The Types of Earthquakes

Explainer · 5-minute read

Not every earthquake has the same cause. Most are tectonic — the plate-driven kind covered in what is an earthquake — but the ground can also shake for volcanic, structural and even human-made reasons. Here are the main types, and the fault movements behind them.

By cause

1. Tectonic earthquakes

By far the most common and the largest. They happen when stress along a fault between (or within) tectonic plates is released as the rock suddenly slips. Nearly every quake you see on the tracker is tectonic, which is why the dots line up along plate boundaries.

2. Volcanic earthquakes

Caused by magma forcing its way through rock beneath a volcano, or by the ground fracturing as a magma chamber inflates or drains. They're usually smaller than big tectonic quakes but are an important warning sign that a volcano is becoming active. Swarms of them often precede an eruption.

3. Collapse earthquakes

Small, local quakes from the sudden collapse of underground spaces — for example the roof of a cave or an old mine. They're felt only nearby and rarely cause wider damage.

4. Induced (human-caused) earthquakes

Human activity can also trigger quakes: filling large reservoirs behind dams, deep wastewater injection, geothermal energy, mining and some types of resource extraction can all change the stress on existing faults. Most induced quakes are small, but a few have been large enough to cause damage, which is why they're carefully monitored.

By fault movement

Tectonic earthquakes are also described by how the two sides of the fault move relative to each other:

• Strike-slip: the blocks slide horizontally past one another, like the San Andreas Fault. Common where plates grind sideways.
• Normal (extensional): the crust is being pulled apart, so one block drops down relative to the other. Common at spreading boundaries and rift zones.
• Thrust / reverse (compressional): the crust is being squeezed, so one block is pushed up over the other. These dominate subduction zones and produce the largest, most tsunami-prone earthquakes.

Why the type matters

The cause and fault type shape how big a quake can get, how deep it is, and whether it can generate a tsunami. Shallow thrust quakes on subduction megathrusts are the ones capable of reaching magnitude 9; most strike-slip and volcanic quakes are smaller. The tracker shows depth and location for each event, which hints at the kind of fault involved.

More: Seismic waves & subduction · What is an earthquake · The magnitude scale.