Seismic Activity Flashcards
(20 cards)
Earthquake
Shaking if earth resulting from sudden release of built up seismic strain energy along a fault
Rock slowly deforms when stress is placed due to litho movement
Slow deformation may take years or centuries - strain energy being stored
Eventually strain becomes too much
-snaps back into original shape but in a new position relative to where it started
Sudden release stored up energy causes ground to shake and waves are sent out from this point - focus
Epicentre point on Earths surface directly above focus
Body waves
Travel through body of Earth in all directions
-including into its interior
Surface Waves
Travel only through lithosphere
Primary Waves
Body
Compressional Wave,
Pushing and pulling substance its travelling through
6kms-1 in litho
13kms-1 through core
Through solid and liquid
Little damage to infrastructure
Secondary Waves
Body
Distortion Wave
Cause movement up and down or side to side perpendicular to direction of travel
3.5kms-1
Do not travel through liquid
Cause more damage to infrastructure
Love Waves
Surface
Distortion Wave
Cause side to side movement perpendicular to travel
90% speed S waves
Only through lithosphere
Most damage to infrastructure
EQ at different margins
Shallow Foci : constructive, conservative and collision
Destructive = distance from trench increases along benioff zone - depth of earthquake foci increases
Foci deeper as plate subducts further
Impacts Of EQ
Seismic Shaking
Liquefaction
Tsunamis
Seismic Shaking
2010 Haiti, poorly constructed buildings - pancaking vertical concrete columns gave way causing floors to collapse flat
Many killed outright, those survived trapped in rubble
Emphasis in more developed countries to make regions life safe - Japan regulations
Intensity Seismic shaking determined by
Intensity partly determined by magnitude - mostly nature of land buildings are built - sediment amplify
Liquefaction
saturated soil loses strength and integrity as a result of shaking. Water pressure increases and sand particles in soil loses strength contact with one another
-soil begins to flow and move like liquid
When shaking stops water pressure falls again and soil becomes solid once more
Mostly coastal areas - saturated, lower density
Impacts Liquefaction
•foundations of buildings compromised beyond repair
•underground pipes and cables damaged
•upward water pressure may allow water to enter buildings via service ducts and cause water damage
•sloping ground may slide liquefied soil layer - large cracks and fissures in ground - damage human structures
•manhole covers may be lifted up above road level by liquefied soil layer
Tsunamis
Occur when eq on ocean floor cause vertical displacement of lithosphere.
Non-subducting plate springed back displacing column of water vertically
Deep Water~
very fast
low wave heights
very long wavelengths
As approach shore~
leading wave crest slows down
due to friction
wave crest behind it build up
wavelength reduced
dramatic increase to wave height
Tsunamis Impacts
Travel many kilometres inland
-destruction infrastructure (power of wave and debris)
Without adequate warnings hundreds of thousands of lives can be lost
-Boxing Day SEA 2004
Devastate coastal ecosystems, coral reefs and mangrove swamps
Attempts to predict
Seismic Gap
Dilation
Seismic Gap
Seismic Gap = section of fault that has not moved seismically over an unusually long time period
Hypothesis = given enough time, all parts of an active fault will move
If most parts of a fault have moved, section still try to move is where next large eq most likely
Seismic Gap Positive
Loma Prieta eq San Andreas Fault 1989.
By mid 80s 3 sections had not moved in over 20 years
San Francisco, Parkfield and Loma Prieta
When fault moved in 1989 it filled the Loma Prieta Gap
Seismic Gap Negative
May be other factors restricting movement.
•Unbroken stretch of fault may be stronger than those surrounding
•Movement in nearby segments relieved strain in seismic gap
E.g. Tokai SW Japan not yet been filled. Overdue movement however Mag 9 eq 2011 happened in Tohaku, NE Japan
Dilation Theory
Observations of rocks undergoing stress reveal that rock begins to dilate (expand) - result of micro cracks that develop.
Dilation = change in showered of seismic waves travelling through, changes in magnetic and electrical properties, increases in release of gas (radon)
If recognisable patterns can be observed prior to eq happening - may be possible to use them to predict future eq
Dilation Theory Test
Parkfield, California following 1989. Previous eq here in 1966.
Seismologists had to wait 15yrs until 2004 for medium-size earthquake - produced no precursors at all
