龙门山断裂带北东段青川断裂晚第四纪活动习性研究

The Longmenshan fault zone is slipping with a very low rate so that it was regarded as one of areas with low seismic risk before the Wenchuan earthquake. However, it was such a fault that hosted the unexpected Wenchuan Ms 8.0 earthquake and generated surface ruptures with a total length greater than 300 km implying its strong seismogenic ability. Studies after the earthquake have revealed that the static Coulomb stress perturbations triggered by the earthquake are obviously positive on both southwestern and northeastern segments of the Longmenshan fault zone and therefore the recurring times for them are advanced, respectively. Five years afterwards, the Lushan earthquake, which occurred to the immediate southwest of the Wenchuan earthquake ruptures, seems to confirm a triggering mechanism and implies a high seismic risk for the northeastern segment of the Longmenshan fault zone. Nevertheless, little attention has been paid to this fault segment and therefore it is so less studied that its rupture behavior remains debatable. Focusing on this problem, this study attempts to learn the rupture behaviors on the Qingchuan fault, the most significant fault in the northeastern segment of the Longmenshan fault zone, such as slip rates and paleoearthquake history, by carrying on high-accuracy topographic surveys of faulted landforms using differential GPS (RTK) and unmanned aerial vehicle (UAV) as well as paleoseismological trench excavations, combined with optically stimulated luminescence (OSL) and radiocarbon dating methods on the basis of detailed remote images interpretation and tectonic landforms mapping in the field. Such a study is expected to bring out great significance in learning the crustal deformation pattern and assessing the seismic risk on the northeastern segment of the Longmenshan fault zone.

由于龙门山断裂带的滑动速率非常低，因而在汶川地震前被认为是地震危险性较低的地区。然而正是这样一条断裂上却发生了汶川Ms 8.0级地震，并产生了超过300 km的地表破裂带，显示了很强的发震能力。震后的研究结果显示，地震在龙门山断裂带的南西段和北东段造成了应力增加，使各断裂段的发震时间都提前了。5年后发生在南西段上的芦山地震似乎证实了断裂带存在级联破裂的触发机制，也意味着其北东段可能具有较高的地震危险性。然而，该断裂段上开展的工作寥寥无几，研究程度也非常低，其活动习性也存在很大疑问。因此本项目拟选取该断裂段上最重要的青川断裂作为研究对象，以遥感影像解译与野外构造地貌填图为基础，利用差分GPS和无人机进行高精度断错地貌测量，并开展古地震探槽开挖，结合光释光和放射性碳等测年手段，精细地研究其滑动速率及古地震历史等活动性参数，为了解龙门山断裂带北东段的地壳变形模式和评价地震危险性提供可靠的依据。

青川断裂位于龙门山断裂带北东段，曾被认为是一条不活动的断裂，因而长期被忽视。然而汶川地震后开展的古地震探槽工作显示，青川断裂全新世以来发生过破裂地表的大地震，是全新世活动断裂。但青川断裂上开展的研究工作很少，对断裂的运动方式及滑动速率等活动性参数仍知之甚少，给了解断裂的活动习性带来了较大的不确定性。为解决这一问题，在遥感影像解译和野外构造地貌填图的基础上，本研究利用无人机高精度测量了青川断裂上断错地貌的位移量，结合光释光等方法测定的断错地貌面形成年龄，首次获得了断裂可靠的晚更新世右旋滑动速率为≥0.65 mm/yr。此外，通过对断层滑动面上擦痕的调查发现，青川断裂在中南段的运动方式为右旋走滑兼具少量逆冲分量，在北东段则表现为右旋走滑兼具正断分量，且正断分量向北东方向逐渐增加。青川断裂在不同段落运动方式的差异可能与断裂北西盘碧口地块的南西端受到青藏高原向东扩展的挤压运动以及在北东端受到汉中盆地拉张活动的影响有关。