Key Terms: Seismology - the study of earthquakes Seismology - the study of earthquakes Seismologist - a scientist who studies earthquakes and seismic waves Seismologist - a scientist who studies earthquakes and seismic waves Seismograph - instrument that detects and records seismic waves Seismograph - instrument that detects and records seismic waves Seismogram - the paper Seismogram - the paper recording of an earthquakes recording of an earthquakes seismic waves made by a seismic waves made by a seismograph seismograph

How do we measure earthquakes? We can measure an earthquake using two different scales. We can measure an earthquake using two different scales. Both scales measure two different characteristics of the earthquake. Both scales measure two different characteristics of the earthquake.

Richter Scale - measures earthquake magnitude Magnitude is the amount of energy released during an earthquake Magnitude is the amount of energy released during an earthquake For every increase of 1 on the Richter Scale, there is a release of 32 times more energy For every increase of 1 on the Richter Scale, there is a release of 32 times more energy For example, an 8.5 on the Richter Scale releases 32 times more energy than a 7.5 on the Richter Scale. For example, an 8.5 on the Richter Scale releases 32 times more energy than a 7.5 on the Richter Scale.

Mercalli Scale - measures earthquake intensity Intensity is how the earthquake feels to humans Intensity is how the earthquake feels to humans

Destruction by earthquakes

Tsunamis Tsunami - a large ocean wave that results from an underwater earthquake (as well as landslide or volcanic eruption) Tsunami - a large ocean wave that results from an underwater earthquake (as well as landslide or volcanic eruption) often responsible for more loss of life than the actual earthquake it stemmed from often responsible for more loss of life than the actual earthquake it stemmed from

More Destruction from Earthquakes Liquefaction - a temporary state in which loose soil and rock materials take on the property of liquid as a result of severe ground shaking Liquefaction - a temporary state in which loose soil and rock materials take on the property of liquid as a result of severe ground shaking Aftershock - a smaller earthquake that follows a larger earthquake (can be up to 1,000 aftershocks after an extremely large earthquake) Aftershock - a smaller earthquake that follows a larger earthquake (can be up to 1,000 aftershocks after an extremely large earthquake)

Liquefaction Damage in Peru, 1970

Earthquake damage in Alaska in 1964 (8.4 on the Richter Scale)

Damage from the San Francisco Earthquake of 1906 (8.3 on the Richter Scale)

Seismogram of the Virginia Earthquake August 23 rd on the Richter Scale

The earthquake originated in a region of VA known as the “Central Virginia Seismic Zone.” The earthquake was created by reverse faulting in this zone.

Damage from the VA Quake

The National Cathedral in D.C.

The Washington Monument

More on the VA Quake ac_l0tKaw&feature=related ac_l0tKaw&feature=related

Earthquakes: Faults and Folds

Two Types of Folds Anticline - an upfold in rock layers Anticline - an upfold in rock layers Syncline - a downfold in rock layers Syncline - a downfold in rock layers

Causes of Earthquakes Fractures - large breaks in the lithosphere Fractures - large breaks in the lithosphere Faults - a break in the lithosphere along which movement occurs Faults - a break in the lithosphere along which movement occurs Forces in the Earth’s crust cause rocks to bend and stretch. When the force becomes too great, and the rocks “elastic limit” is passed, the rock breaks. This is how an earthquake occurs. Forces in the Earth’s crust cause rocks to bend and stretch. When the force becomes too great, and the rocks “elastic limit” is passed, the rock breaks. This is how an earthquake occurs.

Three Types of Faults Normal Faults Normal Faults Reverse Faults Reverse Faults Strike-Slip Faults Strike-Slip Faults

1. Normal Fault found at divergent plate boundaries found at divergent plate boundaries occurs due to the force of tension (pulling rocks apart) occurs due to the force of tension (pulling rocks apart) hanging wall moves DOWN relative to the footwall hanging wall moves DOWN relative to the footwall Example: Mid-ocean ridges Example: Mid-ocean ridges

Normal Fault – Tension

2. Reverse Fault found at convergent plate boundaries found at convergent plate boundaries occurs due to the force of compression (pushing rocks together) occurs due to the force of compression (pushing rocks together) hanging wall moves UP relative to the footwall hanging wall moves UP relative to the footwall Example: Himalayan Mountains Example: Himalayan Mountains

Reverse Fault - Compression

3. Strike-slip Fault found at transform fault boundaries found at transform fault boundaries occurs due to the force of shearing (rocks pushed in different directions) occurs due to the force of shearing (rocks pushed in different directions) NO hanging wall or footwall, rocks slide past each other horizontally NO hanging wall or footwall, rocks slide past each other horizontally Example: San Andreas Fault in California Example: San Andreas Fault in California

Strike-Slip Fault - Shearing

REMINDER: The cause of MOST earthquakes is plates moving over, under, and around each other The cause of MOST earthquakes is plates moving over, under, and around each other

Seismic Waves

Seismic waves - vibrations of the Earth’s crust caused by an earthquake Seismic waves - vibrations of the Earth’s crust caused by an earthquake They are caused by the energy released when rocks break They are caused by the energy released when rocks break Focus - the point below the Earth’s surface where the earthquake first occurs (point at which the rock breaks) Focus - the point below the Earth’s surface where the earthquake first occurs (point at which the rock breaks) seismic waves are produced and travel outward from here seismic waves are produced and travel outward from here depth of the focus can affect the amount of damage depth of the focus can affect the amount of damage

Epicenter - point on the Earth’s surface directly above the focus Epicenter - point on the Earth’s surface directly above the focus most destruction occurs at the earthquake’s epicenter most destruction occurs at the earthquake’s epicenter you must have AT LEAST THREE seismic readings in order to determine the location of the epicenter of an earthquake you must have AT LEAST THREE seismic readings in order to determine the location of the epicenter of an earthquake

3 Types of Seismic Waves Primary Waves (P-wave) Primary Waves (P-wave) Secondary Waves (S-wave) Secondary Waves (S-wave) Surface Waves (L-wave) Surface Waves (L-wave)

1. Primary Waves (P-waves) cause rock particles to move back and forth cause rock particles to move back and forth described as push-pull waves described as push-pull waves fastest seismic wave fastest seismic wave travel through solids, liquids, and gases (faster through solids) travel through solids, liquids, and gases (faster through solids)

2. Secondary Waves (S-waves) cause rock particles to move at right angles cause rock particles to move at right angles slower than p-waves slower than p-waves travel only through solids travel only through solids

3. Surface Waves (L-waves) move outward from the epicenter move outward from the epicenter have an elliptical as well as back and forth motion have an elliptical as well as back and forth motion slowest seismic wave slowest seismic wave cause the most damage cause the most damage travel through solids, travel through solids, liquids, and gases liquids, and gases

Using seismic waves to map the Earth’s interior Scientists have found that at certain depths, the speeds and paths of seismic waves changes. These changes are due to density differences in the Earth’s interior. These changes mark the boundaries of Earth’s layers. Scientists have found that at certain depths, the speeds and paths of seismic waves changes. These changes are due to density differences in the Earth’s interior. These changes mark the boundaries of Earth’s layers. Follow along as we move inside, through the interior of the Earth, from the crust to the inner core…… Follow along as we move inside, through the interior of the Earth, from the crust to the inner core……

Mapping Earth’s Interior Mapping Earth’s Interior Moho Discontinuity - boundary between the crust and upper mantle where seismic waves speed up (they are going through a more dense layer) Moho Discontinuity - boundary between the crust and upper mantle where seismic waves speed up (they are going through a more dense layer) Asthenosphere – P and S waves slow down here, but then speed up again as they are transmitted through the solid upper and lower mantle Asthenosphere – P and S waves slow down here, but then speed up again as they are transmitted through the solid upper and lower mantle

Shadow Zone – wide zone of the Earth, opposite the focus, where seismograph stations do not detect seismic waves (This occurs from the bending of waves at the outer core) Shadow Zone – wide zone of the Earth, opposite the focus, where seismograph stations do not detect seismic waves (This occurs from the bending of waves at the outer core)

Mapping Earth’s Interior Outer Core - S-waves are stopped here because they do not travel through liquid and P-waves are slowed down and deflected creating the shadow zone Outer Core - S-waves are stopped here because they do not travel through liquid and P-waves are slowed down and deflected creating the shadow zone Inner Core - P-waves speed up again (more dense again) Inner Core - P-waves speed up again (more dense again)