Page:Popular Science Monthly Volume 69.djvu/311

Rh the matter at Unalaska, the water was so hot that a vessel could not approach within five miles of the island. It is conceivable that' the great earthquake rift has its center in the area of weakened sea-bottom occupied by the Bogoslofs. It is possible, even probable, that the coincidence of time does not show any real connection between Bogoslof and the earthquake of 1906. Against the connection may be urged the great distance, the great depth of most of the intervening sea, and the alleged facts that the seismograph at Sitka showed a shock from the south, while that at Tokyo indicated waves from the east. It is also stated that no shock was felt at St. Paul, St. George or Unalaska, but that a great shock was felt at Unimak. Unimak, like Unalaska, lies to the east of the supposed line of the fault. In any case, the birth of the third Bogoslof is a matter worthy of thorough investigation, and its approximate coincidence in time with the earthquake in California is very suggestive.

The earthquake of 1906 is receiving the most thorough study possible, and in such a way as to give promise of important practical results. The many previous earthquakes have been recorded, but their most essential feature, the location and extent of the causing fissure has rarely been indicated. In the records we read again and again that 'fissures opened in the ground,' but whether these were rifts in the crust or mere slumps of soft ground as a rule has escaped attention. The great earthquake of 1868 opened rifts at intervals from Tomales Bay to Carisa Plain, and also a fissure on the east side of San Francisco Bay, where a straight crack about ten miles long extended from Haywards toward the south. One side of this rift showed a lateral displacement of about four feet. To this short rift, rather than to the Portolá-Tomales fissure, the shock in San Francisco in 1868 may have been due. The shock in that year was more violent in Oakland than in San Francisco and most violent about San Leandro and Haywards, to the south of Oakland. It is conceivable that the shock of 1865, having its center in the Portolá fault, not far from San Francisco, gave that city a degree of immunity in 1868. Other destructive earthquakes, as recorded by Holden ('Catalogue of Earthquakes on the Pacific Coast, 1769 to 1897') are as follows:

1800. This earthquake was severe about San Juan Bautista, but whether in the Portolá fault or the Pajaro fault is not clear.

1812. This earthquake wrecked the mission of San Juan Capistrano in southern California, and was felt along the line of the southern missions. It had its center possibly in the Santa Catalina Channel.

1818. This earthquake injured the mission of Santa Clara; hence it may have been along the Portolá fault.

1836. This was said to be similar to the shock of 1868, its center along the Portolá line; 'great fissures were made in the earth.'

1839. This was severe from Redwood to San Francisco, 'a great fissure opened to Mission San José.' It was probably also in the Pájaro fault.