In situ stress measurements to 3.5 km depth in the Cajon Pass Scientific Research Borehole: Implications for the mechanics of crustal faulting
In situ stress measurements to 3.5 km depth in the Cajon Pass Scientific Research Borehole: Implications for the mechanics of crustal faulting
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DOI：
10.1029/91jb02175
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发表时间：
1992-04
影响因子：
--
通讯作者：
M. Zoback;J. Healy
中科院分区：
文献类型：
--
作者：
M. Zoback;J. Healy
利用水力压裂技术对Cajon Pass研究钻孔的原位应力方向和大小进行了测量，测量深度为0.9-3.5 km。这些测量结果支持了关于地壳断裂力学的两个重要结论。首先，测量的地应力大小表明剪应力与正应力的比值。这与基于莫尔-库仑理论和实验室确定的摩擦系数在0.6-1.0范围内的预测相一致，假设静水孔隙压力（这通常被称为拜耳定律）。因此，应力测量表明，圣安德烈亚斯断层附近地壳的摩擦强度高（即与实验室推导的摩擦值一致），并且圣安德烈亚斯断层附近地壳的剪切应力水平主要受其摩擦强度控制。然而，在1.75 ~ 3.5 km （N57oE + 19 o）范围内的钻孔最大水平压缩方向数据表明，圣安德烈亚斯断层一定非常弱，因为在平行于-- - N60oW走向的圣安德烈亚斯断层的平面上完全没有右侧剪切应力。在与圣安德烈亚斯断层平行的平面上缺乏右侧剪切应力，这一点尤其令人惊讶，因为卡洪山口位于圣安德烈亚斯断层的一段，自1812年以来没有发生过大地震，因此可能在地震周期中相当“晚”。然而，井中测量到的应力方向和大小都与钻探点周围区域的活动断层类型一致，最明显的是与钻探点附近的圣安德烈斯平行的Cleghorn断层的正断层和第四纪左旋滑动（Meisling和Weldon, 1982; Weldon, 1986； R. J. Weldon等人，未发表的报告，1981）。我们认为，在Cajon山口地区观测到的应力状态（和第四纪断层偏位）只有在圣安德烈亚斯以与地震应力下降相当的低剪应力而不是Byerlee定律预测的高得多的剪应力移动时才可能存在，这一结论与沿着圣安德烈亚斯系统缺乏摩擦产生的热流一致（例如，Brune等人，1969；Henyey和Wasserburg, 1971； Lachenbruch和Sass， 1973, 1980）。总的来说，卡洪山口的地应力和热流测量（本期Lachenbruch和Sass）支持了圣安德烈亚斯系统的概念模型，其中圣安德烈亚斯相对于周围的地壳非常弱。
Measurements of in situ stress orientation and magnitude at the site of the Cajon Pass research borehole have been made from depths of 0.9-3.5 km using the hydraulic fracturing technique and analysis of stress-induced well bore breakouts. The results of these measurements support two important conclusions about the mechanics of crustal faulting. First, the magnitudes of measured in situ stresses indicate ratios of shear to normal stres. s on favorably oriented fault planes that are consistent with predictions based on Mohr-Coulomb theory and laboratory-determined coefficients of friction in the range of 0.6-1.0 assuming hydrostatic pore pressure (this is commonly known as Byerlee's law). Thus the stress measurements indicate that the frictional strength of the crust adjacent to the San Andreas fault is high (i.e., consistent with laboratory-derived friction values) and that the level of shear stress in the crust adjacent to the San Andreas is principally controlled by its frictional strength. However, data on the orientation of maximum horizontal compression in the borehole from 1.75 to 3.5 km (N57oE + 19 o) indicate that the San Andreas must be quite weak as a complete absence of right-lateral shear stress resolved on planes parallel to the --N60oW striking San Andreas fault is observed. The lack of right-lateral shear stress on planes parallel to the San Andreas fault at this site is especially surprising as Cajon Pass is located along a section of the San Andreas which has not had a major earthquake since 1812 and is thus presumably quite "late" in the earthquake cycle. Nevertheless, both the orientation and magnitudes of stresses measured in the well are consistent with the style of active faulting in the area surrounding the drill site, most notably normal faulting and Quaternary age left-lateral slip on the Cleghorn fault that parallels the San Andreas in the vicinity of the drill site (Meisling and Weldon, 1982; Weldon, 1986; R. J. Weldon et al., unpublished report, 1981). We argue that the stress state (and Quaternary fault offsets) observed in the Cajon Pass area could exist only if the San Andreas moved at low shear stresses comparable to seismic stress drops rather than the much higher values predicted by Byerlee's law, a conclusion consistent with the lack of frictionally generated heat flow along the San Andreas system (e.g., Brune et al., 1969; Henyey and Wasserburg, 1971; Lachenbruch and Sass, 1973, 1980). Taken together, the Cajon Pass in situ stress and heat flow measurements (Lachenbruch and Sass, this issue) support a conceptual model of the San Andreas system in which the San Andreas is extremely weak with respect to the surrounding crust.