15.1 Forces Within Earth

Module 15: Earthquakes

Lesson 1: Forces Within Earth


Stress and Strain

  • 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.


Movement and Earthquakes

  • 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.


Types of Stress Acting on Earth's Rocks

  1. Compression:

    • Decreases the volume of material.

  2. Tension:

    • Pulls material apart.

  3. Shear:

    • Causes twisting in materials.


Deformation and Strain

  • Strain: Deformation of materials in response to stress.

    • No strain, compression, tension, shear.


Rock Behavior Under Stress

  • Rocks can twist, squeeze, and stretch but fracture when stress and strain reach a critical point.

    • Movement at this point releases energy causing earthquakes.


Stress-Strain Relationship

  • 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.


Material Behavior Under Stress

  • Brittle Materials: Fail with little plastic deformation (e.g., wood, glass).

  • Durable Materials: Can undergo significant deformation before failure (e.g., metals, rubber).


Influence of Temperature and Pressure

  • Increased pressure requires greater stress to reach elastic limit.

  • At high temperatures, rock may flow fluidly, reducing stress.


Faults

  • 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.


Types of Faults

  1. 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.

  2. Normal Faults:

    • Movement is partly horizontal and vertical; stretches the crust.

    • Rock on one side moves down relative to the other side.

  3. Strike-slip Faults:

    • Caused by horizontal shear; movement is horizontal in opposite directions (e.g., San Andreas fault).

    • Common at transform plate boundaries.


Earthquake Waves

  • Most earthquakes are caused by fault movements:

    • As stress builds, rocks undergo elastic deformation, leading to failure and earthquakes.


Types of Seismic Waves

  • 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 and Epicenter

  • 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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