Th e prize is the plea sure of fi nding the thing out.

—Physicist Richard Feynman, 1918– 1988

Stretched out on a sun- warmed rock, I admired the hawks circling lazily in

the bright- blue sky. It was a perfect October day in 1997 in Petit Jean State

Park, high in Arkansas’s Ozark Mountains. About 50 yards away, I could see

a fi ve- foot yellow- and- orange wooden tripod topped by a shiny metal disk

that looked like a large Frisbee. Th e disk was an antenna receiving radio signals

from Global Positioning System (GPS) satellites orbiting thousands of miles

above the earth (fi g. 1.1). An expensive, high- precision GPS receiver about the

size of a personal computer recorded the signals and used them to fi nd the an-

tenna’s latitude and longitude to incredible accuracy.

In the early morning chill, I’d carefully set up the tripod over a metal marker

drilled into solid rock. Doing this involved sighting through a lens to position

the tripod over the marker and adjusting the tripod legs to make sure the an-

tenna was level. Th is complicated sequence felt like an intelligence test that

I’d slowly and only barely passed.

Th e GPS receiver was doing a very simple thing— measuring its location—

using incredibly complicated space technology. Fellow geologists and I had

installed 24 markers like this one over a large area in the central U.S. We’d

mea sured their positions in 1991 and 1993 and were now doing it again.

Our goal was to learn more about the mysterious zone of earthquakes called

the New Madrid seismic zone. It’s named after the town of New Madrid,

pronounced “MAD- red,” in the area of southeastern Missouri known as the

“Bootheel.” Th e zone includes parts of Missouri, Arkansas, Tennessee, Ken-

tucky, Illinois, and Indiana. In 1811 and 1812, large earthquakes here shook

the central U.S., and small earthquakes continue in the zone today. A map of

Chapter 1Th reshold

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the recent small earthquakes shows some major patches, which we think

are mostly aftershocks of the past large earthquakes, surrounded by a diff use

“cloud” (fi g. 1.2).

Th ese earthquakes are interesting because they’re in a strange place. Most

big earthquakes happen at the boundaries between the great rock plates that

slide around on the earth’s surface. For example, the San Andreas fault in

California is part of the boundary between the Pacifi c and North American

plates. In contrast, the New Madrid seismic zone is a less active earthquake zone

in the middle of the continent, within the North American plate.

Geologists know surprisingly little about what’s going on here. We don’t

know why the earthquakes occur; when they started; if, when, and where

future large earthquakes will occur; how serious a danger they pose; or how

FIGURE 1.1 GPS antenna at Petit Jean State Park.

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FIGURE 1.2 Locations of earthquakes between 1975 and 2008 in and

around the New Madrid seismic zone. (After University of

Memphis)

Missouri

Illinois

Indiana

Kentucky

TennesseeArkansas

Mississippi Alabama

Memphis

New Madrid

St. Louis

Ohio River

MississippiRiver

Magnitude 5

Magnitude 4

Magnitude 3

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DISASTER DEFERRED4

society should confront them. A big part of the problem is that because large

earthquakes here are much rarer than in many other seismic zones, we don’t

yet have the data to answer these questions. Th is situation made New Ma-

drid a perfect place to use the new GPS method that was quickly becoming

a powerful tool for earthquake studies around the world.

Because we were recording GPS data at each site for 10 hours a day over

three days, I had a lot of time in a beautiful place to think about what we were

learning. Already, it looked like we were on the threshold of something big.

Earthquakes happen when slow motions stored up in the earth over hundreds

or thousands of years are suddenly released. We had expected to see the sites

moving. Surprisingly, we weren’t seeing that, but our fi rst two surveys weren’t

enough to be sure. Th is survey would settle the question.

A few months later, graduate student Andy Newman, who was analyzing

the survey data for his doctoral thesis, brought his fi ndings to my offi ce at

Northwestern University. Th e result was clear. To the accuracy of the GPS

mea sure ments, the ground across the earthquake zone wasn’t moving.

To tell if a monument in the ground is moving, geologists mea sure its

position at diff erent times and see if it changes. Because every mea sure ment

has some uncertainty, we look to see whether the position has changed by

more than that uncertainty. It’s like the way you tell if a diet is working. You

know that there’s some uncertainty in the scale because weighing yourself

several times gives slightly diff erent answers. Th e question is whether over

time your weight changes by more than that uncertainty.

Th e GPS systems used in geology are so incredibly precise that we mea sure

the motions of markers in the ground in millimeters— 1/1000 of a meter—

per year. Because a meter is 39.37 inches, a millimeter is about 1/25 of an inch.

As fi gure 1.3 shows, that’s about the size of the bigger letters on a dime. If the

ground moves that much in a year, GPS can detect it.

Th is precision lets geologists mea sure the slow movements of the earth.

Th at makes GPS the most important new tool we’ve gotten in the past 20

years for earthquake studies. For example, GPS shows that the motion across

California’s San Andreas fault is about 36 millimeters per year. Most of the

time, the fault is “locked” by the friction between the rocks on either side, so

the motion is stored up in the rock. Eventually, the stored motion overcomes

the friction, and the fault moves in a big earthquake. Th is happens about

every hundred years, so in seconds the fault moves about 3,600 millimeters,

or about 12 feet!

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THRESHOLD 5

Our GPS data for New Madrid didn’t show any motion. Specifi cally, they

showed that the ground was moving less than 2 millimeters per year. Th at’s at

least 18 times more slowly than the San Andreas. We were also pretty sure that

the number would get much smaller if we kept mea sur ing for a longer time.

Already it was a lot slower than we’d expect if a big earthquake were com-

ing any time soon. I thought of the joke in which a tourist asks a Maine farmer

“Does this road go to Bangor?” and is told “Nope, stays right where it is.”

During the next few months, all of us in the project talked at length about

what the lack of motion might mean. Conventional wisdom was that the New

Madrid area faced a major earthquake risk. Th e U.S. government claimed

that this risk was as high as in California and was pressuring communities to

make expensive preparations.

Th e GPS data showed that these common ideas about the New Madrid

seismic zone needed serious rethinking. All of us in the project were excited.

Although as scientists we’re trying to solve the earth’s mysteries, much of our

eff ort actually goes into day- to- day chores like trying to fi x computer pro-

grams. Most of the time we do routine studies and fi nd answers that aren’t

too surprising. Th e most exciting times are when a project gives an unexpected

answer that leads to new insight. Sometimes that’s due to clever planning,

but often, as in this case, it’s just unplanned dumb luck. We have only a few

of these moments in our careers, so we trea sure them.

Because a scientist’s most exciting moment of discovery is often his fi rst, I

was especially pleased for Andy. During many years of advising students, I’d

learned that those who make major discoveries while in school generally go

on to make others later. I think that’s because they learn to spot things that

1 inch

1 millimeter

FIGURE 1.3 GPS mea sure ments can tell if a point on the earth moves by

more than a millimeter— about 1/25 of an inch— in a year.

Th at’s the height of a capital letter on a dime.

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DISASTER DEFERRED6

don’t fi t into the accepted picture. Instead of forcing new data to fi t into their

preconceptions, they learn to think outside of conventional wisdom. We all try

to do this but usually fail. Hence, most of our contributions come from the

few “aha” moments when we break free. It feels like a door has just opened,

and there’s so much new to explore.

To working scientists like me, real science is very diff erent from the ideal

“scientifi c method” taught in elementary school. Th at ideal scientist is like a

lone explorer who examines the possible paths to a clearly visible mountain,

chooses the best, and presses on. Real scientists are like a mob of hikers try-

ing to fi nd the way to an unseen lake through dense woods full of swamps,

mosquitoes, and poison ivy. We argue about which routes look best, try dif-

ferent ones, follow them when they seem to be working, and try others when

they aren’t. It’s exciting and fun but also confusing and frustrating. Eventu-

ally, mostly through luck, we reach the lake, often by diff erent routes that get

there about the same time. Once we’re at the lake, we argue about whether

it’s the right lake.

Th e moral is that while searching for the lake, we were all confused and

going in the wrong directions about half the time. We fi nally got there as a

group by combining many people’s eff orts. It’s hard to say who contributed

what because we’re all sure that we played a key role. It’s also not that impor-

tant, because after relaxing in satisfaction for a while, we realize that there’s a

bigger lake somewhere higher up on the mountain, and it’s time to get to work

looking for it.

Because scientists are human, science is a very human endeavor. Scientists

choose problems to study and methods to study them that refl ect their inter-

ests, skills, and sense of where their eff orts will yield useful new knowledge.

Typically, others are exploring diff erent aspects of similar or related problems,

using diff erent methods, and sometimes fi nding diff erent results and draw-

ing diff erent inferences. Even starting with the same set of observations, how

individual scientists interpret them depends in large part on their preconcep-

tions. Although we’d like to be totally objective, we can’t be. It’s like watching

a sports event when it’s unclear what happened in a tricky play; fans see the

result that’s good for their team. Sometimes instant replay convincingly settles

the question, and sometimes it doesn’t.

Eventually, as scientifi c knowledge increases, a clear result emerges that com-

bines many people’s work over many years. Until then, scientists grappling

with a problem often have diff ering views. Th ere’s spirited debate about the

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THRESHOLD 7

meaning of our incomplete results. Th is debate is crucial for progress, as

described by the ancient Jewish sages’ adage “the rivalry of scholars increases

wisdom.”

Th is messy pro cess has been going on since the large earthquakes in 1811

and 1812 brought the New Madrid seismic zone to scientists’ attention. At the

time, geology had just begun as a science, and seismology, the branch of geol-

ogy that studies earthquakes, didn’t exist. As seismology evolved and knowl-

edge about the New Madrid earthquakes increased, it became clear how

unusual they are and how challenging it is to understand them. Hence, many

researchers have been exploring scientifi c and policy issues for the area.

Many of the results I’ll be talking about come from work that graduate

students, friends— mostly from other midwestern universities— and I have

done in the past 20 years. Th is isn’t a formal, structured project. Geologists

in general are individualists who like messy problems that don’t have simple,

clean solutions. Most of us view ourselves— correctly or not— as generalists

with broad ranges of skills rather than as narrow specialists. Th us, instead of

working in large organizations with clear hierarchies and assigned tasks, we

typically work as loose groups of friends with overlapping interests. We’re

very informal, so faculty and graduate students work pretty much as equals.

We share ideas, but all involved have their own take on what’s going on.

Th e results we’ll discuss also show how empirical geology is. Th e earth is

too complicated to have fundamental laws that predict what’s going on, the

way physics does. Instead, geologists observe what’s happening and try to

make sense of it. What we fi nd often surprises us and forces us to change our

ideas. Th e earth regularly teaches us humility in the face of the complexities

of nature. Th us, although we take our science seriously, we generally don’t

take ourselves too seriously.

Th is geologists’ outlook makes it easier to communicate our science to the

public. Many of us do a lot of education and outreach and enjoy it. Whether

through class and public lectures, the media, or just talking to people, I

fi nd wide interest in how our planet works and how its workings aff ect

humanity.

Th is book grew out of that interest. It’s an overview of studies of the sci-

ence and hazards of the New Madrid earthquakes. Th ese studies involve many

people, each of whom views the subject diff erently. Th e book is written from

my perspective, developed through 35 years of studying earthquakes around

the world and 20 years of thinking about New Madrid.

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I’ll show how scientists study the questions surrounding the New Madrid

earthquakes, how far we’ve gotten, and what we still have to do. I’ll explain

what we know and what we don’t, what we suspect, and which is which. Dis-

cussing these questions involves going into some concepts that can seem a

little complicated, because the earth is complicated.

Understanding these concepts lets us look at issues that scientists, engi-

neers, policy makers, and the public are struggling with. Th ese issues are in-

teresting scientifi cally and have practical signifi cance because billions of

dollars are involved. Moreover, if you live in the Midwest, understanding

what we’re learning will reduce your fear of earthquakes.

We’ve learned a lot about the New Madrid earthquakes in the past 20 years,

so the picture coming out is very diff erent from older ideas. Still, there’s a lot

we don’t yet know. Th at’s typical in studying the earth. A college student can

take years of classes in many other sciences before encountering topics that

the instructor admits aren’t understood. In the earth sciences, the fi rst courses

present many unsolved fundamental questions about our planet. An earth

science instructor is like the apocryphal medical school dean who tells incom-

ing students: “Half of what we will teach you in the next four years is wrong.

Th e problem is that we don’t know which half.”

I hope readers— especially students considering possible careers— take away

a sense of the excitement, fun, and opportunities in science. Earth science, in

par tic u lar, off ers the challenge of working on interesting and important prob-

lems like earthquakes in the middle of continents, where what we learn will

help us live better with a complicated and active planet.

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