PrepTest 85, Section 4, Question 24

According to the generally accepted theory of plate tectonics, the earth's crust consists of a dozen or so plates of solid rock moving across the mantle�the slightly fluid layer of rock between crust and core. Most earthquakes can then be explained as a result of the grinding of these plates against one another as they collide. When two plates collide, one plate is forced under the other until it eventually merges with the underlying mantle. According to this explanation, this process, called subduction, causes an enormous build-up of energy that is abruptly released in the form of an earthquake. Most earthquakes take place in the earth's seismic "hot zones"—regions with very high levels of subduction. Contrary to expectations, however, global seismic data indicate that there are also regions with high levels of subduction that are nonetheless nearly free of earthquakes. Thus, until recently, there remained a crucial question for which the plate tectonics theory had no answer—how can often intense subduction take place at certain locations with little or no seismic effect?

One group of scientists now proposes that the relative quiet of these zones is tied to the nature of the collision between the plates. In many seismic hot zones, the plates exhibit motion in opposite directions�that is, they collide because they are moving toward each other. And because the two plates are moving in opposite directions, the subduction zone is relatively motionless relative to the underlying mantle. In contrast, the plate collisions in the quiet subduction zones occur between two plates that are moving in the same general direction—the second plate's motion is simply faster than that of the first, and its leading edge therefore becomes subducted. But in this type of subduction, the collision zone moves with a comparatively high velocity relative to the mantle below. Thus, rather like an oar dipped into the water from a moving boat, the overtaking plate encounters great resistance from the mantle and is forced to descend steeply as it is absorbed into the mantle. The steep descent of the overtaking plate in this type of collision reduces the amount of contact between the two plates, and the earthquake-producing friction is thereby reduced as well. On the other hand, in collisions in which the plates move toward each other the subducted plate receives relatively little resistance from the mantle, and so its angle of descent is correspondingly shallow, allowing for a much larger plane of contact between the two plates. Like two sheets of sandpaper pressed together, these plates offer each other a great deal of resistance.

This proposal also provides a warning. It suggests that regions that were previously thought to be seismically innocuous—regions with low levels of subduction—may in fact be at a significant risk of earthquakes, depending on the nature of the subduction taking place.

According to the generally accepted theory of plate tectonics, the earth's crust consists of a dozen or so plates of solid rock moving across the mantle�the slightly fluid layer of rock between crust and core. Most earthquakes can then be explained as a result of the grinding of these plates against one another as they collide. When two plates collide, one plate is forced under the other until it eventually merges with the underlying mantle. According to this explanation, this process, called subduction, causes an enormous build-up of energy that is abruptly released in the form of an earthquake. Most earthquakes take place in the earth's seismic "hot zones"—regions with very high levels of subduction. Contrary to expectations, however, global seismic data indicate that there are also regions with high levels of subduction that are nonetheless nearly free of earthquakes. Thus, until recently, there remained a crucial question for which the plate tectonics theory had no answer—how can often intense subduction take place at certain locations with little or no seismic effect?

One group of scientists now proposes that the relative quiet of these zones is tied to the nature of the collision between the plates. In many seismic hot zones, the plates exhibit motion in opposite directions�that is, they collide because they are moving toward each other. And because the two plates are moving in opposite directions, the subduction zone is relatively motionless relative to the underlying mantle. In contrast, the plate collisions in the quiet subduction zones occur between two plates that are moving in the same general direction—the second plate's motion is simply faster than that of the first, and its leading edge therefore becomes subducted. But in this type of subduction, the collision zone moves with a comparatively high velocity relative to the mantle below. Thus, rather like an oar dipped into the water from a moving boat, the overtaking plate encounters great resistance from the mantle and is forced to descend steeply as it is absorbed into the mantle. The steep descent of the overtaking plate in this type of collision reduces the amount of contact between the two plates, and the earthquake-producing friction is thereby reduced as well. On the other hand, in collisions in which the plates move toward each other the subducted plate receives relatively little resistance from the mantle, and so its angle of descent is correspondingly shallow, allowing for a much larger plane of contact between the two plates. Like two sheets of sandpaper pressed together, these plates offer each other a great deal of resistance.

This proposal also provides a warning. It suggests that regions that were previously thought to be seismically innocuous—regions with low levels of subduction—may in fact be at a significant risk of earthquakes, depending on the nature of the subduction taking place.