Paleoearthquakes on the Anninghe and Zemuhe fault along the southeastern margin of the Tibetan Plateau and implications for fault rupture behavior at fault bends on strike-slip faults

Accepted Manuscript Paleoearthquakes on the Anninghe and Zemuhe fault along the southeastern margin of the Tibetan Plateau and implications for fault rupture behavior at fault bends on strike-slip faults

Hu Wang, Yongkang Ran, Lichun Chen, Yanbao Li PII: DOI: Reference:

S0040-1951(17)30351-7 doi:10.1016/j.tecto.2017.08.030 TECTO 127604

To appear in:

Tectonophysics

Received date: Accepted date:

25 July 2017 26 August 2017

Please cite this article as: Hu Wang, Yongkang Ran, Lichun Chen, Yanbao Li , Paleoearthquakes on the Anninghe and Zemuhe fault along the southeastern margin of the Tibetan Plateau and implications for fault rupture behavior at fault bends on strike-slip faults. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Tecto(2017), doi:10.1016/j.tecto.2017.08.030

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.

ACCEPTED MANUSCRIPT Paleoearthquakes on the Anninghe and Zemuhe Fault along the Southeastern Margin of the Tibetan Plateau and implications for fault rupture behavior at fault bends on strike-slip faults Hu Wang1,2, Yongkang Ran2, Lichun Chen3, Yanbao Li3 Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University,

PT

1

Chengdu, 611756, China

Key Laboratory of Active Tectonics and Volcano, Institute of Geology, China Earthquake

RI

2

Administration, Beijing 100029, China

Division of medium to long-term earthquake prediction, Institute of Geology, China

Earthquake Administration, Beijing 100029, China.

The Key Laboratory of Crustal Dynamics, the Institute of Crustal Dynamics, China

NU

4

SC

3

MA

Earthquake Administration, Beijing 100085, China

ABSTRACT

D

Fault bends can serve as fault segment boundaries and are used in seismic hazard

PT E

assessment. Recent studies addressing whether rupture propagation would be arrested at such structural complexities have commonly focused on computational modeling. However, multi-cycle paleoseismic rupture observations through fault bends have seldom

CE

been reported. In this study, we used trenching and radiocarbon dating to reveal paleoseismic rupture histories on the southern segment of the Anninghe fault (ANHF)

AC

along the southeastern margin of the Tibetan Plateau to explore multi-cycle surface rupture behavior at an extensional fault bend (with an angle of about 30°) at Xichang between the ANHF and Zemuhe fault (ZMHF). Specifically, nine trenches were opened in a fault depression at Maoheshan site and five paleoseismic events were identified. These have been named E1 through E5 respectively corresponding to events at 1400-935 BC, 420-875 AD, 830-1360 AD, 1295-1715 AD, and 1790 AD-Present. After comparison with the historical records of earthquakes around Xichang and previous paleoseismic results, we suggest that the five seismic events are constrained at: 1365 BC-935 BC, 814 AD, 950 AD- 1145 AD, 1536 AD and 1850 AD, respectively. The average recurrence interval of

ACCEPTED MANUSCRIPT earthquakes along the southern segment of the ANHF is about 700–800 yr. Furthermore, the evidence indicates that surface-faulting events along the southern segment of the ANHF appear to be unevenly spaced in time. Moreover, based on comparisons of seismic events along the ANHF and ZMHF, we find that two fault segments simultaneous ruptured during the 814 AD and 1850 AD earthquakes, event E3 and the 1536 AD earthquake

PT

ruptured the ANHF but not rupture the ZMHF. We suggest that the Xichang fault bend is not a persistent fault boundary, indicating that extensional fault bends with an angle of

RI

about 30° may not effectively terminate seismic ruptures on strike-slip faults. Key words: the Anninghe fault, the Zemuhe fault, paleoseismology, fault bends, segment

SC

boundary

NU

1. Introduction

Fault bends are considered possible segment boundaries along strike-slip faults

MA

and therefore play a role in limiting the rupture length of earthquakes (King and Nabelek, 1985; Mann, 2007). Understanding the role fault bends may play as segment boundaries

D

is important in seismic hazard analysis for estimating the future maximal extent and

PT E

therefore size of earthquakes. In some recent studies, numerical modeling (Kase and Day, 2006; Lozos et al., 2011) and experimental observations (Kato et al., 1999) further suggested that different angles of fault bends may determine whether earthquake rupture

CE

could be arrested or not at fault bends on strike-slip faults. Specifically, Lozos et al. (2011) used a two-dimensional finite element method to simulate rupture propagation on fault

AC

bends and suggested that threshold angles (18° for compressional, 24° for extensional) above which rupture would be highly likely to be arrested. Geologically, fault bends acting as geometrical barriers to earthquake rupture have also been observed in paleoseismologic investigations and historic ruptures (Zhang et al., 1991; McCalpin et al., 1996; Langridge et al., 2002; Elliott et al., 2015). However, the historical rarity of large strike-slip earthquakes limits systematic investigations of the role of fault bends in halting ruptures. Here we explore a paleoseismic study on the Anninghe (ANHF) and integrate previous paleoseismic results on the Zemuhe fault (ZMHF) along the southeastern margin of the Tibetan Plateau to reveal a millennial seismic rupture history at the Xichang fault

ACCEPTED MANUSCRIPT bend, which has an angle of about 30° (Fig. 1, 2). The results have important implications for multi-cycle rupture patterns at fault bends on strike-slip faults, and are also helpful for reassessment of seismic risk in the populated Xichang region. Historical records show that regions of the ANHF and ZMHF have experienced intense seismic activity (Fig. 1). For example, three large earthquakes in 814 AD M7, 1536

PT

AD M7.5, and 1850 AD M7.5 occurred around Xichang City (Department of Earthquake Disaster Prevention, 1995). These historical records provide a rare opportunity for us to

RI

study whether previous earthquakes simultaneously ruptured across the Xichang fault bend between the ANHF and ZMHF. Recently, Wang et al. (2013, 2014) used multiple

SC

trenches at Yuehua and Daqingliangzi sites along the southern segment of the ANHF (south of Mianning) and the ZMHF, respectively, (Fig. 2) to better understand paleoseismic

NU

activity along these faults. Their study integrated previous paleoseismic results along the faults with their research and revealed evidence of five paleoseismic events. However, to

MA

better correlate simultaneous rupture behavior along the ANHF and ZMHF, we must first acknowledge that the previous paleoseismic research gives rise to the following

D

uncertainties: 1) A missing event without the corresponding historical records at the

PT E

Yuehua site has been found (Wang et al., 2014). This event needs to be checked at other sites along the fault; and 2) The Yuehua site is close to the Xichang fault bend (Fig. 2). Scharer et al. (2017) suggest that surface rupturing at the site may extend beyond the

CE

nominated fault segment boundary rather than being the result of simultaneous rupturing on the other side of the boundary. In other words, paleoseismic evidence at the Yuehua

AC

site does not emphatically rule out the possibility of rupturing along the fault trace being the result of an event along the ZMHF rather than separate seismic rupturing of the ANHF. To address the above two uncertainties, we chose a site which is much farther away the Xichang fault bend (about 70 km) along the southern segment of the ANHF at Maoheshan. The Maoheshan site is used to reveal the history of paleoseismic rupturing along the southern segment of the ANHF (Fig. 2). 2. Geologic settings The southeastern region of the Tibetan Plateau is located at the junction of three active tectonic blocks -- the Sichuan-Yunnan, Bayan Har, and South China blocks (Fig. 1).

ACCEPTED MANUSCRIPT The region has shown intense seismic activity in recent years. For example, the 2008 Mw7.9 Wenchuan earthquake, 2010 Ms7.1 Yushu earthquake and 2013 Ms7.0 Lushan earthquake (Fig. 1) all occurred in this region. These earthquakes caused significant economic loss and killed thousands of people. Within this region, are two faults, the ANHF and ZMHF, located on the eastern boundary of the Sichuan-Yunnan block (Fig. 1). The

PT

Sichuan-Yunnan block has been extruded in a southeastward direction by deformation of the Tibetan Plateau, resulting from the collision of the Indian and Eurasian plates

RI

(Tapponnier et al. 1982, 2001; Peltzer & Tapponnier 1988).

The ANHF is a left-lateral strike-slip fault that connects to the Xianshuihe fault

SC

(XSHF) in the north and terminates at the ZMHF in the south (Fig. 1). The ANHF strikes nearly north–south and consists of two fault segments which overlap around Mianning

NU

(Fig. 2) (Ran et al., 2008). The left-lateral slip rates of the ANHF were determined mainly on its northern segment. Ran et al. (2008) estimated a left-lateral Holocene slip rate of

MA

3.6–4.0 mm/yr based on aerial photo interpretations, total station surveys, and radiocarbon dating from multiple trenches. The northern part of the ZMHF connects to the

D

ANHF and forms the Xichang fault bend, which has an angle of about 30° (Fig. 2). The

PT E

left-lateral slip rates of the ZMHF are constrained at 3–5 mm/yr using measurements of deformed landforms and radiocarbon dating of charcoal and peat samples (Ren et al.,

CE

1994; He et al., 2008; Wang et al., 2011).

3. Paleoseismic investigations at Maoheshan site

AC

3.1 Site description To reveal evidence of paleoseismic events, we have conducted detailed geologic and geomorphic investigations along the Maoheshan section of the ANHF, where the presence of a nearly NS-striking fault valley is preserved that favors the development of depression catchments produced by slip along the fault (Fig. 3). The main sediments in the depression were likely transported to the site by runoff from local hills. When investigating the site, we chose the lowest depression as our trenching site to ensure we accessed better preserve evidence of paleoseismic rupturing. The trenching site was about 80 m in length and 25 m in width (28°30'27"N，102°11'57"E) (Fig. 3 and 4). To

ACCEPTED MANUSCRIPT constrain fault distributions, correlate stratigraphic units, and present more evidence of paleoseismic events, we opened nine trenches across the depression. 3.2 Stratigraphy All nine trenches were excavated by hand and named GYTC1 to GYTC6, and GYSNTC1 to GYSNTC3. Trenches GYTC1, GYTC2, GYTC3, GYTC4, GYTC5 and

PT

GYTC6 were excavated near perpendicularly across the fault valley to help reveal structure of faults. The other three trenches GYSNTC1, GYSNTC2, and GYSNTC3 were

RI

dug parallel to the strike of the fault valley to facilitate spatial correlation between stratigraphic units. Two faults named FW and FE were revealed through trenching.

SC

Trenches GYTC3, GYTC4, and GYTC5 revealed branch fault FW. However, during excavation, trenches GYTC1 and GYTC2 could not be opened farther westward by hand

NU

meaning we failed to constrain the branching of fault FW. The distributions of FE were constrained by all trenches except for GYSNTC3. For analysis purposes, each wall was

MA

near vertically excavated, systematically cleaned and gridded into squares of mainly 1 m by 1 m before being photographed. The photographs were used for detailed field mapping.

D

The stratigraphy of the depression was divided into six units, including 14 sub-units. The

PT E

data is summarized from oldest to youngest in Table 1 and Fig. 5 (letter “U” denotes strata units). After reviewing all the collected data for clear evidence of paleoseismic events and stratigraphic units, it was decided that trenches GYTC5, GYTC4, GYTC3 gave the best

CE

evidence of paleoseismic activity. This evidence is presented in Figs. 6, 7, 8, and 9 while data from the remaining trenches is given in Figs. S1-S6 of the supplementary files.

AC

3.3 Evidence of faulting events From trenching along the Maoheshan section, five paleoseismic events are identified and named E5 through E1 from oldest to youngest. Among the various indicators used for recognizing paleoearthquakes in unconsolidated sediments in a strike-slip fault setting, the best and clearest evidence for fault rupturing of the ground surface includes: scarp formation, scarp-derived colluvium, in-filled and void fissures, and sand blows (Sieh, 1978; Weldon et al., 1996; Fumal et al., 2002). Upward fault terminations are generally effective in identifying more recent faulting event, because subsequent ruptures often follow the same plane (McCalpin et al., 2009). Based on the

ACCEPTED MANUSCRIPT above indicators of strike-slip faulting, we show stratigraphic evidence for five paleoearthquakes from oldest to youngest at the site. Figure 5 shows the different stratigraphic units revealed by trenching and the corresponding events labelled E1 (youngest event) to E5 (oldest event). Stratigraphic Unit 1, which corresponds to paleoearthquake event E5, is comprised of 3 layers. These are termed U1-1 to U1-3.

PT

Subsequent events and their corresponding layers are labelled U2-1, U2-2, and so on. Interpretation of the evidence revealed by this study is examined below:

RI

Event E5

First, deformation evidence of older paleoseismic events is not well revealed during

SC

trenching because of trench depth. For event E5, only the southern wall of trench GYTC3 revealed deformation evidence. U1-2 (Stratigraphic Unit) intruded into U1-3 and is

NU

overlain by U2-1 (Fig. 8). U2-1 is a black peaty layer, indicating a quiet-water depositional environment. This is distinct from the coarse-grain sandy deposits of U1-3. This evidence

MA

indicates an event occurring that produced surface rupturing which blocked water flow allowing for the formation of a pond or swamp environment that was water rich and

D

allowed for peaty sediments to form. Therefore, we suggest that an earthquake occurred

Event E4

PT E

between deposition of U1-3 and U2-1.

Deformation evidence produced by event E4 is well revealed on the two walls of

CE

trench GYTC5, and the southern wall of trench GYTC3. In trench GYTC5, eastern branch fault FE ruptured U2-1 and U2-2, which in turn were overlain by U3-2 (Figs. 6, 7, 10). U2-1

AC

was apparently vertically offset by about 10 cm (Fig. 10). In the eastern part of the trench exposure, U2-2 was offset and overlain by a wedge-shaped U3-1, which we interpret as a scarp-derived deposit following an earthquake (Fig. 6, 7). On the eastern part of the southern wall of the trench GYTC3, U1-3 intruded into U2-1, showing an upward folding deformation and was unconformably overlain by U3-2 (Fig. 8). Base on the above evidence, we suggest that event E4 occurred after deposition of U2-2 and before that of U3-1. Event E3 Paleoseismic evidence produced by event E3 is primarily revealed on the southern

ACCEPTED MANUSCRIPT wall of trench GYTC3 (Fig. 8). In the central part of the trench exposure, U2-2 intruded into U3-2 with some nodules consisting of sandy clay from U2-2 being observed in the grey-black peat of U3-2. U4-2 unconformably overlies U3-2 (Fig. 8). Moreover, U3-2 is deformed and overlain by U4-1 with the shape of a scarp-derived deposit on the southern wall of trench GYTC5 (Fig. 6, 11). We suggest that event E3 occurred between the

PT

depositional age of U3-2 and U4-1. Event E2

RI

Deformation evidence produced by event E2 is mainly revealed on the southern walls of trenches GYTC5 and GYTC3. Specifically, U4-3 is offset and overlain by U5-1 on

SC

the southern wall of trench GYTC5 (Fig. 6, 11). U4-2 is offset and overlain by U5-1, which has the shape of a scarp-derived colluvial deposit (Fig. 8). We suggest that event E2

NU

occurred after deposition of U4-3 and before that of U5-1. Event E1

MA

Deformation evidence produced by the latest event E1 is a little ambiguous. The southern wall of trench GYTC5 shows that E1 ruptured U5-1 near the subsurface and

D

produced a fissure that is overlain by scarp-derived deposit U6-1 (Fig. 6, 11). Moreover,

PT E

similar deformation exists such as subsurface wedge-shaped fissures. U5-1 and U5-2 faulting on the southern wall of trench GYTC4 is also revealed (Fig. 9). Combining all these lines of evidence in the trenches, we suggest that the latest event E1 occurred

CE

between depositional age of U5-2 and U6-1. 3.4 Radiocarbon dating of paleoearthquakes

AC

Numerous charcoal and peat samples were found in the trenches. Nine charcoals were sent to Beta Analytic Inc. of America for radiocarbon dating with all charcoal samples being dated by accelerometer mass spectrometer (AMS dating). One peat sample (Y5) was processed at the Laboratory of Neotectonics and Geochronology, Institute of Geology, China Earthquake Administration. The radiocarbon dating results are summarized in Table 2. All ages reported herein are two-sigma (95.4% confidence limits) calendric ages calibrated with the OxCal 4.3 program (Bronk-Ramsey 2009) using the IntCal09 atmospheric model from Reimer et al. (2009). For the calibrated age, probability density functions (PDFs) overlap between different samples. OxCal uses Bayesian statistics to

ACCEPTED MANUSCRIPT re-weigh 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 multi-peaked distributions (Bronk Ramsey, 2009; Lienkaemper et al., 2009). All radiocarbon dates from trenches are generally in correct stratigraphic order, such that younger samples overlie older samples. Samples with much closer dating ages within an

PT

error range of several tens of years or collected within the sub units from the same stratigraphic unit were used in phase correction when establishing sequence correction

RI

during processing of the OxCal 4.3 program (Dawson et al. 2003). For example, we put samples (GYTC310, GYTC301 and GYTC510) from U5-2 into a phase. Similarly, samples

two different trenches are also put into phase.

SC

(GYTC513, GYTC506, GYTC315, and GYTC306) collected in subunits from U2-2 in the

NU

As previously discussed, event E5 occurred after deposition of samples (GYTC310, GYTC301 and GYTC510) in U5-2. Using OxCal for further analysis, the age of event E1 is

MA

well constrained between 1790 AD-Present (Fig. 12). In keeping with this kind of scenario, ages of all events can be constrained by dating samples from scarp-derived colluvial

D

deposits and faulting or disturbed units below. Event E2 occurred before deposition of

PT E

U5-1 and after that of U4-3. Charcoal samples were used to constrain the age of event E4 to between 1295-1715 AD. Event E3 occurred before deposition of U4-1 and after that of U3-2, and events E4 and E5 respectively occurred before deposition of U3-1, U2-1 and

CE

after that of U2-2, U1-3. Event E3 is constrained by samples GYTC312 and GYTC504 from U4-3 and U3-1, respectively, and is dated between 830-1360 AD. Event E4 is

AC

constrained by samples GYTC504 in U3-1 as well as GYTC513, GYTC506, GYTC315 and GYTC306 in U2-2. It is dated to between 420-875 AD. Event E5 is just constrained on its upper boundary by sample Y5 and dated as 1400-935 BC. 4. Discussions 4.1 Comparisons of paleoseismic events and historical recorded earthquakes between the Maoheshan and Yuehua sites on the southern segment of the ANHF Wang et al. (2014) used multiple trenches to reveal evidence of five paleoseismic events and used an OxCal 3.10 program to constrain ages of the events from youngest to oldest at: 1750 AD–present, 1430 AD–1870 AD, 940 AD–1150 AD, 700 AD–1000 AD, and

ACCEPTED MANUSCRIPT 1400 BC-500 BC, respectively. Here, we used the OxCal 4.3 program to recalibrate the event ages to: 1730 AD–present, 1440 AD–1765 AD, 950 AD–1145 AD, 700 AD–995 AD, and 1365 BC-505 BC (Fig. 13), respectively. The Maoheshan trenching site also revealed evidence of five paleoseismic events. The ages are constrained from youngest to oldest at: 1790 AD–present, 1295 AD–1715 AD, 830 AD–1360 AD, 420 AD–875 AD, and 1400

PT

BC-935 BC, respectively (Fig. 12). Comparing the paleoseismic results of the two sites (Fig. 13) indicates that both sets of results are consistent, indicating a complete

RI

paleoseismic sequence.

Records of historical earthquakes for the region mention that three large

SC

earthquakes occurred in 814 AD M7, 1536 AD M7.5, and 1850 AD M7.5 around Xichang (Department of Earthquake Disaster Prevention, 1995). Using similar correlations

NU

between historical earthquakes and paleoseismic events (Wang et al., 2014), we suggest that events E4 and E2 are associated with the 814 AD and 1536 AD earthquakes. As for

MA

event E3, the age of this event is constrained to 830 AD–1360 AD at the Maoheshan site and 950 AD–1145 AD at the Yuehua site. We suggest that the narrower age range

D

between 950 AD–1145 AD better represents the timing of the event. This age is close to

PT E

the older seismic event E4 that is interpreted as the 814 AD earthquake. No historical records around the Xichang region document an earthquake during this time period. However, upon careful examination of the Chinese historical records, we find that this time

CE

range covers the rule of the Dynasty of Five Kingdoms and Ten Countries, when the region experienced frequent wars and invasions of different tribes. Moreover, this event

AC

tightly followed the 814 AD earthquake that caused severe damage and human loss. Thus, the local economy and infrastructure may not have been well developed when this “missing” earthquake occurred so it may not have been recorded. The age of the oldest event E5 is beyond the range of historical records at Xichang. As for the latest event E1, Wang et al. (2013) previously suggested two possibilities for interpreting the event: 1) It was associated with the 1850 AD M7.5 earthquake; or 2) It relates to a mid-sized magnitude

earthquake

of

M6.75

(or

M6.6

determined

by

http://earthquake.usgs.gov/earthquakes/eventpage/iscgem893478#general),

USGS, which

occurred in 1952 AD (Department of Earthquake Disaster Prevention, 1995). We

ACCEPTED MANUSCRIPT reanalyzed both possibilities to correlate existing evidence with these historical earthquakes and suggest that the youngest seismic event revealed from trenching is more likely to be interpreted as the 1850 AD earthquake rather than the 1952 AD earthquke. The three primary reasons are analyzed as follows: 1) Based on multiple surface-rupturing statistics and the corresponding magnitudes of earthquakes summarized by Wells and

PT

Coppersmith (1994), who used an equation to summarize strike-slip faulting where by: M = 5.16(±0.13) + 1.12(±0.08) ∗ lg(SRL), in which M denotes the magnitude of an

RI

earthquake, SRL represents the corresponding surface rupture length. Applying this equation to the moderate sized M6.75 earthquake of 1952 AD would give a

SC

surface-rupture length of 16-45 KM. Evidence at the trenching sites of Maoheshan and Yuehua indicate that it is possible the whole southern segment of the ANHF (south of

NU

Mianning) was ruptured during the youngest event E1. This segment is about 90 km long (Fig. 2), which is much longer than the potentially calculated rupture length by the 1952

MA

AD earthquake. In other words, the magnitude of the youngest event would be much greater than that of the 1952 AD earthquake. 2) There are no studies reporting surface

D

rupturing associated with the 1952 AD earthquake (Department of Earthquake Disaster

PT E

Prevention, 1995). If the 1952 AD earthquake produced surface ruptures, field surface expressions would be easily visible because any coseismic deformation would be relatively fresh. 3) The age of the latest event is constrained at 1790 AD – Present,

CE

historical records around the Xichang documented only two earthquakes, ie. the 1850 AD and the 1952 AD earthquakes during the time range. Since we suggest that it is unlikely to

AC

associate the youngest event with the 1952 AD earthquake, we are more inclined to suggest that the 1850 AD earthquake also ruptured the southern segment of the ANHF. Moreover, the magnitude of the 1850 AD earthquake is much greater than that of the 1952 AD event, and great earthquakes easily produce surface ruptures than small earthquakes (McCalpin 2009). In other words, the 1952 AD earthquake is suggested to not produce ruptures to the surface, which is also evidenced by a recent earthquake in the nearby region with a similar magnitude and seismotectonic setting. Specifically, Jiang et al. (2015) used joint analysis with seismicity relocation and InSAR inversion on the 2014 Kangding M6.3 (or Mw 5.9) along the left-lateral strike-slip XSHF and suggested that the earthquake

ACCEPTED MANUSCRIPT could not produce surface ruptures. 4.2 Characteristics of the paleoearthquake sequence along the southern segment of the ANHF After correlating the paleoseismic results at Maoheshan and Yuehua with those found in the historical record, we suggest that the five seismic events from oldest to

PT

youngest are constrained at: 1365 BC-935 BC, 814 AD, 950 AD- 1145 AD, 1536 AD and 1850 AD, respectively. The average recurrence interval of earthquakes on the southern

RI

segment of the ANHF is about 700–800 yr, which is slightly better constrained than the previously determined 600-800 yr (Wang et al., 2014). The interval between E1 and E2 is

SC

314 yr, and 391–586 yr between E2 and E3. The interval between E3 and E4 is shorter at 136–331 yr, but much longer at 1749–2179 yr between E4 and E5. These surface faulting

NU

events on the southern segment of the ANHF appear to be unevenly spaced in time. Earthquakes reoccurring at such irregular intervals along the southern segment of the

MA

ANHF might be related to the complexity of fault interactions (Rockwell et al., 2000; Dolan et al., 2007; Berryman et al., 2012) in the left-lateral fault system of the eastern and

D

northern boundaries of the Sichuan-Yunnan block. They may also indicate regional

PT E

irregularities for large earthquakes within the southeastern region of the Tibetan Plateau (Wang et al., 2013). In terms of seismic risk assessment, the minimum recurrence interval of 136–331 yr between the events E3 and E4 is probably the most prudent interval in the

CE

southern segment of the ANHF.

4.3 Comparisons of historical earthquakes on the southern segment of the

AC

ANHF and ZMHF and implications for fault rupture behavior at fault bends on strike-slip faults

Wang et al. (2013) used multiple trenches and historical records to suggest that the most recent two large earthquakes on the ZMHF are associated with the 814 AD and 1850 AD earthquakes (Fig. 13). Based on the comparisons of seismic events on the ANHF and ZMHF (Fig. 13), we find that the two fault segments simultaneous ruptured during the 814 AD and 1850 AD earthquakes while the missing event and 1536 AD earthquake ruptured the ANHF but not rupture the ZMHF. The fault distributions of the ANHF and ZMHF at Xichang show that the Xichang fault bend is extensional with an angle of about 30°.

ACCEPTED MANUSCRIPT However, Lozos et al. (2011) suggested that it is highly possible that seismic rupture would be arrested by a threshold angle above 24° along an extensional fault bend. Multi-cycle paleoseismic observations along the ANHF and ZMHF do not appear to be completely consistent with the hypothesis presented by Lozos et al. (2011). In other words, two of the four seismic events show that the Xichang fault bend could not stop seismic

PT

rupturing, which is more complex than the simplified model presented by Lozos et al. (2011). Since the historical record could not determine the exact location of initial rupturing

RI

of paleoearthquakes, we analyze rupture behavior of the ANHF and ZMHF based on the following possibilities and related assumptions:

SC

Possibility A: If initial rupturing of the four seismic events occurred along the ANHF (Fig. 14a), the evidence indicates that the 1850 AD and 814 AD earthquakes jumped the

NU

Xichang fault bend and produced simultaneous rupturing of the ZMHF, while the ruptures of the 1536 AD and missing events were arrested at the fault bend. In other words, there is

MA

an approximately 50% possibility that seismic rupturing could be arrested by the Xichang fault bend. Based on this assumption, we speculate that this rupture behavior through the

D

fault bend might be a random process.

PT E

Possibility B: If initial rupturing of the 814 AD and 1850 AD earthquakes occurred along the ZMHF while the other two earthquakes nucleated on the ANHF(Fig. 14b). It is shown that the 1850 AD and 814 AD earthquakes could jump over the Xichang fault bend

CE

and produce simultaneous rupturing of the ANHF, while the ruptures of the 1536 AD and missing events were arrested at the fault bend. In this case there is an approximately

AC

100% possibility that seismic rupture is arrested by the Xichang fault bend from the ANHF to the ZMHF, with 0% possibility of such an occurrence happening in the opposite direction. This further indicates that when the ZMHF ruptures during an earthquake, the ANHF also ruptures. However, when the ANHF ruptures during an earthquake, the ZMHF may not rupture. Based on this assumption, we speculate that ruptures can jump the Xichang fault bend relatively easily from the ZMHF to the ANHF but are impeded in the opposite direction. Possibility C: If three events nucleated on the ANHF and the other on the ZMHF (Fig. 14c, d), which means two of the three events (a 67% possibility) were stopped by the

ACCEPTED MANUSCRIPT Xichang fault bend from the ANHF to ZMHF, with 0% possibility that seismic rupturing is impeded from the ZMHF to the ANHF. Based on this assumption, we speculate that the potential for seismic rupturing jumped from the ANHF to ZMHF appears to be slightly lower than that from the opposite direction. Comparing the three assumptions, possibilities B and C suggest seismic rupturing

PT

jumping from the ZMHF to the ANHF occurs more readily than it does in the opposite direction (Fig. 14 b, c, d). Though we do not know what mechanism contributes to this

RI

rupture behavior at the Xichang fault bend, it is plausible that the rupture pattern relates to fault movement direction, regional stress status, and fault structures (Poliakov et al., 2002).

SC

Possibility A, on the other hand, suggests an irregular rupture pattern. We cannot rule out that the pattern of seismic rupturing around the Xichang bend is random chance. This

NU

pattern may depend on the state of stress along the ZMHF resulting in random simultaneous rupturing due to coseismic or long-term fault interactions (Stein et al., 1997;

MA

Dolan et al., 2007). In aggregate, the above three possibilities indicate that the Xichang fault bend is not a persistent fault boundary even though it has an extensional fault bend

D

angle of about 30°. The evidence suggests in a strike-slip fault seismic environment such

PT E

an angle is not necessarily effective at terminating seismic ruptures. Furthermore, the rupture patterns at Xichang fault bend indicate that using only one seismic rupture at fault bends to estimate seismic risk underestimates future seismic risk. Therefore, the study of

CE

multi-cycle paleoearthquakes is useful to understanding the present level of seismic risk and rupture patterns associated with fault bends.

AC

5. Conclusion

We used multiple trenching and radiocarbon dating to reveal evidence of five paleoseismic events at a depression at the Maoheshan site along the southern segment of the ANHF on the southeastern margin of the Tibetan Plateau. Ages of the five seismic events are constrained from oldest to youngest at: 1400-935 BC, 420-875 AD, 830-1360 AD, 1295-1715 AD, and 1790 AD-Present. After comparison with the historical earthquake records of the Xichang region and previous paleoseismic results nearby, we further suggest that the five seismic events are constrained at: 1365 BC-935 BC, 814 AD, 950 AD- 1145 AD, 1536 AD and 1850 AD, respectively. This paleoseismic sequence suggests

ACCEPTED MANUSCRIPT that the average recurrence interval of earthquakes along the southern segment of the ANHF is about 700–800 yr. Further, the reoccurrence pattern is unevenly spaced in time with a minimum recurrence interval of 136–331 yr between events E3 and E4. Using such an interval for seismic risk assessment would be prudent along the southern segment of the ANHF. In addition, evidence indicates that the Xichang fault bend is probably not a

PT

persistent fault boundary although it has an angle of about 30°. This result suggests that in a strike-slip fault environment such a fault bend angle cannot effectively terminate seismic

RI

ruptures. Multi-cycle paleoearthquakes are very useful in interpreting seismic hazards and rupture patterns around fault bends

SC

Acknowledgements

This work was supported by the National Science Foundation of China (Grant

NU

41672207). Great thanks to Julian C. Lozos and Jiyang Ye for their helpful discussions.

MA

Thanks to An Li and Liangxin Xu for field work.

References

D

Berryman, K.R., Cochran, U.A., Clark, K.J., Biasi, G.P., Langridge, R.M., Villamor, P., 2012.

1690-1693.

PT E

Major earthquakes occur regularly on an isolated plate boundary fault. Nature 336,

Bronk, R.C., 2009. Bayesian analysis of radiocarbon dates. Radiocarbon 51, 337-360.

CE

Dawson, T.E., McGill, S.F., Rockwell, T.K., 2003. Irregular recurrence of paleoearthquakes along the central Garlock fault near El Paso Peaks, California. J. Geophys. Res.

AC

108, 2356-2385.

Deng, Q.D., 2007. Map of Active Tectonics in China. Seismological Press, Beijing. Deparment of Earthquake Disaster Prevention, China Earthquake Administration (DEDP, CEA), 1995. The catalogue of Chinese historical strong earthquakes, 514 pp., Seismological Press, Beijing, China. Dolan, J.F., Bowman, D.D., Sammis, C.G., 2007. Long-range and long-term fault interactions in Southern California. Geology 35, 855-858. Elliott, A.J., Oskin, M.E., Liu-Zeng, J., Shao, Y.X., 2015. Rupture termination at restraining bends: The last great earthquake on the Altyn Tagh Fault. Geophys. Res. Lett. 42,

ACCEPTED MANUSCRIPT 2164-2170. Fumal, T.E., Weldon, R.J., Biasi, G.P., Dawson, T.E., Seitz, G.G., Frost, W.T., Schwartz, D.P., 2002. Evidence for Large Earthquakes on the San Andreas Fault at the Wrightwood, California, Paleoseismic Site: A.D. 500 to Present. Bull. Seismol. Soc. Am. 92, 2726-2760.

PT

He, H.L., Oguchi, T., 2008. Late Quaternary activity of the Zemuhe and Xiaojiang faults in southwest China from geomorphological mapping. Geomorphology 96, 62-85.

RI

Jiang, G.Y., Wen, Y.M., Liu, Y.J., Xu, X.W., Fang, L.H., Chen, G.H., Gong, M., Xu, C.J., 2015. Joint analysis of the 2014 Kangding, southwest China, earthquake sequence

SC

with seismicity relocation and InSAR inversion. Geophys. Res. Lett. 42, 3273-3281. Kase, Y., Day, S.M., 2006. Spontaneous rupture process on a bending fault. Geophys.

NU

Res. Lett. 33, L10302. doi: 10.1029/2006GL025870.

Kato, N., Satoh, T., Lei, X.L., Yamamoto, K., Hirasawa, T., 1999. Effect of fault bend on the

MA

rupture propagation process of stick-slip. Tectonophysics 310, 81-89. King, G., Nabelek, J., 1985. Role of Fault Bends in the Initiation and Termination of

D

Earthquake Rupture. Science 228, 984-987.

PT E

Langridge, R.,Stenner, H., 2002. Geometry, slip distribution, and kinematics of surface rupture on the Sakarya fault segment during the 17 August 1999 Izmit, Turkey, earthquake. Bull. Seismol. Soc. Am. 92, 107-125.

CE

Lienkaemper, J.J., Ramsey, B.C., 2009. OxCal: Versatile tool for developing paleoearthquake chronologies–a primer: Seismological Research Letters 80,

AC

431-434.

Lozos, J.C., Oglesby, D.D., Duan, B.C., Wesnousky, S.G., 2011. The effect of double fault bends on rupture propagation: A geometrical parameter study. Bull. Seismol. Soc. Am. 101, 385-398. Mann P., 2007. Global catalogue, classification and tectonic origins of restraining- and releasing bends on active and ancient strike-slip fault systems. Geo. Soc. London Special Pub. 290, 13-142. McCalpin, J., Nishenko, S.,1996. Holocene paleoseismicity, temporal clustering, and probabilities of future large (M>7) earthquakes on the Wasatch fault zone, Utah. J.

ACCEPTED MANUSCRIPT Geophys. Res. 101, 6233-6253. McCalpin, J.P., 2009. Paleoseismology. 473 pp., Academic Press, San Diego, California, USA. Peltzer, G., Tapponnier, P., 1988. Formation and evolution of strike-slip faults, rifts, and basins during the India-Asia collision: an experiment approach. J. Geophys. Res. 93,

PT

15085-15117. Poliakov, A.N.B., Dmowska, R., Rice, J.R., 2002. Dynamic shear rupture interactions with

RI

fault bends and off-axis secondary faulting. J. Geophys. Res. 107, B11. doi: 10.1029/2001JB000572.

SC

Ran, Y.K., Cheng, J.W., Gong, H.L., Chen, L.C., 2008. Late Quaternary geomorphic deformation and displacement rates of the Anninghe fault around Zimakua. Seismol.

NU

Geol. 30, 86-98 (in Chinese with English abstract).

Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., Ramsey,

MA

C.B., Buck, C.E., Burr, G.S., Edwards, R.L., Friedrich, M., Grootes, P.M., Guilderson, T.P., Hajdas, I., Heaton, T.J., Hogg, A.G., Hughen, K.A., Kaiser, K.F., Kromer, B.,

D

McCormac, F.G., Manning, S.W., Reimer, R.W., Richards, D.A., Southon, J.R.,

PT E

Talamo, S., Turney, C.S.M., Van der Plicht, J., Weyhenmeyer, C.E., 2009. IntCal09 and Marine09 Radiocarbon Age Calibration Curves, 0-50,000 Years cal BP. Radiocarbon 51, 1111-1150.

CE

Ren, J.W., 1994. Late Quaternary displacement and slip rate of the Zemuhe fault in Sichuan, China. Seismol. Geol. 16, 146 (in Chinese with English abstract).

AC

Rockwell, T.K., Lindvall, S., Herzberg, M., Murbach, D., Dawson, T., Berger, G., 2000. Paleoseismology of the Johnson Valley, Kickapoo, and Homestead Valley Faults: clustering of Earthquakes in the Eastern California Shear Zone. Bull. seism. Soc. Am. 90, 1200–1236. Scharer, K., Weldon, R., Biasi, G., Streig, A., Fumal, T., 2017. Ground-rupturing earthquakes on the northern Big Bend of the San Andreas Fault, California, 800 A.D. to Present. J. Geophys. Res. 122, 2193-2218. Stein, R.S., Barka, A.A., Dieterich, J.H., 1997. Progressive failure on the North Anatolian fault since 1939 by earthquake stress triggering. Geophys. J. Int. 128, 594-604.

ACCEPTED MANUSCRIPT Sieh, K.E., 1978. Prehistoric large earthquake produced by slip on the San Andreas Fault at Pallett Creek, California. J. Geophys. Res. 83, 3907-3939. Tapponnier, P., Peltzer, G., Le Dain, A.Y., Armijo, R., Cobbold, P., 1982. Propagating extrusion tectonics in Asia: New insights from simple experiments with plasticine. Geology 10, 611-616.

PT

Tapponnier, P., Xu, Z.Q., Roger, F., Meyer, B., Arnaud, N., Wittlinger, G., Yang, J.S., 2001. Oblique Stepwise Rise and Growth of the Tibet Plateau. Science 294, 1671-1677.

RI

Wang, H., Ran, Y.K., Li, Y.B., 2011. Growth of a small pull-apart basin and slip rate of strike-slip fault: with the example of Zemuhe fault on the southeastern margin of the

SC

Tibetan Plateau. Seismol. Geol. 33, 818-827 (in Chinese with English abstract). Wang, H., Ran, Y.K., Li, Y.B., Gomez, F., Chen, L.C., 2013. Holocene palaeoseismologic

Plateau. Geophys. J. Int. 193, 11-28.

NU

record of earthquakes on the Zemuhe fault on the southeastern margin of the Tibetan

MA

Wang, H., Ran, Y.K., Li, Y.B., Gomez, F., Chen, L.C., 2014. A 3400-year-long paleoseismologic record of earthquakes on the southern segment of Anninghe fault

D

on the southeastern margin of the Tibetan Plateau. Tectonophysics, 628, 206-217.

PT E

Weldon, R.J., McCalpin, J.P., Rockwell, T.K., 1996. Paleoseismology of strike-slip tectonic environments, in Paleoseismology, J. McCalpin (Editor), Academic Press, San Diego, 271-329.

CE

Wells, D.L., Coppersmith, K.J., 1994. New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement. Bull. Seismol.

AC

Soc. Am. 84, 974-1002. Zhang, P.Z., Slemmons, D., Mao, F.Y., 1991. Geometric pattern, rupture termination and fault segmentation of the Dixie Valley-Pleasant Valley active normal fault system, Nevada, U.S.A. J. Struct. Geol. 13, 165-176.

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig.1. Tectonic and topographic map of the eastern Tibetan Plateau. The white rectangle

CE

shows the ANHF and ZMHF region. Four red stars represent epicenters of the 2001 Mw7.8 Kunlun earthquake, 2008 Mw7.9 Wenchuan earthquake, 2010 Ms7.1 Yushu

AC

earthquake, and 2013 Ms7.0 Lushan earthquake. Small red circles show epicenters of earlier documented (historical and instrumental) earthquakes. Small white rectangles denote cities. KLF, Kunlun Fault, GZ-YSF, Ganzi-Yushu Fault, XSHF, Xianshuihe Fault, LMSF, Longmenshan Fault, ANHF, Anninghe Fault, ZMHF, Zemuhe Fault, DLSF, Daliangshan Fault, XJF, Xiaojiang Fault, HHF, Honghe Fault, JSJF, Jinshajiang Fault. The locations of active faults and earthquake epicenters are from the tectonic activity map of China (Deng et al., 2007).

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

PT E

Fig.2. Fault distributions around the Xichang fault bend region. The two grey rectangles show the trenching sites at Yuehua on the ANHF and Daqingliangzi on the ZMHF, while the white one relates to the Maoheshan trenching site. Red and black lines denote active

CE

and inactive faults, respectively. The inset map shows schematic fault distributions of the ANHF and ZMHF, the white rectangles are correlated to the three trenching sites,

AC

respectively.

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig.3. Photo and high-resolution image (Google Earth) around the trenching site at Maoheshan section. (a) Red arrows show fault traces of the ANHF. The trenches were

CE

excavated in the depression shown by a white arrow. (b) A high-resolution image (Google Earth) shows the trenching site and its simple interpretation. Red arrows show the ANHF.

AC

A sag pond modified as a pool is located south of the trenching site.

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig.4. Surveyed landforms around the trenching site at the Maoheshan section. The

MA

landforms were measured by GPS RTK. Nine trenches were excavated in the depression. Based on the revelation of trench exposures, distributions of the two faults (named FW and

AC

CE

PT E

D

FE) were mapped and shown by the solid red lines. Contour interval is 1 m.

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

D

Fig.5. Stratigraphic units revealed from trenching. The right column summarizes evidence

AC

CE

PT E

of palaeoseismic events in trenches and the related figures.

RI

PT

ACCEPTED MANUSCRIPT

SC

Fig.6. Map of the southern wall of trench GYTC5 and its interpretation. Red lines represent faults, and black lines show stratigraphic contacts, special cases are dashed. U

NU

denotes units. The lines with numbers show sampling locations, labeled by sample

AC

CE

PT E

D

MA

number and corresponding dated radiocarbon ages.

RI

PT

ACCEPTED MANUSCRIPT

SC

Fig.7. Map of the northern wall of trench GYTC5 and its interpretation. Red lines represent faults, and black lines show stratigraphic contacts, specifically while in dashed. U denotes

AC

CE

PT E

D

MA

corresponding dated radiocarbon ages.

NU

units. The lines with numbers show sampling locations, labeled by sample number and

RI

PT

ACCEPTED MANUSCRIPT

SC

Fig.8. Map of the southern wall of trench GYTC3 and its interpretation. Red lines represent faults, and black lines show stratigraphic contacts, special cases are dashed. U

NU

denotes units. The lines with numbers show sampling locations, labeled by sample

AC

CE

PT E

D

MA

number and corresponding dated radiocarbon ages.

SC

RI

PT

ACCEPTED MANUSCRIPT

NU

Fig.9. Map of the southern wall of trench GYTC4 and its interpretation. Red lines represent faults, and black lines show stratigraphic contacts, special cases are dashed. U

AC

CE

PT E

D

MA

denotes units.

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

MA

Fig.10 Close-up views of deformation revealed in the southern wall of the trench GYTC5.

AC

CE

PT E

D

U2-1 was apparently vertically offset by 10 cm, U2-2 was offset and overlain by U3-2.

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

Fig.11 Close-up views of deformation revealed in the southern wall of the trench GYTC5.

AC

CE

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig. 12 Probability density functions for the timing of events from the trenches calculated using OxCal 4.3 (Bronk, 2009), incorporating both the relative age of events (stratigraphic ordering) and posterior calibrated radiocarbon ages from units that bracket event horizons. Phases are the summed probability for units with multiple radiocarbon dates.

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig.13. Comparison of historical earthquake records and paleoearthquakes along the

NU

ANHF and ZMHF. The left column of the figure: the angle of the Xichang extensional fault bend is about 30°. The yellow, green and blue rectangles represent the trenching sites at

MA

Maoheshan, Yuehua on the southern segment of the ANHF and Daqingliangzi on the ZMHF. The right column of the figure: the dashed thick blue horizontal line marks the fault boundary of the ANHF and ZMHF segmented by the Xichang fault bend. Grey solid

D

curves represent the age distributions of the paleoearthquakes with two-sigma error

PT E

modeled by OxCal 4.3 program. Vertical red lines show the precise timing of the historical earthquakes and their corresponding seismogenic structures. The numbers with arrows

CE

denote the calibrated time ranges of the paleoseismic events. The paleoseismic results at Yuehua and Daqingliangzi are from Wang et al. (2013, 2014), respectively. Considering

AC

the age of the oldest event E5 revealed at Yuehua and Maoheshan sites is beyond historical records, we do not analyze this event in the figure.

PT E

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

Fig.14. Schematic models for interpretation of seismic rupture through the Xichang fault

CE

bend between the ANHF and ZMHF. (a) Locations of initial rupture of the four earthquakes are assumed on the ANHF; (b) Locations of initial rupture of the 1850 AD and 814 AD

AC

earthquakes are assumed on the ZMHF while the other two on the ANHF; (c) and (d) Locations of initial rupture of three earthquakes are assumed on the ANHF with the other one on the ZMHF. Red lines represent surface rupturing while black lines show no rupturing; arrows denote rupture propagation by red and black lines, which jump or terminate at the fault bend, respectively.

ACCEPTED MANUSCRIPT Table 1 Unit descriptions from trenching exposures at the Maoheshan site. Description

U1-1

This unit is brown-pink sandy layer, locally contains some weathered and transported bedrocks.

U1-2

This unit is grey-green silty and sandy layer, mainly revealed in the trenches GYTC3 and GYSNTC2

U1-3

This unit varies slightly in different trenches. In the northern part of the trenching site, this unit is mainly brown or pink dusty sandy clay. In the southern part of the trenching site, this unit manly consists of grey-dusty sandy gravel layer.

U2-1

This unit is black peat layer, locally containing small gravels, and represents a low-energy water-rich environment.

U2-2

This unit could be divided into two sub units. The upper part is brown-dust sandy clay and contains small gravels locally. The lower part is brown-pink sandy clay, contains weathered bedrocks locally.

U3-1

This unit is mainly revealed in the trenches GYTC5 and GYTC4, and consisting of peaty sand or collapsed clay nodule interpreted as scarp-derived colluvial deposit.

U3-2

This unit is grey peaty and sandy clay, and widely developed in all trenches.

U4-1

This unit is grey peaty and sandy clay and not widely developed in the trenches. This unit is clearly shown on the southern wall of the trench GYTC5.

U4-2

This unit is grey sandy clay and basically observed in all trenches.

U4-3

This unit is white-dusty sandy clay, we only observed this unit on the southern wall of the trench GYTC5 and speculate that the unit might develope very locally.

U5-1

This unit is grey-dusty sandy clay, locally containing nodule clay and interpreted as scarp-derived colluvial deposit.

U5-2

This unit is dusty sandy clay.

U6-1

This unit is dusty sandy clay, locally containing nodule clay and interpreted as scarp-derived colluvial deposit. This unit is well revealed on the southern wall of the trench GYTC5.

U6-2

A light yellow-dusty sandy clay, this unit might be modified by human.

AC

CE

PT E

D

MA

NU

SC

RI

PT

Unit No.

ACCEPTED MANUSCRIPT

Table 2 Radiocarbon samples from trenching at the Maoheshan site. Calendar years (Cal BP) 13

C/12C

sample

Radiocarbon age

T P

Lab No. (o/oo)

(years B.P. ±σ)

Description

Unit sampled

Peat

U2-1，GYTC5

770-430 BC

Angular charcoal

U2-2，GYTC5

205 BC- 5 AD

Angular charcoal

U2-2，GYTC5

Angular charcoal

U2-2，GYTC3

Angular charcoal

U2-2，GYTC3

Angular charcoal

U3-2，GYTC5

1σ

2σ

Y5*

2795±50

1085-825 BC 865-855 BC (1.3%)

U N

755-680 BC (25.0%) GYTC513

270341

-25.8

2470±40

675-605 BC (20.3%) 600-515 BC (22.9%)

GYTC506

270339

-24.0

2080±40

GYTC315

270334

-25.6

1770±40

D E

165-45 BC

PT

A M

130-355 AD (93.3%)

220-335 AD

E C

I R

SC

1015-895 BC (66.9%)

365-380 AD (2.1%)

395-435 AD (25.9%)

GYTC306

270331

-24.4

C A

1610±40

350-370 AD (2.0%)

450-475 AD (10.5%) 375-550 AD (93.4%) 485-535 AD (31.8%) 720-745 AD (3.5%)

GYTC504

270337

-23.6

1180±40

775-890 AD 765-970 AD (91.9%)

ACCEPTED MANUSCRIPT

GYTC312

270333

-25.2

1275-1310 AD (37.8%)

1265-1330 AD (51.7%)

1360-1390 AD (30.4%)

1340-1400 AD (43.7%)

670±40

Angular charcoal

T P

1520-1575 AD (7.5%)

GYTC310

270332

-24.8

230±40

1640-1680 AD (31.8%)

1585-1590 AD (0.2%)

1760-1805 AD (26.8%)

1625-1695 AD (36.8%)

1935 AD- (9.6%)

1725-1815 AD (38.2%)

C S U

I R

U4-2，GYTC3

Angular charcoal

U5-2，GYTC3

Angular charcoal

U5-2，GYTC3

Angular charcoal

U5-2，GYTC5

1920AD- (12.6%)

N A

1645-1710 AD (20.1%) 1660-1685 AD (11.7%)

M

1715-1820 AD (47.1%)

GYTC301

270330

-23.8

180±40

1730-1810 AD (41.5%)

D E

1830-1885 AD (9.4%)

1925 AD- (15.0%)

T P E

1910 AD- (18.7%)

1695-1720 AD (16.2%)

GYTC510

270340

-24.3

C C

30±40

1690-1730 AD (22.5%)

1815-1835 AD (11.1%) 1810-1925 AD (72.9%)

1875-1920 AD (40.9%)

A

Note: All samples except for Y5 were processed by standard radiometric dating at the eta Analytic Inc. Miami, Florida USA. The sample Y5 was processed at Laboratory of Neotectonics and Geochronology, Institute of Geology, China Earthquake Administration. The calendar age ranges are equivalent to the 2σ age ranges (95.4% confidence). Radiocarbon ages BP are relative to 1950 (with 1σ counting error).