Active faults · Overlay
Fault Lines Map
Plate boundaries are the big picture; faults are the actual structures that break. Switch on the active fault layer and the map fills in with the individual fractures that accommodate plate motion — dense across California, Nevada and the Aegean, sparse where nobody has surveyed, and frequently crowded with earthquakes that line up along them.
What a fault is
A fault is a fracture in rock where the two sides have moved relative to each other. Faults exist at every scale, from centimetres to the thousand-kilometre structures that define plate edges. When accumulated stress finally overcomes friction, the two sides slip suddenly — and that slip is the earthquake.
The distinction from a plate boundary matters. A boundary is a broad zone of deformation; a fault is one fracture within it. The Pacific–North American boundary is not just the San Andreas but a system hundreds of kilometres wide, containing the Hayward, Calaveras, San Jacinto, Elsinore and many more. That is why the fault layer looks like a frayed rope rather than a single line.
The three ways faults move
Strike-slip
The two sides slide horizontally past one another with little vertical motion. These form at transform boundaries, and the San Andreas Fault is the textbook example — the Pacific plate grinding northwest relative to North America. Strike-slip earthquakes are shallow and can be very destructive when they run beneath cities, as the North Anatolian Fault has repeatedly demonstrated in Turkey.
Normal
The crust is being pulled apart, and the hanging wall drops down relative to the footwall. Normal faults dominate at divergent boundaries and in extending continental terrain — the Basin and Range province across Nevada and Utah is an enormous field of them, which is why that region shows a dense mesh of faults on the map despite being far from a plate edge.
Reverse & thrust
The crust is being compressed and the hanging wall is pushed up over the footwall. A reverse fault with a shallow dip angle is called a thrust fault, and subduction zone megathrusts are the largest of all. Every recorded magnitude 9 earthquake has been a subduction thrust event, because these faults present by far the biggest rupture surfaces on Earth.
| Fault type | Stress | Motion | Typical setting |
|---|---|---|---|
| Strike-slip | Shear | Horizontal sliding | Transform boundaries |
| Normal | Tension | Hanging wall drops | Rifts, spreading centres |
| Reverse / thrust | Compression | Hanging wall rides up | Subduction zones, collisions |
Why quakes and mapped faults do not always line up
This is the most common confusion the fault layer produces, and it is worth stating plainly: the absence of a mapped fault does not mean the absence of a fault.
Fault databases record structures that have been identified, surveyed and published. That process overwhelmingly favours dry land in well-funded, well-studied countries. As a result:
- Offshore faults are heavily under-represented, even though a large share of the world's earthquakes happen beneath the ocean.
- Blind faults have no surface trace at all. The 1994 Northridge earthquake occurred on a fault that had not been mapped before it ruptured.
- Coverage density reflects research effort. The dense fault mesh across the western United States partly reflects decades of intensive USGS mapping, not a uniquely fractured crust.
So treat the fault layer as a map of what we know, not a complete map of what exists. Comparing it against the earthquake data is a good way to see the gap.
Try this: search California over several years with a low magnitude floor and the fault layer on. Aftershock sequences and swarms trace linear structures directly along mapped faults — you are watching the fault map being redrawn by earthquakes.
Common questions
What makes a fault active?
A fault is generally classed as active if it has moved during the Quaternary period, roughly the last 2.6 million years, and is considered capable of moving again. Definitions vary between national surveys, and some use a stricter Holocene threshold of about the last 11,700 years.
What is the difference between a fault and a plate boundary?
A plate boundary is the broad zone where two tectonic plates meet. A fault is an individual fracture in rock along which movement occurs. A plate boundary is usually made up of many faults rather than a single clean break, which is why boundary zones can be hundreds of kilometres wide.
Why do some earthquakes appear far from any mapped fault?
Fault datasets only cover faults that have been identified and surveyed, which strongly favours land in well-studied countries. Offshore faults, buried faults with no surface expression, and faults in remote regions are frequently missing, so earthquakes regularly occur where no fault is drawn.