THE EARTHQUAKES OF MARCH 30
AND APRIL 14.

By A. O. LEUSCHNER.

Records of the earthquakes of March 30 and April 14 were obtained at the Students' Observatory, and it has been thought that an account of these would be of general interest. The records published in the daily papers are obtained by the Duplex seismograph, and are of little use for obtaining information of scientific value as to the location of the center of disturbance, the rate and direction of propagation of the earthquake, and the intensity or destructiveness of the stronger vibrations. The Duplex seismograph merely produces a static record, a more or less accurate picture of the horizontal path described by a partiele of the earth's crust during the earthquake: it tells us nothing about the time of its beginning, of its duration, or of the exact instants and duration of the principal vibrations, ordinarily called shocks. While the magnitude and horizontal direction of the various displacements can be determined from static records, as the time during which such displacements were accomplished remains unknown, nothing can be learned about the actual velocity of the ground at a given instant.

In addition to a Duplex, the Students' Observatory is provided with a Ewing three-component seismograph, which supplies the time-element lacking in the Duplex records.

Ewing records made at a single station, therefore, enable us to obtain some of the information desirable, and a comparison of similar records at different stations would give the rest. Our Lick Observatory is, however, the only other station on this coast possessing a three-component instrument, and no comparison of records having as yet been made, only preliminary results deduced from the recent Berkeley records can now be stated. The Students' Observatory would, however, be greatly indebted for reprints of the static records of every station, and for such other observations as may have been secured during the more pronounced earthquakes. It is not intended, however, to collect at Berkeley a list of all the slight shocks felt in the state, as this is done with great care at the Lick Observatory.*

While both earthquakes were recorded on both instruments at Berkeley, the records of the later are much the more satisfactory. Owing to a defect in the electrical arrangements, the disc and the clock of the Ewing instrument failed to start on March 30, while for April 14 all records are complete, save that the exact time of the commencement of the disturbance could not be ascertained.

The earthquake of March 30, according to the consensus of opinion, was the severest experienced in this vicinity since the great earthquake of October 21, 1868. The records of the Students' Observatory begin August 11, 1887, under the direction of Professor Frank Soulé, and this is the most severe recorded. Judging from the newspaper reports, we may safely conclude that Mare Island was near the origin of the disturbance. On the Rossi-Forel scale, by which earthquakes are divided according to their intensity into ten classes-X. denoting great disaster, -the severity of the shock there would be indicated by IX., some buildings being partially or totally

*The Smithsonian Institution has just published the material collected at Mt. Hamilton, in a volume entitled: A Catalogue of Earthquakes on the Pacific Coast from 1769 to 1897. By Edward S. Holden, LL.D., etc.

demolished. In San Francisco, where chimneys were thrown down and walls cracked, the intensity would be denoted by VIII. or less, while in Berkeley, where sleepers were awakened, clocks stopped, and articles overthrown, but no damage reported, it would be indicated by VI. or VII., more nearly the latter.

The large astronomical clock in the Students' Observatory was stopped at exactly 11 h., 42 m., 26 s., Pacific standard time; but it is difficult to say just how many seconds after the beginning of the disturbance this occurred. Mr. Seares noted the time of the disturbance at 11 h., 42 m., 44 s., which he estimates to have been 15 or 20 seconds after the sharpest part of the shock.

Before discussing the instrumental records, which are reproduced in Figs. 1, 2, and 3, from tracings by Mr. Crawford, it may be well to point out that great caution must be exercised in their interpretation, especially in case of those by the Duplex instrument, as for larger shocks the steady point generally has a tendency to return to or near its original position with respect to the supporting frame. This is of little consequence when the vibrations follow each other rapidly, as compared with the time occupied by the steady point in returning to its former position. If, however, the period of an oscillation of the earth's crust is comparable with that of the steady point, then the earthquake record may be much obscured by the motion of the steady point, being nearly doubled if the steady point and the earth's crust vibrate in opposite direction. Other less serious uncertainties are due to friction in various parts of the instrument. The Duplex instrument also has a limited range, the greatest displacement which can be registered being 0.4 of an inch. If this is exceeded, the combination bob will strike the framework and will be sent back. From what has been said above, it is easily seen that such a displacement may correspond to a considerably smaller actual displacement of the earth's crust. Actual displacements of the ground from 0.2 to 0.4 inches are, however, to be expected

in California. In fact, the displacements on March 30 exceeded the limit set by the Duplex instrument in both horizontal components. Yet a careful study of Fig. 1, and a comparison of the same with the Ewing record leads to the conclusion that little or no harm was done on this account, owing to the fact that the instrument happened to be set up in such a manner as to allow more than an average range in the direction in which the principal shocks took place.

Beginning at the starting point in Fig. 1, we see that the first displacement was in a northerly direction with a slight tendency to the west (the records are opposite in direction to actual displacements). At A the general direction changed to southwest and slight tremors set in. At B the direction suddenly changed from southwest to southeast to the point C. Here the pointer was taken off the plate, which, no doubt, was due to a sudden vertical motion of the earth's surface, which in intensity must have exceeded all prior shocks. This is also shown by the Ewing seismograph. The record continues at D, and indicates at Fa succession of minute vibrations in all possible directions, after which the disturbance continued in a southwesterly direction. This was followed by an extensive displacement in nearly the opposite direction. Here an almost circular vibration set in, as indicated by the loop near A. This was followed by a displacement in a westerly direction to G, and again in a southerly direction to where the record ends, the tracing point leaving the plate, and no further records being obtained. This must be considered rather fortunate, as the principal vibrations seem to have been well recorded, while the smaller and less important vibrations, which always follow, tend to obscure the record of the principal ones. The lines in Figs. 1 and 2 are not absolutely true representations, however, of the earth's motion, owing to the above mentioned causes. The greatest E. W. and N.S. displacements in the figure correspond to actual displacements of the ground of 0.53 of an inch (13 mm.) and 0.63 of an inch

(16 mm.). The corresponding figures from the Ewing seismograph are 0.35 of an inch (9 mm.), and 0.73 of an inch (19 mm.), or on the average we obtain for E.W. 0.44 of an inch (11 mm.), and for N.S. 0.68 of an inch (17 mm.), while the maximum vertical displacement, as recorded on the Ewing was 0.81 of an inch (21 mm.).

These figures are unusual, but large displacements are not criteria for the destructiveness of an earthquake, unless the time during which they are accomplished is correspondingly short. Large displacements of long period correspond to a slow vibration of the ground and might not even be felt. The characteristics of this earthquake prove that the period of the principal vibrations was comparatively short, but the actual velocity and acceleration of a particle of the earth's crust could not be determined. A very conservative estimate would give a velocity of about 0.8 of an inch (20 mm.) per second.

The earthquake of April 14 caused no damage in any part of the state. Its intensity at Berkeley was III. on the Rossi-Forel scale. Two distinct shocks were felt by some persons in lower Berkeley, one several minutes before 11:00, the other several minutes after 11:00 p.m., the total interval being about fifteen minutes. At the Students' Observatory only one shock occurred, as is definitely shown by the Ewing seismograph, but it is impossible to say to which of the two it corresponds. Figs. 2 and 3 represent the records obtained with the Duplex and Ewing seismographs respectively, the former being an exact copy of the original record, which multiplies 4.3 times, the latter being reduced in the ratio 1 to 3.1.

As the instrument multiplies the N.S., E.W., and vertical components 3.3, 3.0, and 1.7 times respectively, the displacements as measured from Fig. 3 are to the true displacements in the ratios 1.1 to 1, 1 to 1, and 0.5 to 1, respectively.

The nature of the disturbance is best studied from Fig. 3. The lines drawn to the center designate intervals of one