Faults, Earthquakes & Seismic Waves: Sixth Grade Science
Earthquakes are ground-shaking vibrations produced by the sudden fracture of strained rock along crustal faults, radiating mechanical energy as seismic waves.
The Elastic Rebound Theory
In 1910, American geologist Harry Fielding Reid developed the Elastic Rebound Theory to explain how earthquakes occur. Tectonic forces continuously push plates, but friction along fault interfaces prevents the rocks from sliding smoothly:
- Elastic Strain Accumulation: The locked rock along the fault bends and deforms elastically, storing vast quantities of mechanical potential energy like a drawn bowstring.
- Fault Rupture: When the accumulated tectonic stress exceeds the frictional strength holding the fault locked, the rock fractures catastrophically.
- Elastic Rebound: The bent rock snaps back into an undeformed position in seconds, releasing its stored strain energy as high-energy vibrations called seismic waves.
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| ELASTIC REBOUND MECHANISM |
| |
| 1. Unstressed Rock 2. Elastic Strain Bending 3. Fault Rupture|
| | | \ / | | |
| | | \ / | | |
| ---Fault Line--- ----Fault Line---- ---Fault Line--- |
| | | / \ (Rebound Quake!)|
| | | / \ | | |
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Focus Versus Epicenter
- The Focus (Hypocenter): The actual physical point deep underground within Earth's crust where rock fracture initiates and seismic waves originate.
- The Epicenter: The point on Earth's surface situated directly vertically above the focus. The epicenter typically experiences the most intense ground shaking and structural damage.
The Three Primary Fault Types
A fault is a fracture in crustal rock along which measurable movement has occurred, categorized by the direction of relative slip:
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| THE THREE MAJOR FAULT TYPES |
+---------------------+-------------------+-----------------------------------+
| Fault Class | Dominant Stress | Relative Block Motion & Setting |
+---------------------+-------------------+-----------------------------------+
| Normal Fault | Tension (Pulling | Hanging wall slips DOWNWARD |
| | apart / stretch) | relative to footwall; divergent |
+---------------------+-------------------+-----------------------------------+
| Reverse / Thrust | Compression | Hanging wall is shoved UPWARD |
| Fault | (Squeezing together| relative to footwall; convergent |
+---------------------+-------------------+-----------------------------------+
| Strike-Slip Fault | Shear (Tearing / | Blocks slide past each other |
| | horizontal slip) | horizontally; transform boundaries|
+---------------------+-------------------+-----------------------------------+
The Anatomy of Seismic Waves
When an earthquake ruptures, energy radiates outward in all directions through spherical wavefronts divided into body waves (traveling through Earth's interior) and surface waves (traveling along the exterior crust):
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| SEISMIC WAVE CLASSIFICATION |
+---------------------+-------------------+-----------------------------------+
| Wave Category | Wave Type | Particle Motion & Velocity |
+---------------------+-------------------+-----------------------------------+
| Primary Waves | Body Wave | Longitudinal (Compressional); |
| (P-Waves) | | Fastest (~6-8 km/s); arrives 1st; |
| | | travels through solids & liquids |
+---------------------+-------------------+-----------------------------------+
| Secondary Waves | Body Wave | Transverse (Shearing); |
| (S-Waves) | | Medium speed (~3-5 km/s); 2nd; |
| | | travels ONLY through solids |
+---------------------+-------------------+-----------------------------------+
| Surface Waves | Surface Wave | Rolling elliptical (Rayleigh) and |
| (L-Waves) | | side-to-side (Love); slowest; |
| | | causes maximum structural damage |
+---------------------+-------------------+-----------------------------------+
P-Waves Versus S-Waves: The Race Through Earth
- Primary (P) Waves: Compressional waves that push and pull rock particles in the same direction the wave travels, exactly like sound waves. P-waves are the fastest seismic waves, traveling through crustal rock at roughly 6 kilometers per second. Because P-waves compress matter, they travel through solid rock, liquid magma, and water oceans.
- Secondary (S) Waves: Transverse shear waves that displace rock particles side-to-side at right angles to the direction of wave travel. S-waves travel at roughly 3.5 kilometers per second (about 60 percent of P-wave speed). S-waves cannot travel through liquids because liquids have no shear strength.
- Surface Waves: Move along Earth's surface like ocean ripples, rolling the ground up and down and tearing foundations apart with severe sideways shearing.
Exercises and Solutions
Exercise 1: Decoding the P-S Wave Time Interval. A seismograph station located in California records the arrival of seismic waves from an earthquake. The first compressional P-wave arrives on the seismogram at exactly 10:14:20 AM. The first transverse S-wave arrives at 10:17:50 AM. (a) Calculate the S-P time lag interval in seconds; (b) Explain how this time lag interval changes as distance from the earthquake epicenter increases; (c) If a second seismograph station recorded an S-P lag time of only 45 seconds, which station was located closer to the epicenter?
Solution 1: (a) The S-P time lag is calculated by subtracting the P-wave arrival time from the S-wave arrival time: 10:17:50 minus 10:14:20 equals 3 minutes and 30 seconds, which equals 210 seconds. (b) The S-P time interval increases directly with distance. Because P-waves travel nearly twice as fast as S-waves, the gap between them widens continuously the farther they travel from the focus, exactly like a fast race car pulling steadily ahead of a slower car. (c) The second station (with an S-P lag of 45 seconds) was located significantly closer to the epicenter because the P-wave and S-wave had not had enough travel time to separate widely.
Exercise 2: Analyzing Fault Mechanics Under Differing Tectonic Stresses. Identify the specific fault type (normal, reverse, or strike-slip) that develops under each of the following geological scenarios and describe the relative motion of the crustal blocks: (a) Crustal rock being stretched apart in the East African Rift; (b) Crustal rock being compressed and squeezed during the collision of India and Asia; (c) Tectonic plates grinding horizontally past each other along the San Andreas Fault.
Solution 2: (a) A normal fault develops under tensile stress (pulling apart). The hanging wall block slips downward relative to the footwall block along the inclined fault plane. (b) A reverse fault (or low-angle thrust fault) develops under compressive stress (squeezing). The hanging wall block is forced upward and over the footwall block. (c) A strike-slip fault develops under horizontal shear stress. The two crustal blocks slide past one another horizontally with virtually zero vertical displacement.