Abstract The 2008 Wenchuan M 8.0 earthquake induced local stress perturbations near the Longmenshan (LMS) fault zone. Using in situ measurements and stress tensor inversions from focal mechanisms, we investigated the post-earthquake perturbed stress field at shallow depths (≤1.2 km), in the upper crust above a basal attachment (∼ 5–18 km) as well as in the middle crust beneath a basal attachment (∼ 19–32 km). At shallow depths in the Beichuan-Jiangyou section, the maximum principal stress (σ1) orientations changed from a pre-earthquake orientation of NWW to a post-earthquake orientation of NEE. However, near the Baoxing-An'xian and Qingchuan-Mianxian sections, the pre-earthquake σ1 orientation of NW-NWW was largely unaltered. Before the mainshock, in the upper crust above the basal detachment, the entire LMS fault zone exhibited a consistent σ1 orientation of NW-NWW. However, after the earthquake, some sections, such as the Beichuan-Nanba section, achieved σ1 orientations of NEE that were distinctly different from the pre-seismic stress field. In the middle crust beneath the basal detachment, the stress field was mostly unchanged. The crust beneath the basal detachment, along the eastern margin of the Tibetan Plateau, is dominated by NWW-oriented maximum principle stress and strain directions and a thrust faulting stress regime along the entire LMS fault zone; these observations are representative of the southeastward motion of the Bayan Har block and its ongoing collision with the South China block along the LMS thrust belt. The spatially heterogeneous stress field that occurred in the upper crust (≤18 km) after the mainshock was most likely caused by the co-seismic static stress changes associated with the Wenchuan earthquake. Prior to the onset of seismic activity, the deviatoric stress magnitudes were estimated at less than 22 MPa and 4 MPa at depths of 10 km and 20 km, respectively, suggesting that the mainshock fault zone has a relatively low fault strength to overcome, in order to initiate significant co-seismic rupture and aftershocks, especially in the crust beneath the basal detachment. These smaller deviatoric magnitudes are indicative of the weaker fault materials that resides in the LMS region.

中文翻译：

---

青藏高原东缘龙门山断裂带附近2008年汶川8.0级地震局部应力扰动

摘要 2008年汶川8.0级地震在龙门山（LMS）断裂带附近引起了局部应力扰动。使用来自震源机制的原位测量和应力张量反演，我们研究了浅层（≤1.2 公里）、基底附着物上方的上地壳（~ 5-18 公里）以及地壳中的震后扰动应力场。基底附着物下方的中地壳（约 19-32 公里）。北川-江油剖面浅层最大主应力(σ1)取向由震前NWW取向转变为NEE震后取向。但在宝兴-安县和青川-绵县附近，NW-NWW震前σ1方位基本没有改变。在主震之前，在基底拆离上方的上地壳中，整个 LMS 断层带表现出一致的 NW-NWW σ1 取向。然而，震后北川—南坝等部分断面达到了与震前应力场明显不同的NEE σ1取向。在基底拆离下的中地壳，应力场基本没有变化。青藏高原东缘基底拆离下地壳以NWW向最大主应力应变方向为主，整个LMS断裂带为逆冲断层应力状态；这些观测结果代表了巴颜喀拉地块的东南运动及其沿 LMS 逆冲带与华南地块的持续碰撞。主震后上地壳（≤18 km）出现的空间非均质应力场很可能是由汶川地震引起的同震静应力变化引起的。在地震活动开始之前，估计在 10 公里和 20 公里深度处的偏应力大小分别小于 22 MPa 和 4 MPa，这表明主震断层带需要克服的断层强度相对较低，以便引发显着的同震破裂和余震，特别是在基底拆离下的地壳中。这些较小的偏角表明位于 LMS 区域的断层材料较弱。在 10 公里和 20 公里的深度处，偏应力大小估计分别小于 22 兆帕和 4 兆帕，这表明主震断层带需要克服的断层强度相对较低，以便引发显着的同震破裂和余震，特别是在基底拆离下的地壳中。这些较小的偏角表明位于 LMS 区域的断层材料较弱。在 10 公里和 20 公里的深度处，偏应力大小估计分别小于 22 兆帕和 4 兆帕，这表明主震断层带需要克服的断层强度相对较低，以便引发显着的同震破裂和余震，特别是在基底拆离下的地壳中。这些较小的偏角表明位于 LMS 区域的断层材料较弱。