stress

the force exerted when an object presses on, pulls on, or pushes against another object

Types of stress:

Shear

Compression

Tension

earthquake

shaking of the ground caused by the sudden movementof large blocks of rock along a fault

cause of an earthquake

the sudden release of stress in the lithosphere

the strength of an earthquake depends on what?

• the amount of stress built up before the rocks move along tectonic plates
• the distance that the rocks move along the fault

more earthquake facts

• they occur along fault lines (tectonic plate boundaries) through quick and sudden movements
• 80% of all earthquakes occur along the Pacific Ocean belt
• they occur in the lithosphere

fault

a fracture, or break in the Earth's lithosphere, along which blocks of rock move past each other; they are classified by how the rocks move along them

lithosphere

hard and rigid layer of earth (where rocks can break and move suddenly, causing an earthquake)

asthenosphere

weak or soft layer of earth (earthquakes do NOT usually occur here because of this)

types of faults

• normal
• reverse
• strike-slip

normal fault

 rocks PULL APART from each other; created by tension (ex: Great Rift Valley in Africa)

reverse fault

rock PUSH TOGETHER toward each other; created by compression (collision-zone boundaries)(ex: Himalaya Mountains)

strike-slip fault

rocks move SIDEWAYS (horizontally) on either side of the fault plane; created by sheer stress (ex: San Andreas Fault)

seismic waves

vibrations caused by earthquakes (similar to the ripple effects in a pond when you throw in a stone)

types of seismic waves

• primary (P waves)
• secondary (S waves)
• surface

Primary waves

(P waves)

• the fastest type
• first to reach a particular location after an earthquake occurs
• can travel through solids, liquids, and gasses
• they cause a "push-pull" effect as they pass through objects

Secondary waves

(S waves)

• second to reach a particular location after an earthquake occurs
• travel to the surface at about half the speed of P waves
• can travel through liquids and gasses (NOT solids)
• they cause the "shaken-up" effect as they pass through objects

Surface waves

• slowest moving
• move along Earth's surface (not its interior, like S and P waves)
• they cause a "ripple" effect on the Earth's surface
• cause the largest ground movements and the MOST damage

Focus

the point underground where the rocks begin to move; the starting pointof the earthquake (similar to the point where the rock plunks into the water)

epicenter

the point on the Earth's surface that is directly above the focus

Seismograph

an instrument that constantly records ground movements

seismometer

measures side to side types of movements (ball hanging from a wire)

how scientists locate an epicenter

• 3 seismic stations are needed
• find the difference an arrival times of both P and S waves at EACH of 3 stations
• time difference is then used to determine distance of epicenter from each station (the greater the time difference, the further away the epicenter)
• a circle is drawn around each station with a radius the distance equal to the distance the epicenter was to the station
• the point where the circles meet is the epicenter

earthquake magnitude

the range of energy an earthquake produces

Charles Richter & Beno Guttenburg

scientists that invented the Richter Scale in the 1930's to measure the magnitude of an earthquake

Richter Scale

measure how fast the ground moves at a seismic station when there is an earthquake

moment magnitude scale

• a newer more accurate scale that scientists now prefer
• for each whole number on the scale, an earthquake releases 32 times more energy

Scale measurements and their meanings

• 0-3.9: very minor earthquake; rarely noticed
• 4.0-4.9: light; slight damage
• 5.0-5.9: moderate; some structures damaged
• 6.0-6.9: strong; major damage to structures
• 7.0-7.9: major; some well-built structures destroyed
• 8.0+: great: major to total destruction

aftershock

smaller earthquakes that occur in an area shortly after a larger earthquake

liquefaction

when the shaking of the ground due to an earthquake causes soil to act like a liquid

tsunami

a gigantic wave caused by an earthquake along the ocean floor

How earthquakes can be predicted

Scientists monitor whether stress is building up in rocks along faults by paying special attention to:

• tilts or changes in the elevation of the ground
• slow movements or stretching in rock
• the development of small cracks in the ground

seismic gap

a section of a fault where few earthquakes occur as compared with sections of a fault on either side of the gap... this is an indication that stress is building up in this section and a major earthquake may be forthcoming

design structures so buildings can resist earthquake damage

• base isolators
• cross braces
• sheer core

base isolators

structures placed between a building and its foundation in a layer (like pancakes) to absorb the ground motion of an earthquake so that not as much shaking and vibration reaches that building

cross braces

pairs of braces that form an X shape

help a structure keep its shape while being shaken

sheer core

an elevator or stairwell built into the middle of a building to add strength to its core