Different measures of magnitudes of the same earthquake are in circulation, confusing the people. Some media are describing the quake as “Great Earthquake” while others are simply calling it “Nepal Earthquake” or “Gorkha Earthquake” or “massive”/”devastating” earthquake. The following two articles, one from The Himalayan Times and the other from an Arko.asia blogpost help explain this discrepancy and suggests seismologically more accurate ways to report the magnitude and qualify the quake.
[PAGE UPDATED 15 May 2015]
The same quake has been given different magnitudes. What’s with this number’s game?
Sabitri Dhakal
KATHMANDU: We were all shaken by the earthquake on April 25. We didn’t know at that time that it was a very powerful quake. Yet media have have been giving the very same quake different magnitudes. These are the numbers that have been ingrained in our brains forever — 7.6, 7.8, 7.9, 8.1. So which is the correct magnitude?
All of them are because some media are referring to magnitudes measured by the US Geological Survey (7.9), some to Japan Meteorology Agency (7.3) and some to Chinese (8.1).
There are two basic types of seismic waves generated by an earthquake — body waves and surface waves. These waves cause the shaking that is felt and cause the damage as well. The waves then move on. The magnitude of an earthquake is then measured. The magnitude is a number that characterises the size of an earthquake. It is based on measurement of the maximum motion recorded by a seismograph. Several scales have been defined, but the most commonly used are local magnitude (ML), commonly referred to as Richter scale, surface-wave magnitude (Ms), body-wave magnitude (Mb), and moment magnitude (Mw).
These waves are measured in seismological measurement stations. And this time too there are several interpretations regarding the exact measurement of the April 25 tremblor. The same earthquake is measured as moment magnitude (Mw) of 7.8 Mw or 8.1 Ms (surface wave). People do not pay attention to the units on which they are measured, just look at the figures and try to term them as the Richter scale. “But what people must know is how an earthquake is measured. Whenever we feel seismic waves, the instruments in the local earthquake measurement stations measure them. They are measured as ML, which is the local magnitude — a Richter scale. Therefore, rather than looking for earthquake measurement records given by Americans, Chinese or the Japanese, it will be better to rely on local stations to know the measurement of the earthquake as the waves are felt here and the machines are here too,” shared Lok Bijaya Adhikari, Chief at National Seismological Centre, Lainchaur.
“The Americans have their stations in Hyderabad and Lhasa. As the waves move on accordingly in a frequency, the frequency that has been measured here can’t be the same at another place. Therefore, always look for the local magnitudes if you really want to know the measurement/scale of an earthquake,” he advised.
The earthquake of April 25 was measured at 7.6 on the Richter scale according the National Seismological Centre.
People were even heard saying that an earthquake of magnitude 10 will occur. “But for such occurrence, the earth must crack from its bottom surface,” Adhikari shared. There are no records of an earthquake of such a magnitude. The most powerful earthquake recorded till date is that of Chile that occurred on May 22, 1960 rating 9.5 on the moment magnitude scale.
However, aftershocks can be felt when there are powerful earthquakes. “When there is an earthquake of 9 or 9.5 magnitude, there will be an aftershock of 8 or 7. All these will be disastrous. A smaller earthquake will have smaller aftershocks,” he informed. And as per Adhikari, 144 aftershocks have already occurred since April 25.
“Another wrong report that is doing the rounds is that the recent quake has been termed as a ‘great’ earthquake. It is not a great earthquake. Earthquakes can be termed ‘great’ only if their measurement is greater than eight on the Richter scale. Earthquakes ranging between seven to eight is a ‘major’ quake, while those ranging between six to seven is ‘strong’, five to six is ‘intermediate’, and those below five are smaller ones,” he added.
There are 21 stations in the country that measure the magnitude of the earthquakes. As the data is collected, it is processed and circulated through the media whenever there is an earthquake.
The Himalayan Times, 6 May 2015
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Nepal earthquake: 7.3 or 6.8 Mangnitude?
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 Nepalputs 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.
Arko.asia, 13 May 2015