Most earthquakes result from the movement of Earth’s crust due to plate tectonics.
Along tectonic plate boundaries, rocks in the crust resist movement.
Over time, stress builds up in these rocks.
Stress: Total force acting on crustal rocks per unit of area.
When stress overcomes rock strength, movement occurs along fractures resulting in an earthquake.
Characteristics of earthquakes depend on the:
Orientation of applied stress.
Magnitude of stress.
Strength of involved rocks.
Compression:
Decreases the volume of material.
Tension:
Pulls material apart.
Shear:
Causes twisting in materials.
Strain: Deformation of materials in response to stress.
No strain, compression, tension, shear.
Rocks can twist, squeeze, and stretch but fracture when stress and strain reach a critical point.
Movement at this point releases energy causing earthquakes.
Experiments show a distinct relationship between stress and strain:
A stress-strain curve illustrates different responses to stress:
Elastic Deformation:
Occurs under low stress; material returns to original shape once stress is removed.
Plastic Deformation:
Occurs when stress exceeds elastic limit; deformation remains even after stress is removed.
Point of stress rupture is called failure, represented by ‘X’ on the graph.
Brittle Materials: Fail with little plastic deformation (e.g., wood, glass).
Durable Materials: Can undergo significant deformation before failure (e.g., metals, rubber).
Increased pressure requires greater stress to reach elastic limit.
At high temperatures, rock may flow fluidly, reducing stress.
Crustal rocks fail when stress exceeds rock strength at a weak region called a fault.
A fault is any fracture or system of fractures along which Earth moves.
Fault Plane: Surface of movement can vary from horizontal to vertical.
Reverse Faults:
Caused by horizontal and vertical compression, shortens the crust.
Rock on one side is pushed up relative to the other, seen near convergent boundaries.
Normal Faults:
Movement is partly horizontal and vertical; stretches the crust.
Rock on one side moves down relative to the other side.
Strike-slip Faults:
Caused by horizontal shear; movement is horizontal in opposite directions (e.g., San Andreas fault).
Common at transform plate boundaries.
Most earthquakes are caused by fault movements:
As stress builds, rocks undergo elastic deformation, leading to failure and earthquakes.
Seismic Waves: Ground vibrations produced during an earthquake.
Primary Waves (P-waves): Squeeze and push rocks in the wave direction.
Rock particles move back and forth.
Secondary Waves (S-waves): Move rocks perpendicular to wave direction; slower than P-waves.
Surface Waves: Travel along Earth’s surface, causing most destruction due to up-and-down and sideways ground movement.
Focus: Point of failure in crustal rocks from which seismic waves spread; classified as shallow, mid, or deep focused.
Epicenter: Point on Earth’s surface directly above the focus.
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