3.3.2 Transform plate boundaries

Aside from shear stress, what the opposing movements of plates either side of fault also cause

Cause small thrusts to be developed

What would have happened if the fault had become locked between 1906 and 2016 but then suddenly released

Devastating displacement of 5.5m would be possible. Will happen at some point in future

How Ventura anticline (between Los Angeles and San Francisco) has developed

Developed in sediments which are only 500000 years old. Still rising at rate of 5mm/year, would be 2500m if unconsolidated sediments where not being eroded rapidly at same time

Transform plate boundary

Fault where no crust is created or destroyed

How probability of subsequent earthquake can be assessed

If stress value that causes crustal failure calculated for section of fault line

Where there is seismic gaps

Locked areas, where no earthquakes have occurred to release the stress

Describe the last big earthquake on locked section of the San Andreas fault

Occurred at San Francisco in 1906. 8.3 on the Richter scale, many parts of city flattened and about 700 killed. Part of fault has not moved since

When crustal failure will occur

Occurs once stress exceeds a certain value, which depends on shear stress, normal stress, pore pressure and friction of fault plane

Why California will continue to suffer from severe shallow-focus earthquakes

Plate movement is an unremitting 5cm a year

What happens at conservative plate boundary

Plates slide past each other with a horizontal movement. No subduction, no volcanoes, no creation or destruction of crust

Sections present at the San Andreas Fault zone

Seismically active sections, where small earthquakes are frequent, and locked sections where there is no recent movement

What opposing movement of plates on either side of fault causes

Shear stress in the rocks

What fracturing of an oil well drilled through one thrust shows

Shows that displacement along thrust plane would be 15km in one million years

Difficultly in trying to predict movement of crust prior to earthquake

Some parts of fault are creeping, making many small movements, while other sections are stuck

What happens over many years in locked sections

Strain builds up until there is a sudden break

Tear fault

Strike-slip fault which: dies out without a junction with another fault; has more displacement in the middle of the fault zone; cannot form a tectonic plate boundary

Transform fault

Strike-slip fault which: ends at junction of another plate boundary or fault; shows the same amount of deformation across the fault zone; can form a tectonic plate boundary

What the San Andreas Fault in California is interpreted as

The San Andreas Fault, along with its many associated faults, is now interpreted as a transform plate boundary or transform fault, also known as conservative plate margin

Stress transfer

When section of crust moves during earthquake, stress released. For large earthquakes stress may not be totally used up in displacement of crust, remaining stress transferred to another section of fault, triggering other earthquakes