Which of the following faults occurs when the hanging wall moves down relative to the footwall?

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There are four types of earthquake faults, which are differentiated by the relative position of the fault plane -- that is, the flat surface along which there's a slip during an earthquake.

In a normal fault (see animation below), the fault plane is nearly vertical. The hanging wall, the block of rock positioned above the plane, pushes down across the footwall, which is the block of rock below the plane. The footwall, in turn, pushes up against the hanging wall. These faults occur where the crust is being pulled apart, at a divergent plate boundary.

The fault plane in a reverse fault is also nearly vertical, but the hanging wall pushes up, and the footwall pushes down. This sort of fault forms where a plate is being compressed. A thrust fault moves the same way as a reverse fault, but at an angle of 45 degrees or less [source: USGS]. In these faults, which are also caused by compression, the rock of the hanging wall is actually pushed up on top of the footwall at a convergent plate boundary.

In a strike-slip fault, the blocks of rock move in opposite horizontal directions. These faults form when crust pieces slide along each other at a transform plate boundary. The San Andreas Fault in California is one example of a transform plate boundary.

With all these faults, rocks push together tightly, creating friction. If there's enough friction, they become locked, so that they won't slide anymore. Meanwhile, the Earth's forces continue to push against them, increasing the pressure and pent-up energy. If the pressure builds up enough, it will overcome the friction, the lock will give way suddenly, and the rocks will snap forward. To put it another way, as the tectonic forces push on the "locked" blocks, potential energy builds. When the plates are finally moved, this built-up energy becomes kinetic.

The sudden, intense shifts along already formed faults are the main sources of earthquakes. Most earthquakes occur around plate boundaries because this is where strain from plate movements is felt most intensely, creating fault zones, groups of interconnected faults. In a fault zone, the release of kinetic energy at one fault may increase the stress -- the potential energy -- in a nearby fault, leading to other earthquakes. That's one reason why several earthquakes may occur in an area in a short period of time.

These additional quakes are called foreshocks and aftershocks. The quake with the largest magnitude is called the mainshock; any quakes that occur before the mainshock are called foreshocks, and any quakes that occur after the mainshock are called aftershocks. Most of the time, the worst aftershocks occur within the first 24 hours after the mainshock hits. Bigger earthquakes trigger more aftershocks with larger magnitudes.

In the next section, we'll talk about the waves of energy that earthquakes generate, and the effects that they cause.

By Jennifer Gunner, M.Ed. Education , Staff Writer

Which of the following faults occurs when the hanging wall moves down relative to the footwall?

  • geology San Andreas fault line

  • CampPhoto / iStock / Getty Images Plus

Tectonic plates are always moving under your feet. This constant lithospheric motion results in surface fractures in the Earth’s crust, which are called faults. Large faults also appear in the boundaries between tectonic plates. Keep reading to learn more about the three main types of faults – normal, reverse, and strike-slip faults – as well as places in the world where you can find them.

Faults consist of two rock blocks that displace each other during an earthquake or regular tectonic movement. One block is called the hanging wall, and the other is the footwall. Understanding the parts of a fault can help you identify what type of fault you’re seeing.

The main parts of a fault are:

  • Fault plane - the surface area between two rock blocks created by an earthquake
  • Fault trace - the visible crack in the Earth’s crust that indicates where a fault is
  • Fault scarp - the vertical step that rises during tectonic activity
  • Hanging wall - the rock block that hangs over the fault plane
  • Footwall - the rock block that occurs below the fault plane

The behavior of each of these parts helps earth scientists identify faults as normal, reverse, or strike-slip. Once you know what type a fault is, you can predict what can happen there during an earthquake.

Normal faults, or extensional faults, are a type of dip-slip fault. They occur when the hanging wall drops down and the footwall drops down. Normal faults are the result of extension when tectonic plates move away from each other.

Normal faults create space. These faults may look like large trenches or small cracks in the Earth’s surface. The fault scarp may be visible in these faults as the hanging wall slips below the footwall.

If you’re looking at a mountain that lies on a normal fault, you’ll see that the hanging wall has “dipped and slipped” under the footwall level. This gives the mountain a leaning, sloping look. In a flat area, a normal fault looks like a step or offset rock (the fault scarp).

You may see additional examples of normal faults in these places:

  • Atalanti Fault (Greece) - fault segment between the Apulia and Eurasia plates
  • Corinth Rift (Greece) - marine trench between the Aegean Sea Plate and Eurasian Plate
  • Humboldt Fault Zone (North America) - part of the Midwestern Rift System between Nebraska and Kansas
  • Moab Fault (North America) - canyon and valley zone on the North American Plate in Utah
  • Sierra Nevada Fault (North America) - fault along the eastern edge of the Sierra Nevada mountain range.
  • Wabash Valley Seismic Zone (North America) - series of faults on the North American plate between Illinois and Indiana.

Although reverse faults are also dip-slip faults, they behave the opposite way that a normal fault does. The hanging wall slides up over the footwall during tectonic movement in these faults. Reverse faults with a 45 degree dip (or less) are known as thrust faults, while faults with over 45-degree dips are overthrust faults.

Reverse faults look like two rocks or mountains have been shoved together. Unlike normal faults, reverse faults do not create space. They are found in areas of geological compression.

There are examples of reverse faults in several continents around the world. They are most common at the base of large mountain ranges. Some famous reverse faults include:

  • Glarus thrust (Switzerland) - thrust fault in the Swiss Alps
  • Longmenshan Fault (China) - thrust fault at the Longmen mountains, between the Eurasian and Indian-Australian plates
  • Lusatian Fault (Germany) - overthrust fault between the Elbe valley and Giant Mountains
  • San Ramón Fault (Chile) - part of the west Andean thrust fault system at the base of the Andes mountains
  • Sierra Madre Fault Zone (North America) - compression fault between the Pacific and North American tectonic plates
  • Tacoma Fault (Washington) - part of the Seattle Uplift system between the Juan de Fuca and North American plates

Unlike dip-slip faults which move vertically, rock blocks in strike-slip faults move laterally alongside each other. A fault that moves to the left is a sinistral transcurrent fault, and a fault that moves to the right is a dextral transcurrent fault. Strike-slip faults include transform (which end at another plate boundary) and transcurrent (which end before reaching another plate boundary) fault lines.

So how can you tell if you’re looking at a strike-slip fault? Many strike-slip faults are found on the ocean floor. But if you’re looking at a strike-slip fault, it may look like the land on either side has moved in opposite directions. This movement may cause offset rivers, parallel valleys, and abrupt ends to mountain chains.

The most famous example of a strike-slip fault is the San Andreas Fault. The 1300-kilometer San Andreas Fault stretches across most of California and divides the Pacific and North American tectonic plates. It is responsible for a number of smaller fault systems across the western United States.

Other examples of transcurrent faults include:

  • Anatolian Fault (Turkey) - fault between the Eurasian and the Anatolian plates
  • Alpine Fault (New Zealand) - fault between the Pacific and Indo-Australian plates
  • Hayward Fault Zone (North America) - transform boundary between the Pacific and North American plates; runs parallel to the San Andreas Fault
  • Kunlun fault (Tibet) - fault near the edge of the Eurasian plate
  • Yammouneh Fault (Lebanon) - part of the Dead Sea Transform fault system between the Arabian and African plates

Faults mark the edges of tectonic plates and points of lithospheric stress. But they also create the beautiful mountain ranges and valleys on our planet. If you’d like to learn more about landforms and earth science, check out an article that lists examples of landforms around the world.