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

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 - last

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.

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 kilometers

Steps to Earthquake Safety – Drop, cover and hold!!

Seismic safe structures – resistant to vibrations that occur during an earthquake Page 192-193 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 SafetyKnow your areaSecure your house, gas, water, power shut off

Have food, water, medications, first-aid for at least 1 week