How exactly do you measure the Richter magnitude of an earthquake?

[Simfish]

Supposedly, you use a certain type of seismometer. But in that case, how could a deep earthquake far away register as "higher" than a shallow earthquake that's up close? Sure, the deep earthquake may release more energy, but much of it is dissipated by the time it reaches the seismometer (and the peak ground velocity/acceleration, which is what affects the seismometer, would also be smaller for the deep [but bigger] earthquake).

 

[Gokul43201]

From http://earthquake.usgs.gov/learn/topics/richter.php

The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included for the variation in the distance between the various seismographs and the epicenter of the earthquakes.

Not terribly explanatory, but it's the best I've seen.

 

[davenn]

some clarifications ...

The Richter Scale for determining magnitudes of earthquake, created by Charles Richter
was only originally designed for local (up to ~100km of distance from the recorder) earthquakes, and for Californian quakes, where Mr Richter had done much of his research.
In reality, the Richter scale isn't used by seismologists these days. Its just something that the worldwide media can't shake off in their usual inaccurate reporting.

That magnitude scale was calculated by plotting the maximum amplitude of the P or S wave on the seismogram (from centreline to peak of ink pen movement) with the difference in P and S arrival time and plotting that on a nonogram.
I am not at home at moment so can't show an image of the original nonogram but this quick drawing below will suffice to give you the idea :)

There are a number of magnitude scales
Mb = Body wave magnitude, pretty close to the Richter Mag.
Ms = Surface wave magnitude was used for many years as the primary designation for BIG events as the surface waves didnt attenuate (with distance) as the body waves do (P and S waves)
and the main one used these days by the USGS and other institutions the
Mw = Moment magnitude .

Richter and Mb (MB) get totally saturated on a seismogram with large events M6 and up and often no meaningful data can be gathered about the size of the event.

Ms is great for the huge distant events where masses of surface waves are generated by shallow events <100km deep. but fails with small local events where there may be very little surface wave activity

Mw Moment magnitude was produced to be able to give a much more accurate representation of the actual amount of energy released in a particular event regardless of if it was a Mw3.0 or a Mw9.0 It takes into account the area of fault plane as well as length that slipped amongst other factors to produce an actual release of energy in Newton/metres

cheers
Dave

 

[davenn]

Supposedly, you use a certain type of seismometer. But in that case, how could a deep earthquake far away register as "higher" than a shallow earthquake that's up close? Sure, the deep earthquake may release more energy, but much of it is dissipated by the time it reaches the seismometer (and the peak ground velocity/acceleration, which is what affects the seismometer, would also be smaller for the deep [but bigger] earthquake).

That is mostly answered with my nonogram in my previous post. Deep quakes also suffer from a major lack of surface waves. and you have to rely on body waves to generate a magnitude reading

for your added interest ... we get a difference in time between the P, S and Surface waves because they travel at different speeds.

P waves ~ 7-8 km / sec
S waves ~ 5-6 km / sec
Surface waves ~ 3 km / sec

for local / regional events say up to 300 - 400 or so km it was easy to measure the S-P time in seconds and multiply by 9 to give a close rule of thumb distance to the event.

so say a S-P of 25 sec x 9 = 225km +- ~ 5km

Dave