College, Alaska
1964 Alaska Earthquake
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Courtesy of Jack Townshend, 2005. |
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| The seismograms reproduced above recorded ground
motion via a long-period Sprengnether seismometers and a sensitive
galvanometer. Time progresses from left to right and from top to bottom,
just like text on a printed page. Each line is 60 minutes long with a short
vertical deflection of the trace identifying each minute. Component, magnification, date, and
the beginning and ending time are noted on the left side of the each
seismogram. The original record was made photographically via a light beam bouncing off the galvanometer's mirror and then strategically focused on a sheet of highly sensitive photographic paper. The paper was wrapped on a drum, which rotated once per hour and translated the width of the paper in 24 hours. When the P-waves from the 1964 earthquake arrived, as noted by the red arrow, the beam of light began moving back and forth with large amplitude, going completely off of the photographic paper in each direction. This extreme motion continued for many hours and was so rapid that there was not enough light to register on the photographic paper. Eventually the electrical current generated by the motion of the seismometer exceeded the capacity of the galvanometer and the galvanometer mirror reflecting the light beam became stuck in an extreme position until it was readjusted early the next day. |
