Multiple faulting events revealed by trench analysis of the seismogenic structure of the 1976 Ms7.1 Luanxian earthquake, Tangshan Region, China

Accepted Manuscript Full length article Multiple faulting events revealed by trench analysis of the seismogenic structure of the 1976 Ms7.1 Luanxian earthquake, Tangshan Region, China Hui Guo, Wali Jiang, Xinsheng Xie PII: DOI: Reference:

S1367-9120(17)30310-3 http://dx.doi.org/10.1016/j.jseaes.2017.06.004 JAES 3110

To appear in:

Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

27 November 2016 1 June 2017 7 June 2017

Please cite this article as: Guo, H., Jiang, W., Xie, X., Multiple faulting events revealed by trench analysis of the seismogenic structure of the 1976 Ms7.1 Luanxian earthquake, Tangshan Region, China, Journal of Asian Earth Sciences (2017), doi: http://dx.doi.org/10.1016/j.jseaes.2017.06.004

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Multiple faulting events revealed by trench analysis of the seismogenic structure of the 1976 Ms7.1 Luanxian earthquake, Tangshan Region, China Hui Guo a, Wali Jiang a *, Xinsheng Xie a a

Institute of Crustal Dynamics, China Earthquake Administration, Beijing, China

Abstract The Ms7.8 Tangshan earthquake occurred on 28 July 1976 at 03:42 CST. Approximately 15 hours later, the Ms7.1 Luanxian earthquake occurred approximately 40 km northeast of the main shock. The two earthquakes formed different surface rupture zones. The surface rupture of the Tangshan earthquake was NNE-trending and more than 47 km long. The surface rupture of the Luanxian earthquake was more than 6 km long and consisted of two sections, forming a protruding arc to the west. The north and south sections were NE- and NW-trending and 2 km and 4 km long, respectively. A trench was excavated in Sanshanyuan Village across the NE-trending rupture of the Luanxian earthquake, at the macroscopic epicenter of the Luanxian earthquake. Analysis of this trench revealed that the surface rupture is connected to the underground active fault. The following major conclusions regarding Late Quaternary fault activity have been reached. 1) The Sanshanyuan trench indicated that its fault planes trend NE30° and dip SE or NW at angles of approximately 69 to 82°. 2) The fault experienced four faulting events prior to the Luanxian earthquake at <9.73 ka, 18.42–19.02 ka, 22.73–23.79 ka, and >27.98 ka with an average recurrence interval of approximately 7.5 ka. 3) The Ms7.1 Luanxian earthquake resulted from the activity of the Luanxian Western fault and was triggered by the Ms7.8 Tangshan earthquake. The seismogenic faults of the 1976 Ms7.1 Luanxian earthquake and the 1976 Ms7.8 Tangshan earthquake are not the same fault. This example of an M7 earthquake triggered by a nearly M8 earthquake after more than 10 hours on a nearby fault is a worthy topic of research for the future prediction of strong 1

earthquakes. Keywords: earthquake surface rupture zone; trench; paleoearthquake; faulting events; active fault 1. Introduction The Ms7.8 Tangshan earthquake occurred in China at 03:42 CST on 28 July 1976. On the same day, the Ms7.1 Luanxian earthquake occurred approximately 40 km to the northeast of the main shock at 18:45 CST (Figs. 1a, b, c). The epicenter of the Ms7.8 Tangshan earthquake was located in Lunan District, Tangshan City. It featured a meizoseismal area intensity of XI, a long axis of IX–XI intensity that trended to the NE, and a focal depth of 11 km. The epicenter of the Ms7.1 Luanxian earthquake was located in Sanshanyuan Village, Angezhuang Town, Luanxian County, with an epicentral intensity of IX. The intensity IX isoseismal trended N–S and was superposed on the northeast end of the intensity VIII isoseismal of the Ms7.8 Tangshan earthquake. The focal depth of the latter earthquake was 10 km. The interpreted sequence types of these two strong earthquakes differ among researchers. Some argued that the 1976 Ms7.8 Tangshan earthquake and its aftershocks, which include the Luanxian Ms7.1 earthquake, are characterized by main shock and seismic swarm types (Wu et al., 1982). Others reported that the Luanxian earthquake was an aftershock of the Tangshan earthquake (Lu et al., 1985). Still others argued that the seismogenic fault of the Luanxian earthquake is NWW-trending, according to high-precision positioning of the seismic sequence, and thus that these two strong earthquakes have also been reported as independent main shocks (Liu and Lv, 2011). Among the strong earthquakes that occurred in mainland China during the 20th century, both the 1920 M8.5 Haiyuan earthquake and the 1931 M8.0 Fuyun earthquake had strong aftershocks of M7.0 and M71/4, respectively, following the main earthquakes. 2

The time intervals between the two mainshocks and their corresponding aftershocks were nine days and seven days, respectively. In the 1976 Tangshan earthquake series, the Ms7.1 Luanxian earthquake occurred 15 hours after the Ms7.8Tangshan earthquake. It is critical to determine whether or not the mainshock and aftershock had the same seismogenic structure. 2. Seismogenic Structures of the Tangshan Ms 7.8 and Luanxian Ms 7.1 Earthquakes Since the Tangshan earthquake occurred in 1976, the results of studies of ground deformation, seismic data and surface rupture zones have been used to invert the seismogenic structures of these two strong earthquakes. Scholars reached a consensus about the seismogenic structure of the Ms7.8 Tangshan earthquake. The source rupture strikes NE30°–49° with a dip of 76°–90°, a length of 84–140 km, a maximum right-lateral strike slip of 1.36–4.59 m, and a vertical displacement of 0.5–0.7 m (Chen et al., 1979; Butler et al., 1979; Li et al., 1980; Zhang et al., 1980; Huang, 1981; Zhang et al., 1981; Xie, 1984). However, the seismogenic structure of the Ms7.1 Luanxian earthquake has been interpreted differently by various researchers. Zhang et al. (1980) and Butler et al. (1979) reported that the seismogenic fault trends NE25° or NW60°, with a maximum displacement of 0.6 m. However, other researchers determined that the focal mechanism of the Luanxian earthquake is mostly a normal fault on an EW-trending focal plane (Shedlock et al., 1987; Nabelek et al., 1987; Xie and Yao, 1991; Huang and Yeh, 1997). Many researchers have carried out field investigations to obtain valuable primary data about the surface rupture zones of the Ms7.8 Tangshan earthquake and the Ms7.1 Luanxian earthquake. The surface rupture zone of the Ms7.8 Tangshan earthquake started in the southern region of Tangshan city and extended southwestward. This rupture was 3

approximately 8–11 km long with a right-lateral strike slip of 1.5–2.3 m and a vertical displacement of 0.2–0.7 m (Guo et al., 1977; Yang, 1982; Wang et al., 1981b; Du et al., 1985). Recent studies performed using trench excavation and a composite borehole profile across the rupture of the Tangshan earthquake indicate that the surface rupture zone began in Tangshan city and extended southward to Xihe town, Fengnan County with a total length of more than 47 km (Guo et al., 2011a). The surface rupture zone of the Ms7.1 Luanxian earthquake consists of north and south sections and comprises an arc protruding to the west with a total length of more than 6 km (Yang, 1982; Wang et al., 1981b). The north section, which is trending NNE, is more than 2 km long. The south section is trending NNW, is more than 4 km long and is dominated by a right-lateral strike slip of 0.10–0.25 m. Different theories have been proposed about the activity style of the north section. Du et al. (1985) argued that the left-lateral strike slip is 0.10–0.39 m with a vertical displacement of 0.4 m; however, Wang et al. (1981a) and Yang (1982) reported that the right-lateral strike slip is 0.05–0.10 m with a vertical displacement of 0.3–1.0 m. Sanshanyuan Village, which is located in the bend of the rupture, was the macroscopic epicenter of the Luanxian earthquake. After the Tangshan earthquake in 1976, some small trenches were excavated across the surface rupture zone of the Ms7.8 Tangshan earthquake, which revealed the occurrence of earlier liquefaction phenomena and earlier periods of offset under the surface rupture in the Tangshan area (Fang et al., 1981; Wang and Li, 1984). In recent years, new information about the seismogenic structure of the Ms7.8 Tangshan earthquake has been obtained. Data from the Sunjialou trench in Tangshan city and composite borehole profiles indicate that multiple faulting events occurred on the Tangshan Fault in the Late Quaternary with a recurrence interval of approximately 6.7–10.8 ka (Guo et al., 2011a). Borehole drillings and trench excavations performed as 4

part of the Tangshan city active fault detection project indicate that there was a recurrence interval of approximately 6.9–15.2 ka during the active period on the Tangshan Fault (Liu et al., 2013). Shallow seismic soundings indicate that this fault has broken Pleistocene–Holocene strata (Li et al., 1998; Hao and You, 2001; You et al., 2002). Moreover, studies have also focused on problems related to the seismogenic fault of the Tangshan earthquake (Qiu et al., 2005; Jiang, 2006; Wen and Ma, 2006). To date, little research has been performed on the surface rupture zone and the seismogenic structure of the Ms7.1 Luanxian earthquake. In this study, in order to investigate the surface rupture zone of the Ms7.1 Luanxian earthquake, a trench excavation is conducted in Sanshanyuan village, which was the macroscopic epicenter of the Ms7.1 Luanxian earthquake. 3. Historical Earthquakes and Geological Structure Characteristics in the Luanxian Area 3.1. Historical earthquakes According to the historical record, Luanxian is the most active area in the Jidong region of North China. In contrast to its surrounding areas, the largest earthquake that occurred in Tangshan city prior to 1976 was the 1935 M43/4 earthquake. However, the Luanxian area also experienced two M61/4 earthquakes, which occurred on 17 April 1624 and 23 September 1945 (Gu, 1983). Macroscopic investigation of the 1945 M61/4 earthquake showed that the long axis of the meizoseismal area was NNW-trending with intensity VIII. The macroscopic epicenter was located near Xinglong Village (Seismo-geological Brigade, 1970; Gu, 1983) (Fig. 2a). Therefore, prior to the 1976 Ms7.8 Tangshan earthquake, seismic activity was more prominent in the Luanxian area than it was in the Tangshan area in the Jidong region of North China. 5

3.2. Geological structure characteristics The Tangshan-Luanxian area is situated at the transitional area between the southern margin of Yanshan Mountain and the North China Plain. The most prominent geomorphic feature in this region is the distribution of bedrock relict hills that contain information about tectonic activity that occurred in this region during the late Cenozoic. Previous research has shown that after the 1976 Ms7.8 Tangshan earthquake, Late Quaternary active faults were distributed on both sides of the Weishan-Changshan relict hill located in Tangshan city and its surrounding region. This NNE-trending bedrock relict hill is located at the overturned western limb of the Kaiping synclinorium, which is composed of Mesozoic strata, in the Tangshan area (Fig. 2a). The location of the thrust fault V in the overturned western limb of the synclinorium coincides with the surface rupture zone of the 1976 Ms7.8 Tangshan earthquake (Guo et al., 2011b). Deep exploration data show that the Moho is displaced 2–3 km under Tangshan city (Guo et al., 1977; Liu et al., 1982). The epicenter of the 1976 Ms7.1 Luanxian earthquake was located at the northeast end of the Kaiping synclinorium. Fig. 2a shows that the Sinian strata at the northeast end of this synclinorium have become part of the NW-trending structure as a result of the NW-trending fault near the Luanxian area. It has been speculated that three parallel NW-trending faults occur in the bedrock and extend from Luanxian to Gumazhuang Village. Within these geomorphological and geological structures, in the Luan River Plain in the western region of Luanxian, there is an obvious lineament of NW-trending beaded bedrock relict hills, extending approximately 14 km, which corresponds to the NE-trending bedrock relict hills in Tangshan city (Fig. 3a). This NW-trending relict hill is composed of NW- or NE-striking Cambrian, Sinian, and Archean strata; NW-trending faults are exposed in the relict hill. 6

Before the 1976 Luanxian earthquake, due to the activity of historical earthquakes in the Luanxian area, seismic geologists paid great attention to assessing the strong risk of future earthquakes in the Luanxian area. In the geological map, the NW-trending bedrock relict hills and the fault are distributed; however, there is a lack of Late Quaternary activity of this fault that controls these bedrock relict hills. 3.3. Late Quaternary sedimentary environment The Tangshan-Luanxian region is situated in the alluvial plain of the southern margin of Yanshan Mountain. The Luan River System is the second largest river system in Hebei Province and the largest in Jidong Plain. This river begins in the vicinity of Zhangjiakou city, which is located in the northwestern region of Beijing, winds along the Yanshan Mountain area, and flows through Jidong Plain and into the Bohai Sea. The rivers in this region, from west to east, include the Huanxiang, Dou, Sha, and Qinglong Rivers. These rivers are the paleochannels of the Luan River, which formed the alluvial-proluvial plain (Liu, 1983). Fig. 2b outlines the main features of the Late Cenozoic rock in the Tangshan-Luanxian region (Liu et al., 1982), including the distribution of the bedrock relict hills on the southern margin of Yanshan Mountain, the Quaternary isopach, and the three Late Quaternary pluvial fans in the Luan River alluvial plain. In this figure, the 28 July 1976 Ms7.8 Tangshan earthquake and the Ms7.1 Luanxian earthquake are located in front of the NNE- and NW-trending bedrock relict hills of the southern margin of Yanshan Mountain, at the western and eastern margins of the Luan River alluvial fan, respectively. This alluvial fan formed during the last stage of Late Pleistocene. A better understanding of the Quaternary sedimentary environment in the Tangshan-Luanxian region can help determine the sedimentary characteristics of the 7

strata revealed by trench excavation and provide a qualitative constraint on their stratigraphic age. 4. Strata Characteristics of the Sanshanyuan Trench 4.1. Choice of trench location Before performing the excavation of the Sanshanyuan trench in Luanxian County, we tracked and surveyed the previously reported surface rupture of the 1976 Ms7.1 Luanxian earthquake (Fig. 3a). Due to minor earthquake displacement and changes caused by human activities in recent decades, earthquake relics cannot be currently identified at the surface. In this study, it was difficult to choose the location of the trench across the rupture of the 1976 Luanxian earthquake, particularly because it was challenging to corroborate the NW-trending rupture. Based on the rupture map previously investigated at a scale of 1:1000 (Du et al., 1980), identification by local villagers, and data gained from early drilling detection and small trenches during this project, we ultimately determined the position of the Sanshanyuan trench. Fig. 3b shows a photograph of the NE-trending surface rupture in the northern region of Sanshanyuan village that was captured after the 1976 Ms7.1 Luanxian earthquake (Shaanxi Bureau of Surveying and Mapping, 1981). The Sanshanyuan trench is located approximately 50 m north of Sanshanyuan Village, Angezhuang Town, Luanxian County. The trench crosses the NNE-trending rupture of the Luanxian earthquake (Figs. 3c, d). The NNE-trending bedrock relict hill lies approximately 30 m east of the trench, and the terrain is high in the east and low in the west. The trench trends NW40° and is 90 m long; it has a depth of 7.5 m near the fault plane and an average depth of 6 m. The observed faulting phenomena are concentrated within a 20-m-long section at the southeast end of the trench. Based on the structural phenomena exposed by the trench, 8

we determined the section of the northeast wall at the southeast end of trench to a scale of 1:25 (Fig. 4). The entire section of the southwest wall, except for the southeast end, was cleaned into 13 columnar strips, and the lithologies and boundaries of the strata on each strip were marked. This section of the southwest wall was plotted at a scale of 1:50 (Fig. 5). During its excavation, the trench collapsed near the fault plane at the throw side of the northeast wall (Fig. 6a), likely because the stratum lithology of the throw side is mainly silty sand and fine sand. The length of the collapsed body was approximately 12 m. Because the collapsed body slipped down as a whole unit, the strata were not disturbed. The strata that were covered by the collapse body can also be identified; the boundaries of these strata were constrained by observations of the normal strata at the two ends of the collapse body. 4.2. Strata and samples dating of the trench This trench comprises 17 strata. The uplift side is mainly composed of brown and brick-red clay, whereas the throw side is mainly composed of silty sand and fine sand (Fig. 4). These strata belong to the alluvial fan of the Luan River sedimentary system. The two sections of the trench have the same strata and number of strata (Figs. 4, 5). Three samples were analyzed for their 14C ages by the Beta Analytic Radiocarbon Dating Laboratory, Miami, Florida, USA. Eighteen samples were analyzed for their thermoluminescence (TL) ages by the Geologic Chronology Laboratory of the Institute of Crustal Dynamics, China Earthquake Administration (CEA). The age data and characteristics of this trench are listed in Table 1. The stratigraphic sequence of the Sanshanyuan trench is described, from top to bottom, as follows: Layer 1. Cultivated soil and brown clay with plant roots. This layer is approximately 9

0.2 m thick. Layer 2-1. Brown loam and yellow sandy loam mixture. This layer is approximately 0.6 m thick. Layer 2-2. Brown loam with scattered manganese particles. This layer is approximately 0.8 m thick. Layer 3. Yellow and khaki sandy loam mixture with brown clay lumps, white fine sand strips 1–2 cm wide and 10–20 cm long, and wavy strips of khaki sandy loam. The age determined from sample TL-7 in this layer is 9.73 ± 0.83 ka. This layer is approximately 1.1 m thick. Layer 4-1. Yellow silty sand and white fine sand mixture with sand liquefaction. This layer is approximately 0.2 m thick. Layer 4-2. Brown loam lumps and yellow silty sand mixture, composed mainly of loam lumps. This layer is approximately 0.36 m thick. Layer 5. Pure white fine-grained sand with horizontal bedding. The age obtained from sample TL-3 is 11.68 ± 0.99 ka. This layer is approximately 0.36 m thick. Layer 6. Khaki silty sand with a thin layer of grayish-white fine sand that is 0.5–1 cm thick. The entire layer is approximately 0.36 m thick and contains horizontal bedding. Layer 7. Grayish-brown sandy loam. The ages yielded from samples 14C-2 and TL-2 are 11.97 ± 0.04 ka and 12.01 ± 1.02 ka, respectively. This layer is approximately 0.5 m thick. Layer 8. Pure white fine sand with horizontal bedding and mixed yellow fine sand layers that are 1–2 cm thick. In the SW section of this trench (Fig. 5a), this layer is mixed with two lens-shaped, gray, fine sand bodies with nearly horizontal layers that are named Layer A1 and A2. Five TL dating samples were obtained from this layer. In the NE section, the measured ages of TL-8 (from the top of the layer) and TL-6 (from the bottom of the 10

layer) are 15.94 ± 1.35 ka and 18.42 ± 1.56 ka, respectively. In the SW section, the age of sample TL-13, which was collected 42.6 m from the southeast end of the trench, is 20.01 ± 1.70 ka. Approximately 70 m from the southeast end of the trench, the measured ages of TL-14 (from the top of the layer) and TL-15 (from the bottom of the layer) are 20.55 ± 1.75 ka and 22.52 ± 1.91 ka, respectively. This layer is approximately 1.42 m thick. Layer 9. Khaki silt. The measured ages of samples 14C-1 and TL-1 are 20.04 ± 0.07 ka and 20.73 ± 1.76 ka, respectively. This layer is approximately 0.43 m thick. Layer 10. Pure white fine sand with horizontal bedding. Three samples were obtained from this layer. In the NE section of this trench, the dating results of sample TL-5 (collected from the top of the layer) and TL-9 (collected from the bottom of the layer) are 20.35 ± 1.73 ka and 20.58 ± 1.78 ka, respectively. In the SW section, the age of sample TL-12, which was collected 20 m from the southeast end of the trench, is 22.22 ± 1.89 ka. The thickness of this layer is approximately 2.3 m. Layer 11. Dark-brown clay mixed with a small amount of yellow sandy loam lumps. A large (small) quantity of manganese particles is present in the top (bottom), with cyan clay strips and manganese strips occurring at a distance of 4 m from the fault plane. The measured age of sample TL-8 is 19.02 ± 1.62 ka. Two cracks, named Layer B1 and B2, are also present. The B1 crack is mainly composed of white fine sand with yellow sandy loam. The B2 crack is filled with yellow sandy loam mixed with brown clay lumps containing black manganese particles. The measured age of sample TL-16, which was collected from the B2 crack, is 20.34 ± 1.73 ka. The thickness of this layer is approximately 2.9 m. Layer 12. Gray muddy clay. The measured age of sample

14

C-3 is 22.73 ± 0.08 ka.

The maximum thickness of this layer is 0.43 m. Layer 13. Dark brown and brick-red clay mixture which includes a colluvial wedge 11

named Layer C, with minor X-shaped and approximately 1-cm-wide sand veins near the fault plane. The measured age of sample TL-10 is 22.11 ± 1.88 ka. This layer is approximately 0.36 m thick. Layer 14. Pure white fine sand with horizontal bedding. The measured age of sample TL-11 (collected from the top of this layer) is 23.79 ± 2.02 ka. The age determined from sample TL-18 (collected from the bottom of this layer) is 27.98 ± 2.38 ka. The thickness of this layer is approximately 1.14 m. Layer 15. Dark-brown and brick-red clay mixture including a colluvial wedge named Layer D with small stones and sand veins 10 cm wide near and subparallel to the fault plane. The thickness of this layer is approximately 0.78–1.0 m. Layer 16. Pure white fine sand with horizontal bedding. The visible thickness of this layer is approximately 0.2 m. Layer 17. Brick-red clay with a large quantity of manganese particles, mixed with dark brown clay lumps. This layer contains a small amount of sandstone and some small stones at the bottom. The measured age of sample TL-17 (collected from the top of this layer) is 71.00 ± 6.03 ka. The visible thickness of this layer is approximately 2.4 m. 4.3. Discussion of characteristics of strata and dating results 1) The strata located 0.4–2.5 m beneath the surface revealed by the trench are Holocene strata. Because the strata located between the surface and a depth of 0.4 m had been disturbed by modern-day farming,

the displacement caused by the Ms7.1

Luanxian earthquake was not recorded. 2) This trench contains two sets of strata. Most strata are silty sand with clear bedding that partially contain sandy loam. The southeast end of the trench exposes massive dark-brown and brick-red clay. The two sets of strata are alluvial plain sediments. The silty sand strata are located near the river channel, and the clay strata are still-water 12

sediments located near the margin of the alluvial fan. In this section of the trench, these two sets of sediments are in direct contact with the two sides of the fault plane (Figs. 6a, b, c). 3) Beneath the surface, at depths ranging from 0.4 m to 5.4 m, most strata are of the late stage of Late Pleistocene age. Seventeen measured ages were obtained from these strata. Late Holocene strata, which are 2.1 m thick and have been disturbed by earthquakes, occur at the southeast end of the trench at depths below 0.4 m. Beneath this layer is the late stage of Late Pleistocene clay layer, 2.9-m-thick, which overlies the early Late Pleistocene brick-red clay. One measured age was obtained from each of the two sets of strata. The dating results are consistent with those presented in previous studies, which indicate that the strata of this region represent a late-stage Late Pleistocene alluvial fan of the Luan River (Fig. 2b). 4) In the trench, the lithologies of the strata on the two sides of the fault plane are different. This strata characteristic is more likely to occur in strike-slip faults. 5) Due to the activity of the fault, colluvial or filling cracks occur on one side of the fault while the materials of the cracks appear to have originated from the other side of the fault. For example, the silty sand located in the B1 and B2 cracks in Layer 11 in the southeast wall is from the northwest wall of the fault. In addition, the mixed clay of the C and D colluvial wedges in Layers 13 and 15 of the northwest wall came from Layers 11 and 17, which are located in the southeast wall of the fault. 6) The 21 dated samples from this trench section generally record the ages of the strata. Most of the measured ages are consistent with each other, except for a few unreasonable dating results, which were noted. For example, the deposition rate of the fine sand can be determined based on the measured ages of the tops and bottoms of Layers 8, 14, and 10. The calculated deposition rate of Layer 10 is a hundred times that 13

of Layer 8 and Layer 14. We therefore suspect that the measured age from the bottom of Layer 10 is incorrect. Nonetheless, because the measured 14C ages are relatively credible, those obtained from Layers 7, 9, and 12 can give credence to the TL ages determined from the samples in this trench. The reliability of the age of these strata has thus been improved. 5. Multiple Faulting Events Revealed by the Sanshanyuan Trench in Luanxian County 5.1. Structural phenomena revealed by the trench The structural phenomena revealed by the Sanshanyuan trench are as follows: 1) At the southeast end of the trench, both on the two walls, the direct contact between clay and sand strata record evidence of faulting. The attitude of the thrust fault in the northeast wall is trending NE30° and mainly dipping SE at angles of 65–82° (Fig. 4). The fault in the southwest wall is a normal fault dipping NW (Fig. 5c). In the NE section of the trench, the fault plane extends from the lower section upwards to the center of Layer 2-1; it has broken all of the strata except for the present cultivated soil (Figs. 4, 7a). The fault plane also extends to Layer 2 in the SW section (Fig. 5). Layers 3, 11, and 17, which are located on the southeast side of the fault plane, are in direct contact with Layers 5 to 16 on the northwest side of the fault plane. The southeast side of the fault is uplifted and the northwest side is subsided. 2) The dragging phenomenon recorded in the strata, which is related to fault activity, is well displayed in the two walls of the trench (Figs. 7e, f). In Layer 10, the horizontal sand bed located within approximately 1 m of the fault plane on the northwest side of the fault is depressed downward and then ascends near the fault plane. At the northeast wall of the trench, the largest sloping angle of the sand bed is 50°. The angle between the deformed sand bed and the fault plane is acute (Fig. 7e). All of those phenomena 14

represent the complex pressing and dragging that occurred when the upthrow side was uplifted. 3) On the northeast wall of the trench, the two cracks, Layers B 1 and B2 occur in Layer 11 on the upthrow side of fault (Fig. 6b). The cracks are composed of a white sand and yellow sandy loam mixture and a yellow sandy loam and brown clay mixture, respectively. The shapes of those two cracks look like inverted trapezia and pinch out at the bottom. These characteristics indicate that these cracks are different from the sand veins that were formed by the liquefaction of sand. 4) On both walls of the trench, Layer 12 exists beneath Layer 10. Layer 12 is composed of a 3.7-m-long gray, muddy loam layer that pinches out at both ends, with a maximum thickness of 0.43 m. It was pierced by sand veins during a later period. Layer 13 is a brown and brick-red clay mixture marked by Layer C, 0.36 m thickness, which extends to a length of 2.1 m. “X”-shaped sand veins also occur in this mixed layer (Figs. 7b, d) which indicated the strata near the fault plane were influenced by sand liquefaction. Underlying Layer 13 is Layer 14, which is the material source of the sand veins and is composed of white fine sand. The sphenoid shape, mixed lithology and material source of the Layer 13 represent the characteristics of a colluvial wedge. Layer 12 records siltation features after the colluvial wedge formed. 5) Beneath Layer 14 is Layer 15, which is a brown and brick-red clay mixture that is marked as Layer D and is 0.78–1.0 m thick (Fig. 7c). This layer is mixed with small amounts of yellow sandy loam and sand veins. The lithology of Layer 15 records colluvial characteristics. 6) Layer 6 and Layer 9 have different gradients of 3°–2° and 7°–3°, respectively, along the 90-m-long trench. The dip angles of these two layers are slightly larger within a distance of 20 m from the fault plane and decrease farther away from the fault plane. In 15

the southwest section of the trench, Layer 8 is 0.7 m thick near the fault plane and 2.7 m thick at the northwest end of the trench, thus displaying a difference of 2 m. Two gray, fine sand lens-shaped layers, A1 and A2, mixed in Layer 8, correspond to the different gradients of Layers 6 and 9 (Fig. 5a), respectively, as well as the varied faulting activities of these two layers. 7) In the trench sections, a sand vein rises to the surface at a distance of 20 m from the west side of the fault plane. This phenomenon is a relic of the 1976 Luanxian earthquake. 8) The plane view of the fault between the two walls of the trench reveals a wavy and uneven fault plane (Figs. 8a, b). Some interbedded clay strips and sand veins that both are 1–2 mm wide occur near the fault plane. In the white sand layer, an oblique grayish-black fine sand bed intersected the fault plane, indicating that sinistral slip has occurred (Fig. 8b). 9) The fault plane is dipping mainly to the SE in the northeast wall and to the NW in the southwest wall of the trench. Across the 7–8 m width of the trench, the dipping of the fault plane changes, thus indicating the strike-slip features of the fault. We collected part of the section of the Sanshanyuan trench for preservation, and it is displayed in the hall of our institute (Figs. 8c, d). 5.2. Events and time of faulting Synthetic analysis of the structural phenomena of the Sanshanyuan trench sections revealed five faulting events. Here, we analyze each faulting event from newest to oldest and discuss the timing of the fault activity based on the measured ages of the samples. 1) The youngest faulting event is the 1976 Ms7.1 Luanxian earthquake (E1), which formed the surface rupture and scarps. The surface rupture is up on the east side and down on the west side and is accompanied by left-lateral strike slip displacement. The 16

fault plane extended to the center of Layer 2. Due to the disturbance to surface strata by farming in recent decades, the vertical displacement in the earth’s surface is unclear. The displacements of the bottom of Layer 2-1 in the NE wall and the bottom of Layer 2 in the SW wall, which were formed by the Luanxian earthquake, are approximately 25 cm and 50 cm, respectively. 2) The second event (E2) is the faulting contact between Layer 3 and Layers 5 through 8. The disturbance in Layers 3 and 4 could also have been affected by this earthquake activity. This event occurred after the formation of Layers 3 and 5. The TL age of Layer 3 is 9.73 ka. The ages determined by TL dating in Layers 5, 7, and 8 are 11.68 ka, 12.01 ka, and 15.94 ka, respectively, and the measured

14

C age of Layer 7 is

11.97 ka. Thus, based on the youngest age of the strata, this event (E2) occurred after 9.73 ka. 3) The third event (E3) is revealed by the presence of cracks B1 and B2 in Layer 11 and the dragging phenomena observed near the fault plane in Layer 10. Before this event occurred, the top of Layer 11 was denuded. The cracks consist of mixed silt, sandy loam, and clay lumps from Layers 9, 10 and 11. The aforementioned dragging phenomena observed in Layer 10 near the west side of the fault plane are believed to have been caused by this event. The TL ages of crack B2, Layer 9, and Layer 10 are 20.34 ka, 20.73 ka, and 20.35 ka, respectively. The measured

14

C-1 age of Layer 9 is 20.04 ka, the TL-8

age of Layer 11 is 19.02 ka, and the TL-6 age of the accumulation Layer 8 is 18.42 ka. Thus, based on the youngest age of the faulting layer and the oldest age of the accumulation layer, this event (E3) occurred between 18.42 ka and 19.02 ka. 4) The fourth event (E4) is characterized by the formation of Layers 12 and 13. Layer 13 is a mixed accumulation of clay sediment, while the materials of this layer have originated from Layers 11 and 17 at the uplifted wall of the fault. Layer 12 is mud 17

accumulated in low-lying areas after the formation of Layer 13. These two sets record that the nature of the deposited material changed from coarse to fine after the earthquake occurred. The

14

C-3 age of Layer 12 is 22.73 ka, and the measured TL ages of Layer 13

and the top of Layer 14 are 22.11 ka and 23.79 ka, respectively. Therefore, this event (E4) occurred between 22.73 ka and 23.79 ka. 5) The main feature of the fifth event (E5) is the formation of Layer 15. Layer 15 is a mixed accumulation of sediments, while the materials have originated from Layers 11 and 17 at the uplift wall of the fault. Due to depth and width restrictions during the trench excavation, the shape of Layer 15 is incompletely revealed. The measured TL age of the bottom of Layer 14 is 27.98 ka. Therefore, this event (E5) occurred before 27.98 ka. The characteristics of the youngest layer of the faulting, the layer formed by fault activity, and the accumulation layer, in addition to the timing of every event revealed by the Sanshanyuan trench, are summarized in Table 2. Fig. 9 shows an evolution graph of the faulting events revealed by the Sanshanyuan trench. In this figure, a1, b1, c1, d1, and e1 represent the layer dislocations of the five faulting events, respectively, and a2, b2, c2, d2, and e2 represent the layer depositions occurring after each faulting event, respectively. Layers 11, 16 and 17 were faulted by the fifth event (Fig 9e1). After this event, colluvial wedge D was deposited at the downthrown wall. The material of colluvial wedge D came from Layers 11 and 17 in the uplifted wall of the fault. Layer 14 was covered by this wedge at the downthrown wall (Fig 9e2). Layer 14 and wedge D were faulted by the fourth event (E4) (Fig 9d1). Then, colluvial wedge C was deposited at the downthrown wall. The material of colluvial wedge C mainly came from Layer 11 in the uplifted wall of the fault. Layer 12 represents the accumulation of mud in a standing water environment. Layers 9 and 10 covered on wedge C and Layer 12 at the 18

downthrown wall (Fig 9d2). Before the third event (E3) occurred, the top of Layer 11 was denuded. Layers B1 and B2 formed on the surface of Layer 11 when the third event occurred (Fig 9c1). These cracks consist of mixed silt, sandy loam, and clay lumps that the materials have originated from Layers 9, 10 and 11. Layers 3 to 8 covered on Layers 9 and 11 on the two walls of the fault (Fig 9c2). Fig 9b1 shows that Layers 3 and 5 were faulted by the second event (E2). Layers 3 and 4 were also disturbed when this event occurred. Layer 3 was denuded and Layer 2 covered two walls (Fig 9b2). The youngest faulting event is the 1976 Ms7.1 Luanxian earthquake (E1). This earthquake formed a visible surface rupture and a scarp. The scarp is up on the east side and down on the west side and is accompanied by sinistral slip displacement (Fig 9a1). Because of the disturbance of surface strata by farming in recent decades, the vertical displacement of the earth’s surface caused by this event is unclear (Fig 9a2). The seismogenic fault is a strike-slip fault that is located at the margin of the pluvial fan. This fault provides a reasonable explanation for the direct contact between Layers 16 and 17 and Layers 10 and 11 at the trench, which record different lithologies on the two sides of the fault plane. 5.3. Discussion of fault displacement Because the strata on the two sides of the fault plane do not have the same layers, it is difficult to discuss the vertical displacement of the faulting events. Based on the phenomena that are revealed by the trench, the following conclusions have been reached. 1) At the downthrow side of the fault, between Layers 16 and 14 and Layers 14 and 10, colluvial wedges occur as Layer 15 and Layer 13 and are named D and C, respectively. And the mud sediment in Layer 12 lies above Layer 13. The thickness of colluvial wedge D is 0.78–1.0 m. Layers 13 and 12 are approximately 0.6 m thick. The vertical displacements of the fourth and fifth fault events at the fault plane are more than 19

0.6 m and 0.78 m, respectively, which are greater than the thicknesses of the two colluvial wedges. 2) In addition to calculating the vertical displacement of faulting activity based on the thickness of the colluvial wedges, another approach is to find the provenance of the colluvial wedges and compare the lithologies of their strata. In the sections of the trench, the provenances of Layer 15 and Layer 13 are Layer 17 and Layer 11. On the two sides of the fault plane, the vertical displacement of the top of Layer 13 and Layer 11 is 3.5 m and the vertical displacement of the top of Layer 15 and Layer 17is 4.4 m. Thus, given the 3.5 m vertical displacement between the top of Layer 13 and Layer 11, the fault plane underwent four faulting events after Layer 13 was formed. The average vertical displacement of those four events is 0.88 m. Deducting the displacement of events E 1 and E4, the average vertical displacement of events E2 and E3 is 1.25 m. 3) The silty sand bed dragging phenomenon observed in Layer 10 records the thrusting activity of the third faulting event. However, it is difficult to determine the vertical displacement based on only the 20-cm dragging of one side of the fault plane. 4) The different dips of the high-angle fault plane in the different sections of the trench and the different lithologies of the strata in contact with the fault plane serve as evidence that this fault is a strike-slip fault. However, we were unable to determine the strike-slip displacement of this fault from the trench section. Based on the data of Du et al. (1985), the ratio between the vertical and strike-slip displacement of the NE-trending surface rupture of the 1976 Ms7.1 Luanxian earthquake at Sanshanyuan is 1:1. According to the data of Wang et al. (1981a) and Yang (1982), the sinistral displacement of the NW-trending surface rupture is 0.25 m, the vertical displacement is 0.1 m, and the ratio between them is 2.5:1. 5.4. Comparison of the faulting events on the Luanxian Western Fault and the 20

Tangshan Fault After determining the five faulting events of the Luanxian Western Fault during the Late Quaternary, it is important to compare these results with those of the Tangshan Fault. The vertical displacement of each event on the Luanxian Western Fault revealed by the Sanshanyuan trench is given in Table 3. The vertical and strike-slip displacements of the 1976 Ms7.1 Luanxian earthquake were measured in a field investigation by Du et al. (1985). The strike-slip displacements of events E2 to E5 on the Luanxian Western Fault were obtained from the ratio between the strike-slip and vertical displacements of the 1976 Luanxian earthquake. The vertical displacements of the four seismic events on the Tangshan Fault were revealed by the Niumaku borehole profile (Guo et al., 2011a). The vertical and strike-slip displacements of the Tangshan earthquake are recorded in the Niumaku earthquake relic. The strike-slip displacements of events E2 to E4 were calculated from the ratio between the strike-slip and vertical displacements of event E 1, which is 2.4:1 (Table 3). Fig. 10 compares the timings and strike-slip displacements of the strong Late Quaternary earthquakes on the Luanxian Western Fault, which is the seismogenic structure of the 1976 Ms7.1 Luanxian earthquake, and the Tangshan Fault, which is the seismogenic structure of the 1976 Ms7.8 Tangshan earthquake. Fig. 10 shows that before 1976, the paleoearthquakes of the Luanxian Western Fault and the Tangshan Fault are non-synchronous. The displacements recorded from events E2 to E5 on the Luanxian Western Fault are more than twice that caused by the 1976 Luanxian earthquake. 6. Conclusions In this study, through an investigation of the surface rupture zone of the Ms7.1 21

Luanxian earthquake and the excavation of the Sanshanyuan trench, the following conclusions about the seismogenic structure of this earthquake were obtained. 1) The surface rupture zone of this earthquake, from the NW-trending south section to the NNE-trending north section, is 6 km long. The former is approximately 4 km long, and the latter is approximately 2 km long. 2) The location of the surface rupture zone of this earthquake is different than the north section of Luanxian-Laoting Fault, as reported by Guo et al. (1977). Rather, it distributed along the west side of the NE- and NW-trending relict hills in this area and coincides with the location of the Luanxian Western Fault reported by Wang et al. (1981a). 3) The Sanshanyuan trench in this study is situated at the NE-trending surface rupture zone near the epicenter of the 1976 Ms7.1 Luanxian earthquake. According to the analysis of the phenomena revealed by the Sanshanyuan trench, four faulting events occurred on this fault prior to the 1976 Ms7.1 Luanxian earthquake. The times of these four events are <9.73 ka, 18.42–19.02 ka, 22.73–23.79 ka, and >27.9 8ka. Including the 1976 Ms7.1 Luanxian earthquake and using the median values for calculation, the recurrence intervals of the five faulting events are 9 ka, 9 ka, 4.5 ka, and >4.7ka, respectively. The average interval of the first three recurrence intervals is approximately 7.5 ka. According to the faulting offset analysis of the trench, the earthquake magnitudes of the four faulting events prior to the 1976 Ms7.1 Luanxian earthquake are greater than that of the 1976 Ms7.1 Luanxian earthquake. 4) The surface rupture of the 1976 Ms7.1 Luanxian earthquake is located 40 km from the surface rupture of the 1976 Ms7.8 Tangshan earthquake. The faulting phenomena revealed by the Sanshanyuan trench further demonstrate that the seismogenic faults of the 1976 Ms7.1 Luanxian earthquake and the 1976 Ms7.8 Tangshan earthquake 22

are not the same. The 1976 Ms7.1 Luanxian earthquake resulted from the activity of the NW-trending Luanxian Western Fault, which was in turn triggered by the Ms7.8 Tangshan earthquake. 7. Discussion of the seismogenic structure of the Ms7.1 Luanxian earthquake Since the 1976 Ms7.1 Luanxian earthquake, based on the results of previous investigations, many scholars have reported their interpretations of the seismogenic fault of this earthquake. Some concluded that the seismogenic fault of this earthquake is the Luanxian Western Fault, which is NNE- to NNW-trending (Wang et al., 1981a). Others reported that the NW-trending Luanxian Western Fault and the NNE-trending Luanxian-Lulong Fault are the seismogenic faults (Yang, 1985). Still others concluded that the seismogenic fault of this earthquake is the NE-trending Shangang-Leizhuang Fault (Cui et al., 1981; Du et al., 1985). The results of Guo et al. (1977) indicated that the Ms7.1 Luanxian earthquake is located in the Luanxian-Laoting Fault, which is NW-trending. Several questions about the seismogenic structure of the 1976 Ms7.1 Luanxian earthquake remain. It is inconclusive whether the structure comprises a pair of NE- and NW-trending conjugate faults (Zhang and Diao, 1992) or mainly an NW-trending fault that became NE-trending at the northern end of the fault with an overall left-lateral strike-slip fault movement. The results of this study indicate that the latter is more likely. The Sanshanyuan trench revealed an active fault beneath the surface rupture of the 1976 Ms7.1 Luanxian earthquake. The faulting phenomena revealed by the trench indicate the importance of continuing this trench investigation across the NW-trending surface rupture. In mainland China, the activity rate of the active fault is low compared to that in the western region. Moreover, the recurrence interval of strong earthquakes is long, and the 23

geomorphologies of active structures are not obvious in North China (Deng, 1996). However, this study of the Sanshanyuan trench demonstrates that data from trench and borehole profiles obtained across the surface rupture zone can reveal the seismogenic structures of strong earthquakes in North China.

Acknowledgments This work was supported by the National Natural Fund Youth Fund of China [grant number 41202157]; and the Fundamental Research Funds of Institute of Crustal Dynamics, China Earthquake Administration [grant number ZD2013-19]. During this study, the Luanxian County Seismological Bureau and Sanshanyuan Village Committee provided support for the trench excavation; Prof. Yipeng Wang, Zhuen Yang, Fangmin Song, Bihong Fu, Jun Shen, Youli Li, and others provided guidance and discussions of the field work; and Prof. Ming Lu captured photographs of the trench. We are grateful for their help. We thank the professional editors at Editage and Elsevier, which is a language editing service, for improving the language of the manuscript. Reference Butler, R., Stewart, G.S., Kanamori, H., 1979. The July 28,1976 Tangshan Earthquake----A complex sequence of intraplate events. Bulletin of the Seismololgical Society America 69, 207-220. Chen, Y., Lin, B., Wang, X., Huang, L., Liu, M., 1979. A dislocation model of the Tangshan earthquake of 1976 from the inversion of geodetic data. Chinese Journal of Geophysics 22, 201-217. Cui, Z., Du, C., Li, X., Bian, Z., 1981. Relationship between Tangshan earthquake and active tectonic system. Journal of Chinese Academy of Geological Sciences, the 562 comprehensive group subsidiary. Volume Two, 69-80. 24

Deng, Q., 1996. Active tectonics in China. Geological Review 42, 295-299. Du, C., Meng, X., Chen, S., 1980. The surface rupture and tectonic stress field of Tangshan earthquake. Archive of the Institute of Crustal Dynamics, No.03308. Du, C., Meng, X., Chen, S., 1985. Ground fissures of the Tangshan earthquake, In: Liu, H. (Eds.), Disaster of Great Earthquake (1). China Seismological Press, pp. 174–189. Fang, H., Wang, Z., Zhao, S., 1981. Building scientific research report: Research on seismic engineering and geology in Tangshan strong earthquake area. Prospecting Institute of Technology, China Academy of Building Research, pp.32-34. Gu, G., Lin, T., Shi, Z., 1983. Catalogue of Chinese earthquakes (1831 BC-1969AD). China Science Press, pp. 347-348. Guo, H., Jiang, W., Xie, X., 2011a. Late-Quaternary strong earthquakes on the seismogenic fault of the 1976 Ms7.8 Tangshan earthquake, Hebei, as revealed by drilling and trenching. Science China (Earth Science) 54, 1696-1715. Guo, H., Jiang, W., Xie, X., 2011b. New evidence for the distribution of surface rupture zone of the 1976 Ms7.8 Tangshan earthquake. Seismology and Geology 33, 506-524. Guo, S., Li, Z., Cheng, S., Chen, X., Chen, X., Yang, Z., Li, R., 1977. Discussion on the regional structure background and the seismogenic model of the Tangshan earthquake. Chinese Journal of Geology 12, 3-19. Hao, S., You, H., 2001. A detailed detection of the Tangshan active fault using shallow seismic survey. Seismology and Geology 23, 93-97. Hebei Earthquake Administration, 1980. Catalogue of Tangshan earthquakes (July 1976December 1979). China Seismological Press, pp. 1. Huang, B., Yeh, Y., 1997. The fault ruptures of the 1976 Tangshan earthquake sequence 25

inferred from coseismic crustal deformation. Bulletin of the Seismololgical Society America 87, 1046-1057. Huang, L., 1981. An analysis of the horizontal displacement field of Tangshan earthquake with generalized inverse method and a comparison with the classic method. Crustal Deformoation and earthquake 1, 3-13. Jiang, W., 2006. Discussion on seismogenic fault of the 1976 Tanghsan earthquake. Seismology and Geology 28, 312-318. Li, J., Hao, S., Hu, Y., Yu, Z., Zhu, B., 1998. A study on activity of the seismogenic fault for the Tangshan earthquake of 1976. Seismology and Geology 20, 27-33. Li, Q., Zhang, Z., Jin, Y., Yu, X., Li, Z., 1980. Focal mechanisms of Tangshan earthquakes. Seismology and Geology 2, 59-67. Liu, G., 1983. Formation of Luan River proluvial fan. Site Investigation Science and Technology, 6-16. Liu, G., Guo, S., Liu, C., 1982. Seismogeologic background. In: China Earthquake Administration 1976 Tangshan earthquake Edit Group (Ed.), 1976 Tangshan earthquake. China Seismological Press, pp. 71-130. Liu, K., Qu, G., Chen, J., Wang, W., Ning, B., 2013. Recurrence characteristics of major earthquakes in the Tangshan area, North China. ACTA Geologica Sinica 87, 254-271. Liu, P., Lv, X., 2011. On the causes of the spatial distribution complexity of the 1976 Tangshan earthquake sequence. Earthquake 31, 1-14. Lu, Y., Chen, Z., Wang, B., Liu, P., Liu, W., Dai, W., 1985. Seismological method for earthquake predictio. China Seismological Press, pp. 113. Nabelek, J., Chen, W., Ye, H., 1987. The Tangshan earthquake sequence and its implications for the evolution of the North China Basin. Journal of Geophysical 26

Research 92, 12615-12628. Qiu, Z., Ma, J., Liu, G., 2005. Discovery of the great fault of the Tangshan earthquake. Seismology and Geology 27, 669-677. Seismo-geological Brigade, 1970. Seismic intensity map of the Luanxian earthquake on September 23, 1945. Archive of the Institute of Crustal Dynamics, No.03308. Shannxi bureau of surveying and mapping, 1981. Building scientific research report: Earthquake engineering geological research of Tangshan strong earthquake area appendix 1. Prospecting Institute of Technology, China Academy of Building Research, pp.15. Shedlock, K., Baranowski, J., Wen, X., Liang, H., 1987. The Tangshan aftershock sequence. Journal of Geophysical Research 92, 2791-2803. Wang, J., Chen, G., Zheng, W., Yang, W., 1981a. A study on the surface rupture and reason of Tangshan 7.8 earthquake and Luanxian 7.1 earthquake. Journal of Chang’an University (Earth Science), 61-72 Wang, J., Zheng, W., Chen, G., Yang, W., Chen, G., Pan, Z., 1981b. A study on the principal surface fracture belt and the cause of occurrence of the Tangshan earthquake. Journal of Seismological Research 4, 437-449. Wang, T., Li, J., 1984. The recurrence intervals of the strong earthquake in Tangshan. Seismology and Geology 6, 79-85. Wen, X., Ma, S., 2006. The influence of Tangshan earthquake on the earthquake recurrence for the adjacent fault. Progress in Nature Sci. 16, 1346-1350. Wu, K., Wang, Z., Lv, P., 1982. Seismic parameters and sequences. In: China Earthquake Administration 1976 Tangshan earthquake Edit Group (Ed.), 1976 Tangshan earthquake. China Seismological Press, pp. 69-70. Xie, J., 1984. Some features of crustal elastic rebound of the Tangshan earthquake. 27

Journal of Seismological Research 7, 137-146 Xie, X., Yao, Z., 1991. The faulting process of Tangshan earthquake inverted simultaneously from the teleseismic waveforms and geodesic deformation data. Physics of the Earth and Planetary Interiors 66, 265-277. Yang, L., 1982. Seismic intensity and damage. In: China Earthquake Administration 1976 Tangshan earthquake Edit Group (Ed.), 1976 Tangshan earthquake. China Seismological Press, pp. 1-32. Yang, L., 1985. Geological tectonic background and seismogenic structure of Tangshan earthquake. In: Liu, H. (Ed.), Disaster of Great Earthquake (1). China Seismological Press, pp. 33-39. You, H., Xu, X., Wu, J., He, Z., 2002. Study on the relationship between shallow and deep structures in the 1976 Tangshan earthquake area. Seismology and Geology 24, 571-582. Zhang, S., Diao, G., 1992. The tectonic process of the Tangshan earthquake sequence. Earthquake Research in China 8, 73-80. Zhang, Z., Li, Q., Gu, J., Jin, Y., Yang, M., Liu, W., 1980. The fracture processes of the Tangshan earthquake and its mechanical analysi. ACTA Seismologica Sinica 2, 111-129. Zhang, Z., Xie, J., Xu, F., Peng, S., 1981. Vertical deformations associated with the 1976 Tangshan M=7.8 earthquake. Chinese Journal of Geophysics 24, 182-191.

28

Fig. 1. Distribution of the isoseismal lines and geographical conditions surrounding the region of the 1976 Ms7.8 Tangshan earthquake and the Ms7.1 Luanxian earthquake (Liu et al., 1982); the distribution of faults was modified slightly from the original map. (a) Landform map of China. The red box shows the location of Figure 1b. (b) Isoseismal lines and surface rupture zones of the Tangshan and Luanxian earthquakes. The black lines represent the isoseismal lines of the Tangshan earthquake, and the dotted lines represent those of the Luanxian earthquake. The red lines indicate the surface rupture zone of the Luanxian earthquake, and the pink lines show that of the Tangshan earthquake. The red circles represent the epicenters of the Tangshan and Luanxian earthquakes. (c) Aftershock sequence of the Tangshan earthquake from July 1976 to December 1979 (Hebei Earthquake Administration, 1980). Fig. 2. (a) Map of the geological structure and distribution of historical earthquakes in the Tangshan-Luanxian region (adapted from Liu et al. (1982)). The lines represent faults, red circles indicate the epicenters of the 1976 Ms7.8 Tangshan and Ms7.1 Luanxian earthquakes, and green circles represent the epicenters of historical earthquakes. (b) Distribution of the Luan River pluvial fan in modern times (adapted from Liu et al. (1982)). The dotted lines represent the fault; brown lines and numbers represent the Quaternary isopach and the thicknesses of the Quaternary deposits, respectively. Fig. 3. (a) Distribution of the NW-trending bedrock relict hills. The red lines represent the surface rupture of the 1976 Luanxian earthquake; the dotted lines show the faults. The green triangle is the location of photograph b. (b) Photograph of the rupture of the 1976 Ms7.1 Luanxian earthquake, north of Sanshanyuan Village; the camera is facing NNE (Shaanxi Bureau of Surveying and Mapping, 1981). (c) Google Earth image of the trench. The gray square shows the location of the trench. (d) Plane of the trench. The red line is the location of the fault revealed by the trench; the blue lines show the surface 29

rupture of the 1976 Ms7.1 Luanxian earthquake. Fig. 4. SE end of the NE section of the Sanshanyuan trench, Luanxian County. (a) Photograph of the SE end of the NE wall of the trench. (b) The SE end of the NE section of the trench. The red line shows the fault plane, black squares represent

14

C sampling

sites, and black triangles represent TL sampling sites. Fig. 5. SW section of the Sanshanyuan trench, Luanxian County. (a) SW section of the trench. The black box is the location of figure c. (b) Photograph of the SE end of the SW wall of the trench. (c) The SE end of the SW section of the trench. The red line shows the fault plane; the black triangles represent TL sampling sites. Fig. 6. Photographs of the trench and its two walls. (a) Entire view of the Sanshanyuan trench, trending NW40°. The camera is facing NW. (b) NW wall of the Sanshanyuan trench. White fine sand occurs at the west side of the fault plane, which is the downthrow wall, and brown clay appears at east side of the fault plane, which is the uplift wall. The fault plane has a high dip angle. The cracks B1 at the uplift wall, are indicated by a stick in this photograph. The holes in the section are the locations of the collection of dating samples. At the bottom of this photograph, a colluvial wedge is shown. The camera is facing NE. (c) Stereoscopic map of the fault plane of the Sanshanyuan trench. The main trend of the fault plane in the NE wall of the trench is SE and that in the SW wall is NW. The camera is facing NE. Fig. 7. Photographs of part of the Sanshanyuan trench. (a) Close-up photograph of the upper section of the fault plane at the NW wall. The fault plane extends up to Layer 2-1. The bottom of the Layer 2-1 was displaced by nearly 25 cm. The camera is facing NE. (b) Close-up photograph of the fault plane between colluvial wedge C and Layer 17. It clearly shows that the fault plane between the two layers is marked by several small sand veins. The lithologies of Layer 12, colluvial C and Layer 17 are different. Layer 12 is 30

gray muddy clay. Layer 17 is brick-red clay with dark brown clay lumps. Colluvial wedge C is a mixture of dark brown and brick-red clay. This wedge was pierced by sand veins in later periods that indicate that liquefaction by an event occurred. The camera is facing NE. (c) Close-up photograph of two colluvial wedges at the NW wall. The camera is facing N. (d) Close-up photograph of colluvial wedge C at the SW wall. The camera is facing SW. (e) At the NE wall of the trench, the dragging in the white fine sand bed, with horizontal bedding at the NW side of the fault plane, can be observed. The white fine sand bed was dragged into a concave shape. The dipping angle is 50°, and the camera is facing NE. (f) Close-up of sand bed deformation near the fault plane at the SW wall. The SE side of the fault plane is brown clay, and the NW side is composed of white fine sand. Grayish-black fine sand bed dragging appears in the fine sand strata. The camera is facing SW. Fig. 8. Photographs of part of the Sanshanyuan trench. (a) Map view of the fault in the Sanshanyuan trench. The fault plane is slightly wavelike. Near the fault plane, thin brown clay and sand strips are alternatively intercalated, and clay lumps and white fine sand are mixed. (b) Map view of the fault in the Sanshanyuan trench. Brown clay and white fine sand are in faulting contact, showing thin, millimeter-sized brown clay and fine sand strips in the fault plane. Some fine sand strips form lambda-type structures in white sand strata. (c) The collection process of part of the Sanshanyuan trench. The direction of the stick on the platform of the trench is S–N. The camera is facing NW. (d) The collected section of the Sanshanyuan trench, which was placed in a frame. This specimen is displayed in the hall of our institute. Fig. 9. Evolution graph of the faulting events revealed by the Sanshanyuan trench. a1, b1, c1, d1, and e1 represent the dislocation of the five faulting events, respectively, and a2, b2, c2, d2, and e2 represent the layers deposited after each faulting event, respectively. 31

Layers D, C, and B1 and B2 labeled in this profile are the results of colluvium and cracks associated with events E5, E4, and E3, respectively. Fig. 10. Comparison of strong Late Quaternary earthquakes on the NE-trending Tangshan Fault and the NE–NW-trending Luanxian Western Fault. The blue (red) line represents paleoearthquake events of the Tangshan Fault (Luanxian Western Fault) in the Late Quaternary. The strike-slip displacements of these events are shown in Table 3.

32

33

34

35

36

37

38

39

40

Table 1 Dating results of samples from the Sanshanyuan trench, Luanxian Countya No.

Sample No.

Depth/m

Section of trench

Layer No.

Lithology

Age（ka）

Khaki silt

20.04±0.07

1

14

4.0

NE

Layer 9

2

14

2.3

NE

Layer 7

3

14

C-3

5.5

NE

Layer 12

Brown mucky clay

22.73±0.08

4

TL-1

4.0

NE

Layer 9

Yellow silt

20.73±1.76

5

TL-2

2.5

NE

Layer 7

6

TL-3

1.8

NE

Layer 5

White sand

11.68±0.99

7

TL-4

2.9

NE

Layer 8

White sand

15.94±1.35

8

TL-5

4.4

NE

Layer 10

White sand

20.35±1.73

9

TL-6

3.7

NE

Layer 8

White sand

18.42±1.56

10

TL-7

1.7

NE

Layer 3

Yellow sandy loam

9.73±0.83

11

TL-8

2.4

NE

Layer 11

Dark brown clay

19.02±1.62

12

TL-9

4.9

NE

White sand

20.58±1.78

13

TL-10

5.3

NE

Brown clay

22.11±1.88

14

TL-11

5.7

NE

White sand

23.79±2.02

15

TL-12

6.8

SW

Layer 10

White sand

22.22±1.89

16

TL-13

3.8

SW

Layer A2 in

Grayish yellow fine

Layer 8

sand

17

TL-14

3.0

SW

18

TL-15

5.7

SW

19

TL-16

3.7

NE

20

TL-17

5.0

NE

21

TL-18

7.8

NE

C-1 C-2

Bottom of Layer 10 Colluvial wedge C Top of Layer 14

Top of Layer 8 Bottom of Layer 8 Splitting wedge B2 Layer 17 Bottom of Layer 14

Grayish brown sandy loam

Grayish brown sandy loam

11.97±0.04

12.01±1.02

20.01±1.70

White sand

20.55±1.75

White sand

22.52±1.91

Yellow sandy loam

20.34±1.73

Brick-red clay

71.00±6.03

White sand

27.98±2.38

a 14

C dating was done by the Beta Analytic Radiocarbon Dating Laboratory (Miami, Florida, USA).

Thermoluminescence (TL) dating was done by the Geologic Chronology Laboratory of the Institute of Crustal Dynamics, CEA.

41

Table 2 Layers and timing of every fault event revealed by Sanshanyuan trench Faulting event

E1

The

The

newest

The

formed accumulation Dating results of layer samples

layer

layer

layer

Layer 2

-

Layer 1

Layer E2

The times of the events 18 o’clock 45 points,

-

July 28, 1976, Ms7.1

Layer 3:TL-7(9.73±0.83)ka,

3～Layer

-

Layer 2

Layer 5:TL-3(11.68±0.99)ka,

TE2 < (9.73±0.83)ka

14

Layer 7: C-2(11.97±0.04)ka

8

Layer 9: TL-1(20.73±1.76)ka, 14

C-1(20.04±0.07)ka

Layer 9， Layer B1 Layer 8-5,

E3

Layer 10 and B2

Layer 3

Layer 10: TL-5(20.35±1.73)ka

(18.42±1.56)ka < TE3 < (19.02±1.62)ka

Layer 11：TL-8(19.02±1.62)ka Layer 8:TL-6(18.42±1.56) Layer 14: TL-11(23.79±2.02)ka,

E4

Layer 14

E5

Layer 13

Layer 12,

Layer C

Layer 10

Layer 15

Layer 16

Layer 14

Layer D

Layer 13: TL-10(22.11±1.88)ka

(22.73±0.08)ka < TE4 < (23.79±2.02)ka

Layer 12：14C-3(22.73±0.08)ka The bottom of Layer 14:

(27.98±2.38)ka < TE5

TL-18(27.98±2.38)ka

Table 3 Comparison of displacements and times of Late Quaternary events on the Tangshan Fault and the Luanxian Western Fault Tangshan Fault (Guo et al., 2011a) Events

Vertical

Strike-slip

displacement displacement E1

0.5 m

1.2 m

Ratio 1:2.4

Luanxian western Fault Vertical

Time

Strike-slip

displacement displacement

1976 Ms7.8

0.4 m

0.39 m

Ratio 1:1

Time

1976 Ms7.1

E2

1.0 m

2.4 m

7.61-8.13ka

1.25 m

1.25 m

<9.73ka

E3

2.6 m

6.2 m

>14.57ka

1.25 m

1.25 m

18.42-19.02ka

E4

0.6 m

1.4 m

24.21-26.57ka

>0.6 m

>0.6 m

22.73-23.79ka

E5

-

-

-

>0.78 m

>0.78 m

>27.98ka

42

Graphical abstract

43

Highlights: The trench was excavated across the surface rupture of earthquake. The surface rupture is connected to the underground active fault. The fault experienced five faulting events with a time interval of about 7.5 ka. The seismogenic faults of two earthquakes occurred on the same day are different.

44