How Do Normal Faults Differ From Reverse Faults?

Normal faults and reverse faults are both fractures in the Earth’s crust where rocks have moved, but they move in opposite directions because of different types of stress. In a normal fault, the rock above the fault line (the hanging wall) slides downward relative to the rock below (the footwall). In a reverse fault, the hanging wall moves upward over the footwall. This difference comes down to tension versus compression: normal faults form where the crust is being pulled apart, while reverse faults form where it is being squeezed together.

What Exactly Is a Fault?

A fault is a fracture in the Earth’s crust where blocks of rock have slipped past each other. This movement is rarely smooth. It usually happens in sudden jerks that we feel as earthquakes.

To understand faults, you need to know two terms. The hanging wall is the block of rock that sits above the angled fault surface. The footwall is the block that sits below it. Imagine a mine tunnel following the fault line. You stand on the footwall, and the hanging wall hangs over your head.

The way these two blocks move relative to each other is what defines the fault type. It is not about which side is higher or lower in the landscape today. It is about the direction of movement that caused the fault to form.

How Do Normal Faults Differ From Reverse Faults?

The core difference is the direction of movement. In a normal fault, the hanging wall moves down relative to the footwall. In a reverse fault, the hanging wall moves up relative to the footwall.

This movement is driven by different forces. Normal faults are caused by tensional stress, which pulls the crust apart. Think of stretching a piece of taffy until it snaps. The crust thins and breaks, and one block drops down. Reverse faults are caused by compressional stress, which pushes the crust together. Imagine pushing two cars into each other until one rides up over the hood of the other.

The angle of the fault plane also tends to differ. Normal faults are usually steep, dipping at angles around 60 degrees. Reverse faults often dip at shallower angles, especially in mountain-building zones. When a reverse fault dips at less than 45 degrees, it is called a thrust fault, but the basic movement is the same.

Where Do Normal Faults Form?

Normal faults are found where the Earth’s crust is being extended or stretched. This happens in several distinct settings.

The most famous example is the Basin and Range Province in the western United States. This region covers most of Nevada, parts of Utah, California, and Arizona. Over millions of years, the crust has been pulled apart. Blocks of rock dropped down along normal faults, creating the alternating mountain ranges and flat valleys you see there today.

Normal faults also form at divergent plate boundaries, where tectonic plates move away from each other. The Mid-Atlantic Ridge is a long chain of underwater volcanoes where new crust is created. As the plates separate, normal faults form along the ridge. The East African Rift Valley is another example, where the African continent is slowly splitting apart.

Normal faults can also form at the edges of some mountain ranges where the crust is collapsing under its own weight. The Sierra Nevada in California has normal faults along its eastern edge. The mountains are rising, but the valley floor to the east is dropping along these faults.

Where Do Reverse Faults Form?

Reverse faults form where the crust is being compressed or shortened. The most common settings are convergent plate boundaries, where two plates collide.

When an oceanic plate collides with a continental plate, the oceanic plate dives beneath it in a process called subduction. The compression from this collision creates reverse faults in the overlying continental crust. The Andes Mountains in South America and the Cascades in the Pacific Northwest formed this way.

When two continental plates collide, the crust crumples and thickens. The Himalayas are the best example. The Indian plate is still pushing into the Eurasian plate. Reverse faults and thrust faults stack slices of rock on top of each other, building the highest mountains on Earth. The 2015 Nepal earthquake, which killed nearly 9,000 people, occurred on a thrust fault related to this collision.

Reverse faults also form in smaller compressional settings. Some form along strike-slip plate boundaries where the crust is locally squeezed. Others form in the interior of continents where distant plate collisions send stress through the crust. The Rocky Mountains include some reverse faults that formed during the Laramide orogeny, a period of mountain building that occurred far from the plate boundary.

Why Does the Angle of the Fault Matter?

The angle of the fault plane, called the dip, is a key feature that helps geologists tell these faults apart in the field. It also affects how the fault behaves.

Normal faults typically have steep dips, often around 60 degrees from horizontal. Because the hanging wall drops, the fault surface is exposed at the surface, and the steep angle means the two blocks separate cleanly as the crust stretches.

Reverse faults, especially thrust faults, often have shallow dips. Some thrust faults dip at only 10 to 30 degrees. This shallow angle means the hanging wall can travel a long horizontal distance as it moves up. In the Canadian Rockies, some thrust faults have moved rock tens of kilometers horizontally. The McConnell Thrust, for example, carried ancient rock over much younger rock for a distance of roughly 40 kilometers.

This difference in angle is not just an academic detail. It affects the size and shape of the area that experiences ground shaking during an earthquake. Shallow-dipping reverse faults can create larger areas of strong shaking because the rupture surface is more spread out.

Earthquake Risks From Normal and Reverse Faults

Both types of faults can produce large, destructive earthquakes. The risk depends on the fault’s location, its length, and how much stress has built up.

Normal faults can generate significant earthquakes, though they tend to be smaller than the largest reverse fault events. The 1983 Borah Peak earthquake in Idaho, a magnitude 6.9 event, occurred on a normal fault. It lifted the Lost River Range by several feet and created a visible fault scarp. The 1959 Hebgen Lake earthquake in Montana, magnitude 7.2, also occurred on a normal fault and triggered a massive landslide.

Reverse faults produce some of the largest earthquakes ever recorded. The 2004 Sumatra-Andaman earthquake, magnitude 9.1, occurred on a thrust fault where the Indian plate dived beneath the Burma plate. It generated a tsunami that killed over 200,000 people. The 2011 Tohoku earthquake in Japan, magnitude 9.1, also occurred on a thrust fault in the Japan Trench.

The reason reverse faults can generate larger earthquakes is related to their setting. They form in compressional zones where stress can build over vast areas for thousands of years. When the fault finally slips, it releases enormous amounts of energy. Normal faults, by contrast, often have shorter segments and release stress more frequently in smaller events.

How Can You Identify a Fault in the Field?

Geologists use several clues to identify faults and determine their type. You can learn to spot some of these signs yourself if you know what to look for.

A fault scarp is a step in the landscape created when the ground on one side of a fault moves up or down. Normal faults often create steep, fresh-looking scarps. Reverse faults can create scarps too, but they may be more eroded and harder to see. In active mountain ranges, repeated reverse fault movement creates the steep mountain fronts you see at the base of ranges like the Wasatch Front in Utah.

Another clue is the rock itself. Fault movement grinds and breaks rock, creating a zone of crushed material called fault breccia or a polished surface called slickensides. Slickensides have grooves that show the direction of movement. If you can determine which way the grooves point, you can tell whether the hanging wall moved up or down.

Offset rock layers are the most reliable evidence. If you see a rock layer that is continuous on one side of a fracture but displaced on the other, you can measure the direction and amount of movement. This requires some geologic training, but the basic principle is simple: the rocks tell the story of how they moved.