Deformation and Stress in Geology

Shear stress is applied when tearing something apart, involving twisting and pulling motions.
Deformation is the result of applying stress to an object.
Objects generally respond to stress in a way that reduces it.

Elastic deformation means that after stretching, the object returns to its original shape.
The deformation repairs itself, removing the stress once the stress is released.
Example: Bending a stick and it returns to normal when released.
Elastic deformation is crucial in civil engineering, like in bridge construction. Bridges are designed to deform elastically under stress (e.g., weight of cars or people).
The Golden Gate Bridge deforms elastically. During an event where many people were on the bridge, it flattened noticeably but returned to its original shape after everyone left.
Bridges are intentionally designed to move and respond to stress elastically to prevent them from breaking under pressure.

Plastic deformation involves bending that does not return to the original shape.
Example: Bending a paper clip. It bends, but doesn't return to its original form.
Plastic deformation is bending without breaking, but the original shape is not recoverable.

Brittle deformation involves breaking. If something breaks, it cannot be put back together.
Example: Breaking a stick.

All three types of deformation (elastic, plastic, brittle) occur in rocks on the planet.
Elastic deformation in rocks might be hard to see without a very large piece.
Brittle deformation doesn't necessarily require bending; it can occur from compression or tension.
Wood has good tensile strength (resistance to being pulled apart), while concrete has poor tensile strength but good compressive strength.
Concrete's compressive strength can be increased through prestressing, where reinforcing bars are tightened to put concrete under compression.

Concrete has good compressive strength but poor tensile and shear strength.
Steel has very good tensile strength.
In earthquake-prone areas, concrete structures are reinforced with steel to combine compressive and tensile strengths for better resistance to shear stresses.
Shear stress is often the most efficient way to break something manually.

Earthquakes can result in all three types of deformation. Elastic deformation occurs as rocks bend and return to normal.
Example: During the Loma Prieta earthquake, bedrock experienced elastic deformation in areas where buildings didn't collapse.
Plastic deformation can occur in areas with soft ground or landfill.
Example: The Marina district in San Francisco, built on landfill from the 1906 earthquake, experienced plastic deformation during the 1989 earthquake, causing buildings to collapse.
Brittle deformation on a small scale: A crack in a concrete strip around a yard due to an earthquake.
Brittle deformation on a larger scale: Faults like the San Andreas Fault, where cracks form and displacement occurs.

Displacement: Change in location after brittle deformation.
Example: A fence displaced by the San Andreas Fault during the 1906 earthquake.
San Andreas Fault: A right-lateral fault where, standing on one side, the other side moves to the right.
The Hayward Fault runs through the California Memorial Stadium in Berkeley, with a visible gap in the concrete structure to accommodate movement.
Building codes now prohibit building across faults.

Fault systems consist of a main crack (e.g., the San Andreas Fault) and numerous smaller cracks around it.
The San Andreas Fault is a transform boundary between the Pacific plate and the North American plate.

Stress is force applied over an area: $\text{Stress} = \frac{\text{Force}}{\text{Area}}$
Larger area means the force is distributed over a larger space, resulting in less stress.

How rocks respond to stress depends on various factors.
Composition: Different rocks (e.g., sandstone vs. granite) deform differently under shear stress.
Confinement: Rocks under pressure deform more than those not under pressure.

Low Stress: Slow application of force results in slight bending.
High Stress: Rapid application of force results in movement, noise, and energy release.
Example: Bending a stick slowly results in bending, while bending it quickly can cause it to break more easily.

Long-term, slow strain rates can cause ductile or plastic deformation in rocks.
Even slow strain can eventually cause brittle deformation (breaking).
Folding: Pushing rocks together causes them to bend and fold, potentially leading to fractures.