The seismograms are displayed in static "day-plots" that contain one day's worth of data. These classic seismic plots were historically created using helicorders and are often called helicorder plots. A description of the plots is provided below the plots.
The times used to display the seismograms are in the UTC time system. UTC (Universal Time Coordinated) is a globally recognized time standard that is used as a reference for international timekeeping. UTC does not include any daylight saving time changes.
- Through late fall, winter, and early spring, the local time in central Pennsylvania is Eastern Standard Time (EST), which is UTC-5 hours.
- Through late spring, summer, and early fall, the local time in central Pennsylvania is Eastern Daylight Time (EDT), which is UTC-4 hours.
- In 2023, the time changes occur(red) EST->EDT on Sunday, March 12, 2:00 am and EDT->EST on Sunday, November 5, 2:00 am.
To recover the full group motion, we record the ground movement in three perpendicular directions.
In this case, the directions are "x" (north), "y" east, and "z" vertical.
In the coordinate field below, enter your choice as x, y, or z.
You can read the seismograms much like a page of book, but they are a little bit unusual. The wiggly lines show the movement of the ground at a particular time. The earliest motions are shown in the upper left and across the first "line". The rest of the motions are shown line-by-line, and the last motions to occur are in the lower right, just like words on a page (at least in english).
Earth is never still, so the low-amplitude motions of the lines (almost flat) show the continuous background motions of the ground. When the line departs from horizontal the ground is moving from it's average position. A deflection of the line is the indication that a vibration has passed by the sensor. For the vertical channel, if the line moves up, the ground moved upward, if the line moves down, the ground moves down. The same is true for the x and y channels, but those motions are horizontal (north-south, or east-west).
Time is computed for each line using the distance from the left edge of the plot. Each line has a time reference (we label each hour reference; subsequent lines are 15 minutes later). Each line summarizes 15 minutes of the ground motions. Adjacent lines are shown using different colors so that can be separated when they overlap.