AC.DEMX OF SCONCES] CALIFORNIA 261 



The horizontal displacement of the earth's surface on the trace of the San Francisco fault 

 plane has been frequently described; the occurrence of wavelike undulations in low-tide mud 

 flats at the time of the earthquake and their preservation for a considerable time afterwards 

 may be less familiar. Concerning this curious effect Gilbert wrote as follows: 



Observers of strong earthquakes sometimes report visible progressive undulations of the ground, similar to 

 water waves, and such observations are usually made where the formations are alluvial . . . Not only is there 

 a gradation in physical condition from dry earth through mud to water, but the shaking of a loose formation, 

 whether wet or dry, overcomes the adhesion of particles and thereby imparts for the time being a mobility 

 analogous to that of liquids. It is therefore conceivable that gravity waves, altogether analogous to those of 

 water, may be produced by a violent earthquake in the surface of a loose formation. Certain ridges on soft 

 ground caused by earthquakes in Japan have been inferred by Omori and Kikuchi to represent such soil waves 

 and to indicate a wave length (crest to crest) of 20 to 40 meters. The San Francisco earthquake produced a 

 similar ridging on tidal mud in Tomales Bay [on the outer coast, 50 miles north of the Golden Gate], the average 

 ridge interval being not more than half that of the Japanese examples . . . There is no reason to question the 

 statement that the whole mud plain had the smooth surface common to tidal flats until it was disturbed by the 

 earthquake. Nor do I find any room for doubt either that the ridges originated as waves on the surface of the 

 mud while it was rendered quasi liquid by violent agitation, or that they persisted because the mud promptly 

 resumed its normal coherence when the agitation ceased. It is by no means equally clear that the arrested waves 

 were true gravity waves rolling across the mud plain. Whatever their mechanism and history, they illustrate 

 a mode of response of wet, unconsolidated material to powerful earth tremors, they suggest an explanation of 

 certain wavelike ridges produced on areas of made ground in San Francisco, and they contribute to an under- 

 standing of the peculiar destructiveness of the earthquake in such areas. 



As to the important question of accumulating crustal strain and its expression in the 

 deformation of the surface uear the trace of a fault rift before relief in an earthquake shock, 

 Gilbert wrote briefly in a review 7 of the State report; he regarded the "lack of chronologic 

 unity in the trigonometrical surveys, which were strung along through several decades" in the 

 the San Francisco region as — 



peculiarly unfortunate. . . . This fact made it impossible to discriminate between deformation at the time 

 of rupture and progressive deformation during accumulation of strain before rupture. . . . Nevertheless the 

 results invite the careful attention of geophysicists. To the reviewer the distribution of dislocation, and especially 

 the existence close to the fault, on each side, of a belt of maximum distortion, seems clearly not that which would 

 obtain if the fault passed completely through a solid crust to a liquid substratum. And it also appears that, on 

 the assumption of continuous solidity from the surface downward, the geodetic results might yield to adequate 

 analytic treatment a conception of the order of magnitude of the vertical distance to which the fault penetrated. 



The grounds for this opinion are not stated, and the method of adequate analysis is not 

 outlined; both doubtless were conceived in Gilbert's mind, and if so the layman must regard the 

 depth of his penetration as profound as that of any earthquake fault plane. 



EARTHQUAKE FORECASTS 



The Association of American Geographers elected Gilbert as their president for the year 

 1908, and thus had the pleasure of hearing an address from him at their meeting in Baltimore 

 on January 1, 1909. It was the last address of the land that he ever gave; for although the Geo- 

 logical Society of America elected him for the second time as their president in 1909, he was 

 unable to attend their meeting at the end of that year. The subject of the address before the 

 geographers was "Earthquake forecasts," and in substance and maimer it is one of the finest 

 if not the finest that he ever delivered. Its preparation must have demanded a great amount 

 of study, for it contains not only an ingenuous discussion of many factors determining the place 

 and time of earthquakes, but also much specific information in a field that he had not previously 

 entered. Its treatment is marked by the deliberate and penetrating analysis which char- 

 acterized all his work; the conclusions reached and the moral drawn from them are set forth 

 with a tempered wisdom that even in his own earlier addresses was seldom matched. The 

 style is genial, at times almost conversational; and interest is maintained to the very end. 

 Some of the most striking passages are here quoted, the opening one first: 



The outlook for earthquake forecasting is my theme today. As you are aware I am not a seismologist. 

 My point of view is that of the geologist and general geographer. I speak as a layman, and present impres- 



' Amer Joura. Sci., nvii, 1909, 48-52. 



