MARINE PHENOMENA, 
The effect of the earth movement on the sea-level. — In earthquakes along coastal regions 
the waters of the ocean are usually affected, particularly if there be a displacement of 
the sea-bottom. If the displacement has a considerable vertical component, so that 
one portion of the sea-bottom is dropt relatively to an adjacent portion, the ensuing 
displacement of the prism of water over the region affected will generate a periodic wave, 
which will cause the water along the coast to rise and fall with more or less disastrous 
results. If the dropt portion of the sea-bottom is on the landward side of the fault 
upon which the displacement occurs, the wave will be greater for the same amount of 
displacement than if the drop is on the seaward side. If, however, the vertical com- 
ponent of the displacement is quite small, and the movement is chiefly horizontal, as 
in the case of the fault of April 18, 1906, the sea-wave will be correspondingly insignificant. 
The bottom of the Gulf of the Farallones, which was traversed by the fault from 
Bolinas Lagoon to Mussel Rock, comprizes the inner shallower portion of what is known 
as the 100-fathom plateau off the coast of California. This plateau stretches seaward, 
with an average breadth of 22 miles, immediately off the short line of coast from Pigeon 
Point, in latitude 37° 11’, to the mouth of Russian River, in 38° 26’, a distance of about 
80 geographic miles. The area of this part of the plateau is about 2,500 square miles, 
which includes the area of the Gulf of the Farallones, about 1,200 square miles. On it 
lie the Southeast Farallones, the North Farallones, Noonday Rock, and the Cordell 
Bank, having a northwest and southeast bearing thru 30 geographic miles. The line 
projected southeastward strikes Pigeon Point. (See map No. 4.) The summits of 
the Farallones rise as much as 340 feet above the sea; Noonday Rock has 3 fathoms 
of water over it, and the Cordell Bank has 19 fathoms. Inside of these islets there is a 
very. uniform bottom of sand, with a gradually decreasing depth of water toward the 
shore. Outside of the islets the grade of the bottom rapidly increases. The 100-fathom 
line reaches 5 miles to the southwest of the Southeast Farallones; thence it is 10 miles 
to 500 fathoms and 29 miles to 1,728 fathoms. 
There is no means of directly ascertaining the amount of the vertical component of the 
fault of April 18 for those portions of the fault-trace which le on the sea-bottom across 
the Gulf of the Farallones or in the region to the northward. But where it traverses 
the land to the south of Mussel Rock, there is no evidence of vertical displacement ; 
and to the north of Bolinas Bay, while there is evidence of an uplift on the west side of 
the fault, that uplift is sight, not exceeding 1 or 2 feet. The absence of a periodic wave 
at the Golden Gate indicates that the vertical displacement on that segment of the 
fault which crosses the Gulf of the Farallones, if there was any, was very small. While 
there was no periodic wave of the oceanic water generated by the horizontal displace- 
ment of the sea-bottom, there was an interesting disturbance of the level of the sea, 
shown by the tidal gage near Fort Point on the south side of the Golden Gate, which is 
probably to be classed with the secondary phenomena arising from the displacement. 
The tidal gage yields a record known as a marigram, upon which is chronologically 
indicated the rise and fall of the water in the Golden Gate with the incoming and out- 
going of the tide. The record is said to be sensitive to the impact of waves breaking 
upon the bar outside the heads distant some miles from the gage. It is also sensitive 
to the conflicting volumes of water from the north and south parts of the Bay, when 
these are striving for mastery on the fading tide. Former submarine earthquakes in 
distant parts of the Pacific have generated waves which have been recorded on the 
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