THE CALIFORNIA EARTHQUAKE OF APRIL 18, 1906, 
THE TIME AND ORIGIN OF THE SHOCK. 
DESCRIPTIONS OF THE SHOCK. 
The fact that the California earthquake of April 18, 1906, occurred a little after 5 A. M., 
before people in general were up, is one cause why we have so little reliable information 
regarding the exact time at which it occurred. In answer to questions sent out by the 
Earthquake Commission, a very large number of replies were received, but it is quite 
evident, from the variations among them and from the fact that many only gave the 
time to minutes, that these times are very unreliable. The general descriptions show 
that the earthquake began with a fairly strong movement which continued with increasing 
strength for an interval variously estimated, but which really amounted to about half a 
minute; then very violent shocks occurred, and quiet was restored about 3 minutes later. 
Prof. George Davidson in Lafayette Park, San Francisco, marked time from the begin- 
ning of the shock, which he places at 5° 12™ 00°. He noticed hard shocks until 5° 13™ 
008, a slight decrease to 5° 13™ 308, and quiet again about 5° 14™ 30°. 
Prof. Alexander McAdie, in charge of the Weather Bureau office at San Francisco, 
wrote as follows to Professor Lawson under date of September 8, 1907: 
I have lookt up the record in my note-book made on April 18, 1906, while the earthquake 
was still perceptible. I find the entry “5" 12™’ and after that “Severe lasted nearly 40 
seconds.” As I now remember it the portion “severe, etc.,” was entered immediately after 
the shaking. 
The time given is according to my watch. On Tuesday, April 17, 1908, my error was 
“1 minute slow” at noon by time-ball, or time signals which were received in Weather 
Bureau and with which my watch has been compared for a number of years. The rate of 
my watch was 5 seconds loss per day; therefore the corrected time of my entry is 5" 13" 05° 
A.M. This of course is not the beginning of the quake. I would say perhaps that 6 or 
more seconds may have elapsed between the act of waking, realizing, and looking at the 
watch and making the entry. I remember distinctly getting the minute-hand’s position, 
previous to the most violent portion of the shock. The end of the shock I did not get 
exactly, as I was watching the second-hand and the end came several seconds before I fully 
took in the fact that the motion had ceased. The second-hand was somewhere between 40 
and 50 when I realized this. I lost the position of the second-hand because of difficulty in 
keeping my feet, somewhere around the 20-second mark. 
I suppose I ought to say that for twenty years I have timed every earthquake I have felt, 
and have a record of the Charleston earthquake, made while the motion was still going on. 
My custom is to sleep with my watch open, note-book open at the date, and pencil ready — 
also a hand electric torch. These are laid out in regular order — torch, watch, book, and 
pencil. 
Referring to the fact that his time is about a minute later than that given by other 
observers, he adds: 
However, there is one uncertainty; I may have read my watch wrong. I have no reason 
to think I did; but I know from experiment such things are possible. * * * I have the 
original entries untouched since the time they were made. 

1 The time is given in Pacific standard time, 8 hours slow of Greenwich mean time. 
3 
