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) .

A combination of Geological mapping and two-dimensional electrical resistivity (2D-ER) surveys was applied to study the Unilorin Dam and its environment. The purpose of the study was to investigate the dam for structural anomalies that may compromise the purpose and safety of the dam. Field equipment for the study comprised a SuperSting R8/IP Multi-Electrodes Resistivity Meter, 84 metallic electrodes and the accessories, compass clinometer, and portable GPS equipment. Geological data were processed and plotted to obtain a Geological Map and Rosette Diagram that were used for structural interpretations. The structures interpreted include an asymmetric fold and a strike-slip fault. The 2D resistivity data were processed and tomographically inverted to obtain the resistivity models of the subsurface around the dam. Interpretations of 2D resistivity models showed that the dam reservoir floor is underlain by competent basement rocks, however, the basement rock is weathered in some places. Patches of low resistivities structures interpreted as seepages, fractures and water-saturated cavity were delineated in different sections of the dam. The surface structural elements from geological study support the subsurface structures interpreted in the 2D resistivity models. Weathered structures, fractures, and seepage in the reservoir floor constitute areas of excessive water loss in the dam. The cavity delineated in the dam foundation is a potential threat to the dam’s safety. The dam section with cavity has been referred to structural engineers for detailed study.

Introduction

Safety assessment test on civil engineering constructions such as dams, tunnels, and bridges is a necessary endeavor to forestall sudden failure and collapse of engineering infrastructure. The mechanical properties of old dams that have outlived their life span, or new dams affected by natural hazards might have suffered degradation that can cause some structural damages that may not be obvious on the surface. If such damages are not detected by routine safety checks and addressed in good time, it may deteriorates and consequently lead to the failure of the dam. A dam is a barrier constructed to slow down, direct, or obstruct the free flow of water. The purpose of dam construction includes irrigation, supply of water for human and industry uses, hydro-electric power generation, control of flood, fish rearing, recreation, and navigation (Biswas and Charttergee, 1971; Plata and Iragüen, 1992). In addition to water scarcity, dam failure can lead to flooding and monumental loss of lives and properties. The old method of dam testing for structural damages include drilling and sample testing. Usually, 250–300 mm hole(s) is drilled in some parts of the dam and the samples collected are structurally examined and tested in the laboratory. Despite the cost and time required, drilling provide limited information on the possible structural anomalies/damages in dams (Kepler et al., 2000).