Vol. II

No. 12

The great disaster which befell Messina and Reggio in December, 1908, has been described as the world's most cruel earthquake. The number of killed was estimated at various figures between seventy-five thousand and one hundred thousand. Must Messina now relinquish its title to this sad distinction in favor of Tokio and Yokohama? estimates of the dead in the recent earthquake would seem to answer in the affirmative.

The

While minor earthquakes, and even some of great local intensity, may be due to volcanic eruptions or other causes, all the great world-rocking quakes are caused by the slip of great masses of rock along some plane of faulting. Not long after men began to collect and collate observations about earthquakes with a view to determining the focus of each, it developed that these foci lay along certain lines, more or less straight, and that the frequency of earthquake shocks was greatest at the intersection of two or more of these lines. Further study showed that in many cases these lines were fault lines, i. e., lines along which the solid crust of the earth had been

A paper read before the Associated Engineering Societies of St. Louis, November 21. 1923, at a meeting held under the auspices of the St. Louis Section of the American Institute of Mining and Metallurgical Engineers.

broken, with a consequent displacement of the rocks on either side. Such studies led in time to the modern conception that the outer shell of the earth is divided into great blocks or compartments which are separated from each other by fault planes. These great blocks abut against each other, often times with great pressure, and are sometimes temporarily cemented together by the deposit of other material between them. Two neighboring blocks in this giant mosaic may for a long time be subjected to a differential pressure either vertical or horizontal. This leads to an accumulation of stress which actually causes a slight movement of the rock masses except along their common boundary plane. Here friction prevents any relative movement. But when the stresses become excessive there is a slip along this plane and an elastic rebound of the rock on either side takes place into positions of no strain. This sudden slip is sure to give rise to an earthquake, more or less severe, according to the amount of the movement and the masses of rock concerned. Even if the stresses were produced very suddenly instead of gradually, the same general effect would follow.

The famous California earthquake of April 18, 1906 has been studied more

carefully than perhaps any other. The account given above of the immediate cause of earthquakes is borne out almost to the letter by the results of this study. There was a displacement for about 185 miles along the famous San Andreas rift.

The characteristics of this motion are thus summed up by Hayford and Baldwin in the Report of the Superintendent of the U. S. Coast and Geodetic Survey, Washington, 1907. -"First: Points on opposite sides. of the fault moved in opposite directions. those to the eastward of the fault in a southerly direction, and those to the westward in a northerly. direction. Secondly: The displacements of all points were approximately parallel to the fault. Thirdly: The displacements on each side of the fault were less the greater the distance of the displaced points from the fault. Fourthly: For points on opposite sides of the fault and at the same distance from it, those on the western side were displaced on an average about twice as much as those on the eastern side." The intensity of the disturbance and the damage done were also greatest near the fault plane and rapidly died down with increasing distance from the fault.

From what has been said we should expect earthquakes to be common in regions where faults are abundant and where elevation of great rock masses has taken place in the past and is still going

on.

Such a region are the islands of Japan. They are islands of moderately recent origin, with many mountains and mountain ranges, and elevation is still, apparently, going on. On the convex eastern border of the main group of islands, the land slopes away rather rapidly to the bottom of the Tuscarora Deep, reaching a depth of 27000 feet within a distance from the shore of 110 to 240 miles. On this slope a great majority of the worst Japanese earthquakes originate. Tokio and Yokohama are in a region of high seismicity along a "seismotectonic. line" which extends northward along the eastern border of the main Japanese

Island.

somewhat north of Tokio this le intersects another of similar nature running southward through the interior of ono and of two other islands in the same line. The reports that have come from Japan make it probable that the focus, or foci, of the earthquake lay along this former line.

In both Tokio and Yokohama the destruction was greatest in the densely crowded riverside districts, in alluvial soil or "made" ground. This bears out a fact of general experience. For although the movement of rock particles in the solid earth may be very slight (even in very destructive earthquakes less than an inch), yet the immense energy associated with that movement, when handed on to loose soil, bodies of sand, objects of moderate size or rock monuments, may cause very considerable movements in them. Hence the great heavings and waves in the soil, and the destruction of buildings not firmly braced or well based on solid foundations. Sleepers are sometimes thrown unceremoniously out of their beds, while the earthquake's fashion of handling books in a library is not calculated to soothe the feelings of a bibliophile.

Yet, even in Tokio, only eleven per cent of the houses collapsed under the earthquake: the rest of those destroyed must be laid to the account of the fire. Buildings of reinforced concrete stood the shocks well; brick structures in general fared badly. The native wooden houses stood up well under the swaying of the earthquake, but of course were fuel for the fire.

There is unfortunately no difficulty in detecting a severe earthquake whose focus is close at hand. But if we wish to study the tremors caused by a distant shock, we are faced with a problem. If we could only take up our station outside the earth, but close to it, at a place entirely unconnected with the earth, then we might, given the proper instruments, observe the character and magnitude of the minute quiverings of the earth's surface. But this is impossible. We are on the earth and must

move with it; there is then no relative motion between us and the earth.

We must therefore do the next best thing. We take advantage of the fact that a heavy mass of matter has a great inertia and that, when hung or poised as a pendulum, it will, at least for a short time, remain motionless while the earth vibrates beneath. There will be relative motion between the pendulum and the earth. With proper precautions we can infer from this the magnitude and direction of the earth's movements.

Since the amplitude of the motion is very slight we must arrange to magnify it. This is done by a lever or a system of levers. Let us suppose that a very long pointer is attached to the stationary mass as a fulcrum, and that the shorter end of this lever is made to move by the framework that supports the "steady mass" and is attached to the earth. The longer end of the lever will move in a wide arc thus magnifying the motion of the shorter end.

I have here a heavy weight which I can clamp in any position on a very long light rod terminating in a loop. I fasten the weight about a foot below the loop and, taking hold of the latter. with my hand, I make it vibrate very rapidly through a small arc. The heavy weight remains stationary or nearly so; but the long wire at its further end moves to and fro through a considerable distance. The heavy weight represents the "steady mass" of the seismograph; the loop, the support of this mass, attached to the earth; while the far end of the rod or pointer represents the pen which writes the record of the earthquake. It is plain that within limits I can magnify the earth movement as much as I please.

Let us now place under the pen of the seismograph, thus situated at the end of a lever system, a revolving drum driven by clockwork and carrying around with it a cylinder of smoked paper. The pen will trace on the paper a record, more or less faithful, of the movements of the earth. In practice we should have three such records for each quake, one giving the movement in the North

South plane, or the North-South component, one the East-West component, and one the vertical component.

I said above that the record will be more or less faithful. For the pendulum has a natural period of its own; and when its suspension or point of support is set into vibration the pendulum tends to vibrate in its own period. This tendency will be all the greater, the closer the period of the earth particles approaches that of the pendulum. Hence the necessity for damping out quickly the swings of the pendulum so that it will come quickly to rest and be ready faithfully to tell the story of any subsequent movement of the earth.

When we know the "constants" of the instrument, such as its natural period, the magnification, the damping factor, etc., we can find the amplitude of motion of the earth particles, though this may involve a piece of thorny calculation.

The interpretation of seismograms is an art, calling for a knowledge and application of a large number of scientific principles. From the study of them we have learned that, in general, at least three different classes of earth waves are registered on the seismogram, at a station more than 1000 kilometers away from the center of the quake: (1) Longitudinal waves through the earth's interior, the most rapid, giving us the "first preliminary tremors"; (2) Transverse waves through the earth's interior giving the "second preliminary tremors" and having a speed of about four or five miles a second; (3) Complex surface waves passing around the surface of the earth at a speed of about two miles per second and recording the "long waves" of greatest amplitude.

Extremely interesting indeed are some of the deductions concerning the earth's interior which seismograms have enabled us to make. Knowing the density of the earth from other sources and the speed of the various kinds of earthquake waves we can arrive at a rather exact knowledge of the elasticity of different portions. The earth behaves as a solid far more rigid, at least in some of its parts, than steel. Mathematicians and