Before the theory of plate tectonics, scientists struggled to explain earthquakes by alluding to pressures caused by forces as varied as the tides, the weight of the water on land relative to that of the ocean, sedimentation, and volcanic activity. They preferred a domino-effect explanation where disturbances in one part of the earth caused it to break where the ground was more frail. Today, scientists recognize earthquakes mostly as snaps of the earth in places where moving continental plates come together.
"The earth is in a condition of unstable equillibrium so far as its insides are concerned. The outer crust is solid, but after you get down sixty or seventy miles the rocks are nearly in a fluid condition owing to the great pressure upon them. They flow to adjust themselves to changed conditions, but as the crust cools it condenses, hardens, and cracks, and occasionally the tremendous energy inside is manifested on the surface.
"When the semi-fluid rocks in the interior change their position there is a readjustment of the surface like the breaking up of ice in a river, and the grinding causes the earthquake shocks which are familiar in various parts of the world. The earthquake at San Francisco was probably local, althought the center of the disturbance may have been thousands of miles away from that city."
-- C.W. Hays, Director of U.S.G.S.
"We have a perfect record of this earthquake, although we are thousands of miles away from the actual tremor itself. There were premonitory tremblings, which became at 8:19 [Eastern time] and continued until 8:23 or thereabout. Then there was the severe shock which threw the pen off the cylinder.
"According to our observations here there was a to and fro motion of the earth in the vicinity of Washington amounting to about four-tenths of an inch at the time of its greatest oscillation. These movements kept up in a constantly decreasing ratio until nearly half an hour after noon.
"San Francisco may have been a long way away from the real earthquake and merely have been within the radius of severe action so as to produce disastrous results. It is quite likely, in fact, that the greatest disturbance may have taken place beneath the bed of the Pacific Ocean.
"If it resulted in an oscillation of the earth of only a few inches there would be no likelihood of a great tidal wave. If, however, there was produced a radical depression in the bed of the ocean, the sinking of an island, or some other extraordinary disturbance, a tidal wave along the Pacific Coast would almost certainly be one of the events of this great disaster.
"There are apparently three distinct weak spots in the United States, which are peculiarly subject to earthquake shocks, and we are likely sooner or later to hear from all of them in connection with the shock at San Francisco. There is one weak area along the southern Atlantic coast in the vicinity of Charleston, another is in Missouri, and the third includes the Pacific Coast from a point north of San Francisco down to and beyond San Diego."
-- Willis L. Moore, Weather Bureau
"I figure that the earthquake which caused so much damage in San Francisco came from what we call the focus of disturbance. This focus at San Francisco is seven miles below the surface of the earth. As the Sierra Madre mountains grow, a phenomenon which is constantly going on, the blocks of earth below change positions; as a large block falls a series of shocks travels up and down much the same way as the rings in the water travel out from the point at which a pebble strikes. When the vibration reaches the surface crust a severe shaking of the country adjacent is the result."
-- J. Paul Goode, University of Chicago
"While much concerning the origin of earthquakes is still a matter of doubt in the minds of scientific men, it is now generally conceded that the real cause is the sudden slipping and readjustment of the strata of rocks with the crest of the earth. As the earth is slowly cooling a very slow contraction of the earth's crust is constantly going on, and as this crust consists very largely of stratified layers of rock, the enormous forces arising from this contraction are resisted by the solid rock.
"Notwithstanding the apparent irresistable nature of these layers of rock, they slowly yield to the enormous lateral pressure of contraction and gradually huge folds are pushed up in long mountain ranges. Usually this process goes so slowly and gradually that the yielding of the rock masses takes place without noticeable jar, but occasionally a sudden slip occurs under the gigantic forces, and an earthquake is the result. This slip is usually only a few inches, but when two continents fall together for only a few inches enormous energy is developed.
"Such slips usually occur along the line of an old fissure previously formed, and the depth below the surface of the earth varies from one to twelve miles. Thus placeds situated near these old internal fissures are more likely to experience earthquakes than those farther away. It is a well known geolofical fact that the Pacific coast in California contains several of these fissures and earthquakes are more common there. The entire western part of the United States has been slowly rising for many centuries, and the shifting of soil due to erosion and transportation doubtless contributes to produce these seismic disturbances....
"It is probably true that a critical condition of stress between two gigantic and contending forces may be touched off, as it were, by any feeble force originating at a distance. Thus a distant volcanic eruption or earthquake shock may determine the climax of stress in a given portion of the earth, which will produce an earthquake. Observations show that more earthquakes occur near the full and the new moon than at other times. This is probably due to the fact that at these times the gravitation of the sun and moon are combined, and their effect upon the earth is greater. We can see this effect in the higher tides at new and full moon. But these forces, it will be seen, are the occasions, and not the causes of earthquakes.
"The probably reoccurence of the San Francisco earthquake is a matter of great uncertainty. In general, whenever the internal stress of the forces that give rise to earthquakes is relieved there is usually a long period of quiescence in the strata of the earth, but in the course of time, especially in regions of recent and rapid geological changes, such as in the case on the Pacific Coast, there is almost certain to be recurrences of earthquake shocks from time to time.
"The geological forces may, however, gradually adjust themselves, and it may be many centuries before such a dynamic crisis will arise as that which has just convulsed a continent."
-- Otis Ashmore
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