • When stress is placed on rocks it deforms, or changes. • This is called Elastic Deformation • Rock is stretched and bent until it can no longer take the stress. • When enough stress builds up in the rock it, it slips and energy is released. • The rock then returns to its original shape. This is called Elastic Rebound. Think of a rubber band, you can only stretch it so far until it breaks and return to its original shape. • This energy is felt as an earthquake. 3 Types of Faults Associated with Earthquakes PLATE MOTION FAULT TYPE Transform : plates move past each other Strike –Slip Fault: Fault Blocks move past each other Convergent: Plates move together Reverse Fault: Fault Blocks move together Divergent: Plates Move Apart Normal Fault: Fault Blocks Move Apart Strike- slip Fault occurs at a Transform Boundary Reverse Fault Occurs at a Convergent Boundary