The California earthquake of April 18, 1906 : report of the State Earthquake Investigation Commission, in two volumes and atlas.

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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 component 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 lie 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 slight, 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 displacement 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 outgoing 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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marigram at the Golden Gate. The marigram near Fort Point, for April 18, 1906, shows (fig. 67) a depression of the water-level in the Golden Gate at the time of the earthquake, or rather a little subsequent to that event. The amount of the depression was slightly in excess of 4 inches. The marigram shows a blurring of the pencil mark from the direct action of the earthquake agitation, and this bearing serves to give approximately the time of the shock. It shows that the running clock of the gage was probably too slow, and that the depression of the water-surface did not begin instantaneously, but followed after an interval which may have been from 9 to 10 minutes. Before the shock the gage had had a small vertical movement, ascribed by the officers of the Coast and Geodetic Survey to an imperfect oscillation across the Golden Gate. This minor vertical movement continued during the drop in the level of the water after the shock. The time for the lowering of the water was 9 minutes, as near as can be read from the marigram. It immediately began to recover, and the record shows that the water level rose without minor oscillations, to the normal level within 7 minutes, the total interruption in the normal marigram curve due to this depression being 16 minutes. After full recovery to normal level, the depression was not followed by a complementary rise of the water-surface, and in this sense the movement was not periodic. The minor oscillations referred to above ceased when the maximum depression was reached, and do not appear in their characteristic forms on the marigram curve for some hours after. They were replaced, however, after 6 o'clock, by 2 or 3 oscillations having a period of about 40 to 45 minutes and an amplitude of 1 to 2 inches. These probably correspond to oscillations in San Francisco Bay.

The Tidal Division of the Coast and Geodetic Survey very kindly computed the time which would be required for a wave generated at the fault-line on the bottom of the Gulf to reach Fort Point, and found that it would require 9 minutes, on the assumption that Fort Point is 6 statute miles distant from the fault-trace in a direction normal to it. The position of the gage is, however, 1.3 miles distant from Fort Point within the Golden Gate, so that the time necessary for the wave to reach the gage would be somewhat longer. Now the time at which the gage began to fall is between 9 and 10 minutes

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after the first interruption and blurring of the record by the shock itself, and this coincidence in time suggests that the fall in the water near Fort Point was due to a negative oscillation generated at the line of the fault. The effect produced would have been brought about had there been a slight drop of the sea-bottom on the outer side of the fault. But there is independent evidence, to the north and south of this particular segment of the fault, that there was no drop on the west side, so that this explanation can not very well be entertained. ^[1] It is also possible that the effect observed might have been brought about by a slight expansion of the confines of the Gulf of the Farallones, due to the differential movement along the fault, but this would not explain the coincidence in time. The period of the east-west oscillation of the waters in the Bay of San Francisco, between West Berkeley and Fort Point, has also been computed by the Tidal Division of the Coast and Geodetic Survey to be about 40 minutes. This agrees fairly well with the two or three oscillations recorded by the gage after 6 o'clock, and indicates that the drop of the water-surface outside of the Golden Gate generated an east and west oscillation in the Bay of San Francisco.

Tidal observations conducted at Fort Point for a period of 1 year from the date of the earthquake indicate that there was no change of the relative altitude of sea and land at that point, as compared with the conditions prevailing during the 3 years preceding. A review of the observations for the past 9 years, by the Coast and Geodetic Survey, reveals, however, the interesting fact that in that period of time there has been an apparent subsidence of the coast at that point of 4.8 inches, practically all of this having been accomplished in the first 6 years of this period. There has been no movement in the last 3 years. (April 18, 1907.) The only other tidal gage maintained on the coast of California is that at San Diego, and the marigram obtained there shows no abnormal movement of the surface of the sea referable to the earthquake.

The only other report indicating that the level of the ocean was affected along the coast is by W. W. Fairbanks, of Point Arena, who says: "I have endeavored to learn of any unusual action of water along the sea-coast, and can relate but one instance of anything approaching the character of a tidal wave. On the day of the shock I traveled by wheel and on foot from Albion to Point Arena, 25 miles. At the mouth of Navarro River, at 8 o'clock on the morning of the 18th, I learned from reliable sources that a section of about 10 acres of low, flat land about the mouth of this river was entirely submerged for some minutes immediately after the shock."

The shock felt by ships. — Information regarding the perception of the shock on ships at sea or in harbors has been collected by Prof. George Davidson, and the following notes are chiefly the result of his inquiries:

The U. S. T. S. Pensacola, moored to the pier at the U. S. Naval Training Station, Yerba Buena Island, San Francisco Bay, felt the shock on the morning of April 18, 1906. Surgeon L. W. Curtis reports that while in bed on the Pensacola he felt a vibratory shock lasting about 30 seconds, with one heavy jar about the middle period of the shock. A gentle rumbling sound coincided with the shock. The phenomenon closely resembled vibrations which are at times set up in the ship's hull on starting the dynamo, and it was mistaken for that, tho much more active and exaggerated than ever before observed. The vibration shook down some loosely piled books and papers from a table.

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The pilot-boat Gracie S. was lying in 18 fathoms of water near the lightship off the San Francisco Bar. She was suddenly struck by a seaquake which caused her to quiver as if the chain were running out of the hawser pipe. When the pilot boarded the German Cosmos steamship Nyada, the captain reported that his vessel had been shaken as if she had struck on rocks. The pilot-boat Pathfinder was lying in the vicinity, in 20 fathoms, and reported the same effect.

The steam collier Wellington, inward bound, between Fort Point and Point Diablo, in 50 or 60 fathoms, reported that the vessel was struck as if she were upon rocks. (Personal report of Captain Hayes, of the Board of Pilots.)

The steamer Alliance, of Cape Mendocino, reported by Mr. H. H. Buhne, of Eureka: The captain said she was struck a hard blow, as if she had run on a rock at full speed; time, 5^h 11^m. Mr. Buhne states that all ships in the harbor at Eureka felt the quake, but in South Bay it was heaviest. One vessel was hurled against the wharf time and again, throwing down piles of lumber and shingles.

The schooner John A. Campbell felt the shock at sea, off Point Reyes. The following is a memorandum of the event by Capt. C. J. S. Svenson: "Ship's local apparent time April 18, 1906, 5^h 15^m A. M. Lat. 38° 00' N. Long. 126° 06' W.; 145 miles true west of Point Reyes. Weatherfine; sky clear; wind fresh from north-northwest; sea moderate; ship's course southeast; speed 7 miles per hour. The shock felt as if the vessel struck lightly forward and then appeared to drag over soft ground, and when aft a slight tremor was felt; the whole lasting only a few seconds." The depth of water in the vicinity of the ship's position is 2,400 fathoms.

The steamship National City was approximately in lat. 38° 24' N. and long. 123° 57' W; 29 geographical miles distant from the nearest point on shore and about 31 miles from the fault-trace along the valley of the Gualala River. The vessel felt the shock at 5^h 03^m A. M., April 18, 1906, ship's time. James Denny, the chief engineer, supplies the following comment: "The ship seemed to jump out of the water; the engines raced fearfully, as though the shaft or wheel had gone; then came a violent trembling fore and aft and sideways, like running at full speed against a wall of ice. The expression `a wall of ice' is derived from my experiences in the Arctic." In this vicinity the chart has several soundings, as follows: 911 fathoms over clay and mud at 11.5 miles on the line to Gualala Point; 1,586 fathoms over clay and ooze 8 miles north by compass; 1,821 fathoms over clay and ooze 14 miles N. 54° W. by compass.

The wharfinger at Santa Cruz reports that he heard a rumble before the shock, coming from the southeast, and saw the seismic wave traveling shoreward, causing a great rattling and crashing when it struck the town. Two distinct sets of vibration were felt, the latter being the harder. There was very little surf, the water looking like that in a tub when jarred. The wharf, extending southeast, seemed to pitch lengthwise. A steamer between Santa Cruz and Monterey, also one at Monterey wharf, felt the shock; it jarred them as if they had struck bottom.

Shocks felt at sea subsequent to April 18, 1906. — The ship Alex Gibson, at 7^h 05^m P. M. August 3, 1906, when in lat. 25° 35' N., long. 110° 06' W., experienced a tremendously heavy seaquake, lasting about 40 seconds and shaking the ship from stem to stern as if she were bumping over a ledge of rocks. It shook tools out of the racks in the carpenter shop; threw pots and pans down in the galley, cups and pitchers from hooks in the pantry, and all lamp glasses off the lamps. The crew came running aft not knowing what was the matter, and the captain thought the yards were coming down. The sea at the time was perfectly smooth, the wind light from the southwest, no land in sight, and all sail set in fine, clear weather. At 7^h 10^m P. M., ship's time, another light shock was felt, of about 15 seconds duration; and from 8 to 12 midnight two more very light shocks were felt, but the time was not noted. The captain states that he had experienced

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an earthquake at sea on a former occasion, but the one felt before was nothing compared to this one, either in force or duration. (Hydrographic Bureau.)

The bark St. James, Capt. F. O. Parker, while in lat. 26° 19' N., long. 110° 25' W., in the Gulf of California, on August 26, 1906, was shaken by a seaquake at 12^h 15^m P. M. The shock lasted a minute, and the sensation was as if the vessel were striking upon sunken rocks. Upon arrival at Guaymas, the captain learned that no shock had been experienced at or about the time noted. ( San Francisco Chronicle, Sept. 16, 1906.)

The bark Agate, Capt. C. H. McLeod, while off the northwest coast in lat. 43° 10' N., long. 128° 50' W., 100 miles west of Coos Bay, experienced a heavy shock on September 2, 1906, at 3^h 45^m A. M. The shock lasted nearly 1 minute. The sensation was as if the vessel had struck a coral reef or rock. The wind was light, the weather clear, and the sea smooth. At 3^h 55^m A. M., another shock was felt, not so severe nor so prolonged as the first. ( San Francisco Chronicle, Oct. 2 and 9, 1906. Hydrographic Bureau.)

The ship Robert Searles, Capt. J. H. Piltz, while in lat. 41° 78' N., long. 125° 52' W., 85 miles northwest of Cape Mendocino, experienced a severe shock on September 14, 1906, which occasioned a panic among the crew. The cargo (lumber) and upper works of the vessel were shaken. The shock lasted 25 seconds. ( San Francisco Chronicle, Sept. 17, 1906. Hydrographic Bureau.)

The American schooner Stanley, Capt. K. Petersen, while in the calm center of a cyclone, in lat. 46° 09' N., long. 125° 22' W., 55 miles west of Cape Disappointment, on November 6, 1906, felt a sharp shock that lasted 2 or 3 seconds. Immediately afterwards, when looking toward the southwest, the captain saw 3 mountainous waves coming; when they struck, the ship began to pitch and roll violently, and he thought every minute she would be swamped. (Hydrographic Bureau.)

The schooner Melrose, Capt. M. McCarron, while in lat. 37° 35' N., long. 123° 35' W., felt a seaquake on February 3, 1907. The first shock was at 10^h 30^m A. M., lasting about 8 seconds; and the second at 10^h 50^m A. M., lasting about 5 seconds. Neither shock was violent, but each caused a decided trembling of the vessel. The motion was from east to west. The sky was overcast and the sea was smooth, with light westerly winds. The position of the vessel was 28 geographical miles S. 73° W. from the Southeast Farallon. The nearest sounding on the chart is 5 miles north of this position, where there is shown 1,726 fathoms of water.