Gilbert — The California Earthquake. 51 



monic horizontal motion was given to an open box containing 

 some loose material such as sand. A block resting on and 

 anchored to the upper part of the sand, so as to share its motion, 

 was found not to have harmonic motion, but motion of a dis- 

 tinct type which varied with the conditions of the experiment. 

 Under certain conditions the amplitude of its motion was 

 greater than that of the motion of the box, and its maximum 

 acceleration — the factor corresponding to earthquake inten- 

 sity — very much greater. These novel experiments are not 

 only valuable in their immediate results, but of signal import- 

 ance as indicating a line of study which should develop a com- 

 plete theory of the phenomena of the emergence of earthquake 

 waves. 



The marine phenomena were in accord with the terrestial 

 in that they indicated no bodily movements of the ground 

 except in a horizontal sense. Vessels at sea experienced a 

 shock ; there were boilings of water near the shore ; a small 

 seiche was started in San Francisco bay ; a wave several feet 

 high washed the east shore of Tomales bay, a narrow sheet of 

 water traversed by the fault ; but there was no great sea wave 

 such as accompany vertical dislocations of the ocean bed. 



The main shock, which was of about one minute duration, 

 was reported by many observers as consisting of two parts, or 

 having two maxima, but by others as continuous. Consider- 

 ing the improbability that movement was synchronous and 

 similar over the entire plane of rupture, it is to be assumed 

 that the vibration had different characters at different places, 

 but the observations are not discussed with reference to geo- 

 graphic distribution. There are many records of preceding 

 or accompanying sounds, all of low pitch. The after shocks 

 were of normal character, diminishing with time in frequency 

 and average strength, and continuing for at least ten months. 

 The report enumerates more than 100 in the Urst 24 hours ; 

 about 300 in the first month ; and for succeeding months, 

 71, 21,44, 28, 14, 11, 13, 15, 21, 2, 3, 1, 2. The record is 

 recognized as fragmentary, and the actual number of sensible 

 shocks was probably much larger. There was somewhat volu- 

 minous testimony to the occurrence of visible undulations of 

 the surface of the ground, the speed of which was much slower 

 than that of the elastic waves in rock. 



Cracks opened in many places near the fault ; from several of 

 these were large temporary discharges of water or of water and 

 sand; the circulation of underground water was seriously and per- 

 manently deranged, springs being destroyed, created or changed 

 in volume ; landslides and earthflows were precipitated in great 

 number. Alluvial lands slumped toward stream channels, and 

 soft ground was in some localities left with a wavy surface. 



