Geology

Earthquake belts and distribution. Earthquakes occur in well‐defined belts that correspond to active plate tectonic zones. The circum‐Pacific belt (also called the Rim of Fire) follows the rim of the Pacific Ocean and hosts over 80 percent of the world's shallow and medium‐depth earthquakes and 100 percent of the deep earthquakes. Other earthquake zones are the Mediterranean‐Himalayan belt and the midoceanic ridges that split the crust at the bottom of the world's oceans.

Plate boundaries and associated earthquakes. Distribution plots reveal that many earthquakes are associated with andesitic volcanic action and oceanic trenches that occur over subduction zones in the circum‐Pacific belt. Oceanic trenches are narrow, deep troughs that mark where two plates converge, usually along the edge of a continent or island are where andesitic volcanoes typically occur. Earthquakes originate in Benioff zones, zones that slope downward from the trenches and under the overlying rocks at 30 to 60 degrees. Benioff zones are closely associated with the subduction of a crustal plate below an adjacent plate.

Almost all earthquakes occur at the edges of the crustal plates. The constant bumping, grinding, and lateral movement along crustal boundaries can create sudden movements that result in earthquakes. Each of the three types of plate boundaries—convergent, divergent, and transform—has a distinctive pattern of earthquakes.

There are two kinds of convergent boundaries: subduction and collision. A subduction boundary is marked by the oceanic crust of one plate that is being pushed downward beneath the continental or oceanic crust of another plate. A collision boundary separates two continental plates that are pushed into contact; the suture zone is the line of collision. Both types of boundaries have distinctive earthquake patterns.

Earthquakes associated with a collision boundary define shallow, broad zones of seismic activity that form in complex fault systems along the suture zone. Earthquake patterns in subduction zones are more complex. As the oceanic crust begins to descend, it begins to break into blocks because of tension stress. Shallow earthquakes in the upper part of the subduction zone are a result of shallow‐angle thrust faults, in which slices of plates slide like cards in a deck that is being shuffled. Earthquakes also periodically occur as the plate continues to subduct up to a depth of about 670 kilometers (400 miles). First‐motion studies of these earthquakes suggest they result from both compressional and tensional forces on the subducting plate.

Earthquakes are relatively abundant in the first 300 kilometers (180 miles) of a subduction zone, are scarce from 300 to 450 kilometers (180 to 270 miles), and then increase slightly again from 450 to 670 kilometers (270 to 400 miles). It is possible that these deepest quakes are related to sudden mineral transformations and resultant energy releases or volume changes. It has been theorized that earthquakes do not occur at depths greater than 670 kilometers because the subducting plate is not brittle anymore and has become hot enough to flow plastically.

The distribution of earthquake foci along a subduction zone gives an accurate profile of the angle of the descending plate. Most often, plates start subducting at a shallow angle, which becomes steeper with depth. The angle of subduction is proportional to the density of the plate material, the amount of faulting and thrusting, and the tearing or crumpling of the descending plate.

Divergent boundaries are those at which crustal plates move away from each other, such as at midoceanic ridges. These huge underwater mountains often have a central graben feature, or rift valley, that forms at the crest of the ridge. The formation of new ocean crust that is pushed away from both sides of the ridge fault creates a tensional setting that results in the formation of the graben. Earthquakes are located along the normal faults that form the sides of the rift or beneath the floor of the rift. Divergent faults and rift valleys within a continental mass also host shallow‐focus earthquakes.

Shallow‐focus earthquakes occur along transform boundaries where two plates move past each other. The earthquakes originate in the transform fault, or in parallel strike‐slip faults, probably when a frictional resistance in the fault system is overcome and the plates suddenly move.