EARTHQUAKES . Earthquakes 8.1 What Is an Earthquake? Focus is the point within Earth where the...

EARTHQUAKESEARTHQUAKES

Earthquakes

Earthquakes

8.1 What Is an Earthquake?

• Focus is the point within Earth where the earthquake starts.

• Epicenter is the location on the surface directly above the focus.

An earthquake is the vibration of Earth produced by the rapid release of energy

Focus and Epicenter

• Faults are fractures in Earth where movement can occur. Usually where two plates meet.

Faults

Focus, Epicenter, and Fault

Slippage Along a Fault

Cause of Earthquakes


Elastic Rebound Hypothesis• rocks are deformed at faults due to pressure as

tectonic plates move past, subduct or converge. Eventually the rocks ‘slip’ releasing energy via the process of elastic rebound.

• When the strength of the rock is exceeded, it suddenly breaks, allowing the release of energy and causing the vibrations of an earthquake.

• Most earthquakes are produced by the rapid release of elastic energy stored in rock that is under great amounts of pressure

Elastic Rebound Hypothesis


• An aftershock is a small earthquake that follows the main earthquake. The plates or rocks readjust themselves after the ‘slip’ or elastic rebound has occurred. Can cause further damage to buildings weakened by main quake.

• A foreshock is a small earthquake that often precedes a major earthquake. Can serve as a warning, but may happen years before a major earthquake.

Aftershocks and Foreshocks

Earthquake Waves

8.2 Measuring Earthquakes

Seismographs are instruments that record earthquake waves, use information from the seismometer.

Seismometer: used to measure seismic waves. Measure waves along the x,y, and z axis.

Seismograms are the paper record of the earth’s movement

Seismograph

Seismogram

Earthquake Waves


Body Waves

• P waves (primary wave- first to arrive)

• Identified as P waves or S waves

- Have the greatest velocity of all earthquake waves (4-7km/s)

- Are push-pull waves that push (compress) and pull (expand) in the direction that the waves travel (like pushing a stretched slinky)

- Travel through solids, liquids, and gases

Seismic Waves

Earthquake Waves


Body Waves• S waves (secondary waves- second to arrive)

- Seismic waves that travel along Earth’s outer layer

- Slower velocity than P waves (2-5km/s)

- Shake particles at right angles to the direction that they travel (like a shaking a stretched rope)

- Travel only through solids

Seismic Waves

Earthquake Waves


Surface waves (the slowest and most destructive

• Love Waves

• Identified as Love waves or Rayleigh waves

- Tend to knock buildings off their foundations and highways bridges off their supports due to side to side movement

- Move from side to side perpendicular to the direction of the wave

- Travel through solids

Seismic Waves

Earthquake Waves


Surface waves (the slowest and most destructive

• Rayleigh waves

• Identified as Love waves or Rayleigh waves

- Incredibly destructive as they produce more ground movement and take longer to pass

- Behave like rolling ocean waves

Seismic Waves

Seismogram

Locating an Earthquake


Earthquake Distance

• Travel-time graphs from three or more seismographs can be used to find the exact location of an earthquake epicenter.

• The epicenter is located using the difference in the arrival times between P and S wave

recordings, which are related to distance.

• About 95 percent of the major earthquakes occur in a few narrow zones.

Earthquake Direction

Earthquake Zones



Measuring Earthquakes


Historically, scientists have used two different types of measurements to describe the size of an earthquake—intensity and magnitude.

Intensity

• a measure of the energy released during the earthquake, calculated by height of wave

• a measure of the earthquakes effect on people and buildings

Magnitude

Measuring Magnitude


Richter Scale

• Does not estimate adequately the size of very large earthquakes (above 6)

• 10 base logarithmic scale based on the amplitude of the largest seismic wave

• each whole number jump (ie 56) is a 10 fold increase.

• so a jump from 13 is 100 fold increase, 14 is 1000 fold increase

Measuring Intensity


Modified Mercalli Intensity Scale

• intensities are expressed as roman numerals

• higher number equals higher damage to buildings and infrastructure

• Drawbacks are that intensity lessens with distance, different distances will report different intensities

• As well, building damage can change depending on the geologic structure of area

Measuring Earthquakes- New Method


Momentum Magnitude

• Derived from information about the strength of rock, surface area of the ‘slip’ and the amount of rock displaced

• Moment magnitude is the most widely used measurement for earthquakes because it is the only magnitude scale that estimates the energy released by earthquakes.

• Measures very large earthquakes accurately

Measuring Earthquakes- New Method


Momentum Magnitude

• take a look at your text book, pg 83, notice that there are differences between the Richter magnitude and moment magnitude

• despite the differences, magnitude is always a better measurement because it can determine earthquake strength even when no buildings or people are present

• scientists use readings from different seismograms when reporting earthquake magnitudes

Earthquake Magnitudes

Some Notable Earthquakes

Earthquake hazards in canada

Damage can take several forms

8.3 Destruction from Earthquakes

Ground motion: buildings and bridges collapse Fire: gas lines can break or oil tanks can ignite Landslides Displacement of land Aftershocks: can cause weakened structures

to collapse Tsunamis: underwater earthquakes can cause

massive waves that damage coastal areas

Earthquake Damage

Seismic Vibrations


ground motion

- The design of the structure

- Materials used to build it; stone or brick buildings are the most dangerous

- Nature of the material upon which the structure rests

• Factors that determine structural damage

- Intensity of the earthquake

Seismic Vibrations


ground motion

- The taller the building, the more flexible it is

- Shorter buildings are ‘stiffer’ and resist swaying making them more susceptible to fracturing

• seismic waves cause buildings to sway with the energy of the waves

- Tall buildings are safer than short ones

Seismic Vibrations


Ground motion

Building Design- base isolators

As the ground movesback and forth, the base isolatorsdistort and absorb the motion,significantly decreasing the motionof the building.

Base isolatorStandard foundation

Seismic Vibrations


base isolators

Seismic Vibrations


Building location

- Bedrock absorbs more wave energy and are the most stable

- Softer soils transfer more energy to the buildings causing more damage

- If softer soils have water in them, they can become a little like quicksand during an earthquake.

Seismic Vibrations


Resonant frequency

• If a seismic wave hits a building with a frequency that matches that structure's natural sway then damage increases!

• In physics terms, the building has the same resonant frequency as the wave.

• When this happens, multiple waves at the resonant frequency pass through the structure, amplifying each other. Making a very destructive force.

Seismic Vibrations


Liquefaction

• buildings may sink

• Underground objects (rocks or water) may float to surface

• seismic waves can squeeze saturated soil causing it to behave like a liquid

Effects of Liquefaction



Liquefaction during the 1964 Niigata, Japan earthquake (7.6)



• With many earthquakes, the greatest damage to structures is from landslides and ground subsidence, or the sinking of the ground triggered by vibrations.

Landslides

• In the San Francisco earthquake of 1906, most of the destruction was caused by fires that started when gas and electrical lines were cut.

Fire

Landslide Damage

Date post:	24-Dec-2015
Category:	Documents
Upload:	peter-vincent-kennedy
View:	249 times
Download:	0 times

EARTHQUAKES . Earthquakes 8.1 What Is an Earthquake? Focus is the point within Earth where the...

Documents