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.

 

More rarely there are volcanic events sometimes on the Live Seismic Monitor with a few different spectro signatures like the volcanic tremor we saw from Bogoslof volcano on seismic stations at Okmok and Makushin in Alaska. Being at a distance of around 60 km to 70 km from the seismic stations, the tremor appeared as a bright yellow, thick band at the bottom of those stations. The higher frequencies above 7 Hz had been absorbed or attenuated, and so were not present in this case because of the distance to the stations.

Another possible type of volcanic tremor, known as harmonic tremor, is pictured here:

Source: IGN-Spain

http://www.ign.es/web/ign/portal/vlc-senales-sismicas/-/senales-sismicas/getInfo?fecha=2011-10-10&tipo=2&estacion=CHIE&tabResult=AyerYHoy

 

Note the strong reds near the bottom of the channel, around 2 Hz and below. Note the horizontal banding rising up from the more solid red, in somewhat even multiples. And this is what the word "harmonic" in the phrase "harmonic tremor" means. It's when you have ordered, evenly distributed, harmonic banding accompanying the tremor. It is usually associated with some type of magma, water or gas movement near or at a volcano. And often when it is about to erupt or is already erupting.

 

Non-harmonic tremor looks similar, except without the horizontal banding. Dependant largely on the distance to station and on the source, volcanic tremor and harmonic tremor can appear differently as the distance to the seismic station increases. The further the distance the less higher frequencies will appear, because the high frequencies (from10 Hz up) get attenuated, or absorbed, sooner by the earth. Low frequencies below 10 Hz travel further, and the lowest frequencies (<1 to 4 Hz) travel further still. As the distance increases to about 70 km, most of the frequencies above 5 Hz will have disappeared, leaving one with only a 5 Hz and below band that shows up 70 km away. This explains what we saw on Okmok and Makushin stations during tremor episodes from Bogoslof volcano erupting.

 

Tremor at volcanoes can occur with the solid reds centered around 1 Hz or 2 Hz, 2 to 4 Hz, and more rarely, even higher. Most volcanic tremor looks like a long and high wall going by on a scrolling spectrograph, with reds at the bottom, and bright yellows rising up from the red. Some volcanic tremor will also appear more erratic looking than the more orderly harmonic tremor pictured above. Still other volcanic tremor is interlaced with small earthquakes, which rise up as spikes from the tremor as shown in the right hand picture above. So in that second picture, that is harmonic tremor interlaced with small earthquakes.

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:

https://pnsn.org/spectrograms/what-is-a-spectrogram

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