Research Paper GEOSPHERE Geomorphic offsets along the creeping Laohu Shan section of the Haiyuan fault, northern Tibetan Plateau GEOSPHERE, v. 14, no. 3 Tao Chen1,2, Jing Liu-Zeng1, Yanxiu Shao1, Peizhen Zhang1,3, Michael E. Oskin4, Qiyun Lei1, and Zhanfei Li1 1State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing, 100029, China doi:10.1130/GES01561.1 2Department of Science and Technology, China Earthquake Administration, Beijing, 100036, China 3School of Earth Science and Geological Engineering, Sun Yan-Sen University, Guangzhou, 510275, China 11 figures; 3 tables; 1 set of supplemental files 4Department of Earth and Planetary Sciences, University of California–Davis, Davis, California 95616, USA CORRESPONDENCE: [email protected] ABSTRACT and varying over the seismic cycle (e.g., Peng and Gomberg, 2010; Barbot et al., 2012). It ranges from episodic offsets during earthquakes to continu- CITATION: Chen, T., Liu-Zeng, J., Shao, Y.X., Zhang, High-resolution topographic or imagery data effectively reveal geomorphic ous aseismic slip, or other modes in between, such as slow-slip events and P.Z, Oskin, M.E., Lei, Q.Y., and Li, Z.F., 2018, Geo- morphic offsets along the creeping Laohu Shan sec- offsets along faults that can be used to deduce slip-per-event of recurrent rup- low-frequency earthquakes. These modes of slip ultimately determine the seis- tion of the Haiyuan fault, northern Tibetan Plateau: ture events. Documentation of patterns of geomorphic offsets is scarce on mic potential of individual faults. Evidence for past coseismic offsets may help Geosphere, v. 14, no. 3, p. 1165–1186, https://doi.org faults that undergo both creep and coseismic rupture. In this paper, we used to identify multimodal slip (e.g., Lienkaemper et al., 2006; Barbot et al., 2012), /10.1130 /GES01561.1. newly acquired high-resolution light detection and ranging (LiDAR) data to improving our understanding of the spatial and temporal complexity of fault compile geomorphic offsets along the Laohu Shan section of the Haiyuan behavior, and contributing to the development of physics-based earthquake Science Editor: Raymond M. Russo Associate Editor: Colin Amos fault, in the northern Tibetan Plateau, where interferometric synthetic aperture hazard evaluation and forecasting techniques. radar (InSAR) data suggest creep presently occurs over a 35-km-long stretch Quantification of past fault slip events is usually based on reconstructing Received 22 May 2017 at a rate comparable to the long-term geological slip rate, despite evidence offset landforms along a fault trace, such as deflected stream channels, al- Revision received 14 January 2018 for past coseismic fault rupture. Numerous offset gullies identified using the luvial fans, and offset ridge lines (Sieh, 1978; McGill and Sieh, 1991). Recent Accepted 16 March 2018 Published online 17 April 2018 LiDAR data yield a range of offsets from less than 2 m up to 50 m. These offsets advances in remotely sensed high-resolution topography and optical imag- have well-separated probability density peaks at 2–3 m, ~7 m, and ~14 m, with ing have accelerated this approach. For example, light detection and ranging increments of 2–3 m, 4–6 m, and 5–7 m. The sequence of paleoseismic events (LiDAR) systems can acquire high-resolution (as high as centimeter-scale) along the Laohu Shan section indicates that the gullies with offsets of 2–3 m topographic data, enabling the detailed characterization of landforms and are likely related to surface rupture of the historical 1888 Jingtai earthquake, small displacements. Recent investigations have highlighted the scientific po- plus subsequent creep. Offset increments of 4–6 m and 5–7 m may represent tential of high-resolution topographic data sets for accurately documenting coseismic slip in past paleoseismic events plus creep during the interseismic past fault slip distribution, reconstructing slip history, and formulating earth- period. The creeping Laohu Shan section preserves numerous discrete cumu- quake recurrence models (Hudnut et al., 2002; Haugerud et al., 2003; Zielke et lative offsets, with an offset clustering pattern indistinguishable from that on al., 2010, 2012, 2015; Salisbury et al., 2012, 2015; Haddon et al., 2016). a locked fault with recurrent earthquake ruptures. Association of offset incre- Airborne LiDAR topographic data were acquired in 2011 along 130 km of ments with known paleoseismic events yields a slip rate of 3–5 mm/yr during the Haiyuan fault on the northern Tibetan Plateau to illuminate offset land- the past 200 years, roughly similar to the ~5 mm/yr creep rate. If the ratio of forms (Liu et al., 2013). This survey included both sections that ruptured in the OLD G surface creep rate to the total fault slip rate has been continuous, then seismic 1920 M ~8 Haiyuan earthquake to the east of Jingtai, and sections within the moment release by brittle ruptures, and thus seismic hazard, would be much Tianzhu seismic gap to the west of Jingtai (Fig. 1; Gaudemer et al., 1995). The reduced on the Laohu Shan section of the Haiyuan fault. Alternatively, the cur- survey point cloud had an average shot density of 5 points/m2 after filtering rent high creep rate may be a transient phenomenon, perhaps after slip follow- and classification, which was used to generate 1-m-resolution digital eleva- OPEN ACCESS ing the 2000 Jingtai Mw 5.6 earthquake or in response to the adjacent 1920 tion models (DEMs). Based on the LiDAR-derived digital model of the section M ~8 Haiyuan earthquake rupture that terminated immediately to the east. of the fault that ruptured during the 1920 M ~8 earthquake, Ren et al. (2016) documented the clustering of offsets and discussed the implications of this clustering for slip repetition and fault segmentation. INTRODUCTION Here, we present LiDAR-derived slip measurements along the Laohu Shan section to the west of Jingtai. This 55 km stretch of the Haiyuan fault includes This paper is published under the terms of the Slip accumulation along faults can be achieved through different fault a 30-km-long section adjacent to the western termination of the 1920 rupture, CC-BY-NC license. behavior modes, depending on the frictional properties of the fault plane, where Cavalié et al. (2008) and Jolivet et al. (2012, 2013) used interferometric © 2018 The Authors GEOSPHERE | Volume 14 | Number 3 Chen et al. | Geomorphic offsets along the Laohu Shan creeping fault 1165 Research Paper ( ( ( A ( ( ( ( ( ( ( ( " Wuwei 38°N ( ( ( ( ( ( ( ( ( ( 05/23/1927 ( ( ( ( ( ( ( ( ( ( ( M=8-8.3 ( ( ( Zhongwei QilianLLL Shan " ( " ( ( Gulang ( ( ( ( ( ( ( ( ( ( Z ( JQH 11/02/1888 hon ( ( gwe ( ( ( i ( - ( ( T ( ( ( ( M=6.2 ( o ( ( n ( ( ( ( ( ( g ( ( " xin ( MMS Jingtai ( fa ( ( ( u ( ( "( ( lt ( " ( (LHS( Ha 37°N Tianzhu Fig.2A iyu Tongxin 10/20/1990 an ( ( fau " Mw=5.8 lt 12/16/1920 ( ( M= ~8 Yongdeng 06/06/2000 Haiyuan ( XINING ( Mw=5.6 " ( ( ( ( ( ( r ( ( ( ( ( ( ( ( ( ( ( ( ( 80° 90° 100°( 110°E ( ( llow Rive ORDOS Ye ( ( ( ( B ( 40° Liupan Shan faul ( ( Fig.1A 36°N ( ( LANZHOU TARIM gh Fault Karakorum ( ( ( Haiyuan Faul Liupan Shan ( Altyn Ta ( ( Kunlun Fault ( ( ( t 35° ( ( ( ( Fault T I B E Xia ( ( nshu( i " T He ( ( Jiali-Red Rive Linxia ( ( ( Fault( System 30° ( ( SOUTH( Himalaya 0 50km CHINA ( ( n ( t r ( Fronta Faul ( st ( l Thru ( INDIA t 25°N 102°E103°E 104°E 105°E 106°E Faults Seismicity after 1970 Strike slip Surface ruptures of the 1888, 1920 &1927 EQs. Normal Lenglong Ling MMS Maomao Shan 2.0-2.9 3.0-3.9 4.0-4.9 5.0-5.9 6.0-6.9 Reverse LLL JQH Jinqiang He LHS Laohu Shan Inferred Figure 1. (A) Tectonic setting of the Haiyuan fault showing the location of major active faults and historical earthquakes (M >2) in adjacent regions. Surface ruptures associated with the 1920 and 1927 M ~8 earthquakes (EQs) are shown in orange, and the Tianzhu seismic gap is highlighted in red (after Gaudemer et al., 1995). (B) Distribution of active faults in and around the Tibetan Plateau. GEOSPHERE | Volume 14 | Number 3 Chen et al. | Geomorphic offsets along the Laohu Shan creeping fault 1166 Research Paper synthetic aperture radar (InSAR) to identify fault creep during 1995–1998 and ing fabric in trench exposures (Liu-Zeng et al., 2007). No significant surface during 2003–2009. The creeping behavior of the Haiyuan fault was unknown ruptures were reported for the 2000 Mw 5.6 earthquake (Rong et al., 2001). prior to its discovery via InSAR geodesy. The Tianzhu seismic gap is not associated with any large historical (M >7) Interrogation of airborne LiDAR-based DEMs generated 146 offset measure- earthquakes over the past few centuries, despite this area containing clear ments, among which 128 displacements were also identified and measured geomorphic evidence of Holocene seismicity (Gaudemer et al., 1995). Yuan et during field work in the same area. We used the resulting slip distribution data to al. (1994) excavated five trenches at several locations along the Laohu Shan reconstruct the most recent event, as well as earlier events, with timing from cor- fault (Fig. 3) and identified eight paleoearthquake events, suggesting recur- relations to previously published paleoseismic and historical records. Our offset rence interval of ~1000 yr, although the published trench logs are interpreta- measurements bear on the questions of (1) whether the present, rapid creep of tive sketches and are therefore difficult to evaluate. Excavations at a site west the Haiyuan fault is transient or persistent over the approximately thousand-year of that of Yuan et al. (1994) corroborated that the recurrence interval of large seismic scale, and (2) whether a partially creeping fault section can preserve cu- surface-rupturing events in this area is 800–1000 yr (Liu-Zeng et al., 2007). The mulative slip per earthquake in a fashion similar to that of locked fault sections.