خسارات غارسنگ مربوط به زمین لرزه
Abstract
1- Introduction
2- Geological setting
3- Methods and data
4- Field observations
5- Discussion
6- Conclusions
References
Abstract
A speleoseismological study has been conducted at over a dozen cave sites along the Longmen Shan fault zone of the eastern Qinghai-Tibet Plateau. The aim was to assess the damage inflicted on speleothems by the Mw 7.9 Wenchuan earthquake. Results show that the earthquake led to either partial or complete collapse of the caves. ‘Soda straws’ are shown to be particularly vulnerable to earthquake damage, but statistical analyses indicate that the spindle and slender shapes are also very likely to break. Cave depth is also shown to play an important role in the fracture development during the earthquake. The measured orientations of fallen stalactites are preferentially aligned to the coseismic surface offset peaks and therefore to the direction of earthquake wave propagation. Several such damaged speleothems resulting from sudden co-seismic movements were observed. The direction of ceiling (hanging wall) movement caused by the Wenchuan earthquake is NW-NNW, consistent with block motion on the footwall of the Yingxiu-Beichuan fault. We inferred that the faults in the caves were not co-seismic structures of the Wenchuan earthquake; but instead they are likely normal faults as the result of gravity creeping induced by the earthquake.
Introduction
Speleothems are cave deposits forming from dripping of mineral -rich water. They are quite common in karst landforms and include stalagmites, stalactites and flowstones , etc . Speleothems are usually regarded as good proxies for high -resolution palaeoclimatic reconstructions, in that carbon and oxygen isotopes preserved in speleothems document climatic oscillations throughout the Quaternary (Wang, 2001; Tan et al., 2006; Fairchild et al., 2006; Calaforra et al., 2008; Sorin et al., 2010) . In tectonically active regions , deposition of speleothems is sometimes interrupted by earthquake s , forming damaged speleothems that are usually called as speleoseismites. Therefore, speleothems have also been used for palaeoseismi c research (Forti, 2001; Lacave et al., 2004; Kagan et al., 2005) and a concise overview of this topic has been given by Forti and Postpichl (1984) and Becker et al., (2006) . Earthquake -related speleoseismites include deformed (broken or offset) (stalactites and stalagmites) (Lemeille et al., 1999; Delaby, 2001; Šebela, 2008) , irregular stalagmite growth (Postpischl et al., 1991; Forti, 2001; Shao et al., 2014; Rajendran et al., 2016) , cave sediment deformation, rock falls in cave s , fault displacement in cave walls and ceilings (Becker et al., 2006; Forti, 2007; Camelbeeck et al., 2012; Camelbeeck et al., 2018) , and the termination and re -initiation of stalagmite growth (Panno et al., 2009; Shao et al., 2014; Panno et al., 2016) .