Is earthquake incidence on the rise? The short answer is "no." On a global basis, earthquakes are happening more or less in line with their long-term statistics.
But the longer, and perhaps more pertinent answer is that quake-prone regions sometimes enter periods of increased activity. This seems to be happening in California, where the increased incidence of moderate quakes is of concern to seismologists. It may also be happening in the Mediterranean, although two such quakes in close time succession and in roughly the same region do not necessarily constitute a trend.
By geological standards, the magnitude 6.8 earthquake that devastated southern Italy Nov. 23 was moderate and not at all unusual. Typically, several dozen quakes of comparable severity occur around the world yearly. But the fact that two other such quakes have occurred in populated areas within the past two months -- Algeria, Oct. 10, magnitude 7.3 and northern California, Nov. 9, magnitude 7 -- raises the question of whether earthquake incidence is on the rise.
Waverly Person, of the United States Geological Survey (USGS) National Earthquake Information Center, points out that the occurrence of quakes in populated areas can give a false impression. Many quakes happen in remote areas without being noticed. Thus, when several earthquakes in succession do occur in populated regions, people may be misled into thinking that earthquakes are on the rise.
As a yearly average around the world, geologists expect approximately 100 quakes greater than magnitude 6, including 19 quakes of magnitude 7 or greater, plus one "great earthquake" of magnitude 8. The actual number of quakes may vary considerably from this in any one year. Last year, for example, there were only 12 quakes of magnitude 7 or larger compared to 17 in 1978. The 1979 total included an 8.1 magnitude shock, the first great earthquake in over two years.
This year, so far, there have been 11 quakes of magnitude 7 or larger worldwide and one of magnitude 8, which hit the Santa Cruz Islands in the Pacific Ocean. Thus there is nothing in the global statistics to suggest any general rise in earthquake incidence.
However, while earthquakes worldwide were relatively sparce in 1979, their incidence rose 30 percent in the United States. Indeed, the ocurrence of moderate earthquakes has stepped up so much in California that seismologists have been wondering if this is a prelude to a truly major shock -- one of magnitude 8 or larger. The Nov. 9 quake was preceded by a series of somewhat less-intense quakes near San Francisco beginning Jan. 24 which, in turn, followed a quake in November 1979 in California's Imperial Valley.
Now in the Mediterranean, two moderately strong earthquakes have occurred in rapid succession.
Both regions are zones of strong crustal activity. Earth's outer layer is broken into a dozen or so major plates, plus a number of smaller units. These bump together, move apart, slide past, or override one another. The action is rarely smooth. Plates lock together, building up strains until rock breaks and the pent-up energy is released in earthquakes.
The earthquake magnitude scale reflects the differences in energy involved. Each step up the scale represents roughly a 30-fold increase in energy released. Thus quakes in the 6 to 7 magnitude range do not begin to release the energy of a magnitude 8 quake. That is why seismologists who believe that the California fault systems have a lot of pent-up energy think that the string of moderate quakes may be the beginning of a series that will lead to a major energy release -- that is, a great earthquake. They do not see them as being a mechanism for relieving the main part of the strain relatively gently.
In California, the strain builds up as two giant plates try to slide past each other. These are the plates carrying the pacific Ocean and North America, respectively. In the Mediterranean, the situation is more complex. The African plate seems to be pushing up against Europe and overriding the European plate. Also, the crust is broken into several smaller plates. It would be premature to conclude that two moderate quakes in this system are the start of an earthquake series. But they at least are a warning against complacence.