The Core Idea
Stress Builds Silently, Then Releases All at Once
Earthquakes occur according to elastic rebound theory, first proposed by Harry Fielding Reid in 1906: rock on either side of a fault gradually bends and stores elastic strain energy as tectonic forces slowly push the two sides in different directions, while friction keeps the fault itself locked in place. Once the accumulated stress finally exceeds the fault's frictional strength, the fault suddenly slips, releasing the stored energy all at once as an earthquake — and the rock on either side snaps back toward its original, unstrained shape.
This means earthquakes aren't a sudden, unpredictable event so much as the sudden release of a long, gradual, largely invisible buildup — understanding this buildup-and-release cycle is central to modern earthquake hazard assessment and forecasting efforts.
💡 Memory Trick
Picture bending a wooden ruler slowly with your hands: as you apply steady pressure, the ruler bends further and further, storing more and more elastic energy — but it doesn't break gradually, it holds until suddenly it snaps all at once, releasing all that stored energy in a single instant. This is exactly elastic rebound theory: rock along a fault bends slowly under tectonic stress, stores that strain silently, and then suddenly 'snaps' — the earthquake — releasing the accumulated energy all at once, with both sides of the fault snapping back toward their original shape.
Key Earthquake Concepts
Location, Depth, Faults, and Magnitude
1
Focus vs. Epicenter
The focus (or hypocenter) is the actual underground point where fault rupture begins; the epicenter is the point on Earth's surface directly above the focus.
Example: news reports of an earthquake's location almost always refer to the epicenter, since that's the relevant surface location for most people, even though the actual rupture occurred underground at the focus.
2
Depth Categories
Shallow-focus earthquakes (less than 70 km deep) are the most destructive, since their energy is released closest to the surface. Intermediate (70–300 km) and deep-focus (over 300 km) earthquakes occur exclusively at subduction zones, where the Benioff zone extends the fault surface to much greater depths.
Example: this connects directly to the Benioff zone concept covered in the Subduction lesson, where earthquakes trace the subducting slab's path to depths of up to 700 km.
3
Fault Types
Normal faults form under extension (tension pulling apart), reverse/thrust faults form under compression (pushing together), and strike-slip faults involve horizontal, side-to-side motion typical of transform boundaries.
Example: California's San Andreas Fault is a right-lateral strike-slip fault, meaning the block on the opposite side of the fault appears to move to the right from either side's perspective.
4
Moment Magnitude (Mw)
The modern standard for measuring earthquake size, having replaced the older Richter scale. Each whole-number increase in magnitude represents roughly 32 times more energy release.
Example: a magnitude 9.0 earthquake releases roughly 1,000 times more energy than a magnitude 7.0 earthquake, since the scale is logarithmic rather than linear.
Beyond the Shaking
Tsunamis as a Secondary Earthquake Hazard
Tsunamis form specifically when an earthquake causes vertical (rather than purely horizontal) displacement of the seafloor, physically displacing the entire water column above it and generating a wave that can travel across an entire ocean basin. This is exactly why megathrust subduction zone earthquakes — which involve substantial vertical displacement along their fault interface — are so strongly associated with major tsunamis, while purely horizontal-slipping strike-slip earthquakes (like most along the San Andreas Fault) typically do not generate significant tsunamis.
🖥️ Applied Scenario
A seismologist is asked to explain why a recent subduction zone earthquake generated a major tsunami, while an equally large strike-slip earthquake elsewhere did not.
1
The seismologist explains that the subduction zone earthquake involved substantial vertical seafloor displacement along the fault, physically displacing the ocean water above it and generating the resulting tsunami.
2
By contrast, the strike-slip earthquake involved primarily horizontal fault motion, with minimal vertical displacement of the seafloor, and therefore did not significantly displace the overlying water column.
3
The seismologist concludes that tsunami generation depends specifically on vertical displacement, not simply on overall earthquake magnitude — explaining why two similarly sized earthquakes can have dramatically different tsunami outcomes based purely on fault type.
📌 Exam Application
Exams frequently ask you to distinguish focus from epicenter, name the three fault types and their associated stress (tension, compression, shear), or explain what conditions are necessary for tsunami generation — always specify vertical displacement as the key requirement for tsunami formation.
⚠️ Most Common Earthquakes Mistakes
Don't assume all large earthquakes generate tsunamis — tsunami generation specifically requires vertical seafloor displacement, meaning large strike-slip earthquakes (horizontal motion) typically do not produce major tsunamis even at high magnitude. Also don't confuse focus (underground rupture point) with epicenter (surface point above it) — a very commonly tested distinction.
✓ Quick Self-Test
1) Explain elastic rebound theory in your own words. 2) Name the three fault types and the type of stress associated with each. 3) What specific condition is required for an earthquake to generate a tsunami?
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Earth's Interior — Evidence
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