OhioSeis Helicorder Displays
A helicorder is a digital display of the live seismic signal recorded at a seismic station that depicts a record of ground motion during a period of time. It is a digital representation of the old, but familiar, paper drum recordings of ground motion. Ohio Seismic Network stations also provide helicorder station data for the last 24 hours of ground motion at that station divided into 12-hour segments—current 12 hours and previous 12 hours. The display window for each OhioSeis station is identified by the four-letter station code. These station data are useful to see if an earthquake has occurred at a particular time in the last 24 hours.
Reading the display time
Hours, as seen on the left side of the helicorder display, are given in a 24-hour sequence in Universal Coordinated Time (UTC). UTC is five hours ahead of Eastern Standard Time (EST) and four hours ahead of Eastern Daylight Time (EDT). For example, if an earthquake occurred at 3 p.m. EST, it would show up on the helicorder at the 20th hour UTC. A chart showing the correlation between UTC and local time can be found here: http://hurricanes.noaa.gov/zulu-utc.html
Each UTC hour reads from left to right and is divided into four 15-minute segments shown along the horizontal axis at the bottom of the display. The four different-colored lines for each hour each represent 15 minutes of time but have no signficance except to make them easily distinguishable. The display for the current 12 hours updates once per minute as data from the last minute are added to the last line. For example, if a local earthquake occurred at 20:37 UTC, it would show up on the third line (green) at the seventh minute as depicted on the minute scale at the bottom of the display.
Reading the helicorder display
All OhioSeis stations use a vertical, broadband seismometer; that is, the seismometer records vertical ground motion in a broad range of frequencies. This is so that local or regional high-frequency earthquakes are recorded as well as large, distant (teleseismic) earthquakes with lower-frequency (long period), high-amplitude surface waves. With very large teleseismic earthquakes, the surface waves may continue for an hour or more.
Each horizontal line in the seismograph is typically “squiggly” in appearance from low-amplitude, five-second period microseismic “noise”. As most OhioSeis station seismometers are in buildings, they typically will show noise from cultural activities, particularly during the day. This seismic noise may contain spikes or show other large variations in the signal.
During times when large storms such as hurricanes are present in the North Atlantic Ocean, for example, the amplitude of the microseismic noise will increase dramatically. Blasts from limestone and sandstone quarries will show up as brief bursts of high-frequency energy, whereas strip-mine blasts, which typically use more explosives than quarries, consist of high-frequency energy but are larger in amplitude and duration. OhioSeis stations record strip-mine blasts from the coal fields in eastern Ohio, Pennsylvania, West Virginia, and Kentucky. Typically, however, the mine and quarry blasts occur during the day and usually on weekdays.