A motion picture display of the seismicity of the Earth during the period 1961-1967 has been produced at the Seismological Observatory of the University of Michigan. The movie displays in chronological order, in appropriate geographic location, and with corresponding magnitude representation, all epicenters listed in the ESSA-USCGS compilation of seismic events for the seven-year period. With some minor exceptions, the data set is that used by Barazangi and Dorman (1969). For convenience in viewing, the data has been divided into three geographic regions. The first is of the circum-Pacific belt from 110°E to 60°W; the second, from 0 to 120°E, includes the Alpide-Himalayan belt, the African continent, and the Indian Ocean; the third, from 0 to 105°W, is of the Atlantic Ocean, and contains a repeat of the Central and South American segments of the circum-Pacific belt.
Since the motion picture format emphasizes the magnitude of events as well as chronology and location, some revisions of and additions to the data catalog were necessary. Many events in the catalog have no listed body-wave magnitude. These events fall into two general categories. The first includes events for which data was insufficient to determine a magnitude. These events were arbitrarily assigned a body-wave magnitude of three; they occur predominantly during 1961-1963, an interval during which the WWSSN was still in the installation stage and only partially completed. The second category includes some events equal to or greater than magnitude six, for which no body-wave magnitudes were reported; these events are rather uniformly distributed over the seven-year period. For events in the latter category, body-wave magnitudes from individual stations as listed in the monthly Seismological Bulletin of the USCGS were used. When body-wave magnitudes were not available, surface-wave magnitudes from the same Bulletin were used.
Each frame of the film represents one day's activity, with the events occurring on that day being initially plotted as symbols in their appropriate geographic locations on a Mercator projection. The brightness, in relative units, of an epicentral symbol was governed by the magnitude of the event via the following conversion: twice the magnitude rounded to the nearest integer, with the result diminished by four, thus yielding two brightness units for a magnitude three event, three units for a magnitude 3.5 event, and so on. In each successive frame, the brightness diminishes by one unit, until the symbol disappears. Thus the larger the magnitude of an event, the brighter its symbol initially appears and the longer the symbol is retained through successive frames.
Prints of the movie can be purchased at cost by writing to the following address:
Seismological Observatory
Department of Geology and Mineralogy
The University of Michigan
Ann Arbor, Michigan 48104