I. Not felt
(a) Not felt, even under the most favorable circumstances.
(b) No effect.
(c) No damage.
II. Scarcely felt
(a) The tremor is felt only at isolated instances (
(b) No effect.
(c) No damage.
III. Weak
(a) The earthquake is felt indoors by a few. People at rest feel a swaying or light trembling.
(b) Hanging objects swing slightly.
(c) No damage.
IV. Largely observed
(a) The earthquake is felt indoors by many and felt outdoors only by very few. A few people are awakened. The level of vibration is not frightening. The vibration is moderate. Observers feel a slight trembling or swaying of the building, room or bed, chair, etc.
(b) China, glasses, windows and doors rattle. Hanging objects swing. Light furniture shakes visibly in a few cases. Woodwork creaks in a few cases.
(c) No damage.
V. Strong
(a) The earthquake is felt indoors by most, outdoors by few. A few people are frightened and run outdoors. Many sleeping people awake. Observers feel a strong shaking or rocking of the whole building, room or furniture.
(b) Hanging objects swing considerably. China and glasses clatter together. Small, top-heavy and/or precariously supported objects may be shifted or fall down. Doors and windows swing open or shut. In a few cases window panes break. Liquids oscillate and may spill from well-filled containers. Animals indoors may become uneasy.
(c) Damage of grade 1 to a few buildings of vulnerability class A and B.
VI. Slightly damaging
(a) Felt by most indoors and by many outdoors. A few persons lose their balance. Many people are frightened and run outdoors.
(b) Small objects of ordinary stability may fall and furniture may be shifted. In few instances dishes and glassware may break. Farm animals (even outdoors) may be frightened.
(c) Damage of grade 1 is sustained by many buildings of vulnerability class A and B; a few of class A and B suffer damage of grade 2; a few of class C suffer damage of grade 1.
VII. Damaging
(a) Most people are frightened and try to run outdoors. Many find it difficult to stand, especially on upper floors.
(b) Furniture is shifted and top-heavy furniture may be overturned. Objects fall from shelves in large numbers. Water splashes from containers, tanks and pools.
(c) Many buildings of vulnerability class A suffer damage of grade 3; a few of grade 4.
Many buildings of vulnerability class B suffer damage of grade 2; a few of grade 3.
A few buildings of vulnerability class C sustain damage of grade 2.
A few buildings of vulnerability class D sustain damage of grade 1.
VIII. Heavily damaging
(a) Many people find it difficult to stand, even outdoors.
(b) Furniture may be overturned. Objects like TV sets, typewriters, etc., fall to the ground. Tombstones may occasionally be displaced, twisted or overturned. Waves may be seen on very soft ground.
(c) Many buildings of vulnerability class A suffer damage of grade 4; a few of grade 5.
Many buildings of vulnerability class B suffer damage of grade 3; a few of grade 4.
Many buildings of vulnerability class C suffer damage of grade 2; a few of grade 3.
A few buildings of vulnerability class D sustain damage of grade 2.
IX. Destructive
(a) General panic. People may be forcibly thrown to the ground.
(b) Many monuments and columns fall or are twisted. Waves are seen on soft ground.
(c) Many buildings of vulnerability class A sustain damage of grade 5.
Many buildings of vulnerability class B suffer damage of grade 4; a few of grade 5.
Many buildings of vulnerability class C suffer damage of grade 3; a few of grade 4.
Many buildings of vulnerability class D suffer damage of grade 2; a few of grade 3.
A few buildings of vulnerability class E sustain damage of grade 2.
X. Very destructive
(c) Most buildings of vulnerability class A sustain damage of grade 5.
Many buildings of vulnerability class B sustain damage of grade 5.
Many buildings of vulnerability class C suffer damage of grade 4; a few of grade 5.
Many buildings of vulnerability class D suffer damage of grade 3; a few of grade 4.
Many buildings of vulnerability class E suffer damage of grade 2; a few of grade 3.
A few buildings of vulnerability class F sustain damage of grade 2.
XI. Devastating
(c) Most buildings of vulnerability class B sustain damage of grade 5.
Most buildings of vulnerability class C suffer damage of grade 4; many of grade 5.
Many buildings of vulnerability class D suffer damage of grade 4; a few of grade 5.
Many buildings of vulnerability class E suffer damage of grade 3; a few of grade 4.
Many buildings of vulnerability class F suffer damage of grade 2; a few of grade 3.
XII. Completely devastating
(c) All buildings of vulnerability class A, B and practically all of vulnerability class C are destroyed. Most buildings of vulnerability class D, E and F are destroyed. The earthquake effects have reached the maximum conceivable effects.
There is, however, a problem. Scales that describe the intensity of an earthquake positively require buildings to be damaged or people to be hurt—for without being able to employ phrases like those found above—“Hanging objects swing lightly” or “All buildings … destroyed”—there is, quite simply, no scale. But what of earthquakes that occur where there are no people to be hurt, or where no cities exist that are vulnerable, where there are no structures that may be damaged or leveled? Might there be a way of measuring earthquakes that could be applied equally to both populated and unpopulated corners of the earth, and to all places on the planet, no matter how geologically or culturally or architecturally different?
ON MAGNITUDE
The answer to this fundamental problem was first divined in the 1930s, initially by a Japanese seismologist named Kiyoo Wadati, and then, more famously, by California’s aforementioned Charles Richter. Both of these geophysicists came up with the idea that earthquakes could best be measured not by relying on the experience of observers or by reading the records of damage but by the much more dispassionate and mathematically precise means of examining very closely the traces of quakes that were written onto very accurate seismographs. A measure divined in this way would be, both Wadati and Richter decreed, an indication of something far more absolute than the earthquake’s intensity, which would be called the earthquake’s magnitude. Western chauvinism being what it is—and Richter being by far the better known of these two scientists—the measure that was the result of this new brand of rumination came generally to be known as the Richter Magnitude Scale.
The Richter scale, which is arguably among the most familiar of any of the eponymous scales or units of measurements,* is a very different kind of measure from those that deal with earthquake intensity. It is not at all subjective; rather, it is a mathematical indication derived from a mechanical record. And these days, thanks to Richter’s inexhaustible capacity for promoting his creation, it has come to be the single measure that is almost universally used in contemporary public discussion—in the press and in common conversation—as the most reliable indicator of the strength or power of an earthquake.
We speak quite casually of an event as having been of “magnitude 6” or “magnitude 8.” In the specific case of San Francisco, we speak of it not so much as having occurred in 1906—I found in writing this book that the year seems to have been forgotten by all but the most dedicated historians—but as an event that had a specific magnitude: 7.9 or 8.3, depending on how the figure is calculated. That is what seems to be remembered: San Francisco 7.9. No
t San Francisco 1906. And in the case of the tsunami-triggering quake of December 2004 in northern Sumatra, we refer with awe to its size, its power—and thus its magnitude of 9.3. And that is quite probably how we will remember this event in half a century: People will probably speak of Banda Aceh 9.3, and they may well remember where they were when they heard the news of such an enormous event. Few seem to care especially whether it had an intensity designated by a certain Roman numeral, or whether it was rated this or that on the EMS-98 or the MKS scales. They speak, quite simply, of how big it was, of how big according to the scale that was propagated and publicized by that one otherwise long-forgotten man who first created it: Richter.
In essence the calculation of a Richter magnitude is simplicity itself. It depends entirely on two things: on the size of the waves recorded on a seismograph, and on the distance that this seismograph is from the focus of the earthquake that caused them. Once you have those two pieces of information—which anyone with a seismograph will have simply by reading the ink record on the paper drum—then it is perfectly easy to work out the magnitude.
The seismogram illustrated here shows the Sumatran event of December 26, 2004, as it was recorded by an observatory in Pennsylvania.
At the left end of the trace, the pressure, or P-wave—the nature of which was explained in chapter 7—is shown on the seismogram as arriving first, as it always does, about 17 minutes into the record, almost exactly 1 minute after the known time of the quake. (The horizontal scale is time, in minutes; the vertical scale, here omitted, is usually offered in millimeters.)
The shear, or S-wave, then begins to hit the observatory machine at some 27 minutes into the trace—a delay between the arrival of the two sets of waves of almost exactly 10 minutes, or 600 seconds. Since the average velocity of earthquake waves as they are transmitted through the earth is well known, even in this long-distance case where some of the waves pass close to the earth’s core, it is fairly easy to calculate that the earthquake that caused this sudden vibration of the suspended seismograph weight occurred about 9,750 miles—15,700 kilometers—away from Pennsylvania. (By using the traces from three or more seismographic recording stations, as mentioned earlier, it is then possible to show not merely how far away the earthquake was, but exactly where on the surface of the planet it occurred. This is how earthquake epicenters are, in essence, computed. However, we are concerned here more with size than with position.)
To divine the earthquake’s magnitude, it remains now simply to measure the amplitude of the greatest wave that is found on the seismogram—in this case, the first of the surface waves, which appear rather dramatically at about 68 minutes into the record. This ground movement—caused by the suspended weight bouncing as the earth quakes—was quite large, in terms that seismologists understand. It shows a ground movement of about four millimeters—almost sufficient for the Sumatran earthquake actually to have been felt by a few very sensitively minded people in faraway Pennsylvania.
Once the peak amplitude has been accurately determined, together with the length of time that this amplitude endures (maybe 20 seconds, give or take, although the trace would need to be very closely examined to work this out), then all that then has to happen is (for simple numerical reasons that underlie the basis of the calculations) for the amplitude of 4mm to be converted into microns, by multiplying by a 1,000, and then to be compressed by some mathematical device—usually a logarithm, to base 10—to sort out, to damp down, the enormous range of variations in seismogram amplitudes that are recorded by the world’s machines. Finally, the resulting figure has to be recomputed to allow for the quake occurring a notional 100 kilometers (and not the actual 15,700 kilometers) away from the earthquake epicenter. And at this point, Lo!, a number appears. And in this specific case, it is the number 9.1.
And that number is the Richter local magnitude for this event—a number that can be refined and refined and refined, by adding or subtracting new factors and by recomputing with the insertion of otherwise unregarded additional variables, until a whole slew of other more sophisticated modern magnitudes—numbers representing such indicators as moment magnitudes, seismic magnitudes, surface-wave magnitudes, and so on, each with its specific usefulness to geophysicists—appear, as if from a magician’s hat.
The basic number, though, is already there, ready to be broadcast and offered up as the public statement of the size of the event. In short: The earthquake that caused the Middletown Observatory’s machine in Pennsylvania to experience such a dramatic few minutes of bouncing and shaking on that Boxing Day in 2004 was a faraway seismic event, some 9,750 miles away, and it measured 9.1 on the Richter scale. This number was one that Charles Richter had decreed quite simply would be the logarithm to base 10 of the maximum seismic-wave amplitude (in thousandths of a millimeter) recorded on a standard seismograph* at a distance of 100 kilometers from the epicenter.
The Richter figure thus calculated is of a great value to science, because it has an absolute mathematical purity to it. It does not require anyone to observe the event; nor is it necessary for any building to be damaged or for anyone to die or for any cattle or sheep to be scattered and spooked by it. Human perception is of no consequence in the making of the scale, nor in the assigning of the magnitude number to any event. One cannot publish the Richter scale in the same manner that one can publish an intensity scale, and say that a magnitude 9 event will do such and such damage while a magnitude 8 will do something quantifiably different. All one can say is that a 9 is a very great deal more significant than an 8—and that the amount of energy released by the event is proportionately very much more also.
But, having said this, Charles Richter did try to make his scale somewhat user-friendly. He worked his mathematics around the idea that the number zero should represent the smallest kind of episode that could be recorded on a seismometer; that values of around 3 would describe the smallest earthquakes that could be felt; and that values between 5 and 7 would represent the strongest ruptures that the average Californian—and all the early calculations of this very Californian man were specifically aimed at the fears and apprehensions and past experiences of the locals, his neighbors—might ever experience in a lifetime.
The logarithmic nature of the scale can render it unfamiliar to most. A magnitude 5 earthquake is not twice as great as a magnitude 2.5, for example. But 900 earthquakes of magnitude 5, or thirty of magnitude 6, would be needed to release as much energy as a single quake of magnitude 7. And, while small-magnitude earthquakes can be very inefficient at releasing the stored-up energy in the earth, large-magnitude quakes can be devastatingly good: The 7.9 earthquake in San Francisco released 1017 joules of stored energy in the sixty seconds of ground shaking—and only a very small fraction of that went into making the ground move. Large earthquakes have the power to move mountains, quite literally—and Charles Richter’s scale can show how, and offer figures to prove it.
In conclusion: Intensity requires an audience; magnitude requires only the divining powers of machines. The numbers that are offered up by each kind of measurement, and that bear no relation to the other numbers that are offered up for assigning numerical value to the world’s various physical phenomena, inhabit a curious half world of their own, all invented by geophysicists and each enjoying its own utility. To tell of the intensity of a quake is to relate rather romantically something local and subjective and vaguely arbitrary; to record the magnitude, on the other hand, is to note something that is arithmetical, absolute, and able to be presented for instant comparison. So, while intensity may seem the more amusing indicator to relate, it has to be admitted that earthquake magnitude, and the Richter scale that since 1935 has recorded it, are the stuff of history, and of science, and a matter of record. And thus, in summary, they, the magnitude readings, are of a rather more lasting value. Which is why Rossi, Forel, as well as Medvedev, Sponheuer, and Karnik are known by a very few; and why Charles Richter is a famous figure, now and forever more.
GEOLO
GICAL TIME SCALE
This stratigraphical diagram gives the presently recognized time span of each geological interval for the 545-million-year period, or eon, that can be readily dated and delineated using fossils, and which is known as the Phanerozoic.
The earth is, however, very much older than the life that has existed on it, having been created some 4,200 million years ago. This vastly long earlier period, not shown in the diagram, is divided by geology into two eons: the Proterozoic (which stretches from 2,500 million years to the start of the fossil-rich Cambrian, 545 million years ago) and the Archaean (dating from the estimated coalescence of the solid earth, 4,200 million years ago, up to the beginning of the Proterozoic).
Please note the different scales used in the diagram.
For further information on a time scale, which is constantly being refined, I suggest visiting the website of the International Commission on Stratigraphy at www.stratigraphy.org.
WITH GRATITUDE
ONE AFTERNOON IN THE EARLY SPRING OF 2003, AS I was beginning to plan the research for this book, I remarked to my companion that to get the flavor of the city that would be its central subject I really ought to move there. We talked for a while about the details, and then we agreed: It all seemed like an eminently sensible plan.
The idea then hung in the air, but for no more than five minutes, for suddenly the telephone rang. There was an unfamiliar voice, a man’s, on the other end. He introduced himself as Scott Rice, chairman of the Department of English at San Jose State University in Northern California.