So what does a bigger earthquake look like on a spectrograph?


7.1 Magnitude Earthquake 100 km outside Mexico City, 7-19-17. This earthquake occurred about 100 km from the seismic station MX.TLIG, and here is its waveform and spectrograph:

Click on Image above to go to the USGS event page for this earthquake


A key to understanding seismic events on the spectros is that most seismic activity will register at the bottom of the channel at the very least. If you don't see anything registering below 5 Hz with the event, chances are it's just noise, or a vehicle, or something else man made.  But  even noise  can go below 5 Hz sometimes. In that case you might check to see if the event registered on another station. Practice and a short time of experience watching earthquakes on SpectroNet can quickly add up for anyone wanting to better these kinds of skills. You will also find the chat commentary to be very helpful and informative with your short ramp up to using SpectroNet effectively.


Occasionally we provide an additional waveform duplicate of a channel if there is frequent seismic activity registering on it, such as when Yellowstone volcano has earthquake swarms.


Since most seismic activity occurs in the range of 1 to 25 Hz, and/or will register at many more seismic stations, earthquake events are easily spotted and eventually can be estimated fairly accurately using the SpectroNet Live Seismic Monitor.


You can learn more about what you are seeing on spectrographs by visiting this excellent link at PNSN:

What Is A Live, Near Real Time Spectrograph?


A spectrograph is a visual representaion of seismic data in such a way that frequency and amplitude are displayed with position and colors. In each channel you see from about 25 Hz at the top, down to around 1 Hz at the bottom. Seismic amplitude is displayed with varying intensities of colors: Red means more intense seismic power and amplitude, while yellow colors represent less seismic power and amplitude. But beware that yellow is also frequently the color of much ambient noise. So in addition to the intensity of a signal, one must also pay attention to the frequency range of the signal received, and whether the signal registers similarly on other stations/channels.


Real time spectrographs, such as the ones used on the SpectroNet Live Seismic Monitor, are a time series display of continually updating seismic data. Real time, or "right now" in time is represented at the far right of each channel. New data is received at the far right, and pushes the spectrograph to the left, drawing a new image at the end of the previous one.


As with real time waveform displays, real time spectrographs provide new data continually from active seismometers all over the world. Real time spectrographs are simply an additional tool in a broader analysis kit for earthquakes. And what a wonderful tool they truly are. Even without a companion waveform displayed, a considerable amount of  information can be gained with just a glance of an event appearing on a scrolling spectrograph. The larger the event magnitude, the more red is displayed, and the longer the duration and tail of the event will be.


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