When the Republic Day killer earthquake hit Gujarat, the United States Geological Survey recorded the magnitude as 7.9 on the Richter Scale contradicting the measurement of 6.9 by the Indian Meteorological Department. David J Wald, USGS seismologist and visiting associate at the California Institute for Technology, explained the discrepancy and his reading of the Gujarat earthquake in an e-mailed interview to Features Editor (News) Archana Masih.

There is a discrepancy on the magnitude of the Gujarat earthquake -- the IMD says it measured 6.9 while according to the USGS it measured 7.9 on the Richter Scale. Why is there such a huge difference in both estimates?

I believe the updated USGS magnitude is 7.7. That helps, but does remove the discrepancy. The initial magnitude from the USGS was a Surface wave magnitude (7.9). The updated M7.7 is the Energy or Moment magnitude, a more reliable measure, particularly for large earthquakes.

Moment magnitude is now almost universally accepted as the magnitude, though it is not available as rapidly as the surface wave magnitude. Many independent analyses have confirmed a Moment magnitude ranging from 7.6 to 7.7.

I do not know the details of the 6.9 Indian Meteorological Department magnitude, but I assume it was a locally determined magnitude, not a Moment magnitude. Quite often a locally-derived magnitude saturates, or fails to recover the full size of the earthquake.

Is there any way by which an earthquake of such intensity can be predicted?

The relative intensity of shaking can be partially understood by knowing what soil/rock is in an area, and what faults might rupture and with what magnitude. But it is not possible to know when such earthquakes will happen.

Some tremors were felt before the killer earthquake. Can it be said that those tremors were a warning of things to come?

Many times people feel small earthquakes, but no large earthquakes follow. This is a limitation with earthquake prediction. There are many, many small earthquakes and nearly all of them are not followed by larger ones. We have no way of knowing at this time which ones will be followed by larger ones. We can only see that in hindsight.

In Los Angeles this past year, I have felt many small earthquakes, none of which resulted in a larger earthquake. If I assume that they were warnings, I would be mistaken.

Is Kutch in Gujarat under threat of facing such a quake again? There have been almost 500 aftershocks in the region already.

After a large earthquake there are always many aftershocks, and there is always a chance of damaging ones.

In the past, have two earthquakes of almost the same magnitude hit the same area in rapid succession?

This can and has certainly happened in the past, but it is the exception, not the norm.

What lessons does the Gujarat earthquake have for scientists, governments and developers around the world?

Speaking for myself, in terms of the disaster, I am afraid to say that scientifically we will not learn much. From the societal point of view, we have seen this all too often. In areas where priorities are naturally more fundamental than proper seismic construction, these extremely catastrophic disasters will continue to happen.

What environmental factors could have possibly contributed to such a devastating earthquake? Do you think any man-made factors -- like big dams -- could have contributed as well?

Quite simply, no. The size of this earthquake requires huge tectonic forces over which man has little influence.

The flagrant violation of building rules has been widely criticised for the high death toll, but do you think the loss of life and property would still remain high because the intensity of the earthquake was such?

The shaking was undoubtable very great. However, in many areas no building remained standing. That all buildings did so poorly points to either:
1. violation of the code,
2. inadequate code, or more likely, a combination thereof.

What pointers does the Gujarat earthquake provide for earthquake research?

We need to better understand the occurrence of earthquakes away from our plate boundaries.

Do you think lesser developed countries have enough resources towards such research work? The director of the Hyderabad-based National Geophysical Research Institute, told rediff.com that the quake-measuring instruments in his laboratory stopped functioning because the intensity of the tremors was over 7.0, the upper limit of the instruments. Would you say because of budget constraints, the Indian earthquake programme has suffered?

We might have learned quite a lot from this earthquake had the region been better instrumented (strong motion instruments would have been very informative); alas we missed an opportunity and failed to learn much from this disaster. I do not know any of the details of the funding for Indian earthquake programs, but scientists in India, given more funding, would certainly have done better in responding and learning from such a earthquake.

Design: Dominic Xavier. Dr Wald's image: Courtesy the Los Angles Times

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