EARTHQUAKES

1. EARTHQUAKES Chapter19

2. Seismology refers to the branch of Geology that studies earthquakes WHAT IS AN EARTHQUAKE? • When stress builds in rocks, they reach their elastic limit and eventually break. At the breaking point, an earthquake is “born.” • Earthquakes are natural vibrations of the ground caused by movement along gigantic fractures in Earth’s crust. • Earthquakes can also be caused by movement related to volcanic eruptions.

3. Stress – The forces per unit area acting on a material • Three types: 1. Tension – pulls material apart 2. Compression – decreases volume of material 3. Shear – causes a material to twist

4. Stress effects on material • All stresses deform the material. • The deformation of material is called Strain. Types of Strain: • Elastic Strain – material will deform but then go back to it’s original shape when stress is removed (think rubber bands) • Ductile Strain – material will stay deformed when the stress is removed

6. Types of Faults • A fault is a break or fracture in rock that occurs when stress is applied too quickly or is too great. • The type of fault depends on the type of stress • Normal Fault • Reverse Fault • Strike Slip (Lateral) Fault

7. A normal fault results because of tension forces pulling plates of rock apart. Normal Faults are fractures caused by horizontal tension. Movement along Normal Faults are partly horizontal and partly vertical. The horizontal movement along a normal fault extends the crust. Normal fault

8. A Reverse Fault results because of compression forces push plates of rock together. Reverse faults are fractures as result of horizontal compression. Compressional forces result in a horizontal shortening of the crust. Reverse Fault

9. Strike Slip Faults result because of shear forces when plates slide past one another. Movement along Strike Slip Faults are mainly horizontal. A famous Strike Slip Fault, the San Andreas Fault in California, is responsible for many of the earthquakes to strike California. Strike Slip (Lateral) Fault

10. Seismic Waves • The vibrations of the ground during an earthquake are called seismic waves. • There are three types of seismic waves: • Primary Waves (P-waves) • Secondary Waves (S-waves) • Surface Waves (L-waves) • The waves come from the focus of the earthquake, or the point where the earthquake originates. The focus is typically several kilometers below the Earth’s surface. • We can find the point of origination on the Earth’s surface directly above the focus. This is referred to as the epicenter.

11. Primary Waves (P-waves) • Squeeze and pull rocks in the same direction along which the waves are traveling • Pass through Earth’s interior • Also referred to as “body waves” • Always arrive first at a seismic facility, they are the quickest moving waves

12. Secondary Waves (S-waves) • Cause rocks to move at right angles in relation to the direction of the waves • Pass through Earth’s interior but CANNOT travel through liquid • Also referred to as “body waves” • Second type of wave to reach the seismic facility

13. Surface Waves (L-waves) Moves in two directions as they pass through rock Up and down movement, similar to ocean waves, occurs as wave travels through rock Also causes rocks to move from side to side Travel along the Earth’s surface Most destructive of all earthquake waves Last wave to arrive at the seismic facility

14. Locating an Epicenter Can you locate the epicenter? Rules to Remember: • To determine the location of the epicenter, you must determine the travel times of P-waves in relation to the S-waves. • The further apart the P wave is from the S wave, the further from the epicenter the seismograph is. • A seismograph or seismometer is an instrument used by seismologists to measure the waves moving out from an earthquake. • Information is needed from three different seismograms (the record produced) to find the epicenter of an earthquake. Circles are drawn from each of the three stations, and the location where all three intersect is the epicenter.

15. Measuring Earthquake Activity • More than one million Earthquakes occur each year! However, 90% of these are never felt and cause little, if any, damage. • The amount of energy released during an earthquake is its magnitude. • Richter Scale: numerical rating based on the size or amplitude of the largest wave. For every increase of 1 on the scale, wave is 10 times greater in size and the energy released is 32 times greater. (So an 8.5 earthquake will release 32 times more energy than a 7.5 earthquake.) • The size of the fault rupture, the amount of movement along the fault, or the rock’s stiffness is measured on the Moment Magnitude Scale. Several types of waves are used to determine a true reading. • A measure of the intensity of an earthquake is determined using the Modified Mercalli Scale. (How much damage is done to structures.) The higher the roman numeral, the worse the damage.

16. Clues to the Earth’s Interior • Most knowledge and understanding of the Earth’s interior comes from the study of seismic waves. • Seismic waves change speed and direction when they encounter different materials. • When traveling through the mantle, P and S waves follow direct patterns. • P-waves strike core and some move into the core while others are bent outward from the core. The area left to the side of the core, where no P-waves travel, is called the P-wave shadow zone • S-waves also strike the core but cannot travel through liquid so they are deflected off the sides of the core. The area where no S-waves travel is called the S-wave shadow zone

17. So What Do We Know? • Due to the travel pattern and behaviors of the seismic waves within the Earth, we are able to determine the following: • The lithosphere, which includes the crust and upper mantle, is made of primarily igneous rocks granite, basalt, and peridotite and is dense and coarsely grained. • The asthenosphere, or partially melted mantle, is thought to be peridotite. • The lower mantle is solid and is composed of simple oxides such as iron, silicon, and magnesium. • The inner part of the core is extremely dense and made of a mixture of iron and nickel.

18. Earthquake Safety • Earthquakes generally occur in areas that have had them before, are close to plate boundaries, and without warning. • Seismic Risk refers to the probability of an earthquake due to past seismic activity. • Seismic Gaps are sections of active faults that have not experienced an earthquake over long periods of time. • The probability of earthquake’s occurring is based on two factors: • History of Earthquakes in the area • The rate at which strain builds up in the rocks

19. Earthquake Hazards • The damage of an earthquake is directly related to the strength or quality of the structures involved: Structural Failure • Pancaking – when the supporting walls of the ground floor give and the upper levels subsequently fall onto the lower floors. • Building Height – when between 5 and 15 stories collapse due to the shaking of the ground being “on track” with the natural sway of the building. (Ground vibrations are too quick for taller buildings and too slow for smaller buildings.) Land and Soil Failure • Seismic vibrations cause subsurface materials to liquefy and behave like quicksand, generating landslides or “sinkhole” type structures. Fault Scarps • Fault movements associated with earthquakes can produce areas of great vertical offset. Tsunami • A Large ocean wave generated by vertical motions of the seafloor during an earthquake.