Crustal deformation in the New Madrid seismic zone and the role of postseismic processes

Abstract

Global Navigation Satellite System data across the New Madrid seismic zone (NMSZ) in the central United States over the period from 2000 through 2014 are analyzed and modeled with several deformation mechanisms including the following: (1) creep on subsurface dislocations, (2) postseismic frictional afterslip and viscoelastic relaxation from the 1811–1812 and 1450 earthquakes in the NMSZ, and (3) regional strain. In agreement with previous studies, a dislocation creeping at about 4 mm/yr between 12 and 20 km depth along the downdip extension of the Reelfoot fault reproduces the observations well. We find that a dynamic model of postseismic frictional afterslip from the 1450 and February 1812 Reelfoot fault events can explain this creep. Kinematic and dynamic models involving the Cottonwood Grove fault provide minimal predictive power. This is likely due to the smaller size of the December 1811 event on the Cottonwood Grove fault and a distribution of stations better suited to constrain localized strain across the Reelfoot fault. Regional compressive strain across the NMSZ is found to be less than 3 × 10−9/yr. If much of the present‐day surface deformation results from afterslip, it is likely that many of the earthquakes we see today in the NMSZ are aftershocks from the 1811–1812 New Madrid earthquakes. Despite this conclusion, our results are consistent with observations and models of intraplate earthquake clustering. Given this and the recent paleoseismic history of the region, we suggest that seismic hazard is likely to remain significant.

Dense lower crust elevates long‐term earthquake rates in the New Madrid seismic zone

“…The large earthquakes in the NMSZ have puzzled geodesists in recent decades because the rate of strain accumulation is low (~10 −9 /yr) if even detectable [ Newman et al ., ; Calais et al ., ; Frankel et al ., ; Craig and Calais , ; Boyd et al ., ], issues of postseismic deformation notwithstanding. Additionally, the vertical throw on individual faults imaged in seismic data is generally ~100 m or less [ Schweig and Ellis , ; Van Arsdale , ; Odum et al ., ; Hao et al ., , ].…”

“…(c) Regional stress in the CEUS is 0–20 MPa and <10 MPa (likely 0–5 MPa) in the NMSZ. (d) Strain rates normal to the plate boundary predicted from ridge push (~10 −9 /yr) are similar to geodetic estimates [ Calais et al ., ; Frankel et al ., ; Boyd et al ., ].…”

“…Equations are subject to several constraints. The strain rate near New Madrid, ~5000 km from the plate boundary, is at most ~10 −9 /yr, near or below the rate detectable with existing GPS data [ Craig and Calais , ; Boyd et al ., ]. Second, the velocity at the plate boundary is half of the 2.5 cm/yr spreading rate of the Mid‐Atlantic Ridge.…”

“…Paleoseismic studies suggest similar large earthquake sequences around 1450, 900 A.D., and older [ Tuttle et al ., , ]. Nevertheless, geodetic strain rates are 10 −9 /yr or less, near or below the threshold detectable [ Craig and Calais , ; Boyd et al ., ], and Holocene rates of seismicity are 1–2 orders of magnitude greater than on average through the Cenozoic [e.g., Schweig and Ellis , ], consistent with the episodic activity observed on intraplate faults worldwide [e.g., Clark et al ., ]. Therefore, determining whether long‐term stress is elevated in the NMSZ or whether such large but episodic seismicity is expected elsewhere/everywhere in the CEUS is of paramount importance to assessing seismic hazard.…”

“…Pollitz et al [] chose a +150 kg/m 3 cylindrical anomaly (15 km radius). Imposing instantaneous weakening of the lower crust to a viscosity of 1.5 × 10 19 Pa s (below the lower bound of 10 21 Pa s in the NMSZ [ Boyd et al ., ]) allows the dense body to sink, stressing the overlying seismogenic crust by a much more modest 1–3 MPa.…”