CHAPTER 12 EARTHQUAKES MOVEMENTS OF THE EARTH THAT ARE CAUSED BY A SUDDEN RELEASE OF ENERGY WHEN ROCKS MOVE ALONG A FAULT.

San Fransisco, CA (1906)

Anatomy of an Earthquake Focus - the location within Earth where the first motion occurs along a fault. 90% of earthquakes have a shallow focus. (< 70 km deep) Any given shallow focus earthquake will cause less damage than most deep focus earthquakes Shallow focus quakes cause the most destruction overall Epicenter- the point on the earth’s surface directly above the focus.

Seismic Waves As rocks move, the rocks release energy in the form of vibrations called seismic waves. These waves travel outward in all directions from the focus through the surrounding rock.

Two Main Categories of Waves BODY WAVES SURFACE WAVES Surface waves travel along the surface of a body rather than through the middle. Form from motion along a shallow fault or from the conversion of energy when P waves and S waves reach Earth’s surface. Slowest moving Cause the greatest damage Body waves travel through the body of a medium. P Waves or primary waves or compression waves Fastest; fastest waves (1st detected) Cause rock to move back and forth parallel to the direction the waves are traveling Can move through solids, liquids, and gases. Travel faster through more rigid materials. S Waves or secondary waves or shear waves Second fastest waves Arrive after P waves Cause rock to move side to side perpendicular to the direction in which the waves are traveling. Only move through solids.

Seismic Waves and Earth’s Interior Seismic waves can be used to understand earth’s interior. The composition of the material through which P and S waves travel affect the speed and direction of the waves. By studying the speed and direction of seismic waves, scientists can learn more about the makeup and structure of Earth’s interior.

Because the mantle is denser than the crust. Andrija Mohorovicic, 1909…discovered that the speed of seismic waves increases abruptly at 30 km beneath the surface of continents. Because the mantle is denser than the crust. Therefore, this marks the boundary between the mantle and crust. The depth of this boundary varies from 10 km under the oceans to 30 km under the continents. Earth is composed of 3 composition layers: crust, mantle, core Earth is composed of 5 mechanical layers: Lithosphere, asthenosphere, mesosphere, outer core, inner core.

Shadow Zones Shadow zones are locations on earth’s surface where no body waves (P and S) can be detected. Shadow zones exist because the materials that make up the earth’s interior are not uniform in rigidity (some are liquid; some are solid; furthermore, some are rigid and some are “plastic”). A large S wave shadow zone would be found covering the side of earth opposite the earthquake. The S waves cannot pass through the liquid outer core.

Earthquakes and Plate Tectonics Earthquakes are a result of stresses in Earth’s lithosphere. Most earthquakes occur at Convergent oceanic environments Convergent plate boundaries - colliding Divergent oceanic environments Divergent plate boundaries – spreading Continental environments Convergent, divergent, and transform boundaries

Most Earthquakes occur at Plate Boundaries

Earthquakes away from Plate Boundaries Not all earthquakes occur at active known plate boundaries; sometimes ancient fault zones are buried and result in earthquakes. 1811/1812…New Madrid, Missouri Felt as far away as S. Carolina

Studying Earthquakes Seismology Earthquakes are the best tool for investigating Earth’s internal structure and dynamics. Seismometer/Seismograph – equipment that can detect and record the vibrations of an earthquake. Seismogram – the tracing of vibration intensity recorded by a seismograph. P waves are recorded first, then S waves.

Locating an Earthquake Determining the epicenter of an earthquake Triangulation 3 seismic stations are used P Waves travel faster than S Waves The arrival of P Waves and S Waves are recorded The longer the lag time between the P and S Waves, the farther away the earthquake.

Earthquake Measurement Measure the amount of energy released Measure the amount of damage caused Magnitude = the measure of the strength of the earthquake. Richter Scale measures magnitude, 20th century Moment Magnitude Scale, 21st century More accurate for large earthquakes Measures the strength based on the size of the area of the fault that moves, the average distance that the fault blocks move, and the rigidity of the rocks in the fault zone. The larger the number, the stronger the earthquake. Largest recorded…9.5 Magnitudes of less than 2.5 are usually not felt by people.

Intensity: Modified Mercalli Scale Before the development of magnitude scale, the size of an earthquake was determined based on the earthquake’s effects = intensity. Expresses intensity in roman numerals. From I to XII and provides a description of the effects caused by the earthquakes. XII – highest intensity and total destruction. Depends on the earthquakes magnitude, the distance between the epicenter and the affected area, the local geology, and the earthquake’s duration.

Modified Mercalli Intensity Scale

Expected Damages Most damage is caused by the collapse of buildings or falling objects and flying debris. Other damage is caused by landslides, fires, explosions (broken electric and gas lines), and floodwater (collapsing dams). CAN YOU THINK OF ANY OTHER DAMAGE?

Tsunamis Caused by an earthquake that occurs on the ocean floor. The shifting ocean floor creates a Tsunami (giant ocean wave) as energy is transferred.

Earthquake Warnings and Forecasts Scientists are trying to predict where earthquakes might occur. They think that areas along faults that have NOT shown movement (seismic gaps) might be likely areas for future quakes since the stress may be building in these areas.

Prediction of Earthquakes Foreshocks – little earthquakes that may precede the main event. May be a few seconds up to a few weeks prior to main event. Sensors may also detect slight tilting of the ground and may be able to detect the stress and strain building along fault zones. Predicting earthquakes is mostly unreliable.