How Earthquakes Happen: Plate Tectonics to Seismic Waves

Every year, Earth experiences about 500,000 detectable earthquakes — 100,000 can be felt, and 100 cause damage. The largest release energy equivalent to billions of tonnes of TNT. Here's the complete chain: from mantle convection to shaking ground.

1. Moving Plates

Earth's outer shell (lithosphere) is broken into about 15 major tectonic plates that float on the partially molten asthenosphere. Mantle convection, ridge push, and slab pull drive these plates at 1–15 cm/year — roughly the speed your fingernails grow.

Divergent boundaries: Plates move apart. Magma rises to fill the gap (mid-ocean ridges). Shallow earthquakes, low magnitude. Example: Mid-Atlantic Ridge.
Convergent boundaries: Plates collide. Oceanic crust subducts under continental crust, or two continental plates crumple (forming mountains). The most powerful earthquakes occur here. Example: Pacific Ring of Fire, Himalayas.
Transform boundaries: Plates slide past each other. No crust is created or destroyed, but stress builds along the interface. Example: San Andreas Fault.

About 90% of earthquakes occur at plate boundaries. The remaining 10% are intraplate earthquakes — caused by ancient faults reactivated by distant tectonic stresses (e.g., New Madrid seismic zone, central USA).

2. Faults & Stress Accumulation

A fault is a fracture in the crust where the two sides have moved relative to each other. The three main types:

Normal fault: Hanging wall drops down. Caused by tensional (pulling apart) forces. Found at divergent boundaries.
Reverse (thrust) fault: Hanging wall moves up. Caused by compressional forces. Produces the strongest earthquakes. The 2011 Tōhoku M9.1 earthquake was on a megathrust.
Strike-slip fault: Horizontal motion. San Andreas is right-lateral (standing on one side, the other side moves right). The 1906 San Francisco earthquake: 6 m of horizontal offset.

Between earthquakes, the two sides of a fault are locked by friction. Tectonic forces continue to push the plates, bending the rock like a spring. Strain energy accumulates for years, decades, or centuries. When the accumulated stress exceeds the frictional strength of the fault — it breaks.

3. Elastic Rebound

Harry Fielding Reid proposed the elastic rebound theory after studying the 1906 San Francisco earthquake. The process:

Tectonic forces slowly deform the rocks on both sides of a locked fault. The deformation is elastic (like bending a ruler).
When stress exceeds the fault's frictional strength, the fault ruptures. The rupture propagates along the fault plane at 2–4 km/s (70–80% of the S-wave speed).
The rocks snap back to their undeformed shape — releasing all the stored elastic energy as seismic waves and heat.
The fault re-locks. Stress begins accumulating again toward the next earthquake.

Seismic moment (energy measure): M₀ = μ \cdot A \cdot D μ = shear modulus of rock (~30 GPa for crust) A = fault rupture area (m²) D = average slip (m) 2011 Tōhoku M9.1: μ = 30 GPa A \approx 500 km \times 200 km = 10¹¹ m² D \approx 20 m M₀ \approx 30\times10⁹ \times 10¹¹ \times 20 = 6 \times 10²² N\cdotm Moment magnitude: Mw = (2/3)\cdotlog₁₀(M₀) - 6.07 Mw = (2/3)\cdotlog₁₀(6\times10²²) - 6.07 \approx 9.1

4. Seismic Waves

P-waves (Primary): Compressional waves — particles push and pull along the wave direction (like sound in air). Fastest: 5–8 km/s in crust, up to 13 km/s in the core. Travel through solids, liquids, and gases. Arrive first.
S-waves (Secondary): Shear waves — particles move perpendicular to the wave direction. Slower: 3–5 km/s. Cannot travel through liquids (this is how we know the outer core is liquid). More damaging than P-waves due to larger ground motion.
Surface waves: Travel along Earth's surface. Two types:
- Love waves: Horizontal shearing. Fastest surface wave.
- Rayleigh waves: Rolling motion (like ocean waves). Slowest but often most damaging — produce the strongest ground movement felt during an earthquake.

Early warning: P-waves travel faster than S-waves and surface waves. The time gap grows with distance: at 100 km, P arrives ~12 seconds before S. Japan's earthquake early warning system detects P-waves and sends alerts before the destructive S/surface waves arrive — giving seconds to tens of seconds of warning.

5. Magnitude & Intensity

Mw	Description	Energy (TNT)	Frequency	Example
2.0–2.9	Micro	1 kg	~1,000/day	Not felt
4.0–4.9	Light	6 tonnes	~49/day	Glass rattles
5.0–5.9	Moderate	200 tonnes	~4/day	Furniture moves
6.0–6.9	Strong	6,300 tonnes	~120/year	1994 Northridge
7.0–7.9	Major	200 kt	~15/year	2023 Turkey M7.8
8.0–8.9	Great	6 Mt	~1/year	1906 San Francisco
9.0+	Exceptional	200+ Mt	~1/decade	2011 Tōhoku (M9.1)

Each whole number increase in magnitude represents a 32× increase in energy released. An M8 releases 1,000× more energy than an M6. The difference between M9 and M5 is 1,000,000×.

Intensity (Modified Mercalli scale, I-XII) measures the effects at a specific location — how much shaking people feel and how much damage occurs. Intensity depends on distance, soil type, and building construction, not just magnitude.

6. Ground Effects & Hazards

Ground shaking: The primary hazard. Measured in g (acceleration due to gravity). M7 can produce 0.5–1.0 g near the epicentre. Duration (10–60 seconds for large earthquakes) matters as much as peak acceleration.
Liquefaction: Saturated, loose sandy soil loses strength during shaking and behaves like a liquid. Buildings tilt and sink. Major factor in 2011 Christchurch (NZ) damage.
Landslides: Shaking destabilises slopes. The 2008 Sichuan earthquake triggered over 60,000 landslides. Often causes more casualties than shaking itself in mountainous regions.
Tsunamis: Submarine earthquakes (M7.5+) that displace the seafloor generate tsunamis. The 2004 Indian Ocean M9.1 created waves up to 30 m high, killing 230,000 people. Warning systems can provide 10–60 minutes of lead time for distant coasts.
Fires: Ruptured gas lines and downed power lines start fires. The 1906 San Francisco fire destroyed more of the city than the earthquake itself (3,000+ buildings burned over 3 days).

7. Earthquake Engineering

Base isolation: Building sits on rubber-and-steel bearings that decouple it from ground motion. The ground moves; the building stays relatively still. Used in hospitals, bridges, and Japan's National Museum of Western Art.
Dampers: Tuned mass dampers (TMDs) — heavy pendulums or liquid tanks at the top of skyscrapers that sway opposite to the building's motion, reducing amplitude. Taipei 101's 730-tonne TMD is visible to visitors.
Ductile design: Steel frames designed to bend (yield) without breaking. Concrete columns with closely spaced spiral reinforcement prevent brittle collapse. Modern building codes (IBC, Eurocode 8) require buildings to survive the design-level earthquake without collapse.
Shear walls: Reinforced concrete walls that resist horizontal forces. Essential in preventing "soft storey" collapse (where a weak ground floor fails while upper floors remain intact).
Prediction: Currently impossible to predict the exact time, location, and magnitude of an earthquake. Probability-based hazard maps estimate likelihood over decades (e.g., "62% chance of M6.7+ in San Francisco Bay Area within 30 years"). Operational earthquake forecasting uses foreshock sequences and rate changes to assess elevated short-term probability — not prediction.