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