[Skywise]

(05-22-2015, 11:01 PM)zinnanti Wrote: Hi All,

Lay person question here . . . Why do quakes always seem to be downgraded? Maybe it's only my perception, but it seems to be more lately. And, the adjustments to the quakes seem to be more significant.

Well, some would say it's a conspiracy by The Powers That Be to hide the truth from us, that in reality earthquakes are happening more and more often, and bigger. The Powers That Be don't want to alarm the public. They feel we'd all panic if we knew the truth, and that truth is that The Powers That Be know that the end of the world is nigh!!

ahem....

All chain yanking aside, there are some who have expressed that opinion.

Not all quakes get adjusted downward. They sometimes go up as well. Case in point, the Tohoku quake in Japan was originally listed in the high 7's, a significant shortchange on it's ultimate magnitude.

The reason is simple. The first magnitudes given are computer generated. Algorithms read the seismographs (often before the needle has stopped moving), make some quick calculations, and make a best guess at the magnitude. Later, a human seismologist takes a closer look and adjusts the magnitude as needed to be more accurate.

In the case of large quakes, the first reading tends to be low because the seismographs are overwhelmed. The needled is pegged. If the gauge only goes from one to ten, how does it show an eleven?

I am not familiar with the details of determining magnitudes other than the no longer used Richter Scale, but my best guess is that the process is not a simple one, and that the computer algorithms trade accuracy for speed. It is important to get a quick magnitude in order to estimate the need for emergency assistance. The first moments after a large quake are critical to saving lives.

Sometimes it takes months or even years to settle on a 'final' value. But there never really is one. More data and more analysis changes the answer.

Another issue is that there is more than one magnitude scale. They don't always correlate equally, and each one is only accurate under a limited set of conditions.

Brian