Further topics in tectonics: Focal mechanisms Pac NW tectonic setting Shape of mid-ocean ridges...
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Further topics in tectonics:
1.Focal mechanisms
2.Pac NW tectonic setting
3.Shape of mid-ocean ridges (depth of the ocean)
4.Driving forces of plate tectonics
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Elastic Rebound
1. Focal mechanisms
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No offset
No offset
Earthquake break
1. Focal mechanisms
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Volume increase (dilation)
Volume decrease
(compression)
Volume increase (dilation)
Volume decrease
(compression)
1. Focal mechanisms
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Direction of P-wave first motion
1. Focal mechanisms
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Direction of P-wave first motion
1. Focal mechanisms
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Nodal Plane
wave is neither in nor out – it has zero amplitude
1. Focal mechanisms
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C D
D C
Right lateral
1. Focal mechanisms
Two fault planes will produce the same motion
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C D
D C
C
CD
D
Right lateral Left lateral
1. Focal mechanisms
Two fault planes will produce the same motion
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C D
D C
C
CD
D
Right lateral Left lateral
Right and left lateral earthquake sources with perpendicular fault planes have the same first motions
1. Focal mechanisms
Two fault planes will produce the same motion
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1. Focal mechanisms
Orientation of fault plane can be represented by beach balls
Normal or ‘rift’ faulting
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Different orientations of different faults
1. Focal mechanisms
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1. Focal mechanisms
From a network of seismometers can analyze fault orientation
(the more the better)
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1. Focal mechanisms
So can take global map of seismicity and ….
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1. Focal mechanisms
Analyze the fault planes…..
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Nature of faulting depends on style of the tectonic setting
1. Focal mechanisms
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1. Focal mechanisms
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1. Focal mechanisms – normal faulting
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1. Focal mechanisms – thrust faulting
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1. Focal mechanisms – strike slip
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Quakes stop at spreading areashydrophonesseisomometer
2. Our local setting – earthquake locations
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2. Our local setting – focal mechanisms
Offshore, it is mainly normal and strike-slip
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Birth of San Andreas fault: ridge got subducted
2. Our local setting
Juan De Fuca plate a remanant…
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2. Our local setting
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2001 Olympia Mw6.8
From beach balls – 2 possibilities, actually was vertical…
2. Our local setting – Olympia Earthquake
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DIFFERENT KINDS OF QUAKES in the region
2. Our local setting
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Click here
2. Our local setting
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Three types of local earthquake hazard
Subduction earthquakes – brittle rocks
return time 500-600 yr
magnitude 9+
Crustal earthquakes – crustal faults mountain building in Cascades/Olympics
return time – several hundred years
magnitude 7+
Deep earthquakes – mechanism poorly understood, probably due due to minearological changes
return time 50-60 yr
magnitude 7+
2. Our local setting
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3. Shape of ocean spreading ridges (diffusion)
Spreading ridges tend to have a similar profile
What accounts for this?
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Follows cooling curve out to 80 Ma, then flattens out
- General upflow of heat
3. Shape of ocean spreading ridges (diffusion)
Can be plotted as a function of age (if spreading rates are uniform)
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3. Shape of ocean spreading ridges (diffusion)
Age of crust controls depth of ocean
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3. Shape of ocean spreading ridges (diffusion)
aethenosphere
crust
ocean
As new crust is formed and moves away from spreading center, it cools and so thickens.
Go to board to solve equations
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4. What drives plate tectonics?
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Gavitational Geoid is not uniform
4. What drives plate tectonics?
miliGa (1 Gal = 0.001g)
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RED = DENSE (e.g., subduction zones around the Pacific Rim)
4. What drives plate tectonics?
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Topography & Bathymetry
Gravity
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So, Earth split up into plates, sometimes visible
on the surface
San Andreas
Iceland
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Planet can be broken up into major plates
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What is going on?
What is driving this these motions?
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What forces dominate the motion of the plate?
Basal stress (mantle convection)? Ridge push? Slab pull?
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Isolate continental mantle drag..
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WHAT IS THE SOURCE OF THE DRIVING FORCE?
Correlation of plate area, trench length, ridge length
vs plate velocity.
Correlates only with trench length -
n.b. area ~ basal stress; ridge length ~ ridge push; trench length ~ slab pull
As a % of total perimter
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