|SEISMOGRAPH AND MAGNETOGRAPH RECORDS OF THE SAN FRANCISCO EARTHQUAKE, APRIL 18, 19061|
DIVISION OF TERRESTRIAL MAGNETISM, U. S. COAST AND GEODETIC SURVEY.
THE San Francisco earthquake was one of several large earthquakes recorded the world over since the beginning of this year. The writer's prime interest in it as a magnetician is in the record it left behind on the magnetographs at various magnetic observatories of the United States Coast and Geodetic Survey.
It has happened several times within the last few years that earthquakes have occurred in this country which were not recorded for one reason or another, on the existing seismographs, but were indicated by the record of certain magnetographs. The most notable instance was the New England earthquake of March 21, 1904, at about eight minutes after one o'clock in the morning, eastern time. Seismographs of the Milne type at Toronto, Canada, and Baltimore, Maryland, and of the Bosch-Omori type at the Weather Bureau, Washington, D. C, failed to give any record of this earthquake, which was appreciably felt throughout the New England States. The magnetograph at the observatory, Cheltenham, Maryland, sixteen miles southeast of Washington, gave a distinct record at 1h 05s to 1h 17m eastern time. So there have been a number of earthquakes recorded by the magnetograph at Baldwin, Kansas, which were felt in the middle states and reported in the papers. In fact, at this observatory, situated in a region where felt and unfelt local and regional earthquakes are comparatively frequent—note for example the many recent occurrences—more records of earthquakes are obtained on the magnetograph than at any of the other magnetic observatories.
This repeatedly authenticated fact made desirable a concurrent study of seismograph and magnetograph records and hence seismographs have been installed within the last two years at all of the magnetic observatories excepting at Baldwin, Kansas, which was omitted because of its probable early removal on account of the possibility of disturbing influences from electric-car lines. So it happens that the Coast and Geodetic Survey is able at present to contribute the principal portion of the accurate observational data of earthquakes obtained in this country. It was with the expectation that magnetic observatories would also be excellent stations for the installation of