Display Description:
The NSL trace display consists of two panels: the top panel shows approximately the last four hours of vertical-component seismic data from selected stations spread across the NSL network and the bottom panel shows approximately the last five minutes. The station names appear as the middle part of the trace name on the left, for example station BEK is “NN_BEK_HHZ”. The timing for the data is seen along a light blue bar below each panel. It is coded as “yyyyjjj:hh:mm:ss” where “yyyy” is year, “jjj” is day in the year, “hh” is hour, “mm” is minute, and “ss” is second. Day within a year measures the day number from the beginning of the year, such that December 31 is normally day 365, but 366 in leap years. See the table of "day in the year" (For leap years (e.g., 2004) February 29 becomes day 060; then add one day for every date past that.) This is a reasonable day “scale” that seismologists are accustomed to using. The time of day is in reference to the time at Greenwich, England. Formally called Greenwich Mean Time (GMT), it is now usually referred to as Universal Time Code (UTC). UTC Time Conversion Chart It has long been a convention in seismology to apply this time to all seismic recordings so that investigators and agencies all around the globe can exchange data and have the recordings all coordinated, most importantly for earthquake location. UTC is a 24-hour time format, meaning the first hour of the day is 00 and the last hour of the day is 23. If you are in the Pacific Time Zone, for instance, subtract 7 hours from UTC time during the daylight savings part of the year to get local time on a 24-hour format (or 8 hours during the winter part of the year).
What You See:
Each horizontal line represents the earth motion along the vertical axis recorded at a station. Normally, it is simply background noise in the earth. This is analogous to the auditory background noise in a city – it rises and falls with a diurnal rhythm and has much variation over time. There are occasionally strong cultural, or man-made, sources contributing to this background noise, even appearing to be earthquake signals sometimes. (Mining blasts are the far end of this cultural noise and are sometimes difficult to distinguish from real earthquakes.) Most seismic stations are placed outside populated areas to minimize cultural noise. The traces occasionally show various types of electronic noise, such as spikes, sudden offsets, and monochromatic waveforms. Although seismologists are quick to recognize such noise, non-seismologists often mistake it for an earthquake signal. One key to distinguishing electronic noise is that it normally occurs on just one station. Earthquake signals have a definite character and are most likely to be seen at more than one station. See the following description.
Identifying Earthquakes, Near and Far:
Consider now the lower trace display showing about 5 minutes of time. An earthquake within or near the network (“local” event) will be recorded at quite different times on different stations, depending on their distance from the earthquake. Waves emanate out from an earthquake and arrive at the stations much like when a pebble is dropped in water and one observes the waves arriving at different times at various scattered objects sticking out of the water. For an earthquake which is more distant (“regional” event, outside the network and up to 2000 km away), the arrival times may tend to look more closely bunched. Again, picture the wave made by a pebble in water traveling to distant shoreline objects. For earthquakes that are quite distant (“teleseismic” event, at greater than 2000 km away), the first-arriving waves travel paths deeper into the earth and emerge nearly vertically under the seismic network. Thus their times can be nearly coincident across the network, causing the first waves of the event to “line up” over all the traces. Another key to the distance of an earthquake is how spread out the signal appears to be in time. Local event signals appear to be concentrated in a few seconds, decreasing quickly in amplitude. The signal time increases to tens of seconds for regional events and to minutes for teleseismic events. However, the magnitude of the event can make this rule-of-thumb difficult to apply in all cases; for instance, an M 5 local event may cause visible signals for several minutes because of its size. Due to the larger time scale, the upper trace display is of much less help in estimating the distance of an earthquake.
