Earthquake Hazards Along the New Madrid Fault: Using Science to Distinguish Fact from Fiction

NSTA, St. Louis: 2007

Michael Wysession, Professor of Geophysics

Department of Earth and Planetary Sciences Washington University, St. Louis, Missouri

Acknowledgments: Prof. Seth Stein, Northwestern University Prof. Susan Hough, CalTech National Science Foundation Incorporated Research Institutions for Seismology Seismological Society for America U.S. Geological Survey

What is it?

The NMFZ (New Madrid Fault Zone) is a region of elevated seismicity at the intersection of Missouri, Arkansas, Tennessee, Kentucky, and Illinois

Why is it there?

The NMFZ is associated with the Reelfoot Rift – a failed rift zone that was active 750 million years ago.

The Reelfoot Rift is a region of lower elevation (which is why the Mississippi River flows down through it!).

Continents rift apart to make new oceans Ex/ Continent rifts Ex/ New ocean

The Reelfoot Rift began to open when the supercontinent Rodinia split up about 750 million years ago, but then stopped: called a “failed rift”

What happened in 1811-1812?

A swarm of earthquakes occurred. The largest were magnitude 7.0, 7.2, and 7.4

12/16/1811: M7.2

1/23/1812: M7.0

2/07/1812: M7.4

Magnitudes were determined by measuring accounts of

intensity

of shaking (Mercalli Intensity Scale) in historical records and comparing them to

magnitudes

and recorded

intensities

for modern earthquakes [Hough et al., 20000]

A Magnitude 8 (M8) earthquake is much bigger than a M7 earthquake: Roughly 10x greater displacements Roughly 30x more energy released M 6.9

M 7.8

“the big one”

There were no seismometers in 1812, so original sources (accounts) provide the most direct evidence to constrain the magnitude and location of earthquakes, but must be considered in their historic context.

NEW MADRID, MISSOURI: December 16, 1811

"About 2:00 this morning we were awakened by a most tremendous noise, while the house danced about and seemed as if it would fall on our heads. I cried out it was an Earthquake, and for the family to leave the house; which we found very difficult to do, owing to its rolling and jostling about. "At half past 6 o'clock there was another shock, extremely violent - the agitation of the earth was so great that it was with much difficulty I kept my balance -

the motion of the earth was about twelve inches to and fro

. The earth seemed convulsed - the houses shook very much chimneys falling in every direction. The loud hoarse roaring which attended the earthquake, together with the cries, screams, and yells of the people, seems still ringing in my ears.

REELFOOT LAKE, TN "Beginning December 16, 1811, there were violent earthquakes throughout the winter months. On some days the atmosphere was so saturated with sulfurous vapors as to cause total darkness. Trees cracked and fell into the roaring Mississippi. The waters of the river gathered up like a mountain, rising 15 to 20 feet, then receding with such violence that it took whole groves of cottonwoods which edged its borders. Fissures in the earth vomited forth sand and water, some closing again immediately.

”

REELFOOT LAKE, TN

Feb. 19

We have seen a statement by gentlemen from New Madrid, that that place is much torn to pieces by the late Earthquake; so it is almost impossible to get along in any way, but on horseback. Houses of brick, stone and log are torn to pieces, and those of frame thrown upon their sides. The ground for 100 acres has sunk so low that the tops of the tallest trees can hardly be seen above the water; in other places more than half the length of the timber is under water.

What happened away from New Madrid? Not much S. Hough INTENSITY OF SHAKING

Ground motions are usually more severe near bodies of water

Abundant modern evidence reveals that shaking during earthquakes is amplified considerably at sites along major bodies of water, where soils and sediments are generally “loose,” as in the example shown above.

Ste. Genevieve, Missouri, along the Mississippi, was hardly damaged

The New Madrid Earthquakes also triggered large earthquakes in Mississippi, Illinois, and Kentucky

Why were the largest quakes felt as far as the east coast?

Eastern North America is made of old, cold and stiff lithosphere, with no recent tectonic activity, and so it carries waves great distances.

Example: Seismograms from a Texas EQ Mina, NV Mina, NV St. Louis, MO St. Louis, MO

What can we expect for the future at New Madrid?

Don’t know – earthquakes can’t (yet?) be predicted.

Educated guesses – based upon paleoseismicity, currently levels of seismicity, and GPS studies

Seismicity of NMSZ is 1/30-1/100 of the California rate, due to difference in motion rates M>5 ~ every 15 yr M>6 ~ every 150 yr M>7 in 1811-12 There has not been a M6 or greater earthquake in NMSZ since 1800s, nor a M5 or greater in the past 30 years.

PACIFIC NEW MADRID SEISMICITY 1900-2002 NORTH AMERICA

NMSZ FREQUENCY-MAGNITUDE RELATIONSHIP

All seismicity, including total global seismicity follows this general pattern: Example/ Each year, globally: Magnitude Number ≥ 8 1-2 ≥ 7 ≥ 6 ≥ 5 ≥ 4 12 120 1200 12,000 Stein & Newman, 2004

NMSZ FREQUENCY-MAGNITUDE RELATIONSHIP

For New Madrid, combine instrumental seismology with earlier data to explore large earthquake recurrence Large paleoearthquakes occurred at ~ 1450 and 900 AD (Magnitudes unknown)  Gives a M7 every few thousand years.

 Might NEVER get a M8 earthquake Stein & Newman, 2004

Earthquakes occur in the interior of all plates Intraplate earthquakes seem to occur in clusters or swarms, unlike the more consistent plate-boundary earthquakes

Basel: 1356 Some 30 to 40 medieval castles collapsed in the hardest hit area. Many more churches and towers toppled within a 200 kilometer radius of Basel, as the earthquake reached a Mercalli intensity of IX to X Megharoui et al., 2001 Basel 1356 M~6.0-6.5

However, even earthquake recurrence along plate boundaries is highly variable; probabilities hard to assess M>7: mean = 132 yr

s =

105 yr

Sieh et al., 1989

Estimated probability of next earthquake in 30 yrs is 7-51%



Random!

Elastic Rebound (Seismic Gap) Theory?

NY City Bear Gap Hypothesis?

Random seismicity simulation

Earthquakes occur where earthquakes occur Earthquakes occur where seismometers exist

Earthquakes occur where earthquakes occur Earthquakes occur where seismometers exist Almost no seismometers here!!

Notice all the earthquakes EAST of the NMSZ!

The NMSZ might be finished (for now?). It might be time for a different failed rift to undergo some continental creaking.

o

Historical o Instrumental

GPS SITE MOTIONS: Vertical show glacial rebound Horizontal show no pattern

DON’T SEE MOTION AT NEW MADRID OR ELSEWHERE IN EASTERN US: If you don’t bend it, you can’t break it GPS site motions within their ellipse of uncertainty - no motion !

No significant or coherent intraplate deformation visible

GPS CONSTRAINTS ON LARGE EARTHQUAKE MAGNITUDE & RECURRENCE GPS data show little or no (< 2 mm/yr) motion Little or none accumulating for future earthquakes Intersection of Paleoseismology and GPS evidence suggests low M7 earthquakes, but larger events would have VERY long recurrence times. GPS

Newman et al, 1999

Humans are naturally drawn to the dramatic and catastrophic

Example: Since 9/11, when 3000 Americans perished, 150,000 Americans have died from guns and 200,000 Americans have died on highways. We seem comfortable with the 350,000 deaths, but in a panic about the 3000.

U.S. EARTHQUAKES Infrequent, but occasionally major, fatalities and damage Moderate (M 6.7) 1994 Northridge earthquake: 58 deaths, $20B damage Challenge: find mitigation strategy that balances cost of safer construction with benefits, given other possible uses of resources

Humans are naturally drawn to the dramatic and catastrophic

Example: Iben Browning

Humans are naturally drawn to the dramatic and catastrophic

Example: Central U.S. Earthquake Consortium warning for New Madrid

“Seismologists have predicted a 40-60% chance of a

devastating

earthquake in the New Madrid seismic zone in the next ten years. Those odds jump to 90% over the next 50 years. The potential magnitude of a

catastrophic

New Madrid quake dictates that we approach the preparedness on a regional basis"



Unjustified given geologic evidence

Are the seismic hazards as great in NMSZ as California? Of course not.

2% chance of shaking within 50 years: = seismic hazard within next 2500 years

More reasonable assessment: The Global Seismic Hazard Assessment Program (GSHAP), with the support of the International Council of Scientific Unions (ICSU), and endorsed as a demonstration program in the framework of the United Nations International Decade for Natural Disaster Reduction (UN/IDNDR).