Abstract:

The friction coefficient, a critical parameter governing fault sliding, plays an essential role in evaluating fault stability and assessing seismic hazards. Its accurate determination is a fundamental aspect of fault research, and traditional methods for obtaining this coefficient typically rely on invasive techniques such as scientific drilling or friction experiments. These conventional approaches are often costly, time-consuming, and limited by their ability to capture fault zone conditions in a comprehensive and non-invasive manner. In contrast, this study proposes a more straightforward and practical approach for estimating and constraining the sliding friction coefficient of the Longmenshan fault zone, utilizing statistical analysis of calcite mechanical twin crystal density as an indirect indicator of fault slip behavior.

Calcite mechanical twin crystals, which form as a result of faulting processes, are sensitive to the temperature and pressure conditions within the formation environment. The microstructural characteristics of these twin crystals record the maximum stress experienced by different parts of the fault zone, thereby providing a valuable record of fault activity. As a result, stress values near the fault sliding surface can be used to indirectly estimate the friction coefficient of the fault. To achieve this, high-definition images of calcite twin crystals were captured under an optical microscope, followed by processing to derive the twin crystal density values. These measurements were then used in conjunction with corrected stress meter results from prior research, which were obtained through triaxial compression and torsion experiments. The twin crystal density values were incorporated into calculations of the differential stress experienced by the fault zone, and, using the known relationship between differential stress and friction, the friction coefficient was derived. To ensure the accuracy of the results, stress values for the surrounding rock were provided through regional geological survey data.

The statistical analysis revealed that the twin crystal density within the sliding zone increased markedly as the distance from the sliding surface decreased. Specifically, the twin crystal density increased from (87.39±35)mm^-1 in sample DS-1 to (218.63±36)mm^-1 in the immediate vicinity of the sliding surface. The corresponding historical maximum differential stresses were calculated based on previous stress gauge corrections, yielding values of (182.28±25)MPa and (288.30±25)MPa, respectively. Additionally, the maximum burial depth of the exposed strata was estimated to be approximately 5km, which led to the determination of confining pressure (S_V-P₀) of 83.3MPa on the outcrop. Here, S_V represents the stress of the overlying rock layers, while P₀ denotes the pore pressure. Using these values, the friction coefficients of the Longmenshan fault zone during horizontal and reverse faulting were calculated to be 0.61 and 0.13, respectively. These calculated values correspond to the upper limit of the sliding friction coefficient for the fault zone.

This study focuses specifically on the analysis of type Ⅱ mechanical twin crystals of calcite found in the surface outcrops of the Longmenshan fault zone. A particular emphasis is placed on the statistical analysis of twin crystal density the calculation of differential stress and friction coefficients. By integrating previous geological survey data and research on tectonic stress and fault structure within the study area, the calculated friction coefficient was found to range from 0.13 to 0.61. These results are in close agreement with earlier experimental and observational studies, supporting the feasibility and reliability of the method used to estimate fault zone friction coefficients. Based on calcite mechanical twin density, this approach presents a viable, non-invasive, and effective means of estimating friction coefficients in fault zones, with significant implications for understanding fault behavior and seismic risk assessment.

Key words: Longmenshan fault zone, mechanical twinning of calcite, twins density, friction coefficient

摘要：

摩擦系数作为断层滑动的重要参数, 对断层稳定性和地震危险性评估具有指示意义, 摩擦系数的获取是断层研究的重要内容之一。文中通过统计龙门山断裂带地表断层岩中的方解石机械双晶密度, 估算了断层岩所承受的历史最大差应力, 进一步计算了断层的滑动摩擦系数。统计结果显示, 断层岩中的方解石双晶密度随着与滑动面距离的减小呈现明显的升高趋势, 在破碎带内为(87.39±35)mm^-1, 在滑动面附近为(218.63±36)mm^-1, 计算得到的差应力分别为(182.28±25)MPa和(288.30±25)MPa, 该特征表明断层滑动面附近持续承受差应力。用于双晶统计的样品采集于灰岩质断层岩中, 以往的地质调查资料显示这些断层岩的历史最大埋深约为5km。综合上述深度和差应力值, 以及前人对该区域构造应力的研究结果, 计算得到龙门山断裂带在水平和逆冲滑动时的稳态摩擦系数分别为0.61和0.13。上述计算采用的应力值为断层岩所承受的历史最大应力, 因而给出的摩擦系数为稳态滑动摩擦系数的上限。该结果与实验观察结果及科学钻探研究结果一致, 表明通过机械双晶估算断层稳态摩擦系数的方法具有可行性。

关键词: 龙门山断裂带, 方解石机械双晶, 双晶密度, 摩擦系数