Earthquakes Chapter 6

Types of Stress Millions of year and the following forces cause change in shape and volume Tension – stretches rock, occurs where two plates are moving apart Compression – pushes rock together and squeezes until it folds or breaks, occurs when one plate pushes against another Shearing – causes rocks to slip or break apart, pushes rock in two opposite directions

Earthquakes and Plate Tectonics Earths crust is in constant motion due to plate movement Up to a certain point rocks will bend and stretch when a force is applied when force is to much then they break Faults – area where rock move and may break Earthquakes – produced when rocks break and cause vibrations Pacific Ring of Fire – the Pacific plate where 80% of all earthquakes occur

Pacific Ring of Fire

Types of Faults-along plate boundaries 1. Normal Fault – Rock above moves downward in relation to the rock below the fault Ex: Sierra Nevada’s  Hanging wall – block of rock above  Footwall – rock that lies below 2. Reverse Fault – Rock above fault moves up and over the rocks below Ex: Himalayans 3. Strike Slip Fault – Rocks moving past each other without much upward or downward movement Ex: San Andreas Fault of California other Regardless of their magnitudes, most earthquakes are the result of plates moving over, under and around each other

Changing Earth’s Surface Folding Earth’s Crust – collision of two plates  Anticline – fold in a rock that bends upward into an arch  Syncline – fold in a rock that bends downward to form a valley Stretching Earth’s Crust – two normal faults cut through a block of rock  Fault block mountain – parallel faults form Uplifting Earth’s Crust – forces that raise mountains  Plateau – large area of flat land elevated high above sea level

2.2 Seismic Waves – waves of energy produced during an earthquake, carries away Focus – point in Earth’s interior where the release of energy occurs, seismic waves are produced and travel from this point Epicenter – point on Earth’s surface directly above an earthquake’s focus Waves that travel outward from the focus: 1. Primary Waves – occur when the particles in rocks compress and stretch to transmit waves move in same direction - fastest 2. Secondary Waves – particles in rocks move at right angles to the direction of the wave - slowest Waves that move outward from the epicenter Surface Waves – move in an elliptical and back and forth swaying motion - lastlast

Measuring Earthquakes Three commonly used methods of measuring earthquakes of the more than 20 known Mercalli Scale – uses Roman numerals to rank earthquakes by how much damage they cause Richter Scale – measures the magnitude or the energy released from an earthquake based on the seismic waves Moment Magnitude Scale – estimates the total energy released by an earthquake

Measuring Earthquakes Seismologists – scientists who study earthquakes and seismic waves Seismograph – instrument to record primary, secondary and surface waves

The Richter scale is the most widely recognized way of indicating how powerful an earthquake was. The Scale measures seismic waves that are caused by the vibrations from earthquakes. This scale was developed by geophysicist Charles F Richter in 1935 at the California Institute of Technology. It is an open- ended scale, that is, it has no upper limit. Also it is a rare logarithmic scale, which means an earthquake measuring 6 on the scale is 10 times more powerful than one measuring 5, and 100 times more powerful than an earthquake of magnitude 4. The magnitude is measured in fractions of whole numbers eg an earthquake may measure 4.6 or 7.9. The Richter Scale however not used to express damage. An earthquake of magnitude 7.6, if it occurs in a sparsely populated area, will be less harmful than measuring 6.1 in a densely populated area. densely populated area.

Richter Scale The Richter scale is logarithmic, that is an increase of 1 magnitude unit represents a factor of ten times in amplitude. The seismic waves of a magnitude 6 earthquake are 10 times greater in amplitude than those of a magnitude 5 earthquake. However, in terms of energy release, a magnitude 6 earthquake is about 31 times greater than a magnitude 5

Locating an Epicenter Seismograph stations measure seismic waves (figure 14-8) The further apart the waves, the further away the epicenter is Three seismograph stations are needed to measure the epicenter. Distance from the station is measure then with two others it measures the direction of the epicenter Moho Discontinuity – boundary between Earth’s crust and the upper mantle; seismic waves travel slower above the Moho and faster below it due to density differences Shadow Zone – area where seismic waves cannot reach because Earth’s liquid outer core bends primary waves and stops secondary waves

Instruments that Monitor Faults – measures changes in elevation, tilting of the land surface and ground movements along faults Tiltometer – measures vertical movement Creep Meters – measures horizontal movement Laser Ranging Devices – measures horizontal movement GPS Satellites – ground based receivers measure changes in elevation and tilt of the land as well as horizontal movement along a fault

How earthquakes cause damage Shaking – worse over loose sand Liquefaction – turns into liquid mud Aftershocks – smaller earthquakes later Tsunamis- ocean waves generated by earthquakes, also called seismic sea waves  Begin over the earthquake focus, the height of the wave increases as it travels  May travel thousands of miles  Wavelength is several kilometerskilometers

Steps to Earthquake Safety – Drop, cover and hold!! Seismic safe structures – resistant to vibrations that occur during an earthquake  Page  Base Isolated Building - Rubber/steel blocks that buildings sit on  Cement pillars with reinforcing rods wrapped around them for highways Most loss of life occurs when people are trapped in crumbling structures

Earthquake Safety Know your area Secure your house, gas, water, power shut off Have food, water, medications, first-aid for at least 1 week