The 7.9 Scenario

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Press Article: October 9, 2003

What Memphis, Tennessee, Has In Common With The Antelope Valley

Sam Penny, an author and lecturer based in Aguanga, California, recently published his novel Memphis 7.9, a science-based story of what happens when a 7.9 magnitude earthquake strikes within a few miles of a major metropolitan area. The results have much to say about what could happen in The Antelope Valley.

Memphis, Tennessee, rests on the flood plains and bluffs overlooking the Mississippi River, forty-five miles from the southern half of the New Madrid Fault. Its position is similar to the cities of Palmdale and Lancaster. These cities are built on the hills and desert alluvium adjacent to the San Andreas Fault.

The southern extension of the 350-mile long New Madrid Seismic Zone, the part nearest Memphis, hides beneath a mile-thick layer of mud and sand, sediments that fill the Mississippi valley from Illinois to Louisiana. You can find little evidence of the great earthquakes that rocked that region in 1811 and 1812. Five of the temblors measured over 8.0 magnitude. The aftershocks continued for five years. That episode remains the largest seismic event to strike the United States in recorded history.

Seismologists know the fault is there. By measuring and locating the 250 minor temblors that occur each year they have mapped the position of the primary fault. Near Memphis it appears to be a vertical, slip-strike fault with most fractures occurring at depths between six and twenty kilometers.

It does not take a seismologist to see the San Andreas Fault. The twisted sediments along the Highway 14 cut south of Palmdale mark its trace. It is harder to see out in the flatlands where the wash from the mountains have covered its path, but it is there.

The San Andreas is also a vertical slip-strike fault and normally experiences from 250 to 500 minor temblors along any three hundred mile segment of the fault each year. In many ways the southern New Madrid and the San Andreas look very similar in their structure, and can be expected to fracture in the much same way.

Minor earthquakes are of little danger and many do nothing more than to rattle the nerves. What people fear most is a great earthquake. An earthquake is considered great when its magnitude is 7.4 or more. Like the New Madrid there has not been a great earthquake on the southern San Andreas since the 1800s.

Probabilities for great earthquakes on either of the two faults are of the same order of magnitude. Chances are one in five that people alive today will experience or hear of a major earthquake near Palmdale or Memphis within his or her lifetime, but no one knows exactly when.

A great earthquake in this region will result from a long fracture along the fault in the earth's crust. That fracture will be forty miles long for a 7.4 magnitude event, over seventy miles for a 7.9 magnitude event. The epicenter, the surface point above where the fracture starts, will likely be near the center of the fracture.

The most violent shaking often occurs near the end of the fracture furthest away from the epicenter. Crustal fractures propagate close to the speed of the shear or S-waves, and these waves build up into a burst, like the sonic boom of a jet fighter approaching the speed of sound. Structures along or near the trace of the fracture are hit with the accumulated energy in a terrific bang rather than a roar spread out over a longer period of time.

It will take from twenty-two to forty-five seconds for a seventy-mile long fracture to happen, depending upon its symmetry. Places near the fault will be subjected to at least forty-five seconds of violent shaking, longer if there are reflections from major geological structures like the mantle and nearby faults. The longer the shaking the greater the damage done, so longer fractures produce higher magnitudes and greater damage.

Earthquakes do little to change the earth's landscape. The hills and plains have felt earthquakes since their beginnings and are largely in a state of relaxation towards the shaking. Earthquakes do greatest damage to man-made structures, for those are built to defy gravity and press against the environment. The constructs of our society will break, and the more violently the earth shakes, the greater will be the destruction of what man has built. Our society should plan for a great earthquake to happen - someday.

It is important to recognize the risk associated with various kinds of structures, especially when they are located near a major fault. To think that a lesser probability means we can relax our standards is sophomoric. Playing Russian Roulette with a six-shot revolver versus a ten-shot revolver will change the odds, but the risk and ultimate result are the same.

Contingency planning is vital - it may save your life when the inevitable happens and a great earthquake does strike, either on the New Madrid or on the San Andreas.