What causes earthquakes? The short version: rock on either side of a fault wants to move, friction holds it in place, stress builds for decades or centuries, and then friction loses. The rock snaps to a new position in seconds and the stored elastic energy radiates outward as seismic waves. That is the shaking.
The Engine: Moving Plates
Earth’s outer shell is broken into tectonic plates riding on the slowly flowing mantle below. They move at roughly the speed your fingernails grow — a few centimeters per year — driven by heat escaping the planet’s interior.
That sounds gentle, and over a human lifetime it is. But plates are enormous, and the forces involved are staggering. Over a century, a few centimeters per year adds up to meters of accumulated offset that has not yet happened.
Why the Ground Does Not Just Slide Smoothly
If faults were frictionless, plates would creep past each other continuously and earthquakes would be tiny or nonexistent. Instead, faults lock. Rough surfaces and enormous pressure hold the rock in place while the plates keep pushing.
During the locked period, the rock bends elastically — storing energy the way a bent stick does. Surveying equipment can measure this deformation: the ground genuinely tilts and warps near a locked fault over decades.
When the stress finally exceeds what the fault can hold, rupture begins at a point (the hypocenter) and spreads along the fault at a few kilometers per second. The rock on each side springs back toward an unstressed shape. That spring-back is the earthquake.
The Three Main Settings
| Fault type | What the plates do | Example |
|---|---|---|
| Strike-slip | Slide horizontally past each other | San Andreas, California |
| Reverse / thrust | One plate pushed up over the other | Himalayas, Japan Trench |
| Normal | Crust pulled apart, one side drops | Basin and Range, USA |
Thrust faults produce the largest earthquakes, because the compression lets enormous areas of contact lock together before failing. The 2011 Japan earthquake (magnitude 9.0-9.1) was a thrust event on a subduction zone.
What the Magnitude Number Actually Means
The Richter scale people know has been largely replaced by moment magnitude, which measures the total energy released rather than the amplitude on one particular instrument.
It is logarithmic, and this trips everyone up: each whole number is about 32 times more energy, not twice. A magnitude 7 is not slightly worse than a 6 — it releases roughly 32 times the energy. A magnitude 9 is about 1,000 times a magnitude 7.
There is no meaningful upper limit from the scale itself, only from geology: the biggest earthquakes are bounded by how large a fault area can rupture in one go.
Why Aftershocks Happen
The main rupture does not relieve all the stress. It changes the stress distribution on surrounding rock, loading some patches more heavily. Those patches then fail over the following days to years.
Aftershocks follow a predictable statistical pattern: the largest is typically about one magnitude unit below the mainshock, and the rate decays roughly inversely with time. They are expected behavior, not a sign that something worse is coming. That said, the largest aftershock can be damaging in its own right, especially to structures already weakened.
Not All Earthquakes Come From Tectonics
That is what causes earthquakes in the overwhelming majority of cases.
- Volcanic — magma moving underground cracks rock. Usually smaller, but a useful eruption warning sign.
- Induced — human activity, mainly wastewater injection into deep wells and, to a lesser extent, hydraulic fracturing. Also reservoir filling behind large dams.
- Collapse — underground mines caving in. Very local.
Induced seismicity is the fastest-growing category in some regions. The mechanism is not the fluid lubricating a fault so much as increasing pore pressure, which reduces the effective clamping force holding the fault shut.
Can They Be Predicted?
No, and it is worth being blunt about this: no method can reliably predict the date, location, and magnitude of a specific earthquake. Despite decades of research, no precursor signal has proven consistent. Claims otherwise are not credible.
What is possible is forecasting probabilities over decades — “this region has roughly a 1-in-5 chance of a major quake in the next 30 years” — based on fault slip rates and historic recurrence. And early warning is different again: once a rupture starts, electronic signals travel faster than the shaking, so systems can give seconds to tens of seconds of warning. Seconds are enough to stop trains, shut gas valves, and get people under a desk.
Keep Exploring
- What is a volcano? — the other thing plates do at their edges
- How do ocean currents work? — heat moving through a different layer
- Weather vs climate — short-term vs long-term thinking
