Afterslip from the 1450 and 1812 New Madrid earthquakes and rate-and-state frictional parameters of an intraplate fault

Friday, 19 December 2014: 9:45 AM
Oliver Salz Boyd, U.S. Geological Survey, Denver, CO, United States
Present day ground deformation in and near the New Madrid Seismic Zone (NMSZ) is an important factor in our understanding of central U.S. seismic hazard. Here I analyze Global Positioning System data over the period from 2000 to 2013 to assess several potentially active deformation mechanisms including: (1) regional shear; (2) creep on finite dislocations; and (3) post-seismic processes from the 1811–1812 and 1450 NMSZ earthquakes.

A model of regional shear provides virtually no fit to the data and limits right-lateral regional shearing to less than 0.2±0.4 mm/yr. On the other hand, a 40-km long dislocation along the downdip extension of the Reelfoot fault creeping at about 6 mm/yr between 12 and 20 km depth fits the data well, with a 98% chance of being better able to predict the observed ground deformation, having rates of a fraction of a mm/yr, than does a hypothesis where ground deformation has a normalized random distribution. This is consistent with the results of previous authors (Frankel et al., 2012) and can be well explained by a model of afterslip from M7.5 1450 and February 1812 Reelfoot fault events. To successfully model afterslip, the ratio of reference slip rate to fault constitutive parameter is found to be about 10 times smaller than that for the Parkfield segment of the San Andreas fault (Barbot et al., 2009). Creep and afterslip on the downdip extension of the Cottonwood Grove fault and models of viscoelastic relaxation following the December 1811 and February 1812 earthquakes provide a poor fit to the data, in part due to the configuration of the observational network. The fit degrades further for values of lower crustal viscosity less than 1021 Pa-s.

One implication of this modeling is that if much of the present-day surface deformation results from afterslip, it is likely that many of the earthquakes we see today in the New Madrid seismic zone are aftershocks from the 1811­–1812 New Madrid earthquakes. Despite this fact, the afterslip modeling is consistent with models of intraplate earthquake clustering, and when considered with observations of earthquake clustering and the recent paleoseismic history of the region suggests that the NMSZ is in an active period and seismic hazard from 1811–1812-type events is likely to remain significant into the near future.