This is a 24-hour long seismic record.
Time progress left to right and from top to bottom, just like
text on a printed page. Each line is 15 minutes long. The
hour marks are visible as a vertical stripe parallel to the
left edge of the record.

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 about an hour, which is why the record above has a white band
near the center.

Here is a much larger image of the seismogram. It will take a
while to download because its size is 1 MB!