Understanding Seismograms & LISS Helicorders


When you can ‘read’ a seismogram or helicorder you can find out which earthquake made the ‘squiggle’ on the plot.

(Note: Links on this page open in the same page so use ‘back’ to get back to the turotial)

Sometimes when you look at the LISS helicorders you see every one in the list is black and it looks as if the whole world is having a paroxysm. Don’t worry it is not! (This gives rise to all sorts of nonsense on blogs so understanding what is happening is better for your nerves.)

So first of all what makes the squiggles? I am not going to get too technical here, the videos are the best way of learning. All of the videos here are produced by IRIS and you can get more information from their Recent Earthquake Teachable Moments, which uses recent events as teaching opportunities.

There is more after the first three videos.

Seismic wave propagation

Seismic Shadow Zones (Why the signal is delayed at some points)

Exploring Shadow Zones. How are seismic shadow zones like shadows on Earth’s Surface? Addresses reflection and refraction.

So how can we find out which squiggle is which earthquake. Well let’s start by looking at a couple of them and see what we can find. The image below shows 2 earthquakes on the 17th Nov 2010.

ctao2
Click for a larger view.

This is already showing the correct times but just ignore those for a moment and look at the bigger picture. Most of the lines on this plot are ‘flat’ so that makes it fairly easy to spot where the earthquake begins. This is not always the case. Looking at the first smaller squiggle you can sort of guess that it is about 13:42 or 13:43. Now you need to look at the listings to see if you can see the quake. It is going to be in Australia? No probably not. Take a look at the USGS 7 day listing. (Of course this on will not be on there now. That just to show you what it looks like.)

On the day in question the list had the entry I have highlighted in green. This looks like a candidate. It is possibly big enough to show and at 13:29:35 is just a few minutes before the time we are after.

To check it we need to look in two places. These are the phase data and the travel times. To do this click on the date link for that quake. You should now see the quake you are checking, and the top tabs give access to the bits you need.

Click on the Scientific & Technical tab. I have marked the two pages you will need.

If we look at the travel times first we can see on the map that CTAO is round about 4 or 5 minutes from the epicentre.

Santa Cruz P wave travel times

The time of the earthquake is around this so we are on the right track. Now we take a look at the Phase data. This is not as scary as it looks. All you are going to do is look for the station name in the left hand column as a double check of the time.

Station Phase Data

It really is as easy as that. By following these steps you should be able to identify any reasonably large earthquake quickly and accurately and not have to worry that they are going off all over the world. Try this on the next one you see that is visible on many seismos on LISS. You will soon be able to prove to yourself that ALL the squiggles are just one earthquake.

Just for fun see if you can identify the big squiggle in the list – yes it is on that list. When you think you have found it click here to check the details which I have saved for you as they would otherwise be long gone.

Here is a little further reading for you on the subject of seismograms.

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