Paleoseismic Investigation of the Seismic Gap Between The

Paleoseismic investigation of the seismic gap between the seismogenic structures of the 2008 Wenchuan and 2013 Lushan earthquakes along the Longmen Shan fault zone at the eastern margin of the Tibetan Plateau

Hu Wang, Lichun Chen, Yongkang Ran*, Shengxue Lei, and Xi Li KEY LABORATORY OF ACTIVE TECTONICS AND VOLCANO, INSTITUTE OF GEOLOGY, CHINA EARTHQUAKE ADMINISTRATION, BEIJING 100029, CHINA

ABSTRACT

The 2008 Mw 7.9 Wenchuan earthquake occurred along the middle and northern segments of the Longmen Shan fault zone at the eastern margin of the Tibetan Plateau. Five years later, the 2013 Mw 6.6 Lushan earthquake ruptured a section of the southern segment of the Long- men Shan fault zone, leaving a 50-km-long seismic gap between the seismogenic structures of the two earthquakes. In our study, we use trenching and calibrated radiocarbon age models to assess the rupture behavior of the gap over multiple earthquakes. At least two paleoseismic events were identified with age constraints between A.D. 1350–1830 and 525–760 B.C., respectively. Trench stratigraphy suggests the presence of another possible event with an age constraint of A.D. 590–1210. Using cumulative vertical displacement of ~1.5 m for the lowest unit exposed in the trench (U1) and its age of ca. 2500 yr B.P., we estimate the vertical slip rate of the Dachuan-Shuangshi fault, the primary fault along the southern segment, to be ~0.6 mm/yr. The lack of correlation of events between multiple paleoseismic sites along the Dachuan-Shuangshi fault suggests that the seismic gap has a low possibility of rupturing completely during paleoearthquakes. A compari- son of the rupture behavior of the southern segment with the middle segment of the Longmen Shan fault zone indicates that the likelihood of cascading ruptures between the two segments is low.

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INTRODUCTION et al. (2013) integrated methods such as seismic tomography and relocated aftershocks and found a region of anomalously slow seismic wave speeds In the past 5 yr, two devastating earthquakes, the 2008 Mw 7.9 Wench- in the gap. Their results suggest that the weak and ductile crustal materials uan and the 2013 Mw 6.6 Lushan earthquakes, occurred along the Long- at seismogenic depths cannot accumulate enough strain for strong earth- men Shan fault zone at the eastern margin of the Tibetan Plateau (Fig. 1), quakes. To address these opposing views, we use mainly paleoseismologic killing more than ~80,000 people and causing severe damage. The Wen- evidence from trenching and ages from multiple radiocarbon samples to chuan event ruptured the middle segment and northern segment of the reveal a more robust rupture history along the seismic gap. Longmen Shan fault zone (specifically, surface ruptures are along the Yingxiu-Beichuan fault [YBF] and Guanxian-Jiangyou fault [GJF] shown GEOLOGIC SETTING AND PAST PALEOSEISMIC STUDY OF THE in Fig. 1). Seismogenic structure models for the Lushan earthquake (Chen LONGMEN SHAN FAULT ZONE et al., 2013, 2014; X.W. Xu et al., 2013) agree that this event was associated with the Dachuan-Shuangshi fault and a blind reverse fault system along The Longmen Shan fault zone, located at the eastern margin of the the southern segment of the Longmen Shan fault zone. Modeling of seis- Tibetan Plateau, is characterized by elevations of up to 7500 m above sea mogenic structures for the 2008 and 2013 earthquakes shows a 50 km seis- level and by topographic relief of more than 5 km over distances of less mic gap between the active segments of the fault relating to the two events than 5 km, which is the result of an intense collision between the Indian (Fig. 1). Geoscientists have been debating whether this gap has the poten- and Eurasian plates (Kirby et al., 2002). tial to produce a large earthquake in the near future. Wang et al. (2010) used The active Longmen Shan fault zone marks a predominantly conver- the method of balancing seismic moments on faults to suggest that if all the gent boundary with a right-lateral strike-slip component. This fault sys- moment energy were released by a rupture of the whole southern segment, tem was reactivated during late Cenozoic time along a Mesozoic orogenic it could produce an earthquake as large as Mw 7.7 in the next 50 yr. Liu et belt (Burchfiel et al., 1995, 2008; Kirby et al., 2002, 2008). To the west al. (2014) used a similar method to recalculate moment deficit and suggest of the Longmen Shan, East Tibet actively deforms by both right-lateral the seismic gap or the region south of the Lushan rupture zone along the shear parallel to and convergence perpendicular to the Longmen Shan southern segment of the Longmen Shan fault zone could produce a M ~7 fault (King et al., 1997; Chen et al., 2000; Zhang et al., 2004; Shen et al., earthquake. Other studies indicate that Coulomb stress in the gap increased 2005; Gan et al., 2007). Tectonic activity in the Sichuan Basin, east of the considerably after the Wenchuan and Lushan earthquakes, and this repre- Longmen Shan, has been low during late Cenozoic time. Three principal, sents a seismic risk (Parsons et al., 2008; Shan et al., 2013). However, Li subparallel, active faults comprise the northeast-trending Longmen Shan fault zone, named the Yingxiu-Beichuan fault, Guanxian-Jiangyou fault, *Corresponding author: [email protected]. and Wenchuan-Maoxian fault along the middle segment and northern seg-

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Figure 1. Fault distribution of the Longmen Shan fault zone denoted by the blue shading area (modified after Chen et al., 2013). Purple dots denote aftershocks of the 2008 Wenchuan earthquake, and yellow ones show aftershocks of the 2013 Lushan earthquake. Between the seismogenic structures of the two earthquakes, there is a seismic gap with a length of ~50 km denoted by a white open ellipsoid. NS, MS, and SS—northern, middle, and southern segments of the Long- men Shan fault zone; XSHF—Xianshuihe fault zone; YBF—Yingxiu-Beichuan fault; GJF—Guanxian-Jiangyou fault; WMF— Wenchuan-Maoxian fault; GLF—Gengda-Longdong fault; YWF—Yanjing-Wulong fault; DSF—Dachuan-Shuangshi fault. The inset map shows the tectonic background of the Tibetan Plateau, and the two solid red and yellow circles correspond to the two earthquakes mentioned in this study. The red and yellow shaded regions represent the Bayan Har block and Sichuan- Yunnan faulted block, respectively.

ment, and Gengda-Longdong fault, Yanjing-Wulong fault, and Dachuan- geomorphic analysis and radiocarbon dating along the surface ruptures Shuangshi fault along the southern segment (Fig. 1; Zhang et al., 2010). produced by the 2008 Wenchuan earthquake suggest an average recur- Geological investigations suggest that fault motion on the middle rence interval for large earthquakes of ~3000 yr (Ran et al., 2010, 2013). segment and northern segment is dominated by reverse thrusting with a However, there are only a few studies of the paleoseismic behavior along right-lateral component and a vertical slip rate no greater than 1 mm/yr the southern segment of the fault. Densmore et al. (2007) exposed a for the past ~10,000 yr (Ma et al., 2005; Li et al., 2006; Densmore et al., trench at Qingshiping (Fig. 2) on the Dachuan-Shuangshi fault and found 2007). After the 2008 Wenchuan earthquake, numerous geoscientists used evidence of weak deformation, identifying two paleoearthquakes—the a range of methods to measure recurrence intervals of large earthquakes younger event between 930 ± 40 yr B.P. and 860 ± 40 yr B.P. (corrected along these segments of the Longmen Shan fault zone. For example, mod- by OxCal 4.2 to A.D. 1045–1230) and the older prior to 930 ± 40 yr B.P. eling based on geodetic and geological slip rates from interferometric Recently, Chen et al. (2013) excavated a trench and uncovered a natural synthetic aperture radar (InSAR) and global positioning system (GPS) geologic exposure south of Qingshiping (Fig. 2). Their findings suggest inversions estimated the average recurrence interval of large earthquakes that a paleoearthquake occurred around A.D. 1480–1890 (corrected by along the Longmen Shan fault zone to be ~3000–6000 yr (Zhang et al., OxCal 4.2 to A.D. 1515–1885). Moreover, they used results of a previ- 2008; Shen et al., 2009). Multiple trenching investigations integrated with ous scientific report (Institute of Geology, China Earthquake Administra-

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Figure 2. Trenches excavated along the Dachuan-Shuangshi fault (DFS) along the southern segment of the Longmen Shan fault zone. The white ellipsoid denotes the gap between the seismogenic structures of the 2008 Wenchuan and 2013 Lushan earthquakes (EQ). The blue ellipsoid shows the meizoseis- mal region (IX degrees) produced by the 2013 Lushan earthquake. Small colored rectangles in the gap show trenching sites from this study (red), Densmore et al. (2007) (blue), and Chen et al. (2013) (white). Yellows dots represent locations of aftershocks of the 2013 Lushan earthquake. The big solid red circle shows the epicenter of the 2008 Wenchuan earthquake; the smaller one is related to the 2013 Lushan earthquake (L.S. Xu et al., 2013).

tion, 2009) on the 2008 Wenchuan earthquake to place a time constraint between 1390 yr B.P. and 650 yr B.P. (corrected by OxCal 4.2 to A.D. 645–1340) on an earlier event. However, stratigraphic unconformities and Figure 3. Trenching site. (A) Simplified interpretation near the a lack of material for radiometric age dating have resulted in incomplete trenching site, where red arrows show ambiguous and non- continuous linear geometry developed within the fault valley or poorly dated paleoseismic records at the aforementioned trenches on (image downloaded from Google Earth). (B) Geologic section the Dachuan-Shuangshi fault along the southern segment of the Longmen shows Triassic sandstone thrust upon Jurassic sandstone and Shan fault; hence, it is difficult to estimate the average recurrence interval conglomerate; numbers (e.g., 150°/46°) represent dip direc- of paleoearthquakes or the vertical slip rate of the fault segment. tion and angle of bedding. (C) Photo of the fault valley. The red arrows denote the probable location of the active fault. TRENCHING WITHIN THE GAP AT DACHUAN

The Dachuan trenching site is located north of Lushan on the Dachuan- with less vegetation cover and opened two trenches that traversed the fault Shuangshi fault along the southern segment of the Longmen Shan fault valley floor. Trenches A and B are ~30 and 50 m in length, respectively zone (Fig. 2). Geologic mapping in this area suggests that Triassic strata (Fig. DR11). Trench B revealed deposits consisting mainly of mixed grav- were thrusted over Jurassic strata, resulting in typical fault-valley geomor- els and boulders, which may represent a high-energy deposition environ- phology; surface traces of the active Dachuan-Shuangshi fault are not well ment like a debris flow. Trench A revealed three groups of fine-grained expressed, and the linear deformation zone is not continuous (Fig. 3). We clay units that are subhorizontal in the southeast part of the trench and followed the mapping of fault traces of the Dachuan-Shuangshi fault as revealed from the trenching and geomorphic interpretations of Chen et al. 1GSA Data Repository Item 2015010, Figure DR1, is available at www.geosociety.org (2014) to constrain the active fault located within the wooded valley. To /pubs/ft2015.htm, or on request from [email protected], Documents Secretary, search for evidence of surface ruptures, we selected a marshy depression GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.

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warp up to the northwest in the northwest part of the trench. The lower unit within U1 is also warped, in this case with a vertical displacement of stratum (U1) consists of maize-yellow (10YR8/1) fine clay containing ~1.5 m, and also does not change thickness across the deformation zone. some rounded gravel with diameters ranging from 5 to 10 cm. This unit In contrast, the units above (from U2 to U4) wedge westward against the can be divided into two subunits (indicated by the dashed line in Fig. 4). scarp formed by the folding of U1, which indicates that they unconform- The upper subunit has a lighter color and less gravel with a vertical dif- ably overlie the internal stratigraphy within U1 and suggests that they are ference of at least 1.5 m (the minimum value). The middle section of the growth stratigraphic units associated with another event (referred to as event trench stratigraphy (from U2 to U6) is mainly peaty clay containing some E3) that occurred between the deposition of U1 and U2. The interpretation decomposed leaves and interbedded with thin, fine white clays, represent- of E2 is challenging; it may or may not be associated with an earthquake. ing a quiet-water environment. U2 is black and peaty with multiple rotten Support for the earthquake interpretation comes from the observation that leaves and an apparent eastward dip to the bedding. In the northwest end U4 becomes thinner close to the deformation zone; however, away from of the trench, a boulder with a long axis greater than 1 m extends between the scarp, it shows a stable thickness. U3 is a peaty layer indicating a quiet- the base of the upper subunit in U1 and the base of U3. The color of U3 water depositional environment. It is much thicker and mostly deposited on is slightly lighter compared with U2; U3 is possibly divided into at least the hanging wall of the deformation zone. Since the uppermost stratigraphic three subunits, and an obvious difference is that a thin white subunit con- contact between U3 and U4 was originally horizontal, then U4 should have sisting of clay has developed. Possibly disturbed by the boulder, this thin a continuously equal thickness; however, the actual characteristics of U4 white layer is not clear at the northwest end of the trench. U4 is white clay do not support this analysis. Furthermore, as evidence of growth strata, the with a thickness of ~10 cm; however, it pinches out to the west. Units at internal subunit above the thin white layer (its upper contact is shown with the northwest end of the trench are warped upward, forming a buried scarp a dashed line) in U3 apparently does not change thickness across the defor- above an inferred buried trace of the fault. The part of U4 lying closer to mation zone, even though this white layer is not clear on the hanging wall the scarp shows compressed shortening characteristics. The color of U5 is that may have been disturbed by the boulder. Unfortunately, the other wall similar to U3; it contains numerous rotten leaves and has a stable thick- of the trench collapsed quickly owing to the soft nature of the deposits, so ness. U6 is gray peaty clay that thins toward the deformation zone. The no evidence is available from it that would help with the identification of upper layer (U7) contains modern deposits of brown clay. paleoseismic events. Third, the vertical deformation of U3 appears to be Continuity of most units across the trench and a lack of sharp breaks in slightly greater than that of U5. By identifying sudden changes in the strati- stratigraphy suggest that the units in the trench were deformed by folding. graphic components, it is possible to interpret that an earthquake occurred Interpretations of paleoseismic events were therefore based on evidence during this deposition. Conversely, an alternative interpretation is that event of growth strata, stratigraphic warping, and unconformities (Fig. 4). U5 E3 produced a large scarp and units U2, U3, and U4 back filled against the shows warping with a vertical displacement of ~0.4 m, and its thickness scarp. In this case, U4 would be affected by the shape of the large scarp, does not change across the deformation zone, whereas U6 wedges west- but it would not represent an earthquake. The vertical displacement of U3 ward against the scarp formed by the folding of U5. This indicates that U6 (0.6–0.7 m) is slightly greater than the offset of U5 (~0.4 m), as may be unconformably overlies U5 and suggests that it is a growth stratigraphic explained by upward attenuation during the youngest folding. unit associated with a surface-rupture seismic event (referred to as event We found numerous charcoal samples in the trenches. Twelve samples E1) that occurred between the deposition of U5 and U6. Similarly, the sub- were sent for accelerometer mass spectrometer (AMS) dating at Beta

Figure 4. Map and interpretation of the northern wall of the trench. Rounded gray forms represent large boulders. The vertical displacement by warping of U5 is ~0.4 m, located above the older units. Greater degrees of warping such as 0.6–0.7 m and 1.5 m occur for units U3 and U1, respectively. U denotes stratigraphic unit; stratigraphic contact is shown using black lines, dashed where subdivided. Small yellow rectangles represent charcoal, showing locations of radiocarbon samples, labeled by sample number and their corresponding corrected radiocarbon ages. All samples were processed with accelerator mass spectrometry (AMS) radiocarbon dating by Beta Analytic Inc., USA.

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TABLE 1. RADIOCARBON SAMPLES FROM THE TRENCH AT DACHUAN Sample Lab no. 13C/12C Radiocarbon age Calendar age* Mass Sample materialUnit (‰) (yr B.P. ± 1σ) (cal yr B.P., 2σ [95.4%]) (mg) 770–475 B.C. (92.4%) LCG-T2-C7 354270 –28.6 2470 ± 30 465–450 B.C. (1.2%) 2.3Angular fragment charcoal, solidU1 445–430 B.C. (1.8%) LCG-T2-C6 354271 –26.2 2370 ± 30 540–385 B.C. (95.4%)2.5 Tiny piece rotten leaf U1 755–680 B.C. (26.7%) LCG-T2-C29 357462 –27.4 2450 ± 30 670–605 B.C. (15.5%) 3.1Charcoal U2 595–410 B.C. (53.2%) 775–480 B.C. (94.9%) LCG-T2-C30 354273 –26.4 2480 ± 30 3.6 Small portion of wood U2 445–430 B.C. (0.5%) LCG-T2-C27 357468 –28.1 2050 ± 30 170 B.C.–20 A.D. (95.4%)3.0 Charcoal U3 LCG-T2-C4 357463 –28.8 1480 ± 30 535–645 A.D. (95.4%)3.2 Charcoal U3 LCG-T2-C2 354275 –27.0 830 ± 30 1160–1265 A.D. (95.4%)1.6 Charcoal U4 1270–1320 A.D. (60.4%) LCG-T2-C26 357464 –28.8 680 ± 30 2.1Angular fragment charcoal, solidU4 1350–1390 A.D. (35.0%) 1280–1330 A.D. (43.5%) LCG-T2-C12 357466 –28.2 650 ± 30 2.6Charcoal U4 1340–1395 A.D. (51.9%) LCG-T2-C22 354277 –30.0 610 ± 30 1295–1405 A.D. (95.4%)4.0 Large piece of charcoal U5 1680–1740 A.D. (27.1%) LCG-T2-C18 354278 –29.0 100 ± 30 3.1 Small portion of wood U6 1805–1935 A.D. (68.3%) 1680–1740 A.D. (27.1%) LCG-T2-C14 354279 –24.9 100 ± 30 2.3Angular fragment charcoal, solidU6 1805–1935 A.D. (68.3%) Note: All samples were processed by standard radiometric dating at the Beta Analytic, Inc., Miami, Florida, USA. The calendar age ranges are equivalent to the 2σ age ranges (95.4% conﬁdence). Radiocarbon ages B.P. are relative to 1950 (with 1σ counting error).

Analytic, Inc., in the United States. The radiocarbon dating results are sum- marized in Table 1. All ages reported herein are 2σ (95.4% confidence lim- its) calendric ages calibrated with the OxCal 4.2 program (Bronk, 2009), using the IntCal09 atmospheric model from Reimer et al. (2009). For the calibrated age, probability density functions (PDFs) overlap between dif- ferent samples. OxCal uses Bayesian statistics to reweigh the PDFs and account for stratigraphic ordering (overlying ages are younger) or historical age constraints. These statistics result in shifting or trimming the distributions to fewer peaks in multipeaked distributions (Bronk, 2009; Lienkaem- per and Ramsey, 2009). Most of the radiocarbon dates, except for samples LCG-T2-C6 from U1 and LCG-T2-C30 from U2, were in correct stratigraphic order. Sample LCG-T2-C6, collected as a tiny piece of rotten leaf, is possibly slightly younger than the true depositional age. LCG-T2-C30, collected as a small portion of wood, is just slightly older than a sample collected from a lower position in U2 and should be older than the true depositional age. We therefore do not use these two samples during further age constraints. Using the OxCal 4.2 program for further analysis, the ages of events E1 to E3 are constrained to the ranges A.D. 1350–1830, A.D. 590–1210, and 760–525 BC, respectively (Fig. 5). Irrespective of the paleoseismic events revealed in the Dachuan trench along the Dachuan-Shuang- shi fault, unit U1 is observed to have deformed with a cumulative vertical displacement of ~1.5 m (Fig. 4). Taking the age of this unit to be ca. 2500 yr B.P. (Table 1), we estimate the vertical slip rate to be ~0.6 mm/yr.

DISCUSSION AND CONCLUSIONS

Comparisons of Paleoseismic Events in the Seismic Gap Along the Dachuan-Shuangshi Fault Figure 5. Probability density functions for the timing of events from the trench calculated using OxCal 4.2 (Bronk, 2009), incorporating both the relative age of events (stratigraphic ordering) and posterior Chen et al. (2013) constructed a geologic section north of Dachuan, calibrated radiocarbon ages from units that bracket event horizons. ~17 km away from our trench (Fig. 2), and revealed a paleoseismic event Phases are the summed probability for units with multiple radio constrained between A.D. 1515 and 1885 (Fig. 6). Using the vertical offset carbon dates.

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seismic trenches in the gap. Therefore, based on the aforementioned analysis, we suggest that the seismic gap along the Dachuan-Shuangshi fault is unlikely to rupture completely during one earthquake, unless it ruptures together with the middle segment of the Longmen Shan fault zone.

Comparisons of Rupture Behavior Between the Southern Segment and Northern Segment–Middle Segment of the Longmen Shan Fault Zone

The 2013 Mw 6.6 Lushan earthquake occurred along the southern segment of the Longmen Shan fault zone, whereas the 2008 Mw 7.9 Wench- uan earthquake ruptured the whole northern segment and middle segment. Ran et al. (2013) used trenching to reveal evidence of a paleoearthquake that occurred between 3300 and 2300 cal. yr B.P. (or 1350–350 B.C.) that was comparable in magnitude with the 2008 Wenchuan earthquake. They suggested an average recurrence interval of large earthquakes to be Figure 6. Correlations of site probability density functions from OxCal mod- ~3000 yr along the middle segment, with coseismic vertical displacements els for the Dachuan site (our study), Qingshiping site from Densmore et al. of several meters. From the Dachuan trench, assuming the existence of (2007), and the report from Chen et al. (2013). The curves show 2s ranges. event E2, we suggest the average recurrence interval of surface-rupturing earthquakes to be ~1000–1300 yr. If E2 did not occur, the average recurrence interval would be 1875–2490 yr, a much shorter recurrence of 0.4 m produced by the most recent event in the trench at Dachuan and the interval with a smaller coseismic displacement compared with that of the high-angle reverse fault of the Longmen Shan fault (Zhang et al., 2010), the middle segment. Both events E1 and E2 do not correlate to paleoearth- regression analysis (reverse-fault type) of Wells and Coppersmith (1994) quakes along the middle segment. However, the age of E3, constrained was applied to estimate a possible rupture length of ~23 km—or slightly at 760–525 B.C. from trenching at Dachuan, overlaps with that of the more if partial attenuation owing to folding deformation is considered. This penultimate event (1350–350 B.C.) revealed along the middle segment, indicates that the two trenching sites may reveal the same event. Moreover, which appears to suggest that the southern segment primarily ruptured as application of the Z-statistic method (Sheppard, 1975) to the mean ages and a separate segment from the middle segment, although rupture of the two their respective standard deviations (McCalpin, 2009; Biasi et al., 2011), segments together cannot be completely ruled out. i.e., A.D. 1695 ± 95 and A.D. 1580 ± 130 as calculated by the OxCal 4.2 Chen et al. (2014) mapped the distributions of active faults along the program for the two events, gives an estimate for Z of ~0.71. This value southern segment and found more branch faults and secondary folding corresponds to a probability of contemporaneity of ~0.5, meaning that the deformation along the southern segment than have been observed along two events have a high possibility of being the same event. Third, over- the northern and middle segments of the Longmen Shan fault zone. These lapping distributions show that the age ranges of the two events are well structural complexities along the mapped trace of the Dachuan-Shuangshi matched (Fig. 6). However, there is no corresponding rupture evidence of fault may retard through-going rupture on the segment; in other words, this event from the trenching investigation of Densmore et al. (2007) or the rupture of the Dachuan-Shuangshi fault may not extend across the entire report of Chen et al. (2013) at sites ~18 km further to the northeast near southern segment. These findings are more consistent with the southern Qingshiping (Fig. 2). This indicates that surface ruptures in the seismic gap segment having a low ductile strength (Li et al., 2013) compared to models along the Dachuan-Shuangshi fault may be segmented. suggesting that the Dachuan-Shuangshi fault ruptures in large earthquakes. Similarly, even if event E2 does exist, we suggest that a surface rupture produced by the event would be unlikely to have extended to the trenching ACKNOWLEDGMENTS sites of Densmore et al. (2007) and the report of Chen et al. (2013). The This work was supported by the Project of Lushan M7.0 Earthquake Nucleation and its Mecha- nism and Influences—Study on the Tectonic Background of the Bayan Har Block and Nucleation distance of the two sites is ~35 km, i.e., much greater than the length of the Mechanism of the Lushan Earthquake (201408014), the National Science Foundation of China estimated surface rupture produced by event E2 (the cumulative displace- (Grant 41302160), the Project of China Earthquake Administration “Scientific Investigations on ment of U3 is ~0.6–0.7 m, which means the coseismic vertical displace- the 20 April 2013 Lushan, Sichuan Earthquake,” and the Special Projects for Basic Research Work of the Institute of Geology, China Earthquake Administration (IGCEA1304). Thanks go to Fei Han ment produced by event E2 is ~0.2–0.3 m, which is less compared with and Chenglong Liu for the field work. We are grateful to the Seismic Mitigation Bureau of Ya’an that of E1). Additionally, comparing event E2 to the events from Densmore and Lushan for their support of field investigations. Great thanks go to Christopher Madden et al. (2007) and Chen et al. (2013), the Z-statistics method gives Z values Madugo and Eric Kirby, who provided detailed and constructive suggestions on the manuscript. of ~1.26, which correspond to a probability of contemporaneity of ~0.2, and the coseismic vertical displacement for the corresponding event from REFERENCES CITED Biasi, G., Weldon, R.J., and Scharer, K., 2011. Rupture length and paleomagnitude estimates the trenching of Chen et al. 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MANUSCRIPT ACCEPTED 3 OCTOBER 2014 Ran, Y.K., Chen, W.S., Xu, X.W., Chen, L.C., Wang, H., Yang, C.C., and Dong, S.P., 2013, Paleo- seismic events and recurrence interval along the Beichuan–Yingxiu fault of Longmen- PRINTED IN THE USA

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