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
