The death toll of the terrible earthquake that shook India a fortnight ago has climbed to over 20,000. This was the most powerful quake to strike India in 50 years. On January 13, 2001, a killer quake rocked El Salvador in Central America. The loss of life (perhaps as many as 5,000 deaths) was not as great as in the Indian quake, but it was one of the worst earthquakes to hit this nation in decades. The list below presents some of the similarities and differences between the two quakes.

First, a little information about earthquake scales is probably warranted here. Over the years, the most widely used earthquake scale has been the scale developed by Charles Richter in the 1930s. As the number of seismograph stations installed around the world increased, it became obvious that the method developed by Richter was strictly valid only for certain frequency and distance ranges. Because of the limitations of the Richter Scale and other magnitude scales, a new scale, known as moment magnitude, or MW, was formulated. For very large earthquakes, the moment magnitude is considered to give the most reliable estimate of earthquake size.

The general idea behind the various magnitude scales is that a quake having a value of 5 has an amplitude (of the waves recorded on a seismograph) ten times greater than a quake having a value of 6, and a 7 quake has an amplitude ten times greater than a 6. So, each whole-number increase in magnitude represents a tenfold increase in measured amplitude. Also, each whole number step in the magnitude scale corresponds to 31 times more energy than that associated with the preceding whole number.

The severity of an earthquake can be measured in terms of either magnitude or intensity. They're not the same thing, though. Whereas magnitude is related to the amount of energy released at the focus of the quake, intensity is based on the effect the shaking ground has on buildings and natural features. Because the MW or Richter Scales weren't designed to express damage, other scales have been devised that are for this purpose. For instance, the Modified Mercalli Intensity Scale gives information on what a person might feel, or the kind of damage observed from quakes having different intensities. On this scale, at a level VI furniture moves, at level VIII chimneys crumble, at level X big trees sway like reeds and it's difficult to stand, and at a level XII trees are knocked down and rivers change course. The Indian and El Salvador quakes were likely about level X.

Earthquakes occur, in general, at the boundary of two crustal plates, and most often, where continental and oceanic plates collide. Not surprisingly, most major quakes (greater than 7.0 magnitude ) occur in coastal areas. This was the case for both the Indian and El Salvador shakers. However, the epicenter of the January 26 quake in India was in the state of Gujerat, in western India, whereas for the El Salvador quake, the epicenter was offshore in the Pacific Ocean, about 150 miles southeast of the capital city of San Salvador.

The quakes in El Salvador and India registered a 7.6 magnitude and 7.5 magnitude, respectively, using the moment scale, but they occurred at somewhat different depths below the surface (24 km in India and 39 km in El Salvador). By the way, for comparison, the Great San Francisco earthquake of 1906 was also about a 7.7 magnitude, the quake that ripped apart northern Turkey in August of 1999 was a 7.4 magnitude and the 1950 quake that destroyed much of the state of Assam, in eastern India was a 8.6 magnitude (one of the strongest in the world during the 20th century). Last month's Indian quake occurred along a thrust fault - when one portion of the Indian plate pushed northward into the Eurasian plate. For the quake in El Salvador, the main shock appears to have been located within the Caribbean plate above the subducting Cocos plate, and was a normal faulting event.

Although the epicenter of both quakes was within about 12 degrees of the Tropic of Cancer (23 degrees north latitude in India and 12.5 degrees north latitude in El Salvador), they occurred on the opposite side of the globe (70 degrees east longitude in India and 89 degrees west longitude in El Salvador).

Most of the India earthquake-related deaths were in urban areas, such as Ahmadabad, and resulted from buildings toppling and homes collapsing. In El Salvador, landslides and mudslides flowing into rural villages accounted for the majority of the deaths. Both quakes occurred during the morning hours, which likely prevented the number fatalities from being any higher. However, because both El Salvador and India are poor countries, relief efforts were slowed by weak, inefficient infrastructures.

Last year there were 16 earthquakes greater than or equal to a magnitude of 7.0 - the strongest was a 7.9 magnitude trembler on the island of Sumatra in Indonesia. So far, 2001 is off to a rocking start. There have already been 5 quakes with a magnitude of 7.0 or greater - the annual average is 18.

For more about this see http://gldss7.cr.usgs.gov/neis/bulletin/mag7.html  and the science question for February 4, 1999.