348 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
the old fault-trace; and buildings at Berkeley, founded on rock, practically on the line 
of the fault, suffered little or no damage. 
The fault which is so well exposed in the sea-cliff south of Fort Point traverses the city 
of San Francisco in a southeasterly direction for an unknown distance. Along the line 
of its probable course, Mr. Wood has noted evidence of an increase of intensity. The 
fault in its projection seaward probably intersects the San Andreas fault beneath the 
Gulf of the Farallones. It is therefore possible that there was some slight distribution 
of the movement along this intersecting fault. 
The San Bruno fault-scarp, on the peninsula of San Francisco, south of the city, is 
well illustrated in Plate 15, and its structural relations are described in a paper by Andrew 
C. Lawson on the Geology of the San Francisco Peninsula.’ 
The base of the scarp is from 2.5 to 3 miles distant from the San Andreas Rift, and is 
nearly parallel to it. The fault is in two parts: a main fault with a throw of not less 
than 7,000 feet, which drops the Merced (Pliocene) strata against the older Franciscan 
rocks, and an auxiliary fault which drops a wedge of Franciscan strata between the 
main fault and the mass of San Bruno Mountain. The town of South San Francisco is 
on the lower slopes of a rocky spur of San Bruno Mountain, between the two faults, 7.e., 
it is on the dropt wedge of Franciscan rocks. In the water-saturated alluvium and sands 
of Merced Valley, the apparent intensity was high, ranging up to IX of the scale; but 
in South San Francisco, on rock foundation, it was notably lower, as appears from Mr. 
Crandall’s report. The situation of south San Francisco, between the two faults, is such 
that had a movement occurred on either, the damage to structures would have been 
accentuated. But the fact is that the damage was not so accentuated, and there is 
thus no warrant for supposing that any local fault movement occurred. 
One circumstance which, upon first thought, seems to contravene this conclusion, 
was the sudden outgush of water at one point at the base of the San Bruno scarp. This 
remarkable occurrence is described in another place, but may be mentioned here, for 
the purpose of bringing together the facts bearing on the question. The water issued, 
as near as can be determined, at a point on the slope immediately above the fault-trace 
of the auxiliary fault, in the underlying hard rocks, which are there mantled with an 
unknown thickness of sand, possibly 50 feet or more. The outgush of water is indica- 
tive of sudden compression of incoherent water-saturated sand, and does not necessarily 
imply a movement on the deeper fault. Along the line of the fault there are longi- 
tudinal depressions, and it is suggested that one of these was filled with sand, under 
conditions which did not permit of rapid drainage; so that the sand was saturated with 
water, which was expelled as the compressive wave traversed the locality. 
In the region to the southwest of the San Andreas Rift in San Mateo and Santa Cruz 
Counties there are several faults, most of which are represented on maps Nos. 21 and 22. 
No evidence of movement has been detected on any of them, altho the territory has 
been examined quite closely; nor does their presence appear in any way to have affected 
the disposition of the isoseismal curves. They nearly all traverse a country occupied by 
rocky mountainous slopes, and have considerable variation in orientation, altho the pre- 
vailing strike is northwesterly and southeasterly. One fault, however, viz., the San 
Gregorio fault, crosses 2 valleys — San Gregorio Valley and Pescadero Valley — in which 
the intensity of the shock was abnormally high. The independence of this high apparent 
intensity to the fault has been pointed out in another place. 
To the north of Black Mountain, on the northeast side of the San Andreas Rift, 
a branch fault leaves the Rift line a little south of Portola, at an angle of about 25°, 
and is traceable for about 8 miles on the lower northeastern flank of Black Mountain. 
Between this Black Mountain fault and San Andreas Rift there is enclosed a wedge of 
*U.S. Geological Survey, 15th Ann. Report. 

