394 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
no disturbance of the soil on either side of the cavity, even in its immediate vicinity. 
On the shoulder to the southeast, where the trace of the auxiliary fault passes over 
practically bare rock, no evidence of movement was detected on critical examination. 
The expulsion of the water was a purely local phenomenon. In attempting to explain 
the cause of it, or to ascertain the local subterranean conditions which conspired with 
the earthquake shock to bring about the event, it should be noted first that on the line 
of the fault-trace there are longitudinal depressions, which appear to be in part structural 
and in part due to erosion following the fault. If one of these depressions should locally 
have the character of a sink, without free drainage, then the sand which filled it would 
be saturated with water in consequence of the rains of the previous winter. It is believed 
that the compressive action of the earth-wave passing through such a pocket of satu- 
rated sand, and reflected perhaps more than once from the containing rock walls, would 
be adequate to expel 27,000 cubic yards of water from the deeper portion and add it 
suddenly to the more superficial portion of the formation, thus bringing about the 
earth-flow. It may be stated in this connection, although it has no conclusive bearing 
upon the question involved, that the sands of the valley generally are an abundant 
source of well water, and that there is a surface well within a few hundred feet of the 
source of the earth-flow, lower on the slope. There was very little water in the arroyo 
before the earthquake and a very insignificant stream afterwards, the latter being prob- 
ably referable to the drainage from the ejected sand. 
Vicinity of Half Moon Bay (Robert Anderson). — The earthquake shock caused the 
appearance of an unusual amount of water at the surface in many places. This was 
noticeable in the vicinity of San Bruno, where several short streams running into the 
bay were flooded by an unusual volume of water during the first days following the 
earthquake, in spite of the fact that it was perfectly clear weather. Instances have 
been cited in the present writer’s notes on the results of the earthquake in the San 
Francisco Peninsula, of water with a salty taste or milky color issuing from springs 
after the shock, and of streams being muddy and flowing with increased volume. ‘These 
facts, and the fact that water continued to issue after the earthquake at the points where 
earth-flows occurred, and where it had not been in evidence before, and that earth- 
flows occurred sometimes on convex slopes where the concentration of water under 
normal conditions would be unlikely, seem to be explainable only by the theory that 
uriderground conduits were disturbed and made more open, that new channels of escape 
for the water were formed, and that water was actually squeezed out of the hills in some 
‘ases by compressive movements. This flowage of water to the surface, in increased 
amounts and sometimes at new places, caused the formation of the earth-flows. The 
places where these debacles occurred may or may not have been previously points of 
concentration of seepage water, and the soil already in part saturated. But it is sup- 
posed that the content of water was increased by the shock, possibly in extreme cases 
by the gushing up of a large volume; and that this increment of water, with its dis- 
integrating, weighting, lubricating, and direct forcing power, aiding the attack of the 
shock on the soil, was the main cause of the earth-flows. 
There is little evidence as to when the flows were first set in motion; whether at once 
during the shock, or later after the lapse of some brief period of time that may have 
been necessary for the accumulation of the water in extra large quantities. 
EKarth-flows originated in valleys, in gullies, or on hillsides. Where the weight of the 
earth, combined with the weight of the added water, was sufficient and the substratum 
of the soil was rendered plastic, gravity caused it to creep like a lava-stream, leaving 
a hollow in the place from which it came and a fan or tongue of débris down the slope 
below. Movement was especially apt to ensue where the ground had been previously 
wet, the intensity of the earthquake shock being particularly great at such points and 
