Lecture on the New Madrid Earthquakes of 1811-1812

Introduction

• On December 16th, 1811, a massive earthquake shook the United States.
• The tremor was felt in places as far apart as New York and New Madrid, Missouri.
• Series of earthquakes continued for two months until February 1812.
• These intraplate earthquakes are significant because they occurred far from plate boundaries.

Impact and Eyewitness Accounts

• Over 2000 earthquakes occurred, with magnitudes reaching 7 to 7.5.
• Descriptions from William Pierce and Eliza Bryan provide firsthand accounts of the devastation.
• Earthquakes caused massive damage to settlements along the Mississippi River.

Geology and Causes

• Traditionally, earthquakes are linked to plate boundaries, like those along the West Coast.
• The New Madrid earthquakes occurred along intraplate faults.
• These zones can generate significant seismic activity despite being far from plate boundaries.

Historical Context

• New Madrid was a Spanish settlement prior to the Louisiana Purchase.
• Post-1804, it became U.S. territory and a major trade hub before the earthquakes destroyed much.
• The earthquakes led to the first disaster relief bill in Congress, which was poorly executed.

Geological Features of the Mississippi Valley

• Mississippi floodplain is a large sand pit, making it prone to liquefaction during earthquakes.
• Sandblows and soil liquefaction were significant hazards during the earthquakes.

Long-term Effects and Future Risks

• The region has continuous minor seismic activity.
• Historical records suggest a return period for major earthquakes of 200 to 800 years.
• The USGS estimates a 25-40% chance of a magnitude 6+ earthquake in the next 50 years.

Why the Region is Particularly Vulnerable

• Thick, dense crust transmits seismic waves over long distances.
• The region is less prepared for earthquakes compared to areas like California.
• Soil conditions contribute to severe ground shaking and liquefaction hazards.

Conclusion

• The New Madrid earthquakes highlight the potential for devastating intraplate earthquakes.
• Understanding these events is crucial for future preparedness and risk management.

Next Steps

• Further exploration of the geological underpinnings of the New Madrid seismic zone will be covered next time.