installed along the NMSZ, where they were more difficult and costly to set up. It was crucial that each GPS platform remain stationary, a constant problem in the soggy Mississippi Valley. The instruments were anchored with steel rods driven deep into the ground. Each unit was equipped with an SSE receiver and antenna; the receivers were mounted on surveying tripods.
The system was expensive and most USGS funding went to southern California’s network, a continuing source of irritation to Jacobs, but a fact of life. Quake-prone California always got the cake; the other parts of the country got crumbs.
Maybe that’s going to change, Jacobs thought bitterly.
“Walt, we just got through to Atkins on the shortwave,” one of the seismologists shouted. “You’re not going to like what he’s got to say.”
NEAR BLYTHEVILLE, ARKANSAS
JANUARY 13
7:15 P.M.
ATKINS SPOKE QUICKLY, FORCING HIMSELF TO BE concise and brief. He didn’t know how long they’d have a clear communication channel open with Walt Jacobs in Memphis. The static-plagued shortwave band was proving increasingly unreliable.
He’d just spent the last hour with Elizabeth examining readouts of the seismic wave patterns they’d downloaded straight from the seismograph into their laptop.
Dogs were still prowling around the Explorer. They’d heard barks along the bank of the creek, but none of the animals had shown themselves.
As he spoke with Jacobs, Atkins tried to keep his voice calm, professional. The readings had worried him.
“I’ve never seen peak accelerations at this level occurring so long after the main seismic event,” he said into the radio’s microphone.
The seismic tracings indicated a series of sharp peaks and valleys representing the ground’s vertical and horizontal shaking. The secondary and surface waves showed vertical accelerations nearly seventy percent that of gravity. Vertical accelerations of fifty percent the rate of gravity were considered large.
The measurement, known as “acceleration due to gravity,” was nothing more than an attempt to show how fast and hard the ground was shaking by comparing it with known gravitational forces. The baseline measurement was the speed with which a ball falls, an acceleration assigned a value of lg. That’s the same as racing a car 100 meters from a dead stop in four and a half seconds. Moderate earthquakes produced acceleration rates of .05g to .4g. The rate here was .7g
The acceleration rates were troubling enough, but Atkins was also worried about the consistently strong aftershocks—and what they might signify. During the last seven hours, they’d had at least eight quakes that he estimated in the magnitude 5 range, strong enough to rock the Explorer on its axles.
Jacobs confirmed the aftershocks. The seismographs in Memphis had recorded every one of them.
“It looks like the epicenters are bunched roughly forty to fifty miles northeast of us,” Elizabeth said.
They could do only the roughest field calculations. By measuring the time difference between the arrival of the quake’s primary and secondary waves, they were able to compute how far the epicenters were located from them. The calculations were based on the differing speeds of the waves. Both left the earthquake focus at the same moment, but the faster-moving P waves reached the seismograph first, followed by the S waves. The delay in arrival time was proportional to the distance traveled by the waves.
Atkins knew that Jacobs realized what all this meant: the probable existence of another new fault branching out from the New Madrid Seismic Zone. It was the only logical explanation for such a tight bunching of aftershocks that were apparently outside the main fault system.
The radio clicked back on. “John, we’re looking at the same information right now,” Jacobs said. “We’re going to have to confirm it, but it appears the epicenters are clustered in northwestern Tennessee and western Kentucky anywhere from thirty to forty miles east of Kentucky Lake.” The seismographs running nonstop in the library annex had recorded all of them.
Atkins detected the strain in his friend’s voice. His own throat tightened as he explained his concerns. Based on this preliminary data, the NMSZ—once again—had become dramatically larger. Three days earlier, after the magnitude 7.1 event, they’d discovered a new fault running south of Memphis.
And now this.
Elizabeth said, “There’s no evidence the faulting process is slowing down.”
The seismic shock waves were coming far too often for that. The epicenter near Blytheville, the one they were sitting on, continued to generate dozens of microquakes. Then there was the much stronger ground shaking to the northeast. With such instability, the elastic forces in the ground could snap again—at any moment.
“Have you been able to get through to any other seismic stations to nail down the epicenters?” Elizabeth asked.
“We’re working on it,” Jacobs said.
To fix the exact position, data from seismographs at three or more different sites would have to be plotted. The epicenters were the points where the rippling seismic waves overlapped.
Jacobs expected another data transmission soon from the National Earthquake Center in Boulder. Sophisticated computers were analyzing seismic readings from dozens of instruments scattered around the United States and abroad. That data as well as information recorded by seismographs in Memphis—and now near Blytheville, Arkansas—would fix the exact location of each epicenter.
This, in turn, would help them delineate what appeared to be another new fracture jutting off from the main New Madrid Seismic Zone.
Atkins had a map of the NMSZ spread open on his lap. It overlaid a map of the Mississippi Valley. The fault zone was shaped roughly like a hatchet, with the blade running across the intersection of Missouri, Arkansas, Tennessee, and Kentucky. The massive quake they’d just experienced had exploded about midway down the handle, which dipped seventy miles into eastern Arkansas, paralleling the Mississippi River.
The major aftershocks suggested a new branch extending from the top of the hatchet up through northwestern Tennessee well into Kentucky.
If it held up, the expanded fault zone might run roughly four hundred miles. A long S-shaped series of cracks deep in the earth, extending from just below Memphis, crossing the Mississippi from west to east around Caruthersville, and continuing to within 150 miles or so of Lexington, Kentucky.
“It’s the dynamics at work here that worry me,” Elizabeth said. “How one major event, the 7.1 quake of three days ago, triggered a series of aftershocks that set up another major event, and now we’re getting more aftershocks.”
“Liz, what are you saying? That these aftershocks could be leading up to another big one?” Atkins said. Despite his concerns about a possible new fault and the power of the aftershocks, he thought that was going too far. “I know what I just said, but it’s way too premature to start a discussion like that. This could all be part of the normal wind-down after a major earthquake. We could be in for some rough aftershocks for weeks. I know what the seismic history here is. Walt’s told me all about it. But those big quakes happened nearly two hundred years ago. Right now there’s no way we can say all these aftershocks are leading up to another major hit. I’m betting when we start to get some GPS data in, we’re going to see the deformation has started to taper off.”
“I remember how skeptical we all were about Doctor Prable’s data,” Elizabeth said. “If we had moved more quickly, started an analysis when we—”
“Then what?” Atkins said. “What would we have done? Warned the public? Made an earthquake prediction? Come on, Elizabeth. Don’t go second-guessing yourself. We’ll have lots of time later to take a close look at Prable’s data and see what we can learn from it. I’m still not convinced it wasn’t one of the best scientific guesses in history.”
“If we don’t pay attention to those aftershocks, we could find ourselves making the same mistake twice,” Elizabeth said sharply. She didn’t like Atkins’ patronizing tone.
