There is much talk on the web about USGS downgrading earthquakes. But the question is do they? Well yes, but they upgrade them as well just as much. So how do we go about tracking this?
Well first off you need to be aware of the changes that are made during what I will call the ‘public lifetime of an earthquake’. By that I mean the time during which details of the earthquake are available either easily, or with a bit more searching, to the general enquirer.
This period is basically 7 days on the main lists, 8 to 30 days on the following lists and after that you may or may not find the details. Quakes that are deemed to be ‘significant’ or ‘historical’ will still be available, others will slide into oblivion of the web.
If you look at the front page of this blog you will see both upgrades and downgrades recorded, and I try but don’t always succeed to keep track of thinks on this page
I am going to look at this one Balleny Isls region July 21 2011. I can’t of course show you the original, but it was first posted as a mag 5.6. Just to show that it was this link to ATS Quake Watch shows where someone noted the quake. Being small it got forgotten. But then when I came to compile the mag 6 quakes and check that nothing had changed, this one had – by a substantial amount.
Let’s look at what might cause this, or cause a downgrade. I am not going to throw the maths at you but if you are interested in the different scales you can find some information here. You first need to make sure that what you are looking at is actually an increase. For example an recent earthquake – Vanuatu 6.2 – was first posted by EMSC as a 5.8 mb.
Subsequently it was posted as a 6.2 just the same as USGS – but it was 6.2 Mw. If you look at the phase data for the USGS version you will see it says 5.9 mb and 5.8 GS in other words very close to the EMSC figure. Dis EMSC change the magnitude? No. The same happens on USGS as well where sometimes a quake will be published on one scale and then republished on another. You always need to look at what you are looking at!
Of course sometimes, as with Balleny, the magnitude of the quake is changed. Why? Well first off the initial assessment is made by a computer. Computers are remarkably stupid. They only understand 0 and 1 and if the 0’s and 1’s are not in the right place they are too stupid to know better. (That is not a layman’s view – I am a programmer)
The initial assessment may be way out for various reasons, and also can give rise to phantom quakes as I have noted in this post on QVS Data
Once a seismologist has had a look the initial assessment will be updated. But it does not stop there. There are numerous complex calculations to be performed on the phase data received from many different stations. I have to be honest and say I do not know how they finally decide which one is the right one! This process can go on over as long as a few months on occasions.
If we take the quake at VA as another example, this was first put out as a 5.8, then it went to a 5.9, and then back to a 5.8
There is nothing unusual about this. Most of the time changes are small. Just occasionally they are larger, such as Balleny or the changes at the Kermedecs. Go to the changes page and scroll down to the end of June start of July. You will see one Kermedec quake dropped from 6.0 to 5.7, but two uplifted from 5.8 to 6.0
Do they not say the right magnitude will be your next question no doubt. Now that is more tricky. First, despite being a Gubbermint agency, their mission amongst other items is to inform the public about seismic hazards.
I will use much the same argument here as I use against those who say that USGS delete quakes. As far as deletions go it just is not possible! Not only is it a technically difficult thing to do to remove an earthquake from the waveform files of thousands of seismos without leaving a trace, thousands of people see this data and use it from ordinary members of the public such as me to academics and Universities, and other agencies.
Saying they are less than they are is technically not so difficult, but once again thousands of people are viewing the data. If earthquakes were frequently downgraded it would be noticed.
Don’t get me wrong, we often say that they are doing it but in reality it is because we do not have access to the data in it’s fullest form and our perceptions of what should and should not be are coloured.
Call me a Government shill, a disinfo agent or whatever you please I maintain that it is not possible for USGS to systematically lie about the magnitude of earthquakes without those who are knowledgeable of the subject noticing this is going on.
If you have a waveform from an instrument different packages can interpret it different ways but they will all be similar. You can take that waveform and calculate the values manually provided you have all the data, poles and zeros etc.
Consider also that if there was a general perception that they lie – outside of conspiracy forum circles – they could not do their job as no one would listen.