Earthquakes Chapter 12
What’s an Earthquake? • Earthquakes are movements of the ground that are caused by a sudden release of energy when along a fault move. • Occur when rocks under stress shift suddenly along a fault. • Stress is a force that can change the size & shape of rocks.
Elastic Rebound Theory • This theory states --- sudden return of elastically deformed rock to its undeformed shape. • If the fault is locked, stress increases. • When stress reaches passed a certain point, the rocks fracture, separate at weakest point, and spring back or rebound, to original shape. • As they fracture and slip the rocks along the fault release energy in the form of an earthquake.
Anatomy of an Earthquake • Focus --- The area along the fault where slippage first occurs. • Epicenter – The point on the Earth’s surface directly above the focus. • 2 main types of seismic waves: • Body waves – travel through the body of a medium. • Surface waves – travel along the surface of a body rather than through the middle.
Types of Body Waves • Primary waves or P waves – move the fastest; first to be recorded; can travel through solids and liquids; cause rock particles to move together and apart along the direction of waves. • Secondary waves or S waves --- second to be recorded; only travel through solids; cause rock particles to move at right angles to the direction in which waves are traveling. Cause the greatest damage.
Surface waves or Love waves or L waves – cause rock to move side to side & Perpendicular to direction waves are traveling. Rayleigh waves -- cause the ground to move with an elliptical, rolling motion. 2 Types of Surface Waves
Major Earthquake Zones • Most earthquakes occur along or near edges of the earth’s lithospheric plates • Stress is greatest along these moving plate boundaries. • Earth has 3 major earthquake zones: • Convergent Oceanic environments – move toward each other and collide; as the plates move, the overriding plate scrapes across the top of the subducting plate. • Divergent Oceanic environments – Make up the mid-ocean ridges, plates are moving away from each other. Spreading causes earthquakes. • Continental environments – 2 continental plates converge, diverge, or move horizontally in opposite directions.
Fault zones --- Form at plate boundaries because of intense stress that results when plates separate, collide, subduct, or slide past each other.
Recording Earthquakes • Seismograph – records & detects vibrations in the ground. • Consists of 3 separate devices: • One device records the vertical motion of the ground. • The other 2 devices record horizontal motion in the east-west and north-south. • The seismograph records motion by tracing wave-shaped lines on paper and translating the motion into electronic signals (known as a seismogram).
Measuring an Earthquake • Richter scale – measures the ground motion from an earthquake to find its strength. • Moment magnitude is a measurement of a quakes strength based on size of area the fault moves, the average distance that the fault blocks move, and the rigidity of the rocks in the fault zone. • Mercalli scale – expresses the intensity of an earthquake or the amount of damage it causes. Expressed by a roman numeral and a description.