How earthquakes happen
An earthquake is the sudden release of strain that has been building in rock for years or centuries. The outer shell of the Earth is broken into tectonic plates that move a few centimetres a year, and where they meet, friction locks them together while the rock behind keeps pushing. When the stress finally exceeds the friction holding a fault shut, the two sides slip past each other in seconds and the stored energy radiates outwards as seismic waves.
Strain, friction and slip
The mechanism is the same everywhere, from a magnitude 1 nobody notices to the largest events on record. Only the scale differs.
Plates move
Tectonic plates drift at roughly the rate a fingernail grows, a few centimetres a year, driven by heat flow in the mantle beneath them.
A fault locks
Where two blocks of rock meet, friction holds the contact shut. The plates keep moving, so the rock on either side bends and stores elastic strain, like a stick being slowly flexed.
The fault slips
When accumulated stress exceeds the friction, the fault fails. The two sides slip past each other, sometimes by centimetres, sometimes by metres, in a few seconds.
Waves radiate
The stored strain converts to seismic waves that spread outwards. P waves arrive first as a sharp jolt, S waves follow with the stronger side-to-side shaking, and surface waves arrive last and do most of the damage.
The crust resettles
The slip changes the stress on nearby patches of fault, some of which then fail in turn. Those are aftershocks, and they can continue for months.
The three ways a fault moves
Fault type determines what the shaking feels like and whether the sea floor is displaced, which is what generates a tsunami.
| Fault type | How it moves | Where | Example |
|---|---|---|---|
| Strike-slip | Two blocks slide horizontally past each other | Transform boundaries | The San Andreas Fault, California |
| Normal | The crust pulls apart and one block drops down | Where plates diverge or crust stretches | The Basin and Range, Nevada and Utah |
| Reverse or thrust | Compression pushes one block up over another | Subduction zones and collision belts | The Cascadia subduction zone, Pacific Northwest |
Thrust faults at subduction zones produce the largest earthquakes on Earth, because they can rupture along hundreds or thousands of kilometres of locked interface at once. Every recorded magnitude 9 has been one.
They are also the ones that make tsunamis. Vertical movement of the sea floor lifts the water column above it, and that displacement becomes a wave. A strike-slip earthquake of the same magnitude, moving the ground sideways, usually does not.
Can earthquakes be predicted?
- No, and not for lack of trying. Neither USGS nor any other scientific body has ever predicted a major earthquake, and none expects to know how in the foreseeable future. What can be done is forecasting: stating the probability of an earthquake of a given size in a region over years or decades, which is what building codes are based on.
Why do aftershocks happen?
- A slip changes the stress on the surrounding crust, loading nearby patches of fault that were already close to failing. Aftershock rates decay roughly with the reciprocal of time, so the first day produces more than the first month that follows it, but a large earthquake can generate them for years.
Can fracking cause earthquakes?
- Wastewater disposal can, and the effect is well documented. Injecting fluid at depth raises pore pressure and reduces the friction holding an existing fault shut. Oklahoma went from roughly two magnitude 3 or larger earthquakes a year before 2009 to hundreds a year at the peak, and rates fell again after injection volumes were restricted.
Is there earthquake weather?
- No. Earthquakes begin kilometres underground where surface conditions have no influence, and statistical checks find no association with temperature, pressure or season. The belief is ancient and persistent, and it is wrong.