implication for strain accumulation on potentially seismogenic faults
in the central and eastern U.S.
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Large earthquakes within stable plate interiors are direct evidence
that significant levels of strain can accumulate along geologic
structures far from plate boundary faults, where the vast majority
of seismic energy is released. The 1811-1812 New Madrid events in
the Mississippi valley are classical examples of large intraplate
earthquakes (e.g. Nuttli, 1983; Johnston, 1996; Hough et al., 2000).
Because significant intraplate earthquakes are infrequent and the strain rates are so low, neither the rates and pattern of intraplate strain are well constrained, nor are the mechanism(s) responsible for strain accumulation and release on faults inside plates. Unlike plate boundary faults, where far-field plate motion and near-field strain are clearly related, such a relation between far- and near-field motions is not established for plate interiors. Far-field plate motions may contribute to the stress balance that loads seismogenic faults, but other stress sources such as density anomalies in the lower crust or glacial isostatic adjustment and local parameters such as high pore fluid pressure, weakness of pre-existing faults, or density anomalies in the lower crust may contribute equally significantly to the stress budget (see Pollitz et al., 2001, for a review of models in the case of the New Madrid fault zone). |
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 Two collocated GPS antennas at site HCES in the New Madrid seismc zone. Note the different type of monumentation (courtesy G. Mattioli, University of Arkansas). |
Over the past decade, GPS has become an invaluable tool for measuring long-term plate motion and strain accumulation on active faults. In areas of rapid strain accumulation such as the western United States, GPS-derived estimates of strain have been used effectively to help estimate earthquake hazard (e.g., Working Group on California Earthquake Probabilities, 1990). In areas such as the Central and Eastern US, where rates of strain accumulation appear to be slower than ~1 mm/yr, measuring strain from GPS has proven to be significantly more challenging, requiring additional attention to GPS monument stability and the effects of random and correlated sources of errors in GPS station velocities. A better understanding of deformation within plates thus requires geodetic measurements over long periods at both local and plate-wide scales, combined with rigorous attempts to extract the maximum precision from those observations. |
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