ISOSEISMALS: DISTRIBUTION OF APPARENT INTENSITY, | 343 
approaching the base of the hills on this side of the valley. But at the base of these hills 
lies one of the dominant faults of the country — the fault upon which movement took 
place, with rupture of the ground, producing the earthquake of 1868. It would seem 
that, if local earthquakes were to be invoked to explain the high intensity of the alluviated 
valley-bottoms, here was a fine opportunity for an illustration of that doctrine. But the 
seat of the disturbance of 1868 was perfectly passive in 1906. The intensity diminished 
eastward right up to the fault-trace; and there is no suggestion that the disturbance 
along the San Andreas Rift affected it in the slightest degree. This being the case, 
there appears to be no recourse but to ascribe the normal apparent intensity about 
the southern part of San Francisco Bay to the character of the underlying formations 
as in other valleys before described. 
To the west of the San Andreas fault in San Mateo and Santa Cruz Counties, the 
apparent intensity diminishes on the firm rocks more rapidly than to the east of the fault, 
but it rises very notably on the alluvial fan of Pilarcitos Creek at Half Moon Bay, and 
in the alluviated valleys of San Gregorio and Pescadero Creeks, Going westward 
down Pilarcitos Canyon, the apparent intensity drops from X at the fault to less 
than VII within 4 miles of the fault; but along the coastal fringes of alluvium which lie 
between the hills and the sea, it rises again to VIIT at Spanish Town and to IX on the 
flats below the town. In the valleys of San Gregorio and Pescadero Creeks an apparent 
intensity of from VII to VIII extends for 4 miles and 3 miles, respectively, into an area 
of hill lands where the prevailing intensity is from VI to VII. The geology of this 
region, the Santa Cruz Quadrangle, has been mapped by Prof. J.C. Branner, and no fault 
is known at Half Moon Bay. Farther south the San Gregorio fault crosses the mouth of 
San Gregorio Valley and the middle part of Pescadero Valley, with a course parallel to 
the trend of the coast or transverse to the axes of the valleys. But the high apparent 
intensity in the bottoms of these valleys can not be referred to a local earthquake due 
to movement on this fault, since on either side of both valleys, in the immediate vicinity 
of the fault, it drops to below VII; while a few miles farther south on the same fault the 
apparent intensity drops to VI. 
At Santa Cruz a portion of the city is built partly on a series of broad wave-cut terraces 
in the bituminous shale of. the Monterey series and partly on the alluviated bottom- 
lands of San Lorenzo River. The contrast in apparent intensity in these two portions 
of the city is marked. In that portion which is situated upon the terraces the apparent 
intensity ranges from VII to VIII, while on the bottom-lands of the river it rises to 
from VIII to IX. It thus appears again, from a consideration of these four cases on the 
coast extending from Half Moon Bay to Santa Cruz, that the character of the material 
in the alluviated valley-bottoms has exercised a dominant influence in determining the 
apparent intensity of the earthquake shock, and that there is nothing in the facts to 
suggest that any other factor has played an important role. 
The finest illustration of the influence exercised by alluvium in the production of 
high apparent intensity is that afforded by the valley of the Salinas River and its exten- 
sion to the valley of the lower portion of the Pajaro River. The Salinas Valley is one 
of the notable physiographic features of the Coast Ranges. It lies between the Santa 
Lucia and Gavilan Ranges. It is deeply alluviated and strikingly terraced, particularly 
in its lower part. The course of the valley was probably determined originally by the 
fault along the eastern base of the Santa Lucia Range. The river discharges into the’ 
Bay of Monterey about its middle part, a few miles south of the mouth of the Pajaro 
River. On the flood-plain tracts of both rivers, and along the beach of the Bay of Mon- 
terey, the intensity was 1X. This extended up the river for several miles above the town 
of Salinas. There were extensive fissures in the alluvium as far as Gonzales, with slump- 
ing of the ground toward the river trench. Damage of structures, indicating an intensity 
