ISNS 4359
EARTHQUAKES AND VOLCANOES
Spring 2005
Steve Bergman, Instructor
Lecture 11. Historic Earthquakes IV: The New Madrid 1811-1812 Enigmas & Texas EQ
Intraplate earthquakes & the not-so "stable" continental interior
New Madrid 1811-12: Two (M~8) of the >200 damaging earthquakes (M>5) were the largest ever felt in modern
times in the lower 48. [nota bene: much of the following data are inferred, deduced, or estimated using the best information available]
Plate Tectonics & Faults A group of 7 intersecting, reactivated, NE & NW-trending, intraplate (North American Plate
interior >2000 km from the nearest plate boundary), high-angle faults within the NE-trending buried Reelfoot Rift (70 x 200
km), a Late Precambrian to Cambrian ‘failed’ rift system (~600 Ma, reactivated during the Cretaceous 100 Ma); faults were
reactivated by plate boundary stresses transferred to plate interior; possibly related to post-glacial isostatic rebound.
The NW-trending Reelfoot fault (Reverse fault, 32 km long) is the only seismically active fault with surface expression. Last
40 years of deformation measured by GPS ~2-6 mm/yr right lateral offset along NE trending strike-slip faults.
Total length of subsurface rupture zone Unknown, but total length of 7 faults ~200-350 km
Depth to Hypocenter Unknown, probably <20 km
Type of offsets 2 NE-trending right-lateral strike-slip faults linked by a NW-trending, reverse dip-slip step-over fault.
Max Horizontal Offset ~10 m (estimates range from 5-20 m), Vertical Offset <10 m
Time 3 largest EQ: 2AM 12/16/1811; 9AM 1/23/1811; 4AM 2/7/1812 (M w~8). Duration <1-3 min. each.
Felt area >5 million km2: most of central and eastern USA (>27 states), S Canada, NE Mexico; Love waves (felt as far as
2,000 km away in Quebec) caused damage over 600,000 km2 due to low attenuation of the surface waves.
Magnitude & Intensity 18 events over 3 months with ML~5.5-7.5 and MW~5-8, the 3 largest EQ had MW~7-8; total
energy released-MW~8.3; MMI=XI. [total 1,874 shocks felt] {no apparent foreshocks}.
Impact & Other Effects ~1,000? killed; New Madrid (pop. 400) destroyed; [~4000 pop in region]; 2 waterfalls 5-10 m
high over a 3 km course were created across the Mississippi River (1 km N & 8 km S of New Madrid); Mississippi R. flowed
upstream for 30 min.; river tsunami, banks collapsed, liquefaction, fissuring, fountaining, sand blows erupted, swamps
formed, trees snapped, forests destroyed, islands sank; subsidence caused 2 new lakes: Lake St. Francis & Reelfoot Lake.
Chimneys collapsed in Cincinnati; bells rang and clocks stopped in Boston. Jared Brooks in Louisville KY cataloged 1,874
felt EQs. Triggered n=4 M4-5 EQ in KY, IL, MS, & SC in Jan-Feb 1812.
Recurrence Interval Highly Debatable: 100's to 1000's of years (<5% probability of repeat within the next 50 years for
M>8, 90% for M>6). The last large (M>6) EQs to strike the region were in 1843 & 1895 (M>8 @ ~900 & ~1300 AD).
Fig. 1. 1975-1999 Seismicity and Fault Plane Focal Solutions (from C.J. Ammon, Penn State) Fig 2. Block diagram (http://www.eas.purdue.edu/~braile/news/midwest.htm)
New Madrid
Expected
Earthquake
Recurrence
Magnitude
Interval (yr)
4.0
1.2
5.0
10-12
6.0
70-90
7.0
254-500
8.0
550-1200
(from Johnson & Schweig, 1996)
CERI/USGS EQ probabilities
(next 50 years):
Mw=7.5-8
7-10%
Mw=>6
25-40%
Alternative estimates by Seth Stein
from Northwestern Univ:
Based on <10 yrs GPS (<2 mm/yr)
>2500 yr recurrence interval for M>7
Selected references: http://www.hsv.com/genlintr/newmadrd/index.htm,
Gomberg, J & Schweig ES (2002) Earthquake Hazard in the Heart of the Homeland. USGS FS 131-02.
Johnston, AC (1996) Seismic moment assessment of earthquakes in stable continental regions III. New Madrid 1811-1812, Charleston 1886 and Lisbon
1755, Geophysical Journal International, 126, 314-344.
Johnston, AC & Schweig ES (1996) The enigma of the new Madrid earthquakes of 1811-1812. Ann. Rev. Earth Planetary Sciences, 24, 339-84.
Newman, A., S. Stein, J. Weber, J. Engeln, A. Mao, and T. Dixon (1999) Slow deformation and lower seismic hazard at the New Madrid seismic zone.
Science, 284, 619-621.
Seth Stein Web site & links: http://www.earth.northwestern.edu/people/seth/research/nmsz.html
Another Intraplate fault with recent offset of most relevance to Dallas, TX: Meers Fault, OK [WNW trending 30 km long
with 3 m vertical offset; ruptured by M6-7 EQ ~1500 years ago (Recurrence Interval ~thousands of years), occurs along the
frontal fault zone between the Wichita Uplift to the S and the Anadarko Basin to the N.
[El Reno, OK: M5.5EQ of 9Apr1952 (max MMI=VIII) was felt in Dallas (MMI=II-III)].
References: http://ve.ou.edu/weaver/eq_oklahoma/oklahoma_quakes.html, http://geophysics.ou.edu/ok_eq/
Texas Earthquakes
Left map shows probable causes of TX EQs. Solid lines show the four TX regions with historical earthquake activity and
earthquake hazard. Light lines are county boundaries. Three EQ of the last 100 years have been felt over nearly all of TX.
M6 Valentine 16Aug1931; M4.6 2Jan1978 Hobbs; M5.7 Alpine 14Apr1995
http://www.ig.utexas.edu/research/projects/eq/compendium/earthquakes.htm#Figure%2012B