BASIC EARTHQUAKE PRINCIPLES - Geotechnical Earthquake Engineering - page 9

Mid-ocean ridge
Rift

Oceanic crust

Continental crust

FIGURE 2.3 Illustration of a divergent boundary (seafloor spreading). ( From USGS. )

Divergent Boundary. This occurs when the relative movement of two plates is away
from each other. The upwelling of hot magma that cools and solidifies as the tectonic plates
move away from each other forms spreading ridges. Figure 2.3 illustrates seafloor spread-
ing and the development of a mid-ocean ridge. An example of a spreading ridge is the mid-
Atlantic ridge (see Fig. 2.1). Earthquakes on spreading ridges are limited to the ridge crest,
where new crust is being formed. These earthquakes tend to be relatively small and occur
at shallow depths (Yeats et al. 1997).
When a divergent boundary occurs within a continent, it is called rifting. Molten rock
from the asthenosphere rises to the surface, forcing the continent to break and separate.
Figure 2.4 illustrates the formation of a continental rift valley. With enough movement, the
rift valley may fill with water and eventually form a mid-ocean ridge.

Convergent Boundary. This occurs when the relative movement of the two plates is
toward each other. The amount of crust on the earth's surface remains relatively constant,
and therefore when a divergent boundary occurs in one area, a convergent boundary must
occur in another area. There are three types of convergent boundaries: oceanic-continental
subduction zone, oceanic-oceanic subduction zone, and continent-continent collision zone.

1. Oceanic-continental subduction zone: In this case, one tectonic plate is forced
beneath the other. For an oceanic subduction zone, it is usually the denser oceanic plate that
will subduct beneath the less dense continental plate, such as illustrated in Fig. 2.5. A deep-
sea trench forms at the location where one plate is forced beneath the other. Once the sub-
ducting oceanic crust reaches a depth of about 60 mi (100 km), the crust begins to melt and
some of this magma is pushed to the surface, resulting in volcanic eruptions (see Fig. 2.5).
An example of an oceanic-continental subduction zone is seen at the Peru-Chile trench (see
Fig. 2.1).
2. Oceanic-oceanic subduction zone: An oceanic-oceanic subduction zone often
results in the formation of an island arc system, such as illustrated in Fig. 2.6. As the sub-
ducting oceanic crust meets with the asthenosphere, the newly created magma rises to the
surface and forms volcanoes. The volcanoes may eventually grow tall enough to form a
chain of islands. An example of an oceanic-oceanic subjection zone is the Aleutian Island
chain (see Fig. 2.1).
The earthquakes related to subduction zones have been attributed to four different con-
ditions (Christensen and Ruff 1988):

● Shallow interplate thrust events caused by failure of the interface between the down-
going plate and the overriding plate.
● Shallow earthquakes caused by deformation within the upper plate.
● Earthquakes at depths from 25 to 430 mi (40 to 700 km) within the down-going
plate.