ABSTRACT

This research is to study earthquakes in the central and eastern United States. These earthquakes cannot be readily explained by the plate tectonics theory, which predicts earthquakes to be concentrated along the boundary faults of tectonic plates, such as the San Andreas Fault in California. Within plate interiors, no large earthquakes are supposed to occur. The central and eastern United States is within the presumably stable interior of the North American plate, yet earthquakes in this part of the country have been widespread and potentially catastrophic. Three large earthquakes occurred in southeastern Missouri in the winter of 1811-1812, shaking ground as far away as Boston. Geological evidence shows large prehistoric earthquakes in many central and eastern states, and the 2011Virginia earthquake, which shook much of the eastern seaboard, is a reminder of earthquake hazard in the central and eastern US. The researchers on this project will systematically investigate the cause of these earthquakes. Using computer modeling, this research will address three major questions: 1) What causes stress and strain localization in the central and eastern US? Existing hypotheses explain these earthquakes as the results of either weak zones in the Earth?s crust or local tectonic forces. Sophisticated computer models will be developed to investigate how the observed Earth structures interact with tectonic forces to cause clustered earthquakes in the central and eastern US. 2) What triggered the recent seismicity? Present seismicity in Missouri apparently has resulted from geologically recent fault reactivation, perhaps since the last ice age. This research will test the various hypothesized causes to quantify their roles in reactivating faults. 3) Why large earthquakes have repeated in Missouri? Results of this work will improve the assessment and mitigation of earthquake hazards in the central and eastern US. This work will also help understand earthquakes in geologically similar regions such as India, North China, Europe, and Australia. This project will involve undergraduate research and strengthen ties with the earthquake researchers in China, where plate-interior earthquakes are common.

). Intracontinental earthquakes have killed more people than other types of earthquakes combined prior to 2012, yet they remain poorly understood and cannot be readily explained by plate tectonics. Located within the presumably stable interior of the North American plate, the CEUS nonetheless has widespread and potentially catastrophic earthquakes. A cluster of M~7.0 earthquakes in the New Madrid Seismic Zone (NMSZ), southeastern Missouri, between 1811 and 1812 were among the largest earthquakes ever recorded in continental US, and the 2011 Virginia earthquake (Mw 5.8) is a fresh reminder of earthquake hazard in the CEUS. The research consists of three phases of geodynamic modeling, using the state-of-art viscoelasto-plastic finite element codes, to address fundamental questions of earthquakes in the central and eastern United State (CEUS). Existing hypotheses for the clustered seismicity in the CEUS include various weak zones in the lithosphere such as failed rifts, and regional and local driving forces such as ridge push and mafic intrusions in the failed rifts. This research will investigate the dynamic interplay of lithospheric structures and driving forces in a three-dimensional lithospheric dynamic model. The model will be constrained and validated by comparing the predicted and observed stress orientations and stress states in the lithosphere. Present seismicity in the NMSZ and other parts of the CEUS has apparently resulted from recent fault reactivation, perhaps since the last ice age. Various causes of the fault reactivation, including glacial isostatic adjustment and increased erosion, have been explored in different models. The proposed research will quantify the relative roles of these causes in the same mechanical model, using GPS-measured vertical crustal motion in the CEUS as the primary constraint.This research will test the hypothesized causes, such as cyclic loading and viscous relaxation, in a lithospheric dynamic model. This model will also simulate how rupture of other faults in the Mississippi Embayment may have affected loading in the NMSZ, and how the 1811-1812 NMSZ earthquakes may have influenced the instability of neighboring faults, especially those in the Wabash Valley, via stress migration.

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