Earthquakes and Plate Tectonics
The world's earthquakes are not randomly distributed over the Earth's surface. They tend to be concentrated in narrow zones. Why is this? And why are volcanoes and mountain ranges also found in these zones, too?
An explanation is to be found in plate tectonics, a concept which has revolutionized thinking in the Earth's sciences in the last 10 years. The theory of plate tectonics combines many of the ideas about continental drift (originally proposed in 1912 by Alfred Wegener in Germany) and sea-floor spreading (suggested originally by Harry Hess of Princeton University).
Plate tectonics tells us that the Earth's rigid outer shell (lithosphere) is broken into a mosaic of oceanic and continental plates which can slide over the plastic aesthenosphere, which is the uppermost layer of the mantle. The plates are in constant motion. Where they interact, along their margins, important geological processes take place, such as the formation of mountain belts, earthquakes, and volcanoes.
The lithosphere covers the whole Earth. Therefore, ocean plates are also involved, more particularly in the process of sea-floor spreading. This involves the midocean ridges which are a system of narrow submarine cracks that can be traced down the center of the major oceans. The ocean floor is being continuously pulled apart along these midocean ridges. Hot volcanic material rises from the Earth's mantle to fill the gap and continuously forms new oceanic crust. The midocean ridges themselves are broken by offsets know as transform faults.
One of the keys to plate tectonics was the discovery that the Earth's magnetic field has reversed its polarity 170 times in the last 80 million years. As new basaltic material is squeezed up into the midocean cracks and solidifies, it is magnetized according to the polarity of the Earth's magnetic field. If the field reverses its polarity, the strip of new material is magnetized in an opposite sense. As the oceanic floor continues to spread, the new strips of rock are carried away on either side like a conveyer belt.
Using these magnetic strips as evidence of movement, it became obvious that the Earth's surface consisted of a mosaic of crustal plates that were continually jostling one another. If the Earth was not to be blown up like a balloon by the continual influx of new volcanic material at the ocean ridges, then old crust must be destroyed at the same rate where plates collide. The required balanced occurs when plates collide, and one plate is forced under the other to be consumed deep in the mantle.
We now know that there are seven major crustal plates, subdivided into a number of smaller plates. They are about 80 kilometers thick, all in constant motion relative to one another, at rates varying from 10 to 130 millimeters per year. Their pattern is neither symmetrical nor simple. As we learn more and more about the major plates, we find that many complicated and intricate maneuvers are taking place. We learn, too, that most of the geological action - mountains, rift valleys, volcanoes, earthquakes, faulting - is due to different types of interaction at plate boundaries.
How are earthquakes connected with plate tectonics? In 1969, Muawia Barazangi and James Dorman published the locations of all earthquakes which occurred from 1961 to 1967. Most of the earthquakes are confined to narrow belts and these belts define the boundaries of the plates. The interiors of the plates themselves are largely free of large earthquakes, that is, they are aseismic. There are notable exceptions to this. An obvious one is the 1811-1812 earthquakes at New Madrid, Missouri, and another is the 1886 earthquake at Charleston, South Carolina. As yet there is no satisfactory plate tectonic explanation for these isolated events; consequently, we will have to find alternative mechanisms.
Plate tectonics confirms that there are four types of seismic zones. The first follows the line of midocean ridges. Activity is low, and it occurs at very shallow depths. The point is that the lithosphere is very thin and weak at these boundaries, so the strain cannot build up enough to cause large earthquakes. Associated with this type of seismicity is the volcanic activity along the axis of the ridges (for example, Iceland, Azores, Tristan da Cunha).
The second type of earthquake associated with plate tectonics is the shallow-focus event unaccompanied by volcanic activity. The San Andreas fault is a good example of this, so is the Anatolian fault in Northern Turkey. In these faults, two mature plates are scraping by one another. The friction between the plates can be so great that very large strains can build up before they are periodically relieved by large earthquakes. Nevertheless, activity does not always occur along the entire length of the fault during any one earthquake. For instance, the 1906 San Francisco event was caused by breakage only along the northern end of the San Andreas fault.
The third type of earthquake is related to the collision of oceanic and continental plates. One plate is thrust or subducted under the other plate so that a deep ocean trench is produced. In the Philippines, ocean trenches are associated with curved volcanic island arcs on the landward plate, for example the Java trench. Along the Peru - Chile trench, the Nazca plate is being subducted under the South American plate which responds by crumpling to form the Andes. This type of earthquake can be shallow, intermediate, or deep, according to its location on the downgoing lithospheric slab. Such inclined planes of earthquakes are know as Benioff zones.
The fourth type of seismic zone occurs along the boundaries of continental plates. Typical of this is the broad swath of seismicity from Burma to the Mediterranean, crossing the Himalayas, Iran, Turkey, to Gilbraltar. Within this zone, shallow earthquakes are associated with high mountain ranges where intense compression is taking place. Intermediate- and deep-focus earthquakes also occur and are known in the Himalayas and in the Caucasus. The interiors of continental plates are very complex, much more so than island arcs. For instance, we do not yet know the full relationship of the Alps or the East African rift system to the broad picture of plate tectonics.
How can plate tectonics help in earthquake prediction? We have seen that earthquakes occur at the following three kinds of plate boundary: ocean ridges where the plates are pulled apart, margins where the plates scrape past one another, and margins where one plate is thrust under the other. Thus, we can predict the general regions on the Earth's surface where we can expect large earthquakes in the future. We know that each year about 140 earthquakes of magnitude 6 or greater will occur within this area which is 10 percent of the Earth's surface.
But on a worldwide basis we cannot say with much accuracy when these events will occur. The reason is that the processes in plate tetonics have been going on for millions of years. Averaged over this interval, plate motions amount to a several millimeters per year. But at any instant in geologic time, for example, the year 1977, we do not know exactly where we are in the worldwide cycle of strain buildup and strain release. Only by monitoring the stress and strain in small areas, for instance, the San Andreas fault, in great detail can we hope to predict when renewed activity in that part of the place tectonics arena is likely to take place.
In summary, plate tectonics is a blunt, but, nevertheless, strong tool in earthquake prediction. It tells us where 90 percent of the Earth's major earthquakes are likely to occur. It cannot tell us much about exactly when they will occur. For that, we must study in detail the plate boundaries themselves. Perhaps the most important role of plate tectonics is that it is a guide to the use of finer techniques for earthquake prediction.
Scientific American, 1976, Continents adrift and continents aground - Reading from Scientific American: San Francisco, W.H. Freeman and Co., 2 30 p.
Abridged from Earthquake Information Bulletin, vol. 9, no. 6, November - December 1977, by Henry Spall, USGS, Reston, VA.