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Assam-origin seismologist develops improved quake measurement scale

By AJIT PATOWARY
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A new scale to precisely measure earthquakes has been developed by an Assam-origin seismologist named Dr Ranjit Das.

Das hails from Dhekiajuli and works in the National Disaster Research Management Centre in Chile. The scale named �Seismic Moment Magnitude Scale,� which can also be read as the �Das Magnitude Scale,� has been developed to include lower and intermediate magnitudes of the earthquakes in a global context, emphasising the use of body waves, particularly the P waves, in short � Mb.

To improve upon the consistency of the Moment Magnitude, in short � Mw Scale for a wider range, this new scale has been developed as a uniform generalised �Seismic Moment Magnitude Scale,� in short Mwg.

Altogether, 25,708 global earthquake events having mb M0 (P waves and seismic moment) values were taken into consideration while developing this scale. And the data of these earthquakes have been collected from the International Seismological Centre (ISC) and Global Centroid Moment Tensor (CMT) databases for the period between 1976 and 2006.

This new Mwg Scale is also valid for magnitude 3.5 and above, said Dr Ranjit Das, while talking to this newspaper. Since the Mwg scale is computed from low and high frequency spectra, it is consistent for small, intermediate and large earthquake magnitude estimator for all earthquake-related studies. He also claimed that the Mwg Scale would also be helpful for rapid determination of seismic moment for issuing emergency warnings.

It needs mention here that the formulation and validation of the Mw Scale were carried out considering the southern California region for lower and intermediate earthquakes and this scale has so far been referred to by the researchers as the best scale, which matches well with the observed surface-wave magnitudes with Ms greater than or equal to magnitude 7.5 at a global level, Das said.

Going further, he said the greater accuracy of the Mwg Scale over the Mw Scale at different magnitudes (as for example mb or surface-wave magnitude, in short � Ms ) is found to be statistically significant in the range, including smaller and intermediate events. The similarity of the Mwg Scale is also tested on 394 global seismic radiated energy values collected from the 1995 research work titled �Global patterns of radiated seismic energy and apparent stress� authored by GL Choy and JL Boatwright of the US Geological Survey. It was observed that 76 per cent of the estimated radiated energy values obtained through the Mwg Scale show closer agreement to the observed radiated energy values, compared to the Mw Scale.

It may perhaps be pertinent to mention here that magnitude is generally the first source parameter that is calculated quantitatively to determine the strength of an earthquake. American seismologist Charles F Richter derived the first magnitude scale based on records of local shocks in southern California in 1935. Several other magnitude scales then came into practice for estimating the size of an earthquake while accounting for the variations in the earthquake sources and the wave type used.

However, of all the contemporary magnitude scales � that is, body wave magnitude mb, surface-wave magnitude Ms, local magnitude ML, duration magnitude MD, energy magnitude Me� and moment magnitude Mw, the seismic moment-based magnitude Mw Scale has remained the most widely used unsaturated magnitude scale so far, Das said.

The paper on Das� �Seismic Moment Magnitude Scale,� or �Das Magnitude Scale,� was published in the Bulletin of the Seismological Society of America, Vol. 109, No. 4 (pp. 1542�1555), August 2019.

Das, who is engaged in the field of earthquake study for quite a long time, is also a member of the editorial board of the reputed Journal of Geophysics.

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Assam-origin seismologist develops improved quake measurement scale

A new scale to precisely measure earthquakes has been developed by an Assam-origin seismologist named Dr Ranjit Das.

Das hails from Dhekiajuli and works in the National Disaster Research Management Centre in Chile. The scale named �Seismic Moment Magnitude Scale,� which can also be read as the �Das Magnitude Scale,� has been developed to include lower and intermediate magnitudes of the earthquakes in a global context, emphasising the use of body waves, particularly the P waves, in short � Mb.

To improve upon the consistency of the Moment Magnitude, in short � Mw Scale for a wider range, this new scale has been developed as a uniform generalised �Seismic Moment Magnitude Scale,� in short Mwg.

Altogether, 25,708 global earthquake events having mb M0 (P waves and seismic moment) values were taken into consideration while developing this scale. And the data of these earthquakes have been collected from the International Seismological Centre (ISC) and Global Centroid Moment Tensor (CMT) databases for the period between 1976 and 2006.

This new Mwg Scale is also valid for magnitude 3.5 and above, said Dr Ranjit Das, while talking to this newspaper. Since the Mwg scale is computed from low and high frequency spectra, it is consistent for small, intermediate and large earthquake magnitude estimator for all earthquake-related studies. He also claimed that the Mwg Scale would also be helpful for rapid determination of seismic moment for issuing emergency warnings.

It needs mention here that the formulation and validation of the Mw Scale were carried out considering the southern California region for lower and intermediate earthquakes and this scale has so far been referred to by the researchers as the best scale, which matches well with the observed surface-wave magnitudes with Ms greater than or equal to magnitude 7.5 at a global level, Das said.

Going further, he said the greater accuracy of the Mwg Scale over the Mw Scale at different magnitudes (as for example mb or surface-wave magnitude, in short � Ms ) is found to be statistically significant in the range, including smaller and intermediate events. The similarity of the Mwg Scale is also tested on 394 global seismic radiated energy values collected from the 1995 research work titled �Global patterns of radiated seismic energy and apparent stress� authored by GL Choy and JL Boatwright of the US Geological Survey. It was observed that 76 per cent of the estimated radiated energy values obtained through the Mwg Scale show closer agreement to the observed radiated energy values, compared to the Mw Scale.

It may perhaps be pertinent to mention here that magnitude is generally the first source parameter that is calculated quantitatively to determine the strength of an earthquake. American seismologist Charles F Richter derived the first magnitude scale based on records of local shocks in southern California in 1935. Several other magnitude scales then came into practice for estimating the size of an earthquake while accounting for the variations in the earthquake sources and the wave type used.

However, of all the contemporary magnitude scales � that is, body wave magnitude mb, surface-wave magnitude Ms, local magnitude ML, duration magnitude MD, energy magnitude Me� and moment magnitude Mw, the seismic moment-based magnitude Mw Scale has remained the most widely used unsaturated magnitude scale so far, Das said.

The paper on Das� �Seismic Moment Magnitude Scale,� or �Das Magnitude Scale,� was published in the Bulletin of the Seismological Society of America, Vol. 109, No. 4 (pp. 1542�1555), August 2019.

Das, who is engaged in the field of earthquake study for quite a long time, is also a member of the editorial board of the reputed Journal of Geophysics.

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