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 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

CD DC Right lateral 1. Focal mechanisms Two fault planes will produce the same motion

CD DC C CD D Right lateralLeft lateral 1. Focal mechanisms Two fault planes will produce the same motion

CD DC 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

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 – normal faulting

1. Focal mechanisms – thrust faulting

1. Focal mechanisms – strike slip

Quakes stop at spreading areas hydrophones seisomometer 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 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 yr magnitude 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