Factors that affect coseismic folds in an overburden layer

doi:10.1007/s11707-016-0618-8

Front. Earth Sci.

2018, Vol. 12

Issue (1) : 17-23 https://doi.org/10.1007/s11707-016-0618-8

RESEARCH ARTICLE

Factors that affect coseismic folds in an overburden layer

Shaogang ZENG^1,², Yongen CAI¹(

)

¹. Department of Geophysics, Peking University, Beijing 100871, China
². Deep Water Exploration Group, Research Institute, CNOOC Nanhai East Petroleum Bureau, Guangzhou 510200, China

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Abstract

Coseismic folds induced by blind thrust faults have been observed in many earthquake zones, and they have received widespread attention from geologists and geophysicists. Numerous studies have been conducted regarding fold kinematics; however, few have studied fold dynamics quantitatively. In this paper, we establish a conceptual model with a thrust fault zone and tectonic stress load to study the factors that affect coseismic folds and their formation mechanisms using the finite element method. The numerical results show that the fault dip angle is a key factor that controls folding. The greater the dip angle is, the steeper the fold slope. The second most important factor is the overburden thickness. The thicker the overburden is, the more gradual the fold. In this case, folds are difficult to identify in field surveys. Therefore, if a fold can be easily identified with the naked eye, the overburden is likely shallow. The least important factors are the mechanical parameters of the overburden. The larger the Young’s modulus of the overburden is, the smaller the displacement of the fold and the fold slope. Strong horizontal compression and vertical extension in the overburden near the fault zone are the main mechanisms that form coseismic folds.

Keywords ground deformation coseismic fold blind thrust fault finite element method

Corresponding Author(s): Yongen CAI

Just Accepted Date: 28 November 2016 Online First Date: 20 December 2016 Issue Date: 23 January 2018

Cite this article:

Shaogang ZENG,Yongen CAI. Factors that affect coseismic folds in an overburden layer[J]. Front. Earth Sci., 2018, 12(1): 17-23.

URL:

https://academic.hep.com.cn/fesci/EN/10.1007/s11707-016-0618-8
https://academic.hep.com.cn/fesci/EN/Y2018/V12/I1/17

Fig.1 Sketch of the numerical model and seismic overburden fold.

Fig.2 Typical numerical results. (a)?(d) show the coseismic deformation and strain fields near the upper tip of the fault: (a) horizontal displacement field; (b) vertical displacement field; (c) and (d) horizontal and vertical strain fields (positive for extension), respectively; and (e) and (f) horizontal and vertical displacements at the ground surface, respectively. The dashed line represents the model without overburden, and the solid black line represents the model with 80 m of overburden.

Fig.3 Influence of overburden thickness on folding. (a) horizontal displacements; (b) vertical displacements.

Fig.4 Influence of mechanical parameters on overburden folds. (a) and (b) show the influence of Young’s modulus. (c) and (d) show the influence of Poisson’s ratio.

Fig.5 Influence of fault dip angle. (a) horizontal displacements; (b) vertical displacements.

Fig.6 Photographs of fault-related folding at the Wufeng excavation site. (a) The west-facing fault-related fold in the paddy field. (b) A cross-section of the Wufeng excavation site. The red dashed line is the main fault slip of the 1999 Chi-Chi earthquake (Mw=7.6) that occurred in Central Taiwan (Lee et al., 2001; Yang et al., 2014).

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[1]	Shaogang ZENG, Yong’en CAI. The ground deformation field induced by a listric thrust fault with an overburden soil layer[J]. Front Earth Sci, 2013, 7(4): 501-507.

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