EarthquakesEarthquakes

What’s an Earthquake?What’s an Earthquake? Earthquakes are movements of the Earthquakes are movements of the

ground that are caused by a sudden ground that are caused by a sudden release of energy when along a fault release of energy when along a fault move.move.

Occur when rocks under stress shift Occur when rocks under stress shift suddenly along a fault.suddenly along a fault.

Stress is a force that can change the Stress is a force that can change the size & shape of rocks.size & shape of rocks.

Elastic Rebound TheoryElastic Rebound Theory This theory states --- sudden return of This theory states --- sudden return of

elastically deformed rock to its undeformed elastically deformed rock to its undeformed shape. shape.

If the fault is locked, stress increases.If the fault is locked, stress increases. When stress reaches passed a certain When stress reaches passed a certain

point, the rocks fracture, separate at point, the rocks fracture, separate at weakest point, and spring back or rebound, weakest point, and spring back or rebound, to original shape.to original shape.

As they fracture and slip the rocks along As they fracture and slip the rocks along the fault release energy in the form of an the fault release energy in the form of an earthquake.earthquake.

Anatomy of an EarthquakeAnatomy of an Earthquake Hypocenter (Focus) – The area along Hypocenter (Focus) – The area along

the fault where slippage first occurs.the fault where slippage first occurs. Epicenter – The point on the Earth’s Epicenter – The point on the Earth’s

surface directly above the focus.surface directly above the focus.

Anatomy of an EarthquakeAnatomy of an Earthquake A A seismic waveseismic wave is an elastic wave is an elastic wave

generated by an impulse such as an generated by an impulse such as an earthquake or an explosion. Seismic earthquake or an explosion. Seismic waves may travel either along or waves may travel either along or near the earth's surface near the earth's surface

2 main types of seismic waves:2 main types of seismic waves: Body waves – Body waves – a seismic wave that a seismic wave that

moves through the interior of the earthmoves through the interior of the earth.. Surface waves – a seismic wave that Surface waves – a seismic wave that

travels near the earth's surfacetravels near the earth's surface..

Types of Body WavesTypes of Body Waves1.1. A P wave, or A P wave, or

compressional wavecompressional wave, is , is a seismic body wave that a seismic body wave that shakes the ground back shakes the ground back and forth in the same and forth in the same direction and the direction and the opposite direction as the opposite direction as the direction the wave is direction the wave is moving.moving.

2.2. An S wave, or An S wave, or shear shear wavewave, is a seismic body , is a seismic body wave that shakes the wave that shakes the ground back and forth ground back and forth perpendicular to the perpendicular to the direction the wave is direction the wave is moving.moving.

2 Types of Surface Waves2 Types of Surface Waves1.1. Surface waves or Surface waves or

Love waves or L Love waves or L waves – cause rock waves – cause rock to move side to side to move side to side & Perpendicular to & Perpendicular to direction waves are direction waves are traveling.traveling.

2.2. Rayleigh waves -- Rayleigh waves -- cause the ground to cause the ground to move with an move with an elliptical, rolling elliptical, rolling motion.motion.

Major Earthquake ZonesMajor Earthquake Zones Most earthquakes occur along or near Most earthquakes occur along or near

edges of the earth’s lithospheric platesedges of the earth’s lithospheric plates Stress is greatest along these moving Stress is greatest along these moving

plate boundaries.plate boundaries. Earth has 3 major earthquake zones:Earth has 3 major earthquake zones:

1.1. Convergent Oceanic environments – move toward Convergent Oceanic environments – move toward each other and collide; as the plates move, the each other and collide; as the plates move, the overriding plate scrapes across the top of the overriding plate scrapes across the top of the subducting plate. subducting plate.

2.2. Divergent Oceanic environments – Make up the Divergent Oceanic environments – Make up the mid-ocean ridges, plates are moving away from mid-ocean ridges, plates are moving away from each other. Spreading causes earthquakes.each other. Spreading causes earthquakes.

3.3. Continental environments – 2 continental plates Continental environments – 2 continental plates converge, diverge, or move horizontally in converge, diverge, or move horizontally in opposite directions.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 EarthquakesRecording Earthquakes Seismograph – records & detects Seismograph – records & detects

vibrations in the ground.vibrations in the ground. Consists of 3 separate devices:Consists of 3 separate devices:

1.1. One device records the vertical motion of the One device records the vertical motion of the ground.ground.

2.2. The other 2 devices record horizontal motion The other 2 devices record horizontal motion in the east-west and north-south.in the east-west and north-south.

The seismograph records motion by The seismograph records motion by tracing wave-shaped lines on paper and tracing wave-shaped lines on paper and translating the motion into electronic translating the motion into electronic signals (known as a seismogram).signals (known as a seismogram).

Measuring an EarthquakeMeasuring an Earthquake Richter scale – measures the ground motion Richter scale – measures the ground motion

from an earthquake to find its strength.from an earthquake to find its strength. Moment magnitude is a measurement of a Moment magnitude is a measurement of a

quakes strength based on size of area the quakes strength based on size of area the fault moves, the average distance that the fault moves, the average distance that the fault blocks move, and the rigidity of the fault blocks move, and the rigidity of the rocks in the fault zone.rocks in the fault zone.

Mercalli scale – expresses the intensity of Mercalli scale – expresses the intensity of an earthquake or the amount of damage it an earthquake or the amount of damage it causes. Expressed by a roman numeral and causes. Expressed by a roman numeral and a description.a description.