Bad as it Was, It was Small In Comparison

On the accompanying 21-inch-high chart depicting the relative severity of earthquakes, Thursday’s 6.1 temblor is just slightly more than a tenth of an inch from the bottom, while San Francisco’s great quake, estimated to be 8.3, is at the top of the page.

The range of earthquake strengths is so great that Dr. Charles F. Richter, a former professor at Caltech, devised a logarithmic scale bearing his name to measure them in 1935. The scale expresses magnitude in whole numbers and decimals.

The magnitude increases with the wave amplitude of the quake recorded by the seismograph. Each whole number step on the Richter scale represents an increase of 10 times in measured wave amplitude and an increase of 31 times in the amount of energy released by the quake.

That’s why the Mexico City 8.1 earthquake dwarfs Thursday’s Los Angeles episode.

The energy released by a 6.3 magnitude earthquake is about the same as that released by the Hiroshima atomic bomb--or 20,000 tons of TNT. The energy released in an 8.3 magnitude quake is the same as a 15-megaton hydrogen bomb.

Here is how the Thursday’s quake compared with other notable temblors.

Quake Magnitude 1. 1906 San Francisco 8.3 2. 1985 Mexico City 8.1 3. 1952 Kern County 7.7 4. 1980 Eureka 7.0 5. 1983 Coalinga 6.7 6. 1979 Imperial Valley 6.6 7. 1971 San Fernando 6.4 8. 1925 Santa Barbara 6.3 9. 1933 Long Beach 6.3 10. Thursday in Los Angeles 6.1 11. 1986 Palm Springs 6.0

Source: U.S. Geological Survey

AFTERSHOCKS

Time Magnitude 7:45 a.m. 4.1 7:49 a.m. 4.2 8:08 a.m. 3.4 8:12 a.m. 4.4 8:13 a.m. 4.6 8:18 a.m. 3.3 8:20 a.m. 3.1 8:22 a.m. 3.2 8:30 a.m. 3.0 8:55 a.m. 3.0-3.5 9:00 a.m. 4.1 9:01 a.m. 3.0-3.5 9:21 a.m. 3.3 9:33 a.m. 3.0 9:34 a.m. 3.3 10:20 a.m. 3.5 10:47 a.m. 3.6 12:12 p.m. 3.8 1:40 p.m. 3.1

(Later aftershocks were less than 3.0, but the details were not available.)

Source: Caltech and The National Earthquake Center, Golden, Colorado.