54 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
manifest by the appearance of low, abrupt fault-scarps, ranging from less than a foot up 
to 3 feet. Many of these occurred along the slope of somewhat degraded fault-scarps 
due to former movements, and served to revivify them. In other cases the new scarps 
have been developed on slopes where no trace of a previous scarp can be detected. The 
low scarp which was formed on April 18 is by no means a continuous feature, but appears 
at a great many places not widely spaced along the fault-trace, extending often for hun- 
dreds of yards at a stretch, with intervals where no abrupt scarp canbe detected. In 
the latter places it is probable that the differential vertical movement has been distributed 
over a zone of some width, underlain by formations in which the deeper seated fracture 
would be taken up by plastic deformation. The scarp almost invariably faces the north- 
east, but a few cases have been noted in which a fresh scarp on the fault-trace faced the 
southwest for a short distance. These will be mentioned more particularly in the detailed 
descriptions which follow. Associated with the fault-trace, it is quite common to find 
secondary or induced movements of the soil, particularly on steep slopes. These partake 
of the nature of landslides, and very commonly exhibit the characteristic landslide scarp. 
This is usually, however, easy to distinguish from the scarp on the fault proper, or on the 
auxiliary cracks, since it lacks evidence of horizontal displacement, and the broken sod 
is not traversed by diagonal, torsional cracks. ; 
The differential displacement of the earth’s crust above indicated occurred only on 
the northern portion of the Rift. South of San Juan, in Benito County, there is no indi- 
cation along the Rift in the shape of rupture of the soil, or the dislocation of transverse 
structures, which points to the displacement of the underlying formations. It is not, 
however, to be certainly inferred from this that there was no deep-seated rupture south 
of that point. Many earthquakes are known which are referable to sudden slips in the 
earth’s crust for which there is no corresponding rupture at the surface. It is probable 
that the slip, which is so manifest as a surface rupture to the north of San Juan, was 
continued as a subsurface movement for many miles south of that point. 
North of San Juan the displacement on the fault-trace has been followed practically 
continuously to a point on the northern coast of California a little beyond Point Arena, 
a distance of 190 miles. At this point the fault-trace as a continuous feature passes out 
to sea, and the evidence of displacement is lost. At Shelter Cove, in southern Humboldt 
County, however, where as previously stated the Rift features appear again, evidence of 
displacement due to movement on April 18 is also found. The doubt as to whether 
the Rift in Humboldt County is continuous with that which leaves the coast near Point 
Arena, of course also applies to the question of the continuity of the rupture on the day 
of the earthquake. If we assume that the line of rupture is continuous thruout, then its 
full extent from San Juan to Telegraph Hill is about 270 miles. 
Beginning with southern Humboldt County, a somewhat detailed account will now be 
given of the phenomena of the displacement which occurred on April 18, 1906. 
HUMBOLDT COUNTY. | 
We are indebted to the observations of Mr. F. E. Matthes for our knowledge of the 
facts concerning the portion of the coast from Shelter Cove northward. The low 
headland north of Shelter Cove, known as Point Delgada, is traversed by several fissures 
trending roughly parallel with the general sweep of the coast and presenting essentially 
the same surface appearance as the fault fractures observed in Sonoma and Mendocino 
Counties. While it has been found impracticable to demonstrate by actual measurement 
the existence of a horizontal displacement along any of these new fissures —in the 
absence of fences or other objects of sufficiently defined outline — yet it has seemed 
warranted to regard them as true fault or shear fractures, to be classed in the same 
