Different geological survey agencies have presented different data regarding the magnitude of earthquake and aftershocks in Nepal, but who is right?
National Seismological Centre (NSC) with the Department of Mines and Geology in Nepal puts the May 12 aftershock at 6.8 magnitude. While according to the US Geological Survey (USGS) data it is measured at 7.3M.
Data presented by these survey agencies had similar differences in April 25 quake too. USGS had measured it at 7.9M initially and later downgraded to 7.8 M, while NSC said it was 7.6.
Because of the different figures presented by different agencies, there has been some confusion among public as to the real strength of the quake and its aftershocks.
Methods of measuring Earthquakes
Earthquakes are measured in Richter Scale, originally developed by Charles Francis Richter, and its various successor scales as well as other less accurate methods.
Ricter Scale also known as Local magnitude (Ml) is applied for measuring magnitude from less than 600kms from epicentral distance.
The National Seismology Centre in Nepal has the seismometre equipments within Nepal. Therefore the most likely method used to measure recent earthquakes by NSC is modern variation of the Local Magnitude (Ml) scale because of close proximity.
But the Local Magnitude scale saturates when big earthquakes occur, which was the case for NSC when the April 25 quake and the May 12 aftershocks took place and it took some time before they could calculate the magnitude.
For earthquakes that occur at greater epicentral distance, usually more than 600kms, other scales like the Surface Wave Magnitude (Ms) and Body Wave Magnitude (Mb) are used.
Another scale used is Moment Magnitude (Mw). Moment Magnitude is a far more accurate successor to the Richter scale, specially for earthquakes of greater magnitude.
USGS is known to use Moment Magnitude for listing all the earthquake magnitudes on its website.
Moment Magnitude, although more accurate for greater quakes, usually >8M, are designed to be consistent with the Richter scale.
So who is right?
There is no way to answer that. In the end, all methods are estimation of the amount of energy released during the quake. There is no method that can determine quake strength with 100 per cent accuracy for it usually occur deep within the earth crust.
It is easy to assume, the USGS may have better equipments than the Nepalese counterpart. At the same time NSC had their equipment closer to where the earthquake occurred where as USGS probably measured from much greater distance.
What you need to know !
Magnitudes are measured in base-10 logarithmic scale. Which means, the May 12 aftershock measuring 6.8 Ml or ~7.3 M (USGS) has nearly ten times less shaking amplitude than the April 25 quake measuring 7.6Ml or ~7.8M (USGS). In another word the April 25 quake (7.6Ml) released nearly 30 times more energy than May 12 aftershock (6.8Ml).
Factors contributing to the quake impact
Different factors such as geological conditions, depth of earthquake, distance from epicentre and structural integrity of building structures all make the difference on what kind of damage a earthquake will cause.