| A Brief History of Seismology From: Cambridge University Press |
 hen a city that may have taken centuries to build is shaken to the ground in just seconds, contemporary chroniclers tend to be distracted from their usual preoccupation with human power at least long enough to document that the event took place. Such accounts, although often fragmentary, date back to the earliest Greek and Roman historians. By the late 1600s, a few scholars sifting through these old sources began to compile lists of the documented earthquakes; the earliest seems to be Vincenzo Magnati's 1688 list of ninety-one major earthquakes that occurred in the period A.D. 34 to A.D. 1687. Over the next two centuries, a dozen or so others published their own lists, often explicitly restricted to a particular geographical area or a particular period in time (e.g., one chronicles 1,186 shocks in Italy for the period 1783-6). To the extent that these lists overlap, they are often contradictory in relevant details. An equally serious shortcoming is that their entries reflect contemporary population distributions, their geographical accessibility, and the psychology of mass hysteria more than they describe anything approaching an objectified geophysical data base. |
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| The Mercalli scale Few, if any, of these early lists were of permanent significance (see B. F. Howell, Jr., An introduction to seismological research, 1990). Through their inconsistencies, however, such lists did make it clear that a uniform scale for describing earthquake intensity was desperately needed. In 1883, Guiseppe Mercalli (himself one of the list makers) rose to the occasion and proposed the Mercalli scale, a system still based on somewhat subjective observational descriptions. Others adopted it but, before long, began to fiddle with its criteria. The scale was officially modified in 1912, then once again in 1931; in the latter form it is still sometimes used today. An abbreviated version of the Modified Mercalli Intensity Scale is presented in Table 1. |
| The main continuing appeal of the Mercalli scale is that it does not depend at all on the use of scientific instruments but on ordinary human observations. Anyone who can make and assess the required observations can assign a Mercalli intensity to an earthquake. At the higher intensities, one need not even experience the event firsthand to assign the number, for the relevant criteria can be established through an examination of the damage left behind. |
| The Mercalli scale does, however, have its shortcomings: (1) it applies only to populated areas (a fact that becomes obvious as soon as you read the criteria); (2) it does not allow for fractional intensities (in fact, Roman numerals are used, so no one is likely to be tempted on this issue); and (3) it does not give any indication of the strength of the source of the earthquake (a low Mercalli intensity does not distinguish between a mild earthquake nearby and a strong one a greater distance away). |
| Nevertheless, when Mercalli numbers began to be applied to earthquakes, patterns started to emerge from the hodgepodge of lists. Well before 1920, it became clear that the most seismically unstable regions of the earth are associated with surface features where the earth's crust is most severely corrugated--for example, mountains and rifts (whether above or below the sea). Further, there are two broad bands on the globe that together account for more than 90% of the significant earthquakes: one of these bands circles the Pacific Ocean; the other extends in a shallower arc from Indonesia through the Himalayas to the Mediterranean. |
| Scientific approaches Scientists, meanwhile, looked for a more "scientific" way to measure earthquake strength--one linked to the recordings of an unbiased instrument. To design any instrument, however, you first need to have some quantitative understanding of the phenomenon you are trying to measure. This is obviously a Catch-22, for unless you already know something about the relevant characteristics of the phenomenon, you cannot build an instrument that will tell you about the very characteristics you need to know about. Given that earthquakes are sporadic and unpredictable by their very nature, progress in designing seismographs proceeded slowly. |
| The first device specifically designed to record earthquakes was apparently built in China in A.D. 132. This was a circle of eight sculptured bronze dragons, each holding a metal ball in its mouth, and, directly below, a corresponding circle of open-mouthed bronze toads. A strong earthquake would make a dragon drop a ball into the mouth of a toad, and the particular toad was expected to indicate the direction of the earthquake source (an incorrect assumption, it turns out). This device was a beautiful work of art but of dubious value as a scientific instrument, for any earthquake strong enough to be registered in this manner would already be quite apparent to everyone, and the instrument was incapable of supplying any additional information about such an event. |
| By the early 1700s, it had become common knowledge that strong earthquakes disturb the water surfaces of ponds and lakes, and this phenomenon was exploited in several early seismoscopes. Most of these devices used some variation on a vessel of liquid mercury that would spill, or at least slosh around, leaving a record of its motion. None of these devices was sensitive enough to be of much scientific value. |
| A more fruitful approach was to use a pendulum. It had long been noticed that bells in churchtowers often rang spontaneously during a strong earthquake and that pendulum clocks often stopped. Beginning in 1841, James D. Forbes experimented with various pendulum arrangements, and he eventually built a "seismoscope" consisting of a pencil attached to an inverted pendulum, which successfully recorded two earthquakes. Unfortunately, it failed to respond to most of the several dozen other earthquakes that were felt in the area where it was set up (see C. Davison, The founders of seismology, 1927). Meanwhile, geophysicists who were trying to measure the subtle effects of the Sun's and Moon's gravity on Earth were making considerable progress with instruments employing a heavier pendulum, and they were often finding (to their annoyance) that such instruments would go into spasms of uncontrolled jiggling during minor earthquakes. Closer examination revealed that the heavy pendulum itself wasn't jiggling at all; rather, the instruments were recording the vibrations of the ground relative to the pendulum, which, because of its inertia, remained pretty much at rest. |
| In Italy in 1875, Filippo Cecchi put these ideas together and built the first successful seismograph. The device used two heavy pendulums, suspended in such a way that one detected north-south motion and the other detected east-west motion (orientations still used today). At the same time, a third mass suspended on a spring permitted a measurement of the vertical component of the earthquake motion. Over the next few years, John Milne (working in Tokyo) made considerable improvements in the sensitivity of this instrument. Useful seismographic recordings of ground motion date from the Japanese earthquake of November 3, 1880. By the time of the 1906 San Francisco earthquake, scientists were able to compare seismograms of ground motion that had been recorded simultaneously at a number of observatories in different parts of the world. |
| There have been many improvements since, both in terms of sensitivity and in the method of recording data; computer printouts, for instance, have generally replaced the earlier strip-chart recordings. Many modern seismographs no longer measure the motion of the earth relative to a suspended inertial mass; instead they use electronic sensors to measure the strain deformation of the earth directly, usually between two points in a long underground tunnel. In this manner, it is possible to measure crustal movements as small as 0.001 millimeters over lengths of around 25 meters. Problems, nevertheless, remain. Even today, it is difficult to make reliable measurements of very long-period earthquake waves (30-s periods or greater). Moreover, a very strong earthquake will saturate the most sensitive instruments, in the same manner as if you tried to weigh a car on a bathroom scale. As a result, seismographic observatories need to keep a whole array of instruments in continuous operation, some for weak motions and others for strong motions. |
| The Richter Scale The Mercalli Intensity Scale, in one of its three principal versions, was used almost universally for some fifty years. With the progress in instrumentation, however, prospects improved for linking earthquake size to actual seismographic recordings of ground motion. By 1930, it was possible to combine seismographic data from different observatories to pinpoint the geological sources of most earthquakes. What remained was to develop an objective measure of the absolute magnitude, or source strength, of an earthquake. |
| In 1935, Charles F. Richter developed an enthusiastically accepted procedure and numerical scale for assigning earthquake magnitudes on his Richter Scale. An earthquake's Richter magnitude is determined by reading the maximum ground motion recorded by a seismograph, adjusting this value to reflect a "standard" distance from the source (100 km), correcting for any peculiar characteristics of the particular instrument used, then using a mathematical formula to relate the result to a logarithmic numerical scale.  The figure shows the basic relationship in graphical form. The Richter Scale has no top or bottom but can generally be considered to run from 0 to 9. Each increase of 1 on this scale represents a factor of 10 times the ground-motion amplitude, and an increase of 2 represents a factor of 10 x 10, or 100. For example, at a distance of 100 kilometers from the source, a magnitude 8.3 earthquake generates 10 times the shaking amplitude of a magnitude 7.3 earthquake. Similarly, a magnitude 5.6 earthquake shakes the ground with only 1/100 the amplitude of a magnitude 7.6 event. Although these comparisons technically apply only at the standard 100-kilometer distance from the source, until quite recently they were usually treated as a measure of the energy released at the source itself. |
| Earthquakes, it turns out, can differ greatly in many respects: The physical source can be deep or shallow; the source can be a large slip in a concentrated region of a fault or a smaller slip over a more extended region; the source might release greater portions of its energy in shorter--or longer--period waves; and so on. For this reason, it soon became apparent that a single Richter Scale did not give an honest comparison of the energy released in all types of earthquakes. Although it remains common practice to use the Richter magnitude (designated ML) for moderate-sized local earthquakes, a better measure for larger earthquakes seems to be the moment magnitude (MW), which involves a series of different seismographic measurements and a somewhat more involved calculation. These two scales often disagree; the 1964 Alaskan earthquake, for instance, had magnitude ML = 8.6 but MW = 9.2. Moreover, different seismographic observatories often report slightly different ML or MW scores for the same earthquake. It now appears that complete consistency in assigning earthquake magnitudes is probably an unrealistic goal, for when the natural phenomenon itself is inherently fuzzy and irreproducible, no amount of mathematical fiddling can be expected to force all of the data into agreement. |
Future prospects
While these developments in earthquake measurement were taking place, seismologists also made considerable progress in mapping the interior of the earth by analyzing how seismic waves travel between distant parts of the globe. This research helped establish the theory of plate tectonics, which was on a fairly firm footing by the early 1960s and explained why earthquakes are more common in some regions than others. About the same time, researchers began to propose more detailed theories of the mechanisms that produce earthquakes. In 1962, Japanese seismologists adopted earthquake prediction as a formal goal, and in the United States, in 1977 the Earthquake Hazards Reduction Act established prediction as a formal objective of U.S. government-sponsored seismological research. Although to date progress toward this goal of predicting the time, location, and size of earthquakes has been disappointing at best, seismology is still a very young science, and only time will tell where continuing seismological research will lead.
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