Earthquake Waveforms: FAQ
What information is displayed on these plots?
Each of these plots displays seismic waveform traces from stations nearest the earthquake. In red are the picks of the first P-wave arrivals as determined by the real-time processing computer. The labels, PU, PD, or P, indicate whether the first motion is up, down, or undetermined. The pick times are used to calculate a preliminary event time and location which best fits the observations. Based on this event time and location, a predicted arrival time is calculated for each station and is plotted in green. The end of the coda is plotted in blue.
Why do some traces have no pick?
The most common reason for missing picks is a low signal-to-noise ratio. This may be due to a low signal (small earthquake and/or large distance from the earthquake) or high noise (malfunctioning instrument or data transmission difficulties). When there is no pick, the arrivals on the trace may not line up with picked arrivals on adjacent traces. This is because the time delay used to plot the trace is calculated using mean values for the entire state. These may not be accurate for the particular locale of the earthquake.
What is the coda?
After the initial burst of energy in a seismic signal, the signal gradually returns to the background level experienced prior to the earthquake. This tapering off of the seismic energy is the coda. The length of the coda is proportional to the size of the earthquake and is used to determine a quick estimate of magnitude.
Why are many traces clipped even for minor earthquakes?
At present, most of the NCSN stations rely upon 1970s-era analog instrumentation. By today's standards, these instruments have severely restricted dynamic range. In order to detect and monitor small regional earthquakes, they must run at such high gain that nearby events of even minor size exceed the capabilities of their electronics. As we are able to upgrade the instruments to modern broad-band digital instruments, we will suffer less data loss due to clipping.
What information is given in the small box to the left of each trace?
Depending upon room available, from one to three lines of information are printed for each trace.
The first line contains a station identifying name (e.g. MRS), a component identifier (e.g. VHZ, which indicates an analog velocity transducer running at high gain in a vertical orientation) and the distance in kilometers from the station to the earthquake epicenter.
The second line contains the maximum peak-to-peak amplitude for the trace. This is given in units of cm/sec for velocity sensors and in units of %g for accelerometers.
The third line contains the azimuth of the station from the earthquake epicenter in degrees clockwise from north.
Where can I find the location of a station?
Station locations for the stations in the Northern California Seismic Network are tabulated here.
Why are there three plots?
Each plot emphasizes a different feature of the data. The first plot presents the entire waveform for the event. The second plot is a close-up of the first arrivals. The third plot provides a view of more stations at greater distances and uses a reduced time format for positioning each trace in time.
What is reduced time?
If we were to plot each trace beginning at the same time, e.g. the event time, the seismic arrivals would fall later and later as we went to traces at greater and greater distances. This would lead to a display of a lot of rather uninteresting data preceeding the actual seismic arrivals for the more distant stations. To compensate, we start each trace at the event time plus a delay proportional to its distance from the epicenter. This reduces the amount of uninteresting information being presented while preserving a sense of the arrival time differences between traces.
What is the information at the top of each plot?
At the top of each plot is printed the Event ID, the Event date and time (Universal Time) and the Event time (Local Time).