How do you usually describe a plane (with lines)? In ... · • strike is the orientation of the...
Transcript of How do you usually describe a plane (with lines)? In ... · • strike is the orientation of the...
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Describing the fault geometry
How do you usually describe a plane (with lines)?
• strike• dip
In geology, we choose these two lines to be:
strike
dip
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• strike is the orientation of the line where the fault plane intersects the horizontal plane
• dip is the orientation of the line on the fault plane that is perpendicular to strike. It makes the steepest angle with respect to the horizontal (a ball would roll down it).
strike
dip
“N 60° E”
“30° (to the SE)”
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Profile view, as often shown on block diagrams
30°strik
e
“footwall”
“hanging wall”
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Categories of Faults
• “Normal” fault
• “Thrust” or “reverse” fault
“Strike-slip” or “transform”
faultsslip is parallel to fault strike
“Dip-slip” faults:slip is parallel to fault dip
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http://geology.csupomona.edu/janourse/TectonicsFieldTrips.htm
“Thrust” or “reverse” fault
“Strike-slip” or “transform”
fault
“Normal” fault
Assume we are looking along strike (like the diagrams)
This one may be an “oblique” fault: normal plus SS (in and out of the screen)
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http://geology.csupomona.edu/janourse/TectonicsFieldTrips.htm
“Normal” fault
“Thrust” or “reverse” fault
“Strike-slip” or “transform”
fault
Assume we are looking along strike (like the diagrams)
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“Normal” fault? “Thrust” or “reverse” fault?“Strike-slip” or “transform”fault?
1999 Izmit earthquake scarp
there’s just one answer here, can we really tell?
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Right-lateral(dextral)
Left-lateral(sinistral)
Two kinds of strike-slip faults
Stand with your feet on either side of the fault. Which side comes toward you when the fault slips?
Another way to tell: stand on one side of the fault looking toward it. Which way does the block on the other side move?
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Right-lateral(dextral)
Left-lateral(sinistral)
1992 M 7.4 Landers, California Earthquake rupture (SCEC)
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dry stream channel
fault 1 km
1 km in 50,000 years= 20 mm/yr
during this time, ~200major earthquakes
Fault slip over geologic time
LL or RL?
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San Andreas Fault from above
cumulative offset from many earthquakes can be tens of kilometers or more
right lateral or left-lateral?
strike-slip faults can be hard to see from above -
why?
offsets of features with different ages...
choosing where to dig a trench and find the fault
surface
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Close-up view: gouge and breccia in the fault zone (major, active fault)
The Elsinore fault, a strike-slip fault in California: powdery gouge in bedrock
Close-up of breccia: broken up and re-cemented rock. Not as powdered as gouge.
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Where is the Fault?
Is it active?
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Trenching in Seattle: Looking for the fault in soil, and getting timing and size
of prehistoric earthquakes
No road cut? Dig a trench (DIY roadcut)
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http://earthquake.usgs.gov/regional/pacnw/ships/results/tacoma.html
Sometimes we just canʼt see faults,especially if they do not have a recently
formed scarp
LIDAR (LIght Detection And Ranging) imaging strips the trees, houses, etc.
Now we can see the (obscured, partly eroded) Tacoma Fault scarp
This shows geologists where to dig a trench...red lines are branches of the
fault exposed in a trench
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Another LIDAR example from Seattle
??
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How do we know the geometry of faults at depth?
Next week: Prof. Bostock will explain one way we find out a lot about subsurface structure without drilling (examples from our region: Cascadia)
SAFOD: San Andreas Fault Observatory at Depth. Valuable, but prohibitively expensive for most places!