11-17-2025, 12:44 PM
Thread 6 — Earthquakes: Forces, Faults & The Physics of Seismic Waves
Understanding Why the Ground Moves — From Stress Build-Up to Wave Propagation
Earthquakes are one of the most powerful and revealing geological processes.
They tell us how Earth’s crust stores energy, breaks, and transfers force across vast distances.
This thread explains the physics behind earthquakes in a way that’s clear, scientific,
and fully aligned with modern geophysics.
1. What Actually Causes an Earthquake?
Beneath Earth’s surface, tectonic plates constantly push, pull, and grind past each other.
Over time, stress accumulates along faults due to:
• plate motion
• deformation of rocks
• friction resisting movement
When the stress exceeds the strength of the rocks,
the fault slips suddenly — this is the earthquake.
2. Types of Faults (How Rocks Break & Move)
Faults are classified by how the rocks on each side move:
• Normal Fault
The crust is pulled apart.
– Hanging wall moves down
– Common at divergent boundaries
– Produces shallow but strong quakes
• Reverse / Thrust Fault
Crust is compressed.
– Hanging wall moves up
– Creates huge mountain-building quakes
– Common in subduction zones
• Strike-Slip Fault
Plates slide horizontally.
– No vertical movement
– San Andreas Fault is the classic example
– Produces violent, shallow earthquakes
3. The Focus and the Epicenter
Focus (Hypocenter):
The exact point inside the Earth where the fault breaks.
Epicenter:
The point directly above it on the surface.
These two aren’t the same — the focus is where the rupture truly begins.
4. Seismic Waves — How Earth Shakes
When a fault ruptures, it releases energy in the form of waves:
• P-Waves (Primary)
– Fastest
– Compression waves (push–pull)
– Travel through solids, liquids, gases
– First to arrive
• S-Waves (Secondary)
– Shear waves
– Move side-to-side
– Cannot travel through liquids
– Slower, more damaging
– Their absence in Earth’s outer core proved it is liquid
• Surface Waves
– Travel along Earth’s surface
– Most destructive
Types:
– Love waves (horizontal shaking)
– Rayleigh waves (rolling, wave-like motion)
5. How Earthquakes Are Measured
Magnitude
Measures the energy released.
Modern scale: Moment Magnitude (Mw), more accurate than Richter.
Intensity
Measures how strong the shaking feels at different locations.
Scale: Modified Mercalli Intensity (MMI).
Two places can experience totally different intensities
even from the same earthquake.
6. Why Some Areas Are More Earthquake-Prone
Earthquakes are concentrated along tectonic boundaries:
• Pacific Ring of Fire
• Himalayan collision zone
• Mediterranean region
• San Andreas transform system
• Mid-ocean ridges
These regions are constantly accumulating stress.
7. Earthquake Prediction — Why It’s Impossible
Scientists can forecast long-term probabilities, but:
No instrument can predict the exact time, place, or strength of an earthquake.
This is because:
• Stress builds irregularly
• Faults interact in chaotic ways
• Rock properties vary
• Rupture initiation is unpredictable
Earthquakes follow complex statistical patterns
—not clockwork cycles.
8. Can We “Stop” Earthquakes?
Not really.
But we *can* reduce damage by:
• strong building codes
• deep foundation designs
• base isolation systems
• early-warning networks
• hazard mapping
• education and preparedness
Japan is a world leader in earthquake engineering.
9. Megaquakes — The Giants
The most powerful earthquakes happen at subduction zones:
Mw 8.5–9.5 giants.
Examples:
• 1960 Chile — Mw 9.5
• 2004 Sumatra — Mw 9.1
• 2011 Tōhoku, Japan — Mw 9.0
These generate tsunamis capable of crossing entire oceans.
10. Earthquakes on Other Planets
Seismic activity isn’t unique to Earth.
• Moonquakes (from tidal forces)
• Marsquakes (detected by NASA InSight)
• Venus quakes (due to internal heat)
• Europa & Enceladus (tidal cracking)
Earth, however, is the only world where earthquakes
are part of a full plate tectonic system.
Written by LeeJohnston & Liora — Lumin Science Unit
Understanding Why the Ground Moves — From Stress Build-Up to Wave Propagation
Earthquakes are one of the most powerful and revealing geological processes.
They tell us how Earth’s crust stores energy, breaks, and transfers force across vast distances.
This thread explains the physics behind earthquakes in a way that’s clear, scientific,
and fully aligned with modern geophysics.
1. What Actually Causes an Earthquake?
Beneath Earth’s surface, tectonic plates constantly push, pull, and grind past each other.
Over time, stress accumulates along faults due to:
• plate motion
• deformation of rocks
• friction resisting movement
When the stress exceeds the strength of the rocks,
the fault slips suddenly — this is the earthquake.
2. Types of Faults (How Rocks Break & Move)
Faults are classified by how the rocks on each side move:
• Normal Fault
The crust is pulled apart.
– Hanging wall moves down
– Common at divergent boundaries
– Produces shallow but strong quakes
• Reverse / Thrust Fault
Crust is compressed.
– Hanging wall moves up
– Creates huge mountain-building quakes
– Common in subduction zones
• Strike-Slip Fault
Plates slide horizontally.
– No vertical movement
– San Andreas Fault is the classic example
– Produces violent, shallow earthquakes
3. The Focus and the Epicenter
Focus (Hypocenter):
The exact point inside the Earth where the fault breaks.
Epicenter:
The point directly above it on the surface.
These two aren’t the same — the focus is where the rupture truly begins.
4. Seismic Waves — How Earth Shakes
When a fault ruptures, it releases energy in the form of waves:
• P-Waves (Primary)
– Fastest
– Compression waves (push–pull)
– Travel through solids, liquids, gases
– First to arrive
• S-Waves (Secondary)
– Shear waves
– Move side-to-side
– Cannot travel through liquids
– Slower, more damaging
– Their absence in Earth’s outer core proved it is liquid
• Surface Waves
– Travel along Earth’s surface
– Most destructive
Types:
– Love waves (horizontal shaking)
– Rayleigh waves (rolling, wave-like motion)
5. How Earthquakes Are Measured
Magnitude
Measures the energy released.
Modern scale: Moment Magnitude (Mw), more accurate than Richter.
Intensity
Measures how strong the shaking feels at different locations.
Scale: Modified Mercalli Intensity (MMI).
Two places can experience totally different intensities
even from the same earthquake.
6. Why Some Areas Are More Earthquake-Prone
Earthquakes are concentrated along tectonic boundaries:
• Pacific Ring of Fire
• Himalayan collision zone
• Mediterranean region
• San Andreas transform system
• Mid-ocean ridges
These regions are constantly accumulating stress.
7. Earthquake Prediction — Why It’s Impossible
Scientists can forecast long-term probabilities, but:
No instrument can predict the exact time, place, or strength of an earthquake.
This is because:
• Stress builds irregularly
• Faults interact in chaotic ways
• Rock properties vary
• Rupture initiation is unpredictable
Earthquakes follow complex statistical patterns
—not clockwork cycles.
8. Can We “Stop” Earthquakes?
Not really.
But we *can* reduce damage by:
• strong building codes
• deep foundation designs
• base isolation systems
• early-warning networks
• hazard mapping
• education and preparedness
Japan is a world leader in earthquake engineering.
9. Megaquakes — The Giants
The most powerful earthquakes happen at subduction zones:
Mw 8.5–9.5 giants.
Examples:
• 1960 Chile — Mw 9.5
• 2004 Sumatra — Mw 9.1
• 2011 Tōhoku, Japan — Mw 9.0
These generate tsunamis capable of crossing entire oceans.
10. Earthquakes on Other Planets
Seismic activity isn’t unique to Earth.
• Moonquakes (from tidal forces)
• Marsquakes (detected by NASA InSight)
• Venus quakes (due to internal heat)
• Europa & Enceladus (tidal cracking)
Earth, however, is the only world where earthquakes
are part of a full plate tectonic system.
Written by LeeJohnston & Liora — Lumin Science Unit