Verification of the Apparent Crustal Stress Anomaly in the Western Midcontinent

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Two recent independent studies, one in the SE corner of South Dakota, and the other in north central Kansas, indicate a striking departure in the stress regime west of the Midcontinent Rift (MCR) from that established in Wisconsin-Illinois and more eastern states and assumed until now to prevail throughout the Midcontinent stress province. Near-surface in situ stress measurements in South Dakota and a focal mechanism solution of an earthquake in Kansas (depth 1.3 km or greater) both indicate a 90o rotation in the direction of the maximum horizontal stress which is uniformly northeastern in all the previous Midcontinent measurements. The new findings are supported by two earlier focal mechanism solutions suggesting west-northwest stress direction. However, this fascinating and important observation, which may require a redistribution of the stress provinces in the continental U.S., is incomplete in that stress magnitudes representative of the upper crust are difficult to ascertain from near-surface tests, and are impossible to determine from focal mechanism solutions. The goal of this project is to verify the unexpected results west of the MCR, and complement them with credible estimates of the magnitudes of in situ principal stresses, by conducting a comprehensive series of hydraulic fracturing stress measurements in basement granite at depths of about 600 m using the existing Quimby, Iowa Scientific hole.