EARTHQUAKES Dr R B Schultz Global Earthquake Locations

EARTHQUAKES Dr. R. B. Schultz

Global Earthquake Locations

What are Earthquakes? • The shaking or trembling caused by the sudden release of energy • Usually associated with faulting or breaking of rocks • Continuing adjustment of position results in aftershocks • Shaking of earth due to movement of rocks along a fault.

• Rocks under stress accumulate strain energy over time. • When stress exceeds strength of rocks, rock breaks. • Strain energy is released as seismic waves. The longer that energy is stored up and is maintained without release, the more likely that a strong earthquake will occur.

The animation below of a subduction zone, where an oceanic plate is converging with a continental plate, attempts in cartoon fashion to show the deformation of the continental plate that precedes each earthquake.

What is FAULT? • A break or crack in rock along which movement of the earth’s crust occurs. • Can be visible on the crust or can be far underground. • Can be only a meter long or they can extend for hundreds of kilometers.

FAULT

Earthquakes can happen along any type of plate boundary. They also occur along faults which are large cracks in the earth’s crust. Most faults are associated with large plate boundaries where violent earthquakes usually occur.

Movement of rock along a fault can release a tremendous amount of energy: This movement can release three main forces on adjoining areas of the crust: 1. Tension – results when a system of forces pulls or stretches an object from opposite sides. 2. Compression – results when a system of forces pushes an object from opposite sides. 3. Shearing - results when a system of forces is applied to an object in different directions.

What is the Elastic Rebound Theory? All rocks have an elastic limit. If the forces acting on these rocks exceed this limit, or overcome the friction between blocks, the crust snaps and an earthquake occurs.

What is the Elastic Rebound Theory? • Explains how energy is stored in rocks – Rocks bend until the strength of the rock is exceeded – Rupture occurs and the rocks quickly rebound to an undeformed shape – Energy is released in waves that radiate outward from the fault

KINDS OF FAULT 1. Normal Fault § one block can move down and the other can move up, or vice versa. § occurs where forces tend to pull section of the earth’s crust apart. § tends to follow the gravitational pull on the fault blocks involved.

§ The fault plane on the normal fault is generally very steep. In a normal fault the two involved blocks are (by gravity) pulling away from one another causing one of the fault blocks to slip upward and the other downward with respect to the fault plane (it is hard to determine whether both or just one block has moved. ).

§ The exposed upward block forms a cliff-like feature known as a FAULT SCARP. A scarp may range from a few to hundreds of meters in height and their length may continue for 300 or more kilometers (around 200 miles). § FAULT SCARP – landforms that form cliffs and ridges along the fault. (book definition)

Normal Fault

KINDS OF FAULT 2. Thrust Fault § one block moves over another block § occurs where forces tend to push sections of the crust together. § When thrust faults are exposed on the surface overburnden material lies over the main block. They are normally associated with areas of folded surfaces and or mountaineous regions. The dip angles of thrust faults are normally not as steep as a normal fault.

Thrust Fault

KINDS OF FAULT 3. Strike-slip Fault (Transcurrent Fault) § the movement of the blocks is horizontal § known as the San Andreas fault of California. This fault marks the margin line between the Pacific and North American Plates. § On the surface, scarps form as hills crossing the fault zone are torn apart by movement over time.

• Actually anything crossing this fault zone is either slowly torn apart, or offset. • Rivers crossing the fault line are called offset streams and are classic signatures of fault activity along the San Andreas. • These faults can be very long, the San Andreas is nearly 600 miles long.

Strike-slip Fault

Three Types of Faults Strike-Slip Thrust Normal

EARTHQUAKE AND PLATE TECTONICS The lines on the map indicate plate boundaries.

The Focus and Epicenter of an Earthquake • The point within Earth where faulting begins is the FOCUS, or hypocenter • The point directly above the focus on the surface is the EPICENTER

Three Types of Focus 1. Shallow-focus v earthquake originates between the earth’s surface and a depth of 70 kilometers below the surface. v earthquakes in this class are detected at all three types of plate boundaries-trenches, ridges and fault boundaries. v most earthquakes have shallow focus.

Three Types of Focus 2. Intermediate-focus v earthquake originates between 70 and 300 kilometers beneath the surface. 3. Deep-focus v earthquake originates 300 to 700 kilometers below the surface. v Both occurs at trench boundaries, where one plate moves under another.

Detecting Earthquakes • An earthquake’s energy moves from the focus as Seismic Waves. (Siz’mik) • These waves travel to all directions. • An earthquake generates 3 kinds of seismic waves. • These waves are: • P-waves (Primary) • S-waves (Secondary)

Waves • In the illustration below, P-waves have the movement like that of Slinky. While Swaves have the movement of a snake or a rope. L-waves has movements of an ocean wave.

P or Primary Wave • The fastest seismic wave • Can travel through any material on earth- solid or liquid • P-waves compress and expand particles that lie on their path

S or Secondary Wave • It moves at half the speed of P-waves • Can travel only through solids • Movement within the earth resembles to wavelike motion that results when one shakes a stretched rope from side to side.

L or Surface Wave • • The slowest wave Moves along the earth’s surface Causes most damage among all waves Resembles thw motion of ocean waves.

Using Seismographs to Find the Epicenter • Seismograph- A sensitive instrument that measures and records seismic waves.

How does it work? • A seismograph has a heavy weight suspended from a support that is attached to bedrock. During an earthquake, inertia keeps the weight and the pen attached to it still. When a seismic wave passes, the support, base and revolving paper drum shake. Thus, the pen marks zigzag lines on a paper drum, recording the movements of the earth. The recorded lines are the. . .

Seismogram • A recording or the recorded lines of a seismograph. • In the figure below, there are 3 distinct groups of zigzag lines that correspond to the different kinds of seismic waves.

The Epicenter and the Seismogram • Scientists analyze the time difference between the arrival of the P and S waves. • By doing so, they can calculate the distance to the earthquake’s epicenter.

How do they do it? • Note: Please refer to the pictue at the past slide. • Readings from 3 seismograph stations are needed to locate the epicenter of the earthquake. Assume that a scientist calculates the distance from Station A to the epicenter to be 1000 km. The distance is determined on the difference of arrival time of the 2 waves. The scientist knows the distance of A to the epicenter. The scientist draws a circle on a map which has a radius the equivalent of 100 km. Meanwhile, Stations B and C determines the distance to be at 500 and 400 km, respectively. Again, they draw circles. The meeting point of the circles is the epicenter.

Seismic Waves and The Earth’s Interior • The seismic waves that result from an earthquake spread throughout the earth. • Geologists study the speed and the paths of seismic waves to learn about the earth’s interior. • Our knowledge of the earth’s layers relied on the changes in speed and direction of seismic waves.

Seismic Waves and The Earth’s Interior • Early in the century, geologists discovered that large areas on the side of the earth away from the earthquake focus do not receive waves. Look at the image below.

P-Waves and the Earth’s Interior • In the case of P-waves, they can pass through the mantle, the liquid outer core, and the solid inner core. Notice the small shadow zone at the image. It has been stated that P-waves can pass through all particles.

S-Waves and the Earth’s Interior • While at the S-waves, a large shadow zone is formed. Again, it has been staterd that S-waves can only pass through solids. • This discovery has enabled scientists to conclude that the outer core is liquid.

Types of Earthquake 1. Tectonic type- earthquake that result from the upward and downward movements of the crustal plates due to pressure. 2. Volcanic type- earthquakes produced by the pressure generated by magma rising through faults towards the earth’s surface.

Intensity • Intensity- measure of an earthquake’s effect or damage in a place. • Modified Mercalli Scale named after Guiseppe Mercalli is used as a basis for the intensity of an earthquake.

Modified Mercalli Scale Intensity Scale- Description I-Not felt II-Felt by persons at rest III- Felt indoors IV- hanging objects swing V- felt outdoors VI- felt by all

Modified Mercalli Scale Intensity Scale- Description VII- difficult to stand VIII- steering of cars affected IX- general panic X-most masonry and frame structures destroyed with their foundations XI- rails bent gently XII- damage nearly total

Magnitude & Richter Scale Magnitude- measures the energy released during quake by measuring the amplitude of the largest wave recorded on the seismogram Richter Scale- named after Charles Richter used as basis for the magnitude. The scale is logarithmic, one point increases in the scale means tenfold increases

Richter Scale Magnitude Scale- Description 1 - only detectable when an ultrasensitive seismometer 2 - hardly susceptible shocks 3 - very feeble shocks 4 - feeble shocks 5 - earthquakes with moderate strength

Ritcher Scale Magnitude Scale- Description 6 - strong earthquakes 7 -major earthquakes 8 - great earthquakes 9 -never occurred

How are the Size and Strength of an Earthquake Measured? • • Intensity – subjective measure of the kind of damage done and people’s reactions to it – isoseismal lines identify areas of equal intensity Modified Mercalli Intensity Map – 1994 Northridge, CA earthquake, magnitude 6. 7

Measuring Earthquakes The power (magnitude) of an earthquake is measured on the Richter scale, using an instrument called a seismometer. The Richter scale is numbered 0 -10 with 10 being the most powerful. The Richter scale is logarithmic – an earthquake measuring 7 is 10 times more powerful than one measuring 6 and 100 times more powerful than one measuring 5. Up until 2 on the Richter Scale only instruments will detect the earthquake. Earthquakes above 6 cause serious damage and sometimes many deaths The Mercalli scale measures the damage caused by an earthquake. The Mercalli scale goes from I to XII e. g. “VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage. ”

How are the Size and Strength of an Earthquake Measured? Magnitude › Richter scale measures total amount of energy released by an earthquake; independent of intensity › Amplitude of the largest wave produced by an event is corrected for distance and assigned a value on an open-ended logarithmic scale

Effects Of Earthquakes: The Richter Scale • After an earthquake, its description usually includes a number from 1 to 9 in the Richter Scale. • This number indicates the magnitude or the total amount of energy released.

Charles F. Richter: The Inventor • He was a geologist stationed at the California Institute Of Technology. • Before he invented it, earthquakes were measured at a much less scientific scale. Personal observations were made, and scientists had to use this to base their conclusions. • This method greatly reduced the guesswork in determining the magnitude of an

What do earthquakes do to the Earth? Earthquakes do a lot of effects on the earth. These include: • Structural damage Benito Juarez Hospital, Mexico City. (taken in 1985)

What do earthquakes do to the Earth? • Tsunami, a Japanese word meaning harbor wave. These are really different from the actual harbor waves, but can do a significant amount of damage.

What Controls the Level of Shaking? • Magnitude – More energy released • Distance – Shaking decays with distance • Local soils – amplify the shaking

Is there such a thing as “Earthquake Weather”? ? ?

Earthquake Effects - Ground Shaking Northridge, CA 1994

Earthquake Effects Ground Shaking Northridge, CA 1994

Earthquake Effects - Ground Shaking Loma Prieta, CA 1989 KGO-TV News ABC-7

Earthquake Effects - Ground Shaking Kobe, Japan 1995

Earthquake Effects - Surface Faulting Landers, CA 1992

Earthquake Effects - Liquefaction Source: National Geophysical Data Center Niigata, Japan 1964

Earthquake Effects - Landslides Source: National Geophysical Data Center Turnagain Heights, Alaska, 1964 (upper left inset); Santa Cruz Mtns, California , 1989

Earthquake Effects - Fires Loma Prieta, CA 1989 KGO-TV News ABC-7

Earthquake Effects - Tsunamis 1957 Aleutian Tsunami Photograph Credit: Henry Helbush. Source: National Geophysical Data Center

Earthquake Preparedness Before- develop a disaster plan; secure your home; store flammable; pack survival kit lightly; and conduct

Earthquake Duringcover your head; Preparedness look for a safe place to hide temporarily; stay in an open area; go to a higher ground for tsunami alert; and move away from steep elevations.

Earthquake After- exit building calmly; Preparedness see if you have injuries; and do not enter a building in attempt to save someone.

QUIZ 1. Which activities are earthquakes usually associated with? 2. What is a fault? 3 -5. Name the 3 movements of rock. 6. What causes these movements? (2 words. ) 7. Which theory explains the cause why earthquakes occur?

QUIZ 8 -10. Name all types of faults. 11. There are 3 types of focuses, the Shallow, _______, and the Deep. 12. What is the slowest wave? 13. Name the instrument that detects earthquakes and its recordings. 14. At least how many circles are needed to locate the epicenter?

QUIZ 15. Which wave can penetrate the cores? 16. Which wave cannot penetrate the outer core? 17. Who invented a scale used to describe an earthquake? 18 -19. Which main effects were described here? 20. Name at least an engineering technique given.

Thank you for viewing and have a good day!

References: • www. google. com • Integrated Science book

QUIZ Fill in the blanks: 1. Earthquake is the shaking or trembling caused by the sudden release of ____. 2. The point within Earth where faulting begins is the FOCUS, or ______. 3. Shaking of earth due to movement of rocks along a _____.

QUIZ 4. Can be visible on the ____ or can be far underground. 5. ____ is where an oceanic plate is converging with a continental plate. 6. San Andreas fault is what kind of fault? 7. _______ results when a system of forces pushes an object from opposite sides.

QUIZ 9. If the forces acting on these rocks exceed this limit, or overcome the ____ between blocks, the crust snaps and an earthquake occurs. 10. One block can move down and the other can move up, or vice versa is a _______ fault. 11. The point directly above the focus on the surface is the _____.

QUIZ 12. ______ focus earthquake originates between the earth’s surface and a depth of 70 kilometers below the surface. 13. _____ results when a system of forces is applied to an object in different directions. 14. The exposed upward block forms a cliff-like feature known as a ______.