Tectonic Strain in the Interior of the North American Plate?

Calais E., Purdue University, Department of Earth and Atmospheric Sciences, West Lafayette, IN
G. Mattioli, University of Arkansas, Department of Geology, Fayetteville, AR
C. DeMets, University of Wisconsin, Department of Geology and Geophysics, Madison, WI
J.-M. Nocquet, CNRS, Geosciences Azur, Valbonne, France
S. Stein, Northwestern University, Department of Geological Sciences, Evanston, IL
A. Newman, Georgia Institute of Technology, School of Earth and Atmospheric Sciences, Atlanta, GA
P. Rydelek, Center for Earthquake Research, University of Memphis, Memphis, TN

Cite as:
Calais, E., et al. Tectonic strain in plate interiors? Nature 438, doi: 10.1038/nature04428 (2005).

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Geologists have long sought to understand how and why the interior of tectonic plates deform, leading to large, although infrequent, earthquakes. A recently published potential breakthrough [1] suggests that surface deformation in the central U.S. accumulates at rates comparable to those across plate boundaries. However, three analyses of the same dataset find no statistically significant deformation. Therefore, only upper bounds on large earthquake magnitude and repeat time can presently be inferred.

The occurrence of earthquakes in the interior of tectonic plates -- assumed rigid in conventional plate tectonic theory -- indicates that stresses within plate interiors accumulate on faults and are released during large, although infrequent, events. How this cycle relates to the slow deformation of plate interiors is unknown, posing significant difficulties for understanding the associated hazards. Stakes are high, since a number of now densely populated intraplate areas have been struck in the past by large earthquakes, such as the central U.S. in 1811-1812, Basel (Switzerland) in 1356, and Newcastle (Australia) in 1989. Geophysicists are now using the Global Positioning System (GPS) to quantify strain in plate interiors in hope of relating it to stress buildup on seismogenic faults.

Recently, significant strain from GPS measurements have been reported in the New Madrid seismic zone (NMSZ) of the central U.S. and interpreted as indicating deformation rates comparable to those observed at much more seismically active plate boundaries [1]. If confirmed, this result may give insight into the processes that drive the occurrence of large earthquakes in plate interiors and provide new quantitative information for seismic hazard estimation in the New Madrid area [1].

However, independent analyses of the same data performed by three independent groups using different analysis software and processing strategies show no statistically significant site motions or strains, with an average weighted misfit to a rigid plate behavior of 1.4 mm/yr (95% confidence). In particular, the shortening between sites RLAP and NWCC, used by [1] as their primary argument for strain accumulation on the Reelfoot fault, is of marginal significance (1.72.0 mm/yr, 95% confidence) and largely reflects an unexplained offset that occurred between mid-2001 and early 2002. The same analyses, using 156 GPS sites distributed throughout the central and eastern U.S., find no spatially coherent deviation from rigid behavior in the far field of the NMSZ either, except for effects due to the removal of glacial loads, with an average weighted misfit to a rigid plate model of 1.4 mm/yr (95% confidence) as well (additional details available from the authors).

Figure: Velocities and associated uncertainties (95% confidence) from analyses using two different software packages at continuous GPS sites in the New Madrid seismic zone. Site velocities are within their error ellipses and hence show no statistically significant motion. Color circles show regional seismicity (USGS catalogs). Inset: Time series of daily baseline length estimates between sites RLAP and NWCC after removal of a mean. Error bars on daily estimates, omitted for sake of clarity, are on the order of 2-3 mm.

Detecting motion depends critically on the assumed uncertainties of site velocities, which decrease as data span longer times. Hence the present data do not preclude the possibility that a statistically significant tectonic signal may emerge in the future. We will then face the challenge of deciding whether the deformation represents strain accumulating to be released in a future earthquake [1] or long-term relaxation after the 1811-12 earthquakes [2].

Is an upper bound of 1.4 mm/yr of motion across the NMSZ consistent with longer-term data from paleo-earthquakes in the central U.S. [1]? Assuming that characteristic earthquakes repeat regularly in the NMSZ -- a likely oversimplification, one nevertheless used in National Earthquake Hazard maps -- this leads to a minimum repeat time of about 600-1,500 years, consistent with earlier estimates [3] based on the paleoseismic history [4] assuming magnitude 7 earthquakes with 1-2 m of coseismic slip [3].

Although intraplate earthquakes indicate that tectonic stresses within plate interiors accumulate on faults and are released during large infrequent events, deviations from rigid behavior in the central U.S. and several other major plates [6, 7] are below the current resolution of GPS measurements and do not reflect this cycle at least not on a time scale of a decade or less. Longer observation spans and further improvement of geodetic techniques are needed to understand where, why, and how much strain concentrates in plate interiors.

1. Smalley et al., Nature 435, doi:10.1038/nature03642 (2005).
2. Rydelek and Pollitz, Geophys. Res. Letters 21, 2302-2306 (1994).
3. Newman et al., Science 284, (1999).
4. Tuttle and Schweig, Geology 23, 361-380 (1995).
5. Kenner and Segall, Science 289 (2000).
6. Nocquet et al., Geophys. Res. Letters 32, doi:10.1029/2004GL022174 (2005).
7. Beavan et al., J. of Geophys. Res. 107, doi:10.1029/2001JB000282 (2002).

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