Verification of the Apparent Crustal Stress Anomaly in the Western Midcontinent

最近的两项独立研究，一项在南达科他州的东南角，另一项在堪萨斯州的中北部，表明中大陆裂谷（MCR）西部的应力状态与威斯康辛-伊利诺斯州和更东部的州所建立的应力状态有显著的不同，并且直到现在被认为在整个中大陆应力省普遍存在。南达科他州的近地表地应力测量和堪萨斯州地震的震源机制解（深度1.3公里或更大）都表明，在所有先前的中大陆测量中，最大水平应力方向都是一致的东北方向，有90度旋转。这一新发现得到了两种早期震源机制解决方案的支持，即西-西北应力方向。然而，这一引人入胜的重要观察结果是不完整的，它可能需要重新分配美国大陆的应力省份，因为具有代表性的上地壳的应力大小很难从近地面测试中确定，也不可能从震源机制解中确定。该项目的目标是验证MCR以西的意外结果，并通过使用现有的Quimby， Iowa科学孔在600米深处的基底花岗岩中进行一系列全面的水力压裂应力测量，以补充对原位主应力大小的可靠估计。

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.