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，另一项在堪萨斯州中北部，表明中大陆裂谷(MCR)以西的应力制度与威斯康星州-伊利诺伊州和更东部的州建立的制度惊人地背道而驰，到目前为止，人们认为这种制度普遍存在于整个中大陆应力省。南达科他州的近地表地应力测量和堪萨斯州一次地震(深度1.3公里或更深)的震源机制解都表明，最大水平应力方向旋转了90°，在所有以前的中大陆测量中，最大水平应力都是均匀地向东北方向旋转。这一新发现得到了早先提出的两种震源机制解的支持，即西-西北向应力方向。然而，这一引人入胜的重要观测可能需要重新分配美国大陆的应力省份，但它是不完整的，因为代表上地壳的应力大小很难从近地表试验中确定，也不可能从震源机制解中确定。该项目的目标是验证MCR以西的意想不到的结果，并通过使用现有的爱荷华州昆比科学洞在约600米深的基底花岗岩中进行一系列全面的水力压裂应力测量，用对现场主应力大小的可信估计来补充这些结果。

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.