. A history of the earthquake and fire in San Francisco; an account of the disaster of April 18, 1906 and its immediate results. tBerkeley shows that the motion of the earths surfacethere was about half an inch; but according to ProfessorLeuschner it was undoubtedly greater in San A. W. Whitney states (3) that the amplitude ofvibration at Mt. Hamilton was about an inch. The periodof the vibration was about one second. The preliminary tremors were recorded by the in-strument in the Weather Bureau Laboratory at Wash-ington, D. C. for about six minutes before the main
. A history of the earthquake and fire in San Francisco; an account of the disaster of April 18, 1906 and its immediate results. tBerkeley shows that the motion of the earths surfacethere was about half an inch; but according to ProfessorLeuschner it was undoubtedly greater in San A. W. Whitney states (3) that the amplitude ofvibration at Mt. Hamilton was about an inch. The periodof the vibration was about one second. The preliminary tremors were recorded by the in-strument in the Weather Bureau Laboratory at Wash-ington, D. C. for about six minutes before the main preliminary vibrations reached Tokyo about eleven j:(> APPENDIX and a half minutes after they were felt in San Francisco,and lasted nine and three quarters minutes. In Tokyoand in Washington the actual motion of the ground was alittle less than half an inch, but, because of its slowness,it was perceptible only to the instruments. At Birming-ham, England, the preliminary tremors lasted twenty-fiveminutes. (4.) These preliminary tremors occur because some smallwaves of vibration travel through the earth at the rate of. Seismograph Record of the Earthquake APPENDIX 277 about seven miles a second, while the larger waves travelmore slowly—at about two miles per second, apparentlymoving on the surface. It is this preliminary vibrationthat produces the rumbling often heard just previous toearthquake shocks. Such a rumbling sound was heard asfar away as Death Valley, California (more than fourhundred from San Francisco) in connection with theearthquake under consideration. Because of the differentspeeds at which the preliminary and main vibrations travel,the duration of the introductory tremors enable the center of the disturbance to be located. Theobservations at the Lick Observatory on Mt. Hamil-ton (about ten miles south of San Jose) indicate that theshock originated about ninety miles north of that point—that is, in the neighborhood of Tomales
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