Earthquakes

What’s an Earthquake? Earthquakes are movements of the ground that are caused by a sudden release of energy when along a fault move. 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. Occur when rocks under stress shift suddenly along a fault. Stress is a force that can change the size & shape of rocks. 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. This theory states --- sudden return of elastically deformed rock to its undeformed shape. If the fault is locked, stress increases. 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. 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. As they fracture and slip the rocks along the fault release energy in the form of an earthquake.

Anatomy of an Earthquake Hypocenter (Focus) – The area along the fault where slippage first occurs. Hypocenter (Focus) – The area along the fault where slippage first occurs. Epicenter – The point on the Earth’s surface directly above the focus. Epicenter – The point on the Earth’s surface directly above the focus.

Anatomy of an Earthquake A seismic wave is an elastic wave generated by an impulse such as an earthquake or an explosion. Seismic waves may travel either along or near the earth's surface A seismic wave is an elastic wave generated by an impulse such as an earthquake or an explosion. Seismic waves may travel either along or near the earth's surface 2 main types of seismic waves: 2 main types of seismic waves: Body waves – a seismic wave that moves through the interior of the earth. Body waves – a seismic wave that moves through the interior of the earth. Surface waves – a seismic wave that travels near the earth's surface. Surface waves – a seismic wave that travels near the earth's surface.

Types of Body Waves 1. A P wave, or compressional wave, is a seismic body wave that shakes the ground back and forth in the same direction and the opposite direction as the direction the wave is moving. 2. An S wave, or shear wave, is a seismic body wave that shakes the ground back and forth perpendicular to the direction the wave is moving.

2 Types of Surface Waves 1. Surface waves or Love waves or L waves – cause rock to move side to side & Perpendicular to direction waves are traveling. 2. Rayleigh waves -- cause the ground to move with an elliptical, rolling motion.

Major Earthquake Zones Most earthquakes occur along or near edges of the earth’s lithospheric plates Most earthquakes occur along or near edges of the earth’s lithospheric plates Stress is greatest along these moving plate boundaries. Stress is greatest along these moving plate boundaries. Earth has 3 major earthquake zones: Earth has 3 major earthquake zones: 1. Convergent Oceanic environments – move toward each other and collide; as the plates move, the overriding plate scrapes across the top of the subducting plate. 2. Divergent Oceanic environments – Make up the mid- ocean ridges, plates are moving away from each other. Spreading causes earthquakes. 3. 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. Seismograph – records & detects vibrations in the ground. Consists of 3 separate devices: Consists of 3 separate devices: 1. One device records the vertical motion of the ground. 2. 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). 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. 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. 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. Mercalli scale – expresses the intensity of an earthquake or the amount of damage it causes. Expressed by a roman numeral and a description.