Midwest Earthquakes

Lawrence W. Braile

Department of Earth and Atmospheric

Sciences

Purdue University

West Lafayette, IN 47907-1397

(March 2004)

During the past two decades, earthquake data recorded by the New Madrid seismograph network (Figure 1) have provided new insight into the seismotectonics of the New Madrid seismic zone (NMSZ). Prior to the establishment of the network, the NNSZ was characterized by the less-than-200-year-duration historical earthquake record (Nuttli, 1973) and by accounts of the great 1811-1812 earthquakes (Johnston, 1982; Johnston and Schweig, 1996). Seismicity and geophysical data analyzed since 1974 have delineated prominent epicentral trends and tectonic features. Earthquake locations correlate with distinct geophysical anomalies (Figures 2 and 3) indicating that the intraplate seismicity of the NMSZ is associated with an ancient, buried rift that is currently being reactivated by the contemporary, nearly east-west compressional plate-tectonic-generated stresses (Zoback and others, 1980; Braile and others, 1982, 1986, 1997). Positive gravity anomalies in the upper Mississippi embayment (Figures 2 and 3) are interpreted to be caused by high density rocks beneath the embayment that were emplaced during the late Pre-Cambrian to early Paleozoic rifting event or during Mesozoic reactivation of the rift. During the past 150-200 million years, the area has subsided due to the presence of the more dense rocks in the crust resulting in the embayment and the deep burial of the ancient rift structure (Figures 4 and 5). Currently, the buried rift has acted as a “zone of weakness” in the stable continental crust and serves to localize earthquake activity within the Midwest. In addition to the most active portion of the NMSZ that has been the location of large magnitude earthquakes, diffuse seismicity, including events that are generally small than about 5.5 magnitude, is present throughout much of the Midwest. Recent interpretations and discussions of geological and geophysical data in the New Madrid area are contained in papers published in a special issue (Volume 68, Number 4, July/August, 1997) of Seismological Research Letters. More information on the NMSZ and Midwest earthquakes is available in the references listed below and the papers published in the special issue. Recently, new studies of the intensity of shaking of the New Madrid earthquakes has resulted in new estimates of the magnitudes of these events (Hough and others, 2000; Hough and Martin, 2002). The revised magnitudes range between M7 to M7.5. Although the estimated magnitudes are smaller than previous estimates, it is clear that these were very significant events as evidenced by the intensity data (Figures 6-9) and their occurrence indicates a significant earthquake hazard for the Central United States (Figure 10).

Figure 1. New Madrid area network seismicity through time. Magnitudes (m_b) of the earthquakes are indicated by the size of the dots. A. One year, July, 1974 – June, 1975; B. Two years, July, 1974 – June, 1976; C. Five years, July, 1974 – June, 1979; D. Ten years, July, 1974 – June, 1984; E. Twenty years, July, 1974 – June, 1994; F. Twenty year seismicity map and newly defined trends (shaded areas).

Figure 2. Regional Bouguer gravity anomaly map and 1974 – 94 earthquake epicenters (dot size is proportional to magnitude) in the New Madrid seismic zone. Gravity data have been smoothed and adjusted to correct for the effect of the low-density Mississippi embayment sediments. Contours are in mGal. Gravity data from Cordell(1977).

Figure 3. Bouguer gravity anomaly map and 1974 – 94 earthquake epicenters (dot size is proportional to magnitude) in the New Madrid seismic zone. Dashed line shows outline of the interpreted New Madrid Rift Complex. The southernmost portion of this rift complex was recognized in 1975 and named the Reelfoot rift (Ervin and McGinnis, 1975; Hildenbrand, 1985).

Figure 4. Interpreted crustal model and gravity data along a west-to-east profile at 35.5 degrees North latitude. The high density lower crust and thinning of the upper crust are associated with the development of the ancient rift. Subsequently, the area has undergone substantial subsidence resulting in the deposition of the thick section of sedimentary rocks in the Mississippi embayment. Black areas indicate intrusions into the upper crust along the rift margins. Most of the earthquakes in the rift area show strike-slip or thrust mechanisms.

Figure 5. From Braile and others, 1982.

Figure 6. Comparison of Intensity observations for Central and Eastern US and California earthquakes.

map showing differences between a 1994 M 6.7 earthquake in California and smaller (M6.0) earthquake that occured in St. Louis in 1895. The 1895 earthquake was felt over a much wider area and caused damage in a wider area.

Figure 7. Comparison of Intensity observations for Central US and California earthquakes (http://geopubs.wr.usgs.gov/fact-sheet/fs017-03/).

Figure 8. Shake map for the December16, 1811 New Madrid earthquake (from Susan Hough, http://pasadena.wr.usgs.gov/office/hough/).

Figure 9. Shake map comparison of the M7.3 NewMadrid and the M7.3 Landers earthquakes earthquake (from Susan Hough, http://pasadena.wr.usgs.gov/office/hough/).

seismic-hazard map for the entire United States

Figure 10. Earthquake hazard map for the US ( http://geopubs.wr.usgs.gov/fact-sheet/fs017-03/)

References:

Braile, L. W., G. R. Keller, W. J. Hinze, and E. G. Lidiak, An ancient rift complex and its relation to contemporary seismicity in the New Madrid seismic zone, Tectonics, 1, 225-237, 1982.

Braile, L. W., W. J. Hinze, G. R. Keller, E. G. Lidiak, and J. L. Sexton, Tectonic Development of the New Madrid rift complex, Mississippi embayment, North America, Tectonophysics, 131, 1-21, 1986.

Braile, L. W., W. J. Hinze and G. R. Keller, New Madrid seismicity, gravity anomalies, and interpreted ancient rift structures, Seismological Research Letters, 68, 599 – 610, 1997.

Cordell, L., Regional positive gravity anomaly over the Mississippi embayment, Geophysical Research Letters, 4, 285 – 287, 1977.

Ervin, C. P., and L. D. McGinnis, Reelfoot rift: Reactivated precursor to the Mississippi embayment, Geological Society of America Bulletin, 86, 1287-1296, 1975.

Hildenbrand, T., Rift structure of the northern Mississippi embayment from the analysis of gravity and magnetic data, Journal of Geophysical Research, 90, 12607-12622, 1985.

Hough, S.E., J.G. Armbruster, L. Seeber, and J.F. Hough, On the modified Mercalli intensities and magnitudes of the 1811-1812 New Madrid earthquakes, Journal of Geophysical Research, 23,839-23,864, 2000.

Hough, S.E. and S. Martin (2002). Magnitude estimates of two large aftershocks of the 16 December, 1811 New Madrid earthquake, Bulletin of the Seismological Society of America, 92, 3259-3268.

Johnston, A. C., A major earthquake zone on the Mississippi, Scientific American, 246, 59 – 68, 1982.

Johnston, A. C., and E. S. Schweig, The enigma of the New Madrid earthquakes of 1811 – 1812, Annual Reviews of Earth and Planetary Sciences, 24, 339 – 384, 1996.

Nuttli, O. W., The Mississippi valley earthquakes of 1811 and 1812, intensities, ground motion, and magnitudes, Bulletin of the Seismological Society of America, 63, 227 – 248, 1973.

Zoback, M. D., R. M. Hamilton, A. J. Crone, D. P. Russ, F. A. McKeown, and S. R. Brockman, Recurrent intraplate tectonism in the New Madrid seismic zone, Science, 209, 971-976, 1980.

http://pasadena.wr.usgs.gov/office/hough/ (Information on recent studies of the New Madrid earthquakes, links to updated intensity maps and contemporary accounts of the New Madrid earthquakes).

A color version of this document is available online at www.eas.purdue.edu/~braile (click on News).