Historical Seismograms - Evidence from the AD 2000 Izu Islands ...

Pattern Analysis of Small Earthquakes and Explosions 351of the three-component records from SHS are shown in Figures 2 and 3. Timemarks were recorded on each film every 30 seconds and radio time was recorded onthe time strip in order to determine the clock correction and absolute time. Boltand Miller (1975) give a detailed account of the instrumental parameters.3. Analysis of SeismogramsThe film records from the Shasta (SHS) seismographic station are stored at Berkeley.These seismograms were examined and events with maximum trace amplitudes(A,) of 0.5 mm or larger and with a differential S-P travel-time interval (ts-p) of7 seconds or less were recorded. The analysis included reading: 1) the onset timesof the P- and S-waves; 2) the maximum trace amplitudes on each component; 3) thepolarity of first-motion; and 4) the amplitudes of the first swing of the P-wave onr - 1I ' 1I'I. .o 5 io 15 20 25 30secondsFigure 2. SHS recording of a natural earthquake (ML = 2.7) which occurred at 12:33UTC on June 29, 1953.UPt4DI-.IINS19 5 10 15 20 25 30secondsFigure 3. SHS recording of a small explosion (equivalent ML = 1.5) which occurredat 19:31 UTC on August 24, 1962.