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

 

[LudusRex]

The Richter magnitude scale is a logarithmic scale used to measure the strength or magnitude of an earthquake. It is based on the amplitude of the seismic waves recorded by a seismometer, which is a device that measures the movement of the ground during an earthquake.

The amplitude of the seismic waves is affected by several factors, including the distance from the earthquake source, the depth of the earthquake, and the type of rock or soil the waves travel through. The Richter magnitude takes all of these factors into account and provides a single numerical value to represent the overall strength of the earthquake.

It is true that a deep earthquake may release more energy, but it is also important to consider the distance from the earthquake source. While the amplitude of the seismic waves may decrease as they travel through the Earth's layers, the distance from the source also plays a role in the overall strength of the earthquake. A deep earthquake that is far away may still have a higher Richter magnitude than a shallow earthquake that is closer to the seismometer.

Furthermore, the Richter magnitude scale also takes into account the peak ground velocity and acceleration, which are measures of the intensity of the shaking felt at the surface. While these values may be smaller for a deep earthquake, they are still taken into consideration when determining the overall Richter magnitude.

In summary, the Richter magnitude scale is a comprehensive measure of an earthquake's strength, taking into account the distance from the source, the depth of the earthquake, and the intensity of the shaking at the surface. It is a standardized and widely accepted method for measuring and comparing earthquakes, and it allows for accurate and consistent reporting of earthquake magnitudes around the world.