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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