Aftershocks Therapy : Understanding Earthquakes and Their Offspring

For millions of Southern Californians violently awakened in the darkness Jan. 17, the events of 4:31 a.m. will live on in memory as a single, terrible instant in time--the 1994 Northridge earthquake. However, it is important to realize that earthquakes are almost always part of a sequence of related seismic activity.

Seismologists refer to the temblor with the largest magnitude as the mainshock. Smaller quakes before and after the mainshock are called foreshocks and aftershocks, respectively. Although the greatest amount of energy is released in the mainshock, much of the redistributed stress is later released along the fault in the form of aftershocks.

According to seismologists, the mainshock of the Jan. 17 earthquake struck at 4:30:55 a.m. one mile south-southwest of Northridge at a point approximately 11 to 12 miles beneath the earth’s surface. It was initially described as a magnitude 6.6 quake, but Caltech later upgraded the figure to 6.7. Using a different measuring system, the National Earthquake Information Center in Golden, Colo. placed the magnitude at 6.8.

To date, seismologists have identified more than 6,000 aftershocks to the initial earthquake, including 317 magnitude 3.0-3.9, 43 magnitude 4.0- 4.9 and seven 5.0-5.9. How Earthquakes Happen

An earthquake occurs when rocks along a fault in the earth’s crust slip suddenly, releasing energy in the form of seismic waves. Beginning at a single point known as the hypocenter, the quake actually represents a rupture along a varying length of the fault. The epicenter is the point on the earth’s surface directly above the hypocenter.

Living With Aftershocks

The rate of aftershocks declines quickly following the mainshock, the relative number of small to large aftershocks does not. Seismologists have estimated that for every magnitude 5 aftershock in a sequence, there will be roughly 10 magnitude 4s, 100 magnitude 3s and 1,000 magnitude 2s. Therefore, a magnitude 5 aftershock striking weeks after a larger mainshock is entirely consistent with previous recorded seismic activity. In fact, it should be expected.

The magnitude difference between a mainshock and the largest aftershock averages 1.2, so a 6.8 earthquake would have few aftershocks greater than 5.6. A quake is considered an aftershock as long as the rate of seismic activity is greater than the rate preceding the mainshock.

A Quake’s Energy Earthquakes release energy that scientists measure in metric units called ergs. One erg is the amount of energy it takes to move one gram of mass one centimeter in one second. Some examples of an earthquake’s energy force: Energy released (in millions of ergs): Energy released (in millions of ergs): 20,000,000,000,000,000 Magnitude (not to scale): 7 Example or equivalent: Energy generated by Niagara Falls in four months Energy released (in millions of ergs): 600,000,000,000,000 Magnitude (not to scale): 6 Example or equivalent: Energy to launch 2 million NASA space shuttles Energy released (in millions of ergs): 20,000,000,000,000 Magnitude (not to scale): 5 Example or equivalent: Energy created by atomic bomb test in 1945. Energy released (in millions of ergs): 600,000,000,000 Magnitude (not to scale): 4 Example or equivalent: Kiloton of TNT exploding Energy released (in millions of ergs): 20,000,000,000 Magnitude (not to scale): 3 Example or equivalent: Smallest quakes commonly felt

Magnitude 5 Aftershocks

Since Jan. 17, seven aftershocks of magnitude 5.0 or greater have occurred in Los Angeles and Ventura counties, causing additional damage.

Caltech and the U.S. Geological Survey have said that the region has a better than 1-in-3 chance of another magnitude 5 aftershock within the next year.

5.6 Jan. 17, 3:33:30 p.m. 5.2 Jan. 17, 4:43:08 p.m. 5.9 Jan 17, 4:31:58 a.m. 5.1 Jan. 19, 1:09: 28 p.m. 5.1 Jan. 19, 1:11:44 p.m. 5.1 Jan. 29, 3:20:35 p.m. 5.3 March 20, 1:20:12 p.m. Sources: Caltech; U.S. Geological Survey; “Earthquakes” by Bruce A. Bolt; “Environmental Geology” by Carla W. Montgomery; Research by DAVID E. BRADY / Los Angeles Times