Further topics in tectonics: Focal mechanisms Pac NW tectonic setting Shape of mid-ocean ridges...

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

Elastic Rebound

1. Focal mechanisms

No offset

No offset

Earthquake break

1. Focal mechanisms

Volume increase (dilation)

Volume decrease

(compression)

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

C D

D C

Right lateral

1. Focal mechanisms

Two fault planes will produce the same motion

C D

D C

C

CD

D

Right lateral Left lateral

1. Focal mechanisms

Two fault planes will produce the same motion

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

1. Focal mechanisms

Orientation of fault plane can be represented by beach balls

Normal or ‘rift’ faulting

Different orientations of different faults

1. Focal mechanisms

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…..

Nature of faulting depends on style of the tectonic setting

1. Focal mechanisms

1. Focal mechanisms

1. Focal mechanisms – normal faulting

1. Focal mechanisms – thrust faulting

1. Focal mechanisms – strike slip

Quakes stop at spreading areashydrophonesseisomometer

2. Our local setting – earthquake locations

2. Our local setting – focal mechanisms

Offshore, it is mainly normal and strike-slip

Birth of San Andreas fault: ridge got subducted

2. Our local setting

Juan De Fuca plate a remanant…

2. Our local setting

2001 Olympia Mw6.8

From beach balls – 2 possibilities, actually was vertical…

2. Our local setting – Olympia Earthquake

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+

2. Our local setting

3. Shape of ocean spreading ridges (diffusion)

Spreading ridges tend to have a similar profile

What accounts for this?

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)

3. Shape of ocean spreading ridges (diffusion)

Age of crust controls depth of ocean

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

4. What drives plate tectonics?

Gavitational Geoid is not uniform

4. What drives plate tectonics?

miliGa (1 Gal = 0.001g)

RED = DENSE (e.g., subduction zones around the Pacific Rim)

4. What drives plate tectonics?

Topography & Bathymetry

Gravity

So, Earth split up into plates, sometimes visible

on the surface

San Andreas

Iceland

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.

Correlates only with trench length -

n.b. area ~ basal stress; ridge length ~ ridge push; trench length ~ slab pull

As a % of total perimter