Magnitude 9.2 - 30 miles NW of Tatitlek

March 27, 1964   17:36:14 AST
61.0500°N 147.48°W    Depth 14.3 miles

The seismograms reproduced in this image recorded east-west 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 line identifying each minute. Component, magnification, date, and the beginning and ending time are noted on the left side of each seismogram. The original record was made photographically with a light beam focused on a piece of photographic paper. When waves from the 1964 earthquake arrived the beam began moving back and forth so quickly that the photographic paper did not receive enough light to register the position of the trace. This situation continued for many hours following the earthquake, which is why the record from March 27 shows little after the arrival of the P-wave at the red arrow. 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. Photograph by Jack Townshend, 2003. This image and text, plus more 1964 seismograms, are on the USGS website