Shifting Movements of the Earth

By A. C. Lawson, Professor of Geology, University of California

DR. LAWSON: Mr. President, on the printed program I am placed here for a statement as to the views of the ancients. I know little about the views of the ancients, but I will tell you an ancient story. You remember some years ago, before the Southern Pacific Railroad was electrified on the other side of the bay, we had a steam engine running into Berkeley, and there used to be a special engine to pick up passengers for West Berkeley at Shell Mound. One day I was on the train, and there were two young ladies sitting in the car, rather loudly dressed and talking in a loud tone of voice. In front of them there was an old lady, very stout, with a bundle. And these young ladies with the loud-toned dresses were discussing their program for the following Sunday, and one proposed to the other, "Let's go to Blair Park on Sunday." "No, I won't go to Blair Park," said the other, "too many of those Irish there." "Well, let's go to Shell Mound on Sunday." "No, I won't, there are too many Irish, I cannot stand the Irish." And they discussed various places, but the same argument was proposed against every suggestion. Finally the train pulled into Shell Mound and slowed up, and the old lady got up, and before picking up her bundle, she turned to the young women, placed her hands on her hips, and said, "An' it is to Blair Park you won't go; an' it is to Shell Mound you won't go. Sure an' it's to Hell you can go. You'll find no Irish there." (Laughter.)

The point of that story is that the earthquakes are just as ubiquitous as the Irish. Some of you think that we are peculiar in California, in having a lot of earthquakes here, but we are not. Earthquakes are not peculiar to California. They are phenomena that are common the world over; and there is probably no fraction of the earth's surface that has not suffered from the agitation that we call an eathquake. Now, I do not want to expand on that particular topic, but I would like to have you understand that, when we discuss earthquakes in California, we do not do it because we are peculiar here, but because we have certain advantages in the study of earthquakes that make it desirable that those studies should be carried on.

Some of those advantages are found in the surveys that have been made in this State by the United States Government. I have some charts here that will indicate the results of those surveys, and I desire to call your attention, first of all, to those results.

In the year 1854, the latitude and longitude of Mt. Tamalpais was determined with great precision. If the point A on the chart, figure 1,

a

હ

Fig. 1. Mount Tamalpais moved 3.04 meters from A to D between 1854 and 1906 by strain creep, and 1.97 meters from D to C in 1906 by rebound.

represent the geographical position of Tamalpais in that year, then in the year 1882, the peak was found to be at B, a distance of 1.64 meters to the north of the point it had occupied in 1854. It had traveled a certain distance in that interval of time, and the distance traveled and the time being known, we have the rate of movement, the translation of the position of Mt. Tamalpais per year. The rate is about five one-hundredths of a meter a year. Applying that rate to the interval between 1882 and 1906, it being a fair assumption that the motion did not stop suddenly in the year 1882, the position of Mt. Tamalpais must have been at the point D in 1906.

Now, immediately after the earthquake the United States Coast and Geodetic Survey undertook a reoccupation of all these points, a redetermination of their geographic positions, and it was found that Tamalpais was not at D, but was here at the point C. The sudden movement of the station on April 18, 1906, was from D to C. Now, that movement is not parallel to the actual movement that occurred on the 18th of April, 1906. The lines aa', bb' represent the direction of the San Andreas drift along which the movement took place, and Tamalpais, if it moved at all, must have moved in that direction, along the line cc'. But we find it at the point C. So there is a suggestion that Tamalpais suffered not only a sudden back shifting due to the sudden motion at the time of the earthquake, but that it suffered also apparently a transverse shift.

So much for Tamalpais. There is another station up the coast

known as Chaparral. In 1856 its position was determined with great exactness. Here it is at A in the diagram, figure 2. In 1891 it was found to be at B. And from that measurement, the interval of time

Fig. 2

Fig. 2. Chaparral moved 2.61 meters from A to D between 1856 and 1906 by strain creep, and 2.06 meters from D to C in 1906 by rebound.

being known, we get the rate of movement of that station, which is again five one-hundredths of a meter per year. In 1906, it was due at the point D, immediately before the earthquake. Immediately after the earthquake, it was found at the point C; and again, instead of moving parallel to the earthquake shift, it seems to have suffered a transverse shift, so as to bring it on this line cc', instead of being on the line bb', where it was due. Both of those stations are on the east side of the San Andreas fault.

Here is the Farallone light-house in the diagram, figure 3. It is situated on the west side of the fault. In 1860, it was at A. In 1891,

N

Fig. 3

Fig. 3. Farallon moved 2.06 meters from A to D between 1860 and 1906 by strain creep, and 1.29 meters from D to C in 1906 by rebound.

it was redetermined and its position was found to be at B, and the distance and the time of the interval will again give us the rate. The rate we have here is about four one-hundredths of a meter per year. Applying that rate, the Farallone light-house was due at D (indicating), immediately before the earthquake in 1906, but after the earthquake, it was found over here at C (indicating). So that again the point seems to have suffered, on that side of the fault also, a transverse shift.

Again, these three stations, Tamalpais, Chaparral, and Farallones, form the three corners of a triangle, and the determinations represented on these diagrams here indicate very clearly that the whole of the block of ground within that triangle, had moved northerly in accordance with the rate that I have mentioned of about five onehundredths of a meter a year, and that all of the points within that triangle had moved the same amount, at the same rate.

Now, here at Tomales Bay, in that triangle, there are ten stations that were determined in the year 1856. Five of those stations happen to be on one side of the San Andreas fault, and five on the other side, and by grouping these stations together, and taking the means of their positions, we have a very excellent measure of the movement of the earth in the interval between 1856 and the time of the earthquake; since we have the rate of the movement of the whole triangle as determined by these three stations, Tamalpais, Chaparral and Farallone. Applying that rate, the small circle in the diagram, figure 4, stationed at A in 1856 would be due in 1906, just before the earthquake, at the point B. Now, that circle, we may suppose, was located right on the trace of the fault, so that, when the fault occurred, one-half of the circle would be thrown one way, and one-half the other way, and here is where we find the two halves of the circle immediately after the earthquake (indicating), one at c and one at c'.

Now, here again, we have the curious fact that, though the circle we start with is located on the line of the earthquake fault, a narrow thin line, yet neither of the two halves of the circle are on that line. They seem to have suffered a shift transverse to the general direction of the line upon which we might expect them. Now, the explanation of that shift would be an interesting thing, if we could find it. And it seems to lie in this fact: that all of the conclusions that I have been pointing to are based on a triangulation by the United States Coast and Geodetic Survey, and the whole triangulation is based on a line here which is indicated in this chart as a thin blue line, and that line

Fig. 4

a'

Fig. 4. Tomales Bay group. A small circle located at A in 1856 on the San Andreas fault had moved 2.75 meters to B by strain creep up to 1906. By slip and rebound in that year one-half of the circle moved 3.03 meters to C and the other half 3.00 meters to D.

is the base line, stretching from Mt. Diablo to Mocho. All of the work that was done in trying to determine the amount of movement of the earth was based on the assumption that this Diablo-Mocho base line was immobile, that it had not moved. If we take the counter assumption that the base line was not immobile, that it had moved along with the rest of the country, then this transverse shift that I have been speaking about ceases to be real. We can explain the shift by simply making the other assumption, that the base line upon which the whole triangulation is based also participated in the general

movement.

The general conclusion I want to draw from the facts I have recited is that the movements of the earth are not confined to the particular sudden and violent disturbance that we experience at the time of an earthquake shock. The energy that was let loose on the morning of April 18, 1906, had been stored up in the earth's crust as an elastic compression. You cannot imagine a very sudden and violent movement of the earth without the release of some energy, and it