# Earthquake on the East Coast- What was the perceived magnitude?

1. Aug 23, 2011

### RossH

Hi there. So, I was wondering if there is anyway to tell what the perceived magnitude of today's earthquake is, based on my distance from the epicenter. I live around Princeton, so I was about 250 miles north of the epicenter. I was thinking that there might be an equation that describes how quickly the strength of the wave fades or something similar? Just curious.

Thanks!

2. Aug 24, 2011

### davenn

dont get magnitude and felt intensity confused. There is no such thing as perceived magnitude. The magnitude of a quake is a function of distance from epicentre and amplitude of the waves on a seismogram.
Close in, short distance and large amplitude Vs a large distance and a small amplitude is going to produce the same magnitude result. Doesnt matter if you were 10km or 1000km from the event.

Intensity is a perceived report. and that varies according to building type, whether you were in motion ... walking/driving, ground type ... soft ground amplifies the shaking compared to being on solid rock. The Modified Mercalli (MM) scale is used to determine intensity reports from around the region of a quake.
eg MMI ... not felt by anyone through to MMX .... total destruction

Yes there are formula for determining magnitude. Many years ago, Charles Richter Devised the well known Richter Scale ( which has largely been superceeded these days)

He produced a Nonogram ( that I have posted an example of elsewhere in the PF forums) that you use to determine magnitude. This was done by measuring the difference in arrival times of the P and S waves of the quake and plotting that against the distance of the quake from the recording seismometer. This would produce a given magnitude for that event.

These days, its all done digitally, as in the system I run from home. Those calculations are done by the software once I place markers on the P and S wave arrivals on the seismogram.

here's an example from the software I use, this was recorded by a guy in California of a small event in Calif. You can see the P and S markers and in the bottom line of the upper text box you can see the info on the event as calculated by the software. Origin time P arrival; S arrival; S-P difference; distance in degrees and km(miles); magnitude etc

cheers
Dave

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3. Aug 24, 2011

### RossH

Okay, I guess we had a terminology collision. I guess what I was asking is, given that the magnitude of the earthquake was 5.8/5.9 (based on who you believe) at the epicenter, what was the magnitude in NJ near Princeton. Is there any way to calculate the actual magnitude, or do you need a seismograph to find that out? I read some articles that said that it was around 1.5 or 2. Is that accurate? I wasn't really asking about the intensity. And thank you for the excellent and detailed answer.

Last edited: Aug 24, 2011
4. Aug 25, 2011

### davenn

Hi Ross,

As I said the magnitude DOESNT change for a given event, its a M5.8 quake as recorded anywhere in the world for THAT event
If it changed dependant on location then the magnitude scale would be meaningless

actually you were :) you were "sort of" specifying intensity figures in that comment of yours, ie. the way people felt it in Princeton, NJ., but in the preceeding sentance you are talking about magnitude calculation.

You need a seismograph, digital or analog, to do magnitude calculations as I outlined in the first quote of mine in this post.

Intensity calculations are based on personally felt reports and of actual damage observed.
As in my previous post.....
Intensity figures use the Roman Numeral system eg.

I II III IV V VI VII VIII IX X

(1 2 3 4 5 6 7 8 9 10)

there are NO fractional numbers, ie. 1.5 3.4 etc etc in the intensity scale. Whole numbers only as above

Dave

Last edited: Aug 25, 2011
5. Aug 25, 2011

### davenn

Lets look closer at the seismogram and nonogram to determine a magnitude

now these 2 images are unrelated ... they are just examples OK

the seismogram ... in the days of old , ink pens drew a trace on paper wrapped around a cylindrical drum that usually rotated once every 15 minutes.
( there are still a few around ---- my old one is used by a friend in Christchurch City in New Zealand where there has been a series of big quakes over
the last 12 months)

Calculating magnitude
Rule of thumb #1 .... measure the S wave - P wave and get that time in seconds
the example below ~ 10 sec multiply that by 8.5 to get the distance to the event.
(great for quakes under 500km away from the recorder)

So take 2 measurements from the seismogram

1) --- the S - P time = 10 sec.
2) --- the maximum amplitude of the P or S wave ( which ever is bigger) in example below the S wave. lets say its 35mm

transfer those 2 measurements to the nonogram and read off the magnitude.
now the nonogram I have below is one I created for another thread on this forum
so ignore the different figures used. ALSO note this isnt the real one used by Richter. Its one I threw together as a quick example OK ....
so people could see a visual representation :)

you see 3 vertical scales left one is S-P time --- the middle one is Magnitude --- the right one is the max amplitude recorded on the seismogram
You draw a line between the left and right values and where is crosses the middle scale you read off the magnitude

In this example... the S-P was 25 seconds and the Max Amplitude of the trace was 25mm

cheers
Dave

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6. Aug 27, 2011

### davenn

So Ross,

Did that answer you questions ?? you didnt respond

Dave

7. Aug 28, 2011

### Studiot

Well I enjoyed your well laid out presentation.

Thank you.

8. Aug 28, 2011

### davenn

Thanks mate :)

pleased someone got something out of it. is it just me or do you and others also get frustrated when you put a good effort into supplying info only to be basically ignored by the OP ?
maybe I'm just a bit sensitive hahaha

but to be honest there has been times over the months when I have done such with a similar lack of response and just wondered ... " why did I bother ??"

cheers
Dave

9. Aug 28, 2011

### davenn

over the last month have just got a new sensor online and working
is what they call the "Lehman" style at the moment its period is ~ 10 seconds ( when measured undampened)
I'm having a bit of a struggle getting it to the 15 to 20 sec period that it should be capable of. it requires some really fine adjustments and lots of patience :)

There have been a few modifications since that pic , but it will give you a good idea
of the unit

cheers
Dave

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10. Aug 28, 2011

### RossH

Yes it did, thank you. Sorry, for some reason I didn't get an email notification of your post so I didn't realize that you had posted back. Thanks! That was quite a detailed presentation.