# Spatiotemporal Clustering of Large Earthquakes Along the Central-Eastern Sections of the Altyn Tagh Fault, China

Nicolás Pinzon, Yann Klinger, Xiwei Xu, Paul Tapponnier, Jing Liu-Zeng, Jerome van der Woerd, Kang Li, Mingxing Gao

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## To cite this version:

Nicolás Pinzon, Yann Klinger, Xiwei Xu, Paul Tapponnier, Jing Liu-Zeng, et al., Spatiotemporal Clustering of Large Earthquakes Along the Central-Eastern Sections of the Altyn Tagh Fault, China. Journal of Geophysical Research: Solid Earth, 2024, 129, pp.2024JB028912. 10.1029/2024JB028912. insu-05081545

HAL Id: insu-05081545

https://insu.hal.science/insu-05081545v1

Submitted on 24 May 2025

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Distributed under a Creative Commons Attribution 4.0 International License
# JGR Solid Earth

## RESEARCH ARTICLE

10.1029/2024JB028912

†Deceased

**Key Points:**

* Paleoseismic investigations along the Aksay fault section reveal quasi-periodic earthquake recurrence. This rupture pattern is consistent with neighboring fault sections

* Earthquake time correlation over multiple fault sections (regional scale), highlights alternating periods of earthquake clustering and seismic quiescence in the last 8 kyr along the central-eastern Altyn Tagh Fault

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

N. Pinzon,
pinzonmatapi@ipgp.fr

**Citation:**

Pinzon, N., Klinger, Y., Xu, X., Tapponnier, P., Liu-Zeng, J., Van Der Woerd, J., et al. (2024). Spatiotemporal clustering of large earthquakes along the central-eastern sections of the Altyn Tagh fault, China. *Journal of Geophysical Research: Solid Earth*, 129, e2024JB028912. https://doi.org/10.1029/2024JB028912

Received 13 FEB 2024
Accepted 20 NOV 2024

**Author Contributions:**

**Conceptualization:** Yann Klinger,
Paul Tapponnier

**Data curation:** Nicolás Pinzon

**Formal analysis:** Nicolás Pinzon

**Funding acquisition:** Yann Klinger,
Xiwei Xu, Paul Tapponnier, Jing Liu-
Zeng, Jerome Van Der Woerd, Kang Li,
Mingxing Gao

**Investigation:** Nicolás Pinzon,
Yann Klinger, Xiwei Xu, Paul Tapponnier,
Jing Liu-Zeng, Jerome Van Der Woerd,
Kang Li, Mingxing Gao

**Methodology:** Nicolás Pinzon

© 2024 The Author(s).
This is an open access article under the
terms of the Creative Commons
Attribution-NonCommercial License,
which permits use, distribution and
reproduction in any medium, provided the
original work is properly cited and is not
used for commercial purposes.

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## Spatiotemporal Clustering of Large Earthquakes Along the Central-Eastern Sections of the Altyn Tagh Fault, China

Nicolás Pinzon¹ Ⓓ, Yann Klinger¹ Ⓓ, Xiwei Xu² Ⓓ, Paul Tapponnier³†, Jing Liu-Zeng⁴ Ⓓ, Jerome Van Der Woerd⁵, Kang Li³ Ⓓ, and Mingxing Gao⁶ Ⓓ

¹Institute de Physique du Globe de Paris (IPGP), Université Paris Cité, Paris, France, ²China University of Geosciences, Beijing, China, ³National Institute of Natural Hazards, Beijing, China, ⁴Tianjin University, Tianjin, China, ⁵Institute Terre et Environment de Strasbourg (ITES), Strasbourg, France, ⁶Xinjiang University, Urumqi, China

**Abstract** The understanding of the spatial-temporal distribution of past earthquakes is essential to assess the event recurrence behavior and to estimate the size of potential earthquakes along active strike-slip fault systems. However, the scarcity of paleoseismic data remains a major hurdle in this endeavor. This is the case of the longest strike-slip fault in Asia, the Altyn Tagh Fault (ATF). We documented six very likely large earthquakes that potentially ruptured the Aksay section of the ATF. Employing a Bayesian approach, we present modeled date ranges of 6339–5220 BC, 5296–4563 BC, 3026–2677 BC, 1324–808 BC, 314–632 AD, and 915–1300 AD. The mean recurrence time is 1,329 ± 588 years with a coefficient of variation (COV) of ~0.44. In the same fault section, 90 horizontal offsets record an average coseismic slip of 5.1 ± 1.4 m for the last event and suggest four older earthquakes plausibly with a similar slip distribution. Although at the local-scale the COV indicates quasi-periodic rupture behavior, the individual interevent times exhibit significant irregularity, a pattern also observed in adjacent fault sections (Xorxoli, Annanba and Tashi sections). We found that such irregularities are a natural consequence of long-term fault interactions, which allow for synchronized ruptures along the northern and southern strands of the central-eastern ATF. Our rupture model highlights bursty periods of seismic activity with mean interevent times of 475 ± 108 years separated by long-lull periods of 1.1–1.6 kyr. Based on this temporal organization and considering the 401-year elapsed time since the most recent event on the Xorxoli section, there exists a possibility of a forthcoming large earthquake occurring within the next century.

**Plain Language Summary** Identifying earthquake recurrence times and slip distributions over the span of many seismic cycles is key to understand fault behaviors and to better assess the seismic hazard. In this paper, we sought to investigate the spatial and temporal organization of large earthquakes along the central-eastern Altyn Tagh Fault. We began at the scale of an individual fault section studying the past timeline of earthquakes and slip distribution; for this purpose, we used three trenches excavated along the Aksay section and 90 measured offset channels. We identified six large earthquakes breaking through the Aksay section with quasi-periodic recurrences, and an average slip of 5.1 m for the most recent event (MRE). Aiming to gain a broader perspective of the earthquake time history along the ATF, we switch to a regional scale (multiple fault sections) by integrating our findings obtained along the Aksay fault section with previously published investigations on neighboring fault sections. We found that, although, each fault section reveals a quasi-periodic pattern of rupture, large earthquakes in the ATF system appear to synchronize and release strain in less than 1 kyr followed by 1.1–1.6 kyr of seismic quiescence. Long-term large-scale observations are key for understanding the behavior of complex fault systems and anticipating more precisely the timing of future earthquakes.

## 1. Introduction

Several models that assume time-based elastic loading of the crust have been developed to understand earthquake recurrence on major strike-slip fault systems. Broadly, these can be grouped into time-dependent (time and slip predictable) and time-independent models (Shimazaki & Nakata, 1980). The broader the timeframe covered by earthquake records, the better we resolve which model characterizes the rupture pattern along active fault systems. However, instrumental earthquake records and historical chronicles are seldom long enough to robustly represent the rupture behavior, though a few historical time-series do exist for strike-slip faults (Barka & Kadinsky-Cade, 1988; Hubert-Ferrari et al., 2000; Lefevre et al., 2018; Stein et al., 1997). Nonetheless, paleoseismic records
**Project administration:** Yann Klinger, Xiwei Xu, Paul Tapponnier, Jing Liu-Zeng, Jerome Van Der Woerd, Kang Li, Mingxing Gao

**Resources:** Yann Klinger, Xiwei Xu, Paul Tapponnier, Jing Liu-Zeng, Kang Li, Mingxing Gao

**Software:** Nicolás Pinzon

**Supervision:** Yann Klinger

**Validation:** Nicolás Pinzon, Yann Klinger, Xiwei Xu, Paul Tapponnier, Jing Liu-Zeng, Jerome Van Der Woerd, Kang Li, Mingxing Gao

**Visualization:** Nicolás Pinzon

**Writing – original draft:** Nicolás Pinzon

**Writing – review & editing:**

Yann Klinger

offer the possibility to span more seismic cycles and capture temporal patterns based on the analysis of sequential ground rupturing earthquakes (e.g., Berryman et al., 2012; Klinger et al., 2015; Rockwell et al., 2015). Hence, the paleoseismological approach provides key data to explore earthquake recurrence and better elucidate the fault behavior.

This study evaluates the paleoseismic history of the central-eastern side of the Altyn Tagh Fault (ATF) over multiple earthquake cycles. The ATF is a major intracontinental left-lateral strike-slip fault, with an approximate length of 1,600 km. It parallels the northern margin of the Tibetan Plateau (Molnar & Tapponnier, 1975). This makes the ATF one of the main active structures responsible for accommodating the regional deformation due to the convergence between India and Eurasia. Previous studies on the ATF focused on quantifying the slip rate over many seismic cycles (Cowgill, 2007; Gold et al., 2009; Mériaux et al., 2012; Tapponnier et al., 2001; Xu et al., 2005). A minimum quaternary slip rate for the ATF of ~5 mm/yr was proposed based on geological mapping and geomorphic studies (Chinese State Bureau of Seismology, 1992). Based on ¹⁰Be and ²⁶Al cosmogenic dating of alluvial surfaces displaced by the fault, a much higher Holocene slip rate of ~20–30 mm/yr was proposed for the central section (Mériaux et al., 2005; Peltzer et al., 1989). Recently GPS, InSAR, and geological measurements are converging toward a widely accepted slip rate of ~10 mm/yr on the central section (Gold et al., 2011, 2017; He et al., 2013). To the northeast, the fault starts branching and the slip rate on the main strand of the ATF gradually decreases to ~4 mm/yr (Shao et al., 2023; Van de Woerd et al., 2001). Many works on the ATF describe the kinematics of the fault, although its paleoseismic history and recurrence behavior are much less studied and still poorly known.

The central-eastern ATF is divided into four fault sections: Xorxoli section, Annanba section, Aksay section, and Tashi section (Figure 1; Mériaux et al., 2005). In the Annanba section, a 25-km wide restraining bend divides the ATF into a northern and southern strand, which are parallel to each other and are separated by 5–15 km. Located along the northern strand, the Aksay section is a 95 km-long trace that defines the northern edge of the Dangjin Shankou range (Figure 1b; Mériaux et al., 2005). In this study, we report two historical events since 360 AD by inhabitants in the Dunhuang area, located to the northeast of the Aksay section (Figure 1, see discussion section). In terms of paleo-seismological data, the first paleoseismic investigations carried out in the late 1980s were limited by the lack of dating methods (Chinese State Bureau of Seismology, 1992). Washburn et al. (2001, 2003) documented two to four events during the last 3 kyr along the central Altyn Tagh Fault in the Xorxoli section (Figure 1a). Yuan et al. (2018) opened two trenches in the same section (Xorxoli section) of the ATF, documenting nine paleoearthquakes over the last 5.5 kyr. Additionally, five and four well-resolved paleo-events in the last 8 kyr were documented on the north and south strands of the Aksay restraining bend (Annanba section), respectively (Figure 1a; Shao et al., 2018). Luo et al. (2019) reported three well-resolved earthquakes within the last 7 kyr from two trench sites excavated along the Tashi section. Xu et al. (2007) excavated two trenches on the Aksay section, revealing multiple paleo-earthquakes. However, due to insufficient dating samples, the ages of most of the events were not well constrained.

This work explores the spatiotemporal distribution of large earthquakes along the central-eastern ATF at different scales. Firstly, we intend to expand the earthquake time series and determine the rupture behavior at a local scale, or more precisely, at fault section scale (Klinger, 2010). For this purpose, we used new paleoseismic observations and radiocarbon dates from a recent extensive dating campaign at three sites along the Aksay section: western Old Aksay, Banguoba, and Banguoba river trench (Figure 1b; see Trench description). Additionally, we measured coseismic horizontal offsets from deflected channels along 70 km of the fault aiming to assess the slip distribution and the magnitude associated with past-earthquake ruptures along the Aksay section. Secondly, in order to explore the earthquake recurrence patterns at a regional scale, over multiple fault sections, we correlate and integrate event-age probability functions from our sites and previously published paleo-seismic investigations along the Tashi, Annanba, and Xorxoli sections. Thus, we provide a comprehensive reconstruction of the paleoseismic history of the central-eastern ATF, encompassing broader temporal and spatial scales.

## 2. Paleoseismic Investigations Along the Aksay Section

The Aksay fault section strikes N72°E from 93.67 E to 94.75 E longitude (Figure 1). The active fault defines a sharp boundary between coalescing alluvial fans to the north and the Dangjin Shankou range to the south. Most of the alluvial fans and terrace surfaces are covered by loess layers, which are recognized as yellowish deposits on satellite images (Figure 1b). Xu et al. (2007) opened two trenches along the Aksay section, the Banguoba trench
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Overview map of the study area. (a) Shaded relief map of the central-eastern ATF, showing locations of previous paleoseismic studies. Gray and green dots show the locations of trench excavations in the Xorxoli section, and blue dots correspond to trenches in the Annanba section. Orange dot shows the location of two trenches in the Tashi section. White box shows the footprint of part (b) where our sites are located. White band corresponds to Figure 7 where geomorphic offsets were measured. The inset shows the structural map of the Tibetan Plateau and the open red square highlights the central-eastern ATF. (b) Satellite image of the Aksay section. Yellow rectangles show the location of the Banguoba (T1), West Old Aksay (T2), and Banguoba river trench (T3). Local restraining bends, relay zones, and fault partitioning are notable along strike. Upper-left inset highlights deflected channels generated by the left-lateral movement of the ATF. Lower-right inset shows a field view of our master trench T1.</figcaption></figure>

and the West old-Aksay trench; however, the number of charcoal samples collected was too limited to properly constrain the timing of the events at these sites. More recently, we revisited these trenches, and a third trench, the Banguoba river trench, was excavated. We collected a total of 37 charcoal samples from the T1-Banguoba trench (Figure 2), 15 samples from the T3-Banguoba river trench (Figure 3), and 14 samples at the western T2-Old
<table><caption>**Table 1**

Ground Rupture Event Indicators and Corresponding Quality Ranking</caption><thead><tr><td>Quality rank</td><td>Description</td></tr></thead><tbody><tr><td>1</td><td>Faults with no evident offset, non-rooted fractures, subtle thickness changes, and unclear upward termination.</td></tr><tr><td>2</td><td>Faults with minor offsets (&lt;1 cm), gradual to moderate thickness changes possibly due to the event, earthquake horizon located at an erosion contact, likely indicating that the earthquake horizon postdates the event.</td></tr><tr><td>3</td><td>Faults with moderate offsets (&lt;15 cm), fault tips with distinct upward terminations, folded units related to well-rooted faults, colluvial wedge deposits potentially due to the ground shaking during the event, and posterior destabilization of fault scarps.</td></tr><tr><td>4</td><td>Faults associated with substantial vertical deformation, fissures filled with material postdating the earthquake horizon, paraconformities or stratigraphic gaps suggesting an interruption in the sediment transport due to obstruction of rivers or ephemeral channels, growth strata above the earthquake horizon indicative of rapid filling of a structural depression created by the event.</td></tr></tbody></table>

Aksay trench (Figure 4). The photomosaics of these trenches are shown in Figures S1, S2, and S3 of Supporting Information S1.

Evidence of co-seismic disruption in sedimentary units along these trenches includes a range of structures from small fractures and subtle thickness variations to faults with significant vertical deformation. It is important to note that these indicators are strictly associated with secondary deformation formed as a result of the main left-lateral coseismic dislocation. Likewise, the detailed mapping of such features allows the identification of earthquake horizons in the sedimentary sequence (i.e., the ground surface at the time of the paleoearthquake). In order to assess the confidence of each earthquake horizon we used the scheme of Scharer et al. (2007), gathering a series of sedimentological indicators of ground deformation, and associating a quality rank (Table 1). Based on the combination of the quality and frequency of event indicators, earthquake horizons are classified as very likely, likely, and possible. Very likely earthquake horizons are associated with the highest number of observations and at least one event indicator with rank = 4. Likely earthquake horizons are related to a considerable number of observations and at least one indicator with rank = 3. Possible event horizons are restricted to indicators of rank = 2 and/or 1.

## 2.1. Banguoba Trench–T1

### 2.1.1. Site Description and Stratigraphy

The Banguoba trench is located about 9 km east of Aksay old town, at 39.435 Lat and 94.399 Lon. The trench is 23 m long, 4 m wide, and 3–4 m depth (Figure 2). The azimuth of the trench, perpendicular to the local azimuth of the fault, is N20°W. The trench T1 was excavated on a north-sloping alluvial surface that is covered by a mix of loess and sandy gravel deposits. This alluvial surface is cut by the fault, producing a ~50-m-long pressure ridge (Lower-right inset in Figure 1b). The trench was excavated at the eastern side of a pressure ridge that progressively disappears when entering a sag-pond. To the northeast of the site, one 5-m-wide ephemeral stream channel diagonally crosses the fault zone. This channel is likely the main mechanism for the transport and deposition of sediment, including the gravel and sandy deposits exposed in the trench.

The deposits exposed within the trench mainly comprised alluvial units that vary from dark-brown to white-brown in color, with thicknesses ranging from a couple of centimeters to a few millimeters. The different layers consist of gravel to coarse sand, fine sand, and loess. The stratigraphy in this trench has three primary components. South of the fault, between Meter Mark (MM) 1 and MM6 in the lower-middle section, the units exhibit continuous planar-bedding geometries of thick gravel deposits alternating with sandy units. In contrast, the upper part of the trench is composed of interbedded sequences of thinner medium-fine sands and silts with laterally continuous planar-bedding stratification. In the central part (fault zone), between (MM) 6 and MM10, there is a small graben with an uphill-facing scarp against which the fluvial sediments are confined. The small graben is infilled by a succession of units alternating between coarse-medium sand and clay, a bit more than 1 m thick in total. These units exhibit a discontinuous concave bedding geometry. North of the fault, from MM10 to
<figure><img src="image_5.png" /><figcaption>**Figure 2.** (a) Banguoba–T1 trench (east wall). The different sandy units are indicated by letters A to M and they have been colored from orange (older layers) to light yellow (younger layers). Between MM9 and MM6, thin silty sand units are highlighted in yellow to differentiate them from the medium-coarse sandy units in white. Gr1, Gr2 and Gr3 correspond to gravel deposits. Event horizons are labeled from E1 (youngest paleo-event) to E9 (oldest paleo-event). Meter marks (MM) are indicated above the trench. Red lines correspond to fault cracks and radiocarbon samples are indicated by solid red squares. (b) Representation of event indicators according to Table 1. The diameter of the red circle is scaled by the sum of indicators in each rank quality. (c) The inset shows the spatial location of the trench along the Aksay section.</figcaption></figure>

MM17, there is a 7 m wide pressure ridge predominantly composed of matrix-supported fluvial deposits. From MM17 to MM22, some thalwegs, thick scour-and-fill structures, and sandy lenses indicate that paleo-channels were flowing parallel to the fault. Many layers were recognized along the trench that are consistent between the two walls. Sandy, silty, and clay layers are labeled A to M; in some cases we used lowercase to indicate sub-units. Gravel deposits are referred to with the prefix Gr. The sedimentological characteristics of each of these units are described in Figure S4 of Supporting Information S1.

### 2.1.2. Earthquake Ruptures

#### 2.1.2.1. Event E1

Subtle deformation indicators characterize the most recent event observed in T1. Evidence of this possible event comprises two secondary fault strands at MM6, and two additional strands between MM9 and MM10. These cracks bifurcate on both sides of the damage zone from previous faults associated with older events. Both cracks form an upward termination at the top of D. A small depression, formed between both crack systems, is consequently filled by a fine sand layer named unit C. This compact layer and the event horizon (top of D) are sealed by the deposition of unit B.

#### 2.1.2.2. Event E2

The penultimate earthquake is a possible event. E2 is characterized by limited deformation, between MM7 and MM8. Two secondary minor strands branch out from a fault trace, associated with event E3, and cut up to the base of unit e'. Thus, the earthquake horizon of E2 is located at the contact between unit e' and unit E. This contact is slightly offset by some millimeters. The distinct thin layer of whitish silt that makes up unit e' facilitates the unambiguous identification of such small vertical displacement.
#### 2.1.2.3. Event E3

Event E3 is categorized as very likely. The main evidence is located between MM7 and MM8, where one vertical normal fault offsets the units *I* and *H*, then it bifurcates into three secondary faults that offset the silty unit *G* around 10–12 cm vertically. Subsequently, these cracks cut up through unit *f*' and are capped by unit *F*, which starts pinching out and becomes difficult to follow as it gets closer to both walls of the fault zone. Additionally, between MM6 and MM7, a secondary fault branch clearly disrupts *G* and ends at the base of unit *F*. These observations locate the earthquake horizon at the contact between unit *f*' and *F*. In comparison with other recent paleoearthquakes (from E4 up to the youngest one), although this event comprises few indicators, these are associated with a high rank of quality. Moreover, event E3 may also be associated with the Fn fault at MM9 (Figure 2), and considering the low degree of deformation associated with events E2 and E1, the vertical offset of Fn may be more consistent with event E3.

#### 2.1.2.4. Event E4

E4 is a possible event that is primarily characterized by a single fault strand, between MM6 and MM7, which cuts up and offsets the top of *H* by some millimeters. Additional evidence at MM8 includes one individual crack and a set of bifurcating cracks that are not rooted, but clearly show a secondary deformation and offset of the unit *H*. This group of cracks defines the event E4 with an earthquake horizon at the base of the sub-unit *g*'.

#### 2.1.2.5. Event E5

Strong indicators suggest that E5 was very likely a large earthquake. This earthquake may have created a counter-slope scarp of approximately 1 m, suggested by the generation of a gentle ponding effect that allowed the deposition of the sedimentary unit *i*' up to unit *e*'. Once sedimentation reached the same elevation level of unit *J* (from MM10 northward), the current regained its natural flow, resuming northward deposition of recent sediments. Between MM10 and MM13, the fourth event is defined by a group of faults breaking across the pressure ridge up to the top of unit *J*. Though no significant offset can be associated with any of these, an overlapping colluvial wedge (cw) at MM9, probably indicates induced mass-transport during (or shortly after) the event. Based on the smoothed crack pattern observed mostly at MM13, we interpreted that the top of unit *J* was a desiccation paleo-surface that was exposed to the arid conditions. These open cracks were eventually filled and preserved by the more recent sediments of unit *E*.

#### 2.1.2.6. Event E6

The event E6 is a very likely large earthquake that caused a substantial down-drop of the units in the middle-south part of the trench. This event is characterized by a set of distinctive faults with upward termination at the contact between *K* and *J*, which we identify as the earthquake horizon. Layers below this event horizon exhibit a planar-bedding and tabular configuration. These units were abruptly broken into blocks, creating local accommodation space subsequently filled by curved parallel units laterally onlapping on the top of unit *K*. Direct evidence for this event is located between MM6 and MM7, where five bifurcated vertical cracks emerge from two well-rooted faults, and terminate a deformed and irregularly shaped contact between units *K* and *J*. This contact's geometry indicates load casts, possibly formed after the faulting when the uncompacted unit *K*, under excess fluid pore pressure, was subjected to the weight of the overlying coarse sand unit *J*, causing unequal sinking into unit *K*. Additional evidence includes a single fault strand extending through unit *K* at MM5, again with an upward termination at the same irregular contact between units *K* and *J*; consequently, no clear offset is visible at the top. Some internally laminated bedding seems quite disrupted by this fault strand.

#### 2.1.2.7. Event E7

Event E7 was very likely a large event that occurred when unit *L* was at the ground surface. This unit is laterally correlatable across the fault zone based on sedimentary facies such as texture, color, and primary structures (see Figure S4 in Supporting Information S1). Additionally, Unit *L* contains 10 radiocarbon samples that support the stratigraphic correlation and help discern between contemporaneous and older reworked material. The position of the samples within the unit and all their calibrated ages are consistent and do not show out-of-sequence values, thereby ruling out the potential incursion of inherited ages during the sedimentation of unit *L*. Between MM10 and MM12, unit *J* lies almost horizontally above Unit *L*. The absence of an erosional surface and the evident
stratigraphic gap, supported by calibrated C14 ages, indicate a local paraconformity at the contact between units J and L. Event E7 likely produced an up-hill-facing scarp approximately 1–1.5 m high, which resulted in the interruption of the natural river flow, and consequently the non-deposition of units Gr1 and K from MM10 northward. Direct evidence of this event is observed mainly at the base of the trench, between MM8 and MM9, where two cracks offset unit L and extend up to the base of Gr1. It is possible to observe a small offset at the top of unit L, which in fact, was partially eroded since it does not match with the vertical deformation at the base of the unit. This suggests that the event horizon here traced at the contact between L and Gr1 was partly eroded and it postdates the actual event E7. Furthermore, one additional crack between MM5 and MM6 breaks unit L. Although these cracks show no clear offset at the top, the inner lamination of the layer seems slightly disrupted due to event E7.

### 2.1.2.8. Event E8

E8 is designated as a very likely event due to the high-rank indicators of deformation. This event caused a significant down-drop of the central section, visible at the bottom of the trench at MM6, that created some accommodation space later filled by unit Gr2. This event is also distinguished by a set of five faults, all of them dipping northward, between MM1 and MM5. The upward terminations of the faults related to event E8 are located at the contact between *M* and Gr2. There is no evident offset in the stratigraphy between *M* and Gr2, suggesting a period of erosion of the vertical deformation. It is also difficult to identify offset within the underlying gravel Gr3, and the gradual flattening of the cracks at depth makes it difficult to identify deformation at the base of unit *M*. However, the thin pebbly sub-unit within unit *M* records slight vertical disruption associated with E8 between MM1 and MM3. Additionally, one of the faults at MM5 branches out from the previously formed fracture related to E9. The group of fractures related to events E8 and E9 differ from each other by the geometry of the upper part of the faults. On one side, cracks associated with E9 are bent and oriented to the south, while cracks associated with E8 are almost vertical near the top of unit *M*.

### 2.1.2.9. Event E9

There are two possible scenarios to explain the deformation observed in this layer. The first possibility is that a thicker massive sand layer was deposited followed by a thinner pebbly sand deposit, and these two layers were eventually folded and faulted due to an earthquake (E9). This event was followed by the deposition of the upper sub-unit of *M* that would drape the folded surface. Evidence of this hypothesis is observable between MM4 and MM5, where two fault strands have an upward termination at the top of the pebbly layer. The second possibility is that all these faults bent the entire unit *M*, which could have later been eroded prior to the deposition of unit Gr2, suggesting that all the deformation is related to just one earthquake (event E8). Based on the observable upward terminations of the faults, we chose to consider the existence of E9; however, given the limited number of indicators, we classify it as a “likely event.” Additionally, the age of event E9, at least in this trench, is not constrained due to the lack of samples pre-dating the earthquake.

## 2.2. Banguoba River Trench–T3

The T3 trench was exposed by cleaning the east wall of a gully located 60 m to the west of the Banguoba trench (T1). The trench is about 23 m long and 3–4 m deep (Figure 3). The stream channel that shapes the gully cuts perpendicularly across a pressure ridge, then joins a main river channel a few meters to the north. A counterslope scarp, at least 50 cm in height, is the main evidence of surface deformation between the T1 and T3 sites. This scarp extends 45 m to the east before reaching the T1 site. Conversely, to the west of the T3 site, the surface expression of the fault is not very evident. Thus, evidence of offset channels and risers are very subtle, not well-defined, and significantly buried by eolian deposits and eroded by ephemeral tributaries around the site.

In the northern part of the trench, between MM1 and MM9, the units alternate between gravel and sandy units with a well-bedded parallel configuration. Between MM9 and MM11, there are some sub-parallel, more recent silty sand units deposited in a 6-m-wide sag pond. These units are ponded against an uphill-facing fault scarp. The southernmost part of the trench, between MM12 and MM17, reveals interbedded clast-supported gravel deposits with less frequent matrix-supported gravel deposits. Altogether, the trench profile exposed 13 stratigraphic units termed with the prefix T3 (a detailed description of the units is provided in Figure S5 of Supporting Information S1). Given the proximity between the two trenches, such distinctive sedimentary units are correlatable with
<figure><img src="image_6.png" /><figcaption>**Figure 3.** (a) Field-view of T3 site, which is located on the east wall of the channel observed at the right part of the photo. Positive topography between T1 and T3 corresponds to a pressure ridge. (b) Trench log of the Banguoba river site. Stratigraphic units are indicated by letters T3-A to T3-H. Units are color-coded from older (brown-orange to light yellow) to younger (light yellow), and gravel deposits (T3Gr) colored in gray correspond to the succession of gravel deposits. Event horizons are labeled from E1" (youngest paleo-event) to E6" (oldest paleo-event). (c) Representation of event indicators according to Table 1. The diameter of the red circle is scaled by the sum of indicators in each rank quality.</figcaption></figure>

the units present in T1 (Figure S7 in Supporting Information S1). We documented six paleo-seismic events (from young to old E1'' to E6'', respectively), including five very likely and one likely.

### 2.2.1. Event E1''

Event E1'' is a likely event that mainly affects coarser units, making it more difficult to determine the exact location of the event horizon. Between MM11 and MM12, three fault branches offset the units T3-A and T3Gr3 by ~12 cm. These faults, together with one vertical fault at MM13, might go upward to the top of unit T3Gr2 and do not appear to rupture the pebbly unit T3Gr1. Between MM14 and MM16, some non-rooted cracks may also be associated with this event, but since they mainly affect gravel units, their upward terminations cannot be clearly defined.

### 2.2.2. Event E2''

Event E2'' is a very likely event that occurred after the deposition of Unit T3-A. This event created accommodation space for the deposition of units T3Gr3 and T3Gr2. Between MM9 and MM10, four fault branches cut through the silty units from T3-E to the top of Unit T3-A and cannot be traced further upward. The contact between T3-A and T3Gr3 is erosional, which has likely obliterated part of the sedimentary record. Between MM10 and MM11, a set of cracks millimetrically offset the top of Unit T3-C and terminate upward at the top of Unit T3-B. Although the evidence is weak, these non-rooted cracks could be related to a possible earthquake, predating E2'', or they may have formed during Event E2'' or Event E1'' and did not reach the ground surface.

### 2.2.3. Event E3''

High-quality indicators of deformation associated with this event are suggested by the breakage of the sedimentary sequence at the northern part of the trench. This very likely event was strong enough to dislocate the entire strata from T3Gr7 to T3-F, creating accommodation space that allowed for the deposition of units T3-E to T3-A. Three vertical normal faults, between MM5 and MM8, end at the upper part of the unit T3-F and offset the underlying layers by ~20–25 cm. We locate the earthquake horizon where the fault terminations are directly
observable, which, in this case, is at the upper part of Unit T3-F. However, the fact that Unit T3-F outcrops on the surface and may have been subjected to erosion, along with the gap in the stratigraphic record between Unit T3-E and T3-F (see Figure S5 in Supporting Information S1), poses a high degree of uncertainty in the position of the event horizon. The above also implies that if the geological gap is considerably wide, the observable deformation may not necessarily be related to a single earthquake (E3''). At MM4 and MM5, two faults with no evidence of significant displacement, and one main fault between MM5 and MM6, present a lower upward termination just before reaching the top of Unit T3-F. The fact that some upward terminations are not exactly at the same level may be due to lateral variations in thickness during deposition, erosion processes, or substantial lateral slip. Wedge-shaped deposits, between MM10 and MM11 at the base of the trench, mainly consist of sedimentary material from units T3-I to T3-G and indicate a vertical dislocation up to ~1.5 m. Based on lateral thickness variations between these wedge-shaped deposits and the sedimentary units, we believe that the observed vertical deformation results from a summation of vertical displacements related to events E6'', E5'', E4'' and E3'' (which could encompass more than one event).

### 2.2.4. Event E4''

Deformation indicators related to this event are directly observed at MM9, where one south-dipping normal fault cut the units T3-H, T3Gr5, and T3-G. These units are vertically offset by about 15 cm except for the top of T3-G, perhaps due to erosion prior to the deposition of unit T3Gr4, suggesting that the erosional contact postdates event E4''. Between MM7 and MM9, there is a set of fault cracks with upward termination at the contact between units T3-G and T3Gr4. Moreover, right between MM9 and MM10, a fault cutting through T3Gr5 creates a fissure filled with a block derived from unit T3-G. This level of ground disruption supports the high confidence level for this event.

### 2.2.5. Event E5''

Event E5'' is classified as very likely due to the presence of high quality and numerous indicators of deformation. Direct evidence of this event is observable throughout the northern half of the studied exposure. Between MM1 and MM9, numerous fractures cut the unit T3-H, spread through the gravel unit T3Gr5, and are capped by unit T3-G. Between MM4 and MM5, a 25 cm-wide fissure is filled with some debris from T3Gr5 and sedimentary material from unit T3-G. The sedimentological characteristics of T3Gr5 (Figure S5 in Supporting Information S1) complicate the detection of vertical displacements within the unit. However, at MM4, centimetric displacements affecting the base of unit T3Gr5 and unit T3-I are visible. Furthermore, between MM8 and MM9, there are two non-rooted fractures generating a small down-drop of the top part of the unit T3-H. This event produced a fissure at MM8 that is filled with gravel sediments of unit T3Gr5.

### 2.2.6. Event E6''

The high-rank quality suggests that Event E6'' is very likely. The event horizon is interpreted at the erosional contact, between units T3-I and T3Gr6. Between MM5 and MM10, units T3Gr6 and T3-H pinch out against unit T3-I northward, suggesting a significant warping of the entire sedimentary pile up to unit T3-I. At meters MM8 and MM9, two fault strands are capped by unit T3Gr6. These faults branch out from a vertical fault that offsets the unit T3-I. Additionally, at MM7 a single fracture cuts units T3-I and T3-J with no evident offset.

## 2.3. Western Old Aksay Trench-T2

The trench T2, located about 6 km west of Old Aksay Village at 39.391° Lat and 94.189° Lon, has roughly the same azimuth as T1 (N20°W). The trench is about 20 m long and 4 m wide and reaches a maximum depth of 4–5 m (Figure 4). The Western Old Aksay Trench site is characterized by a step-over zone that cuts several alluvial surfaces, producing local pressure ridges and small extension zones. The Western Old Aksay trench was excavated at the southwest tip of an 80–90-m-long and approximately 12-m-wide pull-apart basin located 20 km to the west of the T1 site. Subtle positive topography associated with a pressure ridge defines the northern edge of the basin. To the south of the basin, the fault shows a 10–15° anticlockwise azimuth change, creating a compressional zone expressed on the surface as a topographic high. 150 m to the southwest of the site we reported a cumulative offset of ~15 m on an ephemeral channel (see Table S1 in Supporting Information S2), while at the northeast of the site, several tributaries join a main channel that flows almost parallel to the fault.
<figure><img src="image_7.png" /><figcaption>**Figure 4.** (a) Field view of the T2 site excavated across a pull-apart basin located ~20 km to the west of the T1 site. (b) Trench log of the West Old Aksai T2 (west wall). The different stratigraphic units are indicated by letters T2-A to T2-I, in yellow the succession of silty and sandy layers, and in gray the gravel deposits. Event horizons are labeled from E1' (youngest paleo-event) to E4' (oldest paleo-event). Red lines correspond to fault cracks and radiocarbon samples are indicated by solid red squares. (c) Representation of event indicators according to Table 1. The diameter of the red circle is scaled by the sum of indicators in each rank quality.</figcaption></figure>

The stratigraphy is mainly distinguished by physical properties like texture and color. From the base to the middle-upper part of the trench, the units consist of eolian deposits of fine sand to loess layers, alternating between dark-brown and white-brown in color. More recent units are characterized by pebbly sand units, possibly deposited by surface wash related to storms. Most of these deposits are ponded against a pressure ridge, which consists mainly of matrix-supported gravels. The main units are labeled A to I with the prefix T2 to differentiate nomenclatures from the Banguoba trench (T1). A detailed stratigraphy log with a description of the units is provided in Figure S6 of Supporting Information S1. Four earthquake horizons were identified and categorized from the most recent E1' to the oldest E4'. However, due to the limited charcoal samples in the lower part of the trench, only the two more recent earthquakes could be dated (see Figure S8 in Supporting Information S1).

### 2.3.1. Event E1'

The most recent event documented at this site was very likely a large earthquake. Strong evidence for this event is unambiguously visible only in the northern part of the trench, between MM12 and MM13, where a secondary normal fault bifurcates from faults probably associated with E2' and spreads through T2-C up to the top of T2-C. Additionally, there are two fault cracks, between MM5 and MM6, and some non-rooted fissures between MM8 and MM11. These cracks are sealed by unit T2-B. The event horizon is defined by the contact between T2-C and T2-B, though there is no vertical offset observed.

### 2.3.2. Event E2'

The penultimate event is categorized as very likely and is mainly characterized by five secondary fault branches that splay from fractures formed previously during E3'. One of these cracks, between MM10 and MM11, affects the silty sedimentary units (from T2-H up to T2-F). Between MM11 and MM13, four cracks cut through the gravel units upwards to the top of T2-F, below the gravelly sand deposits of T2-E. From the trench log, it appears that the deposits forming the colluvial wedge (unit T2-E) were likely formed due to the failure of the fault scarp following the earthquake. Farther south, more recent units are onlapping the event horizon.
### 2.3.3. Event E3'

Event E3' occurred between layers T2-G and T2-F and is the most strongly expressed event in this trench, suggesting that it was very likely a large earthquake. It is mostly characterized by a reverse fault at MM10 near the counter-slope scarp. This fault offsets unit T2-H around 25-30 cm. Then, it extends through unit T2-G and is capped by unit T2-F. Additional evidence of E3' includes the bifurcated thrust fault between MM3 and MM5. Based on the offset observed at the base of unit T2-I, this event might have produced a downhill-facing scarp of around 50 cm when unit T2-G was at the surface. However, it seems that a considerable amount of this vertical deformation was eroded prior to the deposition of the gravel unit T2-C.

### 2.3.4. Event E4'

The oldest event is categorized as very likely and direct evidence is mostly observed at MM7. A thrust fault offsets unit T2-I and cuts up through T2-H, which is eroded on its hanging wall and overlain by unit T2-G which significantly thickens onto the footwall toward the north. Based on the upward termination of the fault, the event horizon is located on the erosional contact between T2-H and T2-G. The deformation seems to be accommodated by one reverse fault and some non-rooted fractures located between MM10 and MM11. This deformation is associated with the change in the azimuth of the main fault that, which in turn generates local compression zones.

## 2.4. Paleo-Event Age Determinations

Fifty-eight charcoal samples were collected from T1, T2, and T3 for carbon-14 dating. These samples were prepared and dated using an accelerator mass spectrometry. The analysis was performed at Beta Analytic Lab in Florida and corrected for total fractionation effects according to the mathematical conventions of Talma and Vogel (1993). Then, the radiocarbon age of each sample is calibrated using the curve INTCAL 13 (Reimer et al., 2013; see Tables S1 and S2 in Supporting Information S1). Then, we use a Bayesian approach implemented using OxCal 4.4 (Bronk-Ramsey, 2008) to model the ages of the paleoseismic events. We consider the stratigraphic positions of the samples in the modeling, which results in shifting the PDFs to narrower time intervals. Our calibration model combines phases when a group of samples are stratigraphically unordered within the unit, and sequences when groups of samples are located in a given stratigraphic order within the unit (Lienkaemper & Bronk-Ramsey, 2009). Two samples from T1 and one from T2 were not included in the age models, as they probably correspond to reworked samples. These samples yield ages significantly older than other ages, relative to their stratigraphic position (see Tables S1 and S2 in Supporting Information S1).

In age modeling, the event horizon is typically positioned between two phases (units) containing datable material. However, event horizons can be found above or below units that lack datable material. Since these units were not necessarily deposited instantaneously, they should be considered in the age model. To represent these non-dated units, we used paired boundaries in our models. This method adjusts the PDF of the event age by considering a higher likelihood on the side directly constrained by C14 samples and tapering the probabilities on the side of the non-dated layer (see Section S1 in Supporting Information S1). These considerations were applied in the modeling of paleoevent ages for each of the trenches individually.

The stratigraphy at the Banguoba trench (T1) is quite rich in datable material. A total of 36 samples were considered to model the age of eight out of the nine documented paleoearthquakes. Events E1, E2, E3, E4, E5, E6, E7 and E8 occurred at 1287-1580 AD, 997-1309 AD, 915-1300 AD, 775-869 AD, 314-632 AD, 1324-808 BC, 3026-2677 BC, and 5473-4024 BC, respectively (Figure 5; Table 2). E9 was not modeled due to the lack of samples predating the event. Nonetheless, using the current stratigraphy information, we could assume that it happened before the deposition of the sub-unit at the top of *M* (Figure 2 and Figure S4 in Supporting Information S1), meaning at least before 5374 BC (youngest sample of the unit *M*; Table S1 in Supporting Information S1).

Concerning the most two recent events E1" and E2" documented at T3, we can only ascertain that they occurred after AD 1210 ± 50 (BGR21 sample), as no overlying units contain datable material to constrain these events. E3'' is located at the top of Unit T3-F; from this point, there is a gap in the sedimentary record. Through a litho-chronostratigraphic correlation between Trenches T1 and T3, we found that unit T3-F is correlatable with unit T1-L, and unit T3-E and T3-D correlates with unit T1-J (see Figure S7 in Supporting Information S1). The hiatus in T3 is contemporaneous with units Gr1, K, and perhaps part of *J* (from T1), and it spans approximately 2 kyr. Thus,
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Probability density function (PDFs) for event ages at T1. Bracket below each PDF indicates 95 percent confidence range. Red curves are the modeled ages for paleo-events (E1 to E8) modeled with OxCal v4.4 and IntCal13 calibration curve (Bronk-Ramsey, 2008; Reimer et al., 2013). Light gray envelopes indicate individual calibration and dark gray modeled radiocarbon ages including stratigraphic information.</figcaption></figure>
it is reasonable that E3'' may represent more than one paleoevent, which would have occurred after 2790 ± 110 BC (sample BGR16) and before 303 ± 82 AD (sample BGR12). Events E4'' and E5'' were constrained to have occurred between 6339 to 5220 BC and 5296 to 4563 BC, respectively (Table 2; Figure S9 in Supporting Information S1). E6'' is the oldest event observed in the trench T3; however, its chronological constraint remains uncertain. This is because, between MM5 and MM1, the event horizon becomes difficult to trace laterally due to the massive nature of units T3-J and T3-H. Thus, considering that the sample BGR-43 is from a laminated unit atop T3-I, which might or might not correlate with T3-H, it is unclear whether the sample postdates or predates the event.

Twelve calibrated radiocarbon ages bracket the event time of the two most recent events in T2-West Aksay (Table S2 in Supporting Information S1). Due to the absence of radiocarbon samples at the base of the trench, the earlier two events were not modeled. However, based on the calibrated samples AKT04-27 and AKT04-04, it suggests that events E4' and E3' occurred between 2684 ± 188 BC and 532 ± 105 AD. The two more recent events E2' and E1' likely occurred in the ranges 651–756 AD and 898–1330 AD, respectively (Table 2; Figure S8 in Supporting Information S1).

## 2.5. Large Earthquake Ruptures Along the Aksay Section

Distinctive stratigraphy and high content of organic material (36 radiocarbon dates) constraining the paleo-event ages make the Banguoba trench (T1) the most suitable reference site to characterize the earthquake history along the Aksay section. The Banguoba trench exposes nine surface ruptures along the northern section of the ATF spanning from the Middle to Late Holocene epoch. At this site, we documented five very likely large earthquakes with strong indicators of large deformation (E3, E5, E6, E7, E8), one likely event (E9), and three possible events (E1, E2, E4). The Banguoba river site (T3) allows for the identification of five very likely large events (E2'', E3'', E4'', E5'', E6'') and one likely event E1''. Furthermore, at the west old-Aksay trench (T2) we find four very likely large earthquakes (E1', E2', E3', and E4').

Some earthquakes are likely missed in our trenches, especially within the lower ~2.5 m of T2, where the stratigraphy mostly consists of massive homogeneous loess deposits (see Figure S3 in Supporting Information S1), which provide unclear bedding markers to identify fault groups and to locate event horizons. Hence, we cannot rule out the possibility of missing events. Comparing the ages of samples BGR12 and BGR21 with respect to their stratigraphic position, a very slow sedimentation rate is estimated during the deposition of T3-E to T3-B. Such low sedimentation rates diminish the resolution of the paleoseismic record, so that we are probably missing some event indicators in this section as well. However, since T1 does not show any significant depositional hiatus in the stratigraphy log and in the age distribution, we assume that our reference trench site provides a complete record of the large events that have occurred in the last 8,000 years.

Comparing the age probability distributions of the events, it appears that some age ranges overlap, suggesting that some events likely ruptured the entire fault section in the past. This assumption primarily relies on the temporal overlap between the master trench T1 and T2. However, T2 does not have the same temporal record as T1 due to the lack of exposure of older units. Consequently, the lateral correlation of the oldest earthquakes remains difficult to assess.

From the observation of the three trench sites, all ruptures exhibit different styles and sizes of deformation, leading to differential quality ranges. Klinger et al. (2015) observed from a trench site on the Dead Sea Fault that large deformations are indicative of long ruptures that potentially involved multiple fault segments, supported by a long and rich historical record. Although we lack a comprehensive historical record, we consider that the overlap of age ranges at our three trench sites, along with the quality ranks associated with each event, allow us to establish some benchmarks to evaluate earthquakes that potentially involved rupture of the entire Aksay section and those that may have had a more limited rupture extent. For instance, from the T1 log, we have identified a series of three recent earthquakes (E1, E2, and E3) that overlap with events E1'' and E2'' at T3 and event E1' at T2. Among the three most recent events at T1, E3 exhibits deformation indicators with the highest rank. Event E3 might tentatively correlate with E2'', as both events share high-rank indicators, which is also consistent with the chronological order of events at T3. Furthermore, by comparing T1 with T2 and considering the quality ranks, we propose that event E3 is the most recent earthquake that correlates with E1' and very likely ruptured through the Aksay section (Figure 6). On the other hand, event E1 shares similar moderate-rank qualities and seems to be contemporaneous with E1'' at T3. The lack of evidence of E1 at T2, together with the deformation patterns,
<figure><img src="image_9.png" /><figcaption>**Figure 6.** Comparison of event timing ranges of the three sites (T1, T2, and T3) along the Aksay section. Events with no probability age spectra are represented with blue lines constrained by modeled radiocarbon samples either from the layers overlying or layers underlying the event horizon. Red letters indicate events that likely rupture the entire Aksay section. Red solid stripes represent a strong time correlation, while red-dashed open rectangles represent only potential correlations. Individual earthquake nomenclature based on rank quality VL = Very likely, L = Likely, P = Possible.</figcaption></figure>

suggests that E1 ruptured at least the eastern part of the Aksay section and may have been halted by a 5-km-long double restraining bend located further west (see Figure 7). Similarly, E2 could have ruptured the eastern segment and was not detected in T3 due to the low sedimentation rates or it could be related to shaking-induced effects due to event E1.

Event E5 gathers several indicators of large deformation, such as numerous fault cracks and a collapse of the units in the main damage zone. Although correlation remains difficult, the cracks observed between MM10 and MM11 in T3 could likely relate to this event. Yet, these are non-rooted cracks and none of them displays any significant offset, suggesting that they may have been triggered by event E2'' or E1''. Event E5 seems to overlap with E3', which is documented in T2 and classified as a very likely large earthquake. Although no dates constrain the lower age boundary of E3', its upper boundary is well constrained by the oldest sample of unit T2-F and aligns with the modeled age of E4 (Figure 6).

Earthquake E6 is one of the most strongly expressed in our reference trench T1. As the geological gap in T3 coincides with the modeled age of E3, this earthquake probably falls within the corresponding timeframe of E3''. This event may have created the accommodation space for the deposition of T3-E to T3-A in T3. Event E6 is potentially contemporaneous with the earliest earthquake E4' documented in T2. In Figure 6 the lower boundary for the theoretical age of the E4' is marked based on the sample AKT04-27 associated with the unit T2-I (Table 2, Table S1 in Supporting Information S1). However, this lower age boundary is overestimated since the fault crack cut up through T2-I and T2-H. The true event age is likely younger and more congruent with the E6 time interval (1324–808 BC).

Event E7 exhibits major ground deformation mainly marked by a vertical offset of the units and an abrupt collapse that occurred due to this event. This event perfectly coincides in time with the lower age boundary of E3'', which is pretty well constrained by the samples of the unit T3-F. Lithological characteristics and calibrated ages make units K (T1-nomenclature) and T3-F correlatable (Figure S7 in Supporting Information S1). Thus, the top of unit T3-F may actually be an earthquake horizon encompassed by E3'', which is seemingly contemporaneous with E7.

The age range of E8 overlaps with the dating range of E4'' documented at the T3 site. Additionally, both E8 and E4'' exhibit numerous fault cracks in both sites, a significant down-drop in T1, and vertical offsets on the order of centimeters (~15 cm) at the T3 site.

We classify event E9 as a likely earthquake due to the lack of exposure that limited the amount of evidence supporting the event. However, the T3 site reveals genuine deformation associated with this event. Comparing time ranges between T1 and T3, it is quite clear that the upper age boundary of E9 overlaps well with event E5'' (6339–5220 BC) documented at the T3 site (Table 2). Assuming that those events are the same, we reconsider E9 as a very likely large earthquake, which is strongly expressed by vertical offsets, multiple fault cracks (see T3 site), and folding of the units (see T1 site).
<table><caption>**Table 2**
Summary of Overlapped Earthquakes in the Aksay Section</caption><thead><tr><th rowspan="2">Earthquake number</th><th colspan="2">T1 dates</th><th colspan="2">T3 dates</th><th colspan="2">T2 dates</th></tr><tr><th>95.4% confidence interval</th><th>Weighted mean date</th><th>95.4% confidence interval</th><th>Weighted mean date</th><th>95.4% confidence interval</th><th>Weighted mean date</th></tr></thead><tbody><tr><td>E9</td><td>N/A–5374 BC</td><td>—</td><td>6339–5220 BC</td><td>5545 BC</td><td>No exposure</td><td>—</td></tr><tr><td>E8</td><td>5473–4024 BC</td><td>4794 BC</td><td>5296–4563 BC</td><td>5016 BC</td><td>No exposure</td><td>—</td></tr><tr><td>E7</td><td>3026–2677 BC</td><td>2886 BC</td><td>2931 BC–N/A</td><td>—</td><td>No samples</td><td>—</td></tr><tr><td>E6</td><td>1324–808 BC</td><td>1062 BC</td><td>hiatus</td><td>hiatus</td><td>2496 BC–N/A</td><td>—</td></tr><tr><td>E5</td><td>314–632 AD</td><td>475 AD</td><td>hiatus</td><td>hiatus</td><td>N/A–605 AD</td><td>—</td></tr><tr><td>E3</td><td>916–1236 AD</td><td>1067 AD</td><td>No samples</td><td>—</td><td>898–1330 AD</td><td>1140 AD</td></tr></tbody></table>

Note. N/A no available postdate or predate sample to constrain the earthquake age.

Although the spatial correlation of the three oldest events (E7, E8, E9) is limited due to the lack of exposure of older units at T2, these events exhibit high-rank quality indicators of significant deformation at T1 and T3. Additionally, the quality rank of event indicators associated with E7, E8, and E9 surpass those of E3 (classified as a large event) both in number and in the magnitude of vertical deformation. This suggests that the three oldest earthquakes documented at T1 were very likely large earthquakes that might have ruptured the entire Aksay section.

Six very likely large paleoearthquakes were identified (E3, E5, E6, E7, E8, E9) by integrating the information from the three sites along the Aksay section (Figure 6, Table 2). The average returning time (RI) for these events is ~1,329 years with a standard deviation (σ) of 588 years, with the most recent earthquake reported in ~1096 AD (weighted mean date). In order to have a quantitative measure of recurrence variability, we estimate the coefficient of variation (COV; Goes & Ward, 1994), defined as the standard deviation divided by the average returning time (σ/RI). The Aksay section has a COV of ~0.44.

Previous investigations on the Aksay double restraining bend (Annanba section) reported five well-resolved paleo-earthquakes on the northern strand over the last 8 kyr, with the most recent earthquake occurring approximately 416 AD (Shao et al., 2018). The return time at this site is 1,326 ± 179 years and a COV of ~0.3. For the southern strand, the returning time is 1,433 ± 56 years with a COV of ~0.5, based on four likely large earthquakes that occurred in the last 4 kyr. Farther to the west along the Xorxoli section (Figure 1a), Yuan et al. (2018) documented eight paleo-events over the last 5.5 kyr, with the most recent earthquake documented in ~1615 AD. The calculated RI of 620 ± 410 years in the Xorxoli section indicates recurrence times nearly half of those reported in the Annanba and Aksay sections. Furthermore, a COV of 0.67 suggests that the seismic recurrence along the Xorxoli section is starting to deviate from a periodic behavior, in contrast to the evident quasi-periodic COV reported in the Annanba section (Shao et al., 2018), Tashi section (Luo et al., 2019), and Aksay section (this study). It may be possible that a combination of higher deformation rates (see Figure 1) leading to more frequent events, and a higher level of detection, might be responsible for such short RI and high COV on the Xorxoli section. Although such discrepancy in the COV and RI could provide new insights about a laterally variable seismic behavior, it raises new uncertainties about how earthquakes accommodate deformation along the ATF (see discussion in Section 5).

## 3. Surface Rupture Mapping Along the Aksay Section

In this section, we provide an overview of the seismic deformation through several seismic cycles along the Aksay section. We mapped surface ruptures and measured small offsets to determine the coseismic deformation generated by the most recent earthquake (E3). Additionally, we provide some evidence of cumulative deformation associated with previous earthquakes and discuss the slip distribution and lateral slip variability along the Aksay section.

### 3.1. Offset Measurements and Modeling

Here we use high-resolution Pléiades satellite data (HRS) (~50 cm pixel). The images were processed using Micmac to obtain geometrically refined orthoimages along a 5-km-wide swath straddling ~70 km along the
Aksay section (Figure 7a). Geomorphic piercing lines defined by offset channels (see spatial coordinates in Table S1 in Supporting Information S2) were identified to measure coseismic slip deformation. Using QGIS software (http://www.qgis.org) we measured horizontal offsets by backslip restoration of geomorphic markers along the fault into a proposed initial configuration (Klinger et al., 2011). For each marker, we measure three offset displacements yielding preferred, minimum, and maximum offset values. Our preferred values represent our best reconstruction of the channel thalwegs, while minimum and maximum values correspond to the inner and outer channel margins. This yielded a total of 90 horizontal offset measurements ranging between 1.7 + 0.7/-0.6 and 29.95 + 0.4/-0.7 m. We also categorized the robustness of each measurement based on the sinuosity of the channels and its sharpness (Kurtz et al., 2018; see Table S2 and markers from M1 to M90 in Supporting Information S2).

In order to recognize slip patterns from naturally scattered geomorphic data, we first represent each marker (set of three offset values) as a Probability Density Function using an asymmetric gaussian distribution generated from our preferred ($S_{pref}$), minimum ($S_{min}$) and maximum ($S_{max}$) offsets (see Equation 1 and Table S1 in Supporting Information S2; Klinger et al., 2011; Kurtz et al., 2018; Rizza et al., 2011; Walker et al., 2021; Zielke et al., 2010).

$$f(x) = \frac{\sqrt{2}}{(\sigma - \tau)\sqrt{\pi}} \begin{cases} e^{\frac{-(x-m)^2}{2\sigma^2}} & | \quad x \le m \\ e^{\frac{-(x-m)^2}{2\tau^2}} & | \quad x > m \end{cases} \quad (1)$$

Where $f(x) = PDF$ (probability density function), $m = S_{pref}$, $\sigma = S_{min}$, $\tau = S_{max}$, these two last factors account for uncertainties derived from the offset measurements and encompass the vast majority of possible reconstructions. Hence, we truncated individual PDFs by $S_{min}$ and $S_{max}$ (as in Kang et al., 2020) to better retrieve slip patterns along the fault. Then, we stacked the PDFs to produce a Cumulative Offset Probability Density Function (COPD), which is used to identify main peaks from offset values that may result from individual earthquakes (Klinger et al., 2011; McGill & Sieh, 1991; Zielke et al., 2010, 2015).

## 3.2. Slip Distribution and Variability

The total Cumulative Offset Probability Density (COPD) illustrates five distinct peaks along the Aksay fault section (Figure 7c). The subset with the smallest offsets is likely related to the coseismic slip during the most recent large event (MRE), and the remaining four subsets correspond to the cumulative offsets produced by older events. The average offset attributed to the MRE is 5.1 ± 1.4 m (2σ of uncertainty). Additionally, the subsets of the cumulative probability function tend to decay exponentially with increasing the cumulative offset (Figure 7c). This behavior is a natural consequence of long erosion periods and degradation that offset markers undergo over several seismic cycles (Klinger et al., 2011). The exponential decay from small to large offsets in the COPD is represented by $y \sim e^{-bx}$, where the b parameter denotes the proportion of small offsets with respect to larger ones. This parameter can vary over different faults depending on its external geodynamics (e.g., climate conditions that modify the landscape) (Ludwing et al., 2010; Zielke et al., 2015). Applying the least squares method, we obtained an exponential relationship with b = 0.13; such a value is related to faults in arid/semi-arid climates, where it is more common to record and preserve MRE offsets than large cumulative offsets (Reitman et al., 2023).

The data set is characterized by a high relative standard deviation (RSD); for instance, the 2σ of the subset associated with the MRE is around 34.2% (the other subsets vary around this value). This scattering is largely attributable to the uncertainties derived from the associated quality of the markers (see Table S1 in Supporting Information S2). Using a Discrete Fourier Transform over the total COPD (Figures 7c and 7d), we defined the characteristic wavelength Λ of the slip distribution to be 5.3 m, which is quite consistent with the average slip of the MRE. Thus, one possibility is that the five peaks centered at ~5, ~10, ~15, ~21, and ~26 m could represent at least five large paleoearthquakes, each of them with very similar slip, following a characteristic slip pattern. Nonetheless, the number of geomorphic markers is not significant enough to fully validate this hypothesis. Consequently, it is plausible that the peaks related to the cumulative offsets may not have been generated by a single large earthquake; in other words, we do not rule out the possibility of contributions from smaller or moderate magnitude earthquakes.

Figure 7a depicts classic geometric discontinuities frequently encountered along strike-slip fault systems. Based on the presence of a ~0.7 km wide step-over (A2, west of old-Aksay city in Figure 7a) and a ~0.6 km wide
<figure><img src="image_10.png" /><figcaption>**Figure 7.** Aksay fault section and offset measurements. (a) Fault section (red) mapped from Pléiades satellite image swath. Yellow dots correspond to the location of the trench excavations. (b) Projection along strike of 90 horizontal offset measurements. Error bars represent max and min possible configuration values (see data and methods; Table S1 in Supporting Information S2). The gray stripes represent offsets that are visually grouped. At the bottom part three COPDs corresponding to the eastern Aksay segment (from A1 to A2), central segment (from A2 to A3) and western segment (from A3 to A4). (c) Total COPD corresponding to the entire Aksay section following an exponential decay with the cumulative offset size. (d) Discrete Fourier Transform revealing a coherent characteristic wavelength (5.3 m) of the total COPD.</figcaption></figure>

restraining bend (A3, east of old-Aksay city in Figure 7a), we divided the Aksay section into three structural segments. These segments have an average length of 22.1 ± 1.1 km, in agreement with the characteristic segmentation proposed by Klinger (2010). We calculate the COPD along the three segments (see bottom part of Figure 7b). The western segment (between A1 and A2) is bounded by a compressional relay zone to the east and by the Aksay restraining bend, where the fault strike begins to change on the westernmost side of the segment. The COPD of this segment clearly shows a first peak of 5.0 ± 0.7 m, corresponding to the MRE, and three subsequent peaks related to older earthquakes. The central segment (From A2 to A3) reveals a higher average slip of 6.1 ± 0.6 m for the MRE and some additional ambiguous peaks along the segment. This segment exhibits distinct geometric complexities, including a local double restraining bend (A3, Figure 7a) and a series of slip-partitioned reverse faults between kilometers 35 and 40 (Figure 7a). These features indicate areas of substantial off-fault deformation, as discussed by Sieh et al. (1993) and Klinger et al. (2006).

The COPD of the eastern segment (from A3 to A4; Figures 7a and 7b) exhibits a first peak centered at 2.3 ± 0.7 m. This peak seems to be closely related to the emerging peak at ~2.44 m in the total COPD calculated along the entire Aksay section. Such peak might be explained by the contribution of small to moderate-magnitude earthquakes that rupture a shorter section of the fault. In fact, this peak likely corresponds to the coseismic deformation of E1 and/or E2 (documented at T1 and T3 sites), which we interpret as smaller local earthquakes that possibly involved rupture of just the eastern segment of the Aksay section (see Section 2.5 and Figure 6). We do not exclude the possibility that E1 or E2 could represent the tail of a large event that occurred farther east. The COPD shows another peak centered at 4.8 ± 0.9 m, which is more consistent with the western segments. We believe that this peak corresponds to the average slip of the MRE, whose reduction in slip magnitude with respect to the eastern and central segments might be a response to the partitioning of sinistral displacement along the eastern side of the Aksay section. This decrease has been estimated at about two-thirds (~6 mm/yr) of the total slip rate of the Altyn Tagh Fault (Liu et al., 2020; Shao et al., 2023). Based on terrestrial cosmogenic nuclide dating on fluvial
surfaces and estimations of uplift rates, Van de Woerd et al. (2001), proposed that the strain rate is primarily absorbed by the uplift and shortening of the Tang-He Nanshan mountain range (Figure 9).

# 4. Paleoseismicity Along the Central-Eastern Altyn Tagh Fault-Regional Scale

In this section, we provide a regional-scale perspective (fault system scale) on the spatial and temporal organization of large earthquakes along the Altyn Tagh Fault. For this purpose, we integrate the paleoseismic results from our work along the Aksay fault section with previously published paleoseismic data from the Tashi, Annanba, and Xorxoli fault sections (Shao et al., 2018; Washburn et al., 2001, 2003; Yuan et al., 2018). Hence, from now on we will refer to these four sections as the central-eastern side of the ATF.

## 4.1. Site-To-Site Correlation

Site-to-site correlation aims to highlight potential large earthquakes and resolve multisegment rupture patterns through the combination of available paleoseismic event data. Firstly, paleoearthquake age probability distributions, at each site along the fault, are represented into a single pool of rupture scenarios. Herein, paleoseismic data from 10 trench sites were used to reproduce the event age PDFs (Figures 8 and 9). Starting from the east, two trenches encompass the Saohu Spring site along the easternmost side of the Tashi section. There, based on the sedimentary records and radiocarbon samples, we recreate the age probability functions of the three paleoevents reported by Luo et al. (2019). In this work, three trenches along the Aksay section (Figure 1b) allowed us to identify six large earthquakes and their corresponding probability distribution functions (PDFs). Dating evidence from the Annanba site and Mobaer site, located in the Annanba section, were used to reconstruct event PDFs for the four and five events documented, respectively (Shao et al., 2018). Although Shao et al. (2018) found additional evidence for one more possible event at the Annanba site and two possible events at the Mobaer site, we only include those supported by strong indicators of large deformation. Farther west, three sites along the Xorxoli section were considered, and gaussian PDFs were recalculated using published mean dates and uncertainties. Among these, the Copper Mine Site is the most recent and comprises the most complete record of seismic events. There, Yuan et al. (2018) provide robust evidence for eight paleoearthquakes. Although they identified indicators that could likely be associated with additional ground ruptures, such indicators were not considered as evidence of paleoevents. Based on a reevaluation of the site, we identified one group of these event indicators that, although not numerous, have moderate to high-quality ranks and likely represent an additional event recorded at the Copper Mine Site (CM6.2 event in Figures 8 and 9). The event indicators correspond to well-rooted faults, some with moderate offsets (1–10 cm), fissure-filled blocks, and lateral thickness variations (see Figures S14–S18 in Supporting Information S1). Finally, the Bitter Sea and Camel sites provide strong indicators of two paleo-earthquakes each (Washburn et al., 2001, 2003). Nevertheless, Washburn et al. (2001) do not provide radiocarbon concentrations in years ‘before present’ (standard AMS dates), thus we were unable to compute the earthquake PDFs for the Bitter Sea site.

In order to build the fault rupture scenario each event age PDF is compared with the different PDFs at the neighboring sites (Figure 8). Then, if an overlapping PDF is observed between *n* sites, a single rupture involving these sites could be defined. It is important to note that our rupture model, at the regional scale, only considers those events that might have ruptured multiple segments or fault sections, and it produces the maximum possible rupture lengths that fit the paleoseismic data. Thus, smaller events that may have been more limited in rupture extent, such as E1 and E2 documented at T1, are not represented in this regional chronology; yet today they would still qualify as significant earthquakes.

The number of large ruptures resulting from pooling all the available paleoseismic data will depend on the degree of overlap required to consider them as the same event (Biasi et al., 2002). The overlap calculation consists of the sum of minimum probabilities per time bin in the area of overlap (Biasi & Weldon, 2009). Overlap coefficients range from zero to one, where one indicates full overlap and zero corresponds to no overlap. Biasi and Weldon (2009) suggest a cutoff of 0.3 as a good overlapping standard. Here, we compute the overlap coefficient for each time-neighboring probability distribution. Most of the events yield values equal to or greater than 0.4, indicating a moderate consistency between overlapping time ranges and the shape of the distribution. The minimum overlap coefficient was 0.16 for the case of CM6 and $D_N$ (Figure 9); such a low value in this case is due to the overlapping of two nonuniform PDFs and the fact of measuring the smaller high between them. Thus, despite a relatively low overlap coefficient, we considered CM6 and $D_N$ correlatable due to the similarity in their age ranges. After
<figure><img src="image_11.png" /><figcaption>**Figure 8.** Rupture pool (upper part) covering the Aksay section (this study), Annanba, Xorxoli, and Tashi sections (Luo et al., 2019; Shao et al., 2018; Washburn et al., 2001, 2003; Yuan et al., 2018). The vertical bars represent 95-percent confidence interval of individual earthquake ages from each site. Colored bars correspond to the four most recent earthquakes whose ruptures are mapped on the plot at the bottom. Horizontal red bars highlight overlapping PDFs which are used to compute rupture mean dates (see Figure 9). The events that result from the correlation are numbered from 1 to 10. Event 4 (light blue) corresponds to a major barrier-breaching event documented in every site (~465 km rupture length).</figcaption></figure>

evaluating the quality of our correlations, we applied the rupture mean date method to combine the probabilities into a single PDF that defines the time of the overlapping events (Biasi & Weldon, 2009). This method is based on the sum of the probabilities of each site, bin by bin, and the result is divided by the size of the combined PDF unit area. Red asymmetric gaussian curves in Figure 9 represent the combined probability density functions of the overlapping PDFs found in the rupture pool, and these curves are placed along a single timeline in Figure 10.

## 4.2. Lateral Extent of Large Earthquake Ruptures

The correlation diagram indicates 10 potential large paleoearthquakes over a period extending back to 8 kyr BP (Figure 10). We estimate the seismic moment of these events by using the total rupture length from the site-to-site correlation (Figure 8). Additionally, we use coseismic slip measurements along the Aksay section (Figure 7) and
previously published information along the Xorxoli and Annanba sections (Elliott et al., 2015; Han et al., 2018; Washburn et al., 2001). For the two most recent earthquakes (Event 1 and 2) we used scaling laws relating magnitude, rupture area (A) and average slip (S), $M_0 = \mu \times S \times A$ (Wells & Coppersmith, 1994); we considered $17.5 \pm 2.5$ km as the depth extent of rupture (Ge et al., 2022; Li et al., 2018), and the shear modulus ($\mu$) is taken to be $3.3 \times 10^{10}$Nm$^2$. In the case of event 2, for the multi-section single-event scenario, the magnitude was derived by summing the seismic moments computed for each fault section. Then, we determine the moment magnitude applying the relation $M_w = \frac{2}{3}\log(M_0) - 9.01$ (Kanamori, 1977). Since we have no certainty about the amount of slip related to each of the preceding paleoearthquakes (from events 3 to 10), we estimated their moment magnitudes using log-linear scaling laws between magnitude (Mw) and rupture length (L) for strike-slip faults, $log_{10}(L) = a + bM_w$. Based on the dimensions of the fault system we used the scaling coefficients $b = 0.681$ and $a = -2.943$, proposed by Thingbaijam et al. (2017).

Based on horizontal offsets documented along the Xorxoli section, the rupture length of Event 1 (MRE) was constrained to be at least 140 km-long with an average coseismic slip of 3.2 m (Han et al., 2018). This event is not observed at the Mobaer site due to a limited sedimentary record over the past six centuries, which makes it difficult to assess if the rupture may have broken a much longer section. However, among the several ruptures identified in Copper mine site, event 1 is the most strongly expressed, indicating that it might have ruptured through the entire 200-km Xorxoli section (as suggested by Yuan et al., 2018). This was also suggested and supported by some well-defined and continuous offset measurements between the Xorxoli and Annanba sections (Elliott et al., 2015). These two possible rupture lengths for the MRE might have resulted in magnitudes ranging between Mw7.6 and Mw7.7 (bold values in Table 3).

Event 2 was the MRE documented along the Aksay section. Strong indicators of large deformation observed at T1, T2, and T3, suggest that it could have ruptured the entire Aksay section. Although event $A_S$ seems to correlate with event $A_N$ along the Annanba section, based on the absence of small landform offsets and the recent fault scarps, Elliott et al. (2015) and Shao et al. (2018) argued that event $A_N$ (on the northern strand) is older than event $A_S$ (on the southern strand). Yet, event 2 (mean age ~1096 AD) is still quite contemporaneous with event 2' (mean age ~1092 AD). Thus, there are two potential scenarios: (a) Event 2 and event 2' were two separate events, terminating on either side of the Aksay restraining bend, with rupture lengths of 95 and 230 km, respectively. (b) Alternatively, event 2 may have breached the Aksay bend, but event indicators were obliterated by erosion or somehow eluded detection at the Mobaer site, resulting in a rupture length of ~325 km. We used the average coseismic slip (~5.1 m) from the COPD along the Aksay section (Figure 7) to estimate the magnitude of the event(s) based on these two scenarios (see Table 3).

It is important to note that the magnitude of the most recent earthquakes may be slightly underestimated. Firstly, because farther west of the Xorxoli section, near the Pingdingshan bend (Figure 8), Yuan et al. (2019) widely constrained the most recent earthquake within the time frame of 1220–1733 AD. This range encompasses the possible ages for both event 1 and event 2. Consequently, it implies that either event 1 or event 2 could have formed a doublet with that earthquake, or that one of them managed to breach the Pingdingshan bend and continued propagating farther to the west. Secondly, on the eastern side of the fault system, the Saohu Spring site presents a gap in the sedimentary record spanning over the last 2.5 kyr (Luo et al., 2019). Thus, we cannot evaluate whether event 2 or event 3 were able to jump the Subei stepover and rupture through the Tashi section as well.

From the site-to-site correlation (Figure 8), it is evident that the Aksay restraining bend (Annanba section) plays a crucial role in the rupture propagation (Nielsen & Knopoff, 1998; Wesnousky, 2008). On one hand, there are some earthquakes like events 5, 6, and 8 that propagate along southern strand of the ATF (Xorxoli and Annanba sites), where the restraining bend appears to prevent the rupture propagation onto the northern strand of the ATF. These events yield magnitudes around $M_W$ 7.6 (Table 3). Similarly, the earthquakes that propagate along the northern strand of the ATF, either along the Aksay and Tashi sections (events 7 and 9) or along the Aksay section and the northern strand of the Annanba section (events 3 and 10), are seemingly halted along the eastern side of the restraining bend. These earthquakes exhibit magnitudes between $M_W$ 7.4 and $M_W$ 7.5. However, the Aksay restraining bend may not always stop the rupture propagation (Duan et al., 2019). Event 4 is likely a major earthquake that could have breached the Aksay barrier and propagated along all four fault sections studied here, which means a maximum rupture length of around ~465 km, yielding a moment magnitude of ~$M_W$ 8.2 (Table 3).
<table><caption>**Table 3**

Calculation of Mo and Mw Based on Rupture Parameters</caption><thead><tr><th>Earthquake</th><th>Rupture section</th><th>Length (km)</th><th>Slip (m)</th><th>Mo X 10<sup>20</sup> (N/m<sup>2</sup>)</th><th>Mw</th></tr></thead><tbody><tr><td rowspan="2">Event 1</td><td rowspan="2">Xorxoli section</td><td>140<sup>a</sup></td><td>3.2<sup>b</sup></td><td>1.48</td><td><strong>7.58</strong></td></tr><tr><td>200</td><td></td><td>2.11</td><td><strong>7.68</strong></td></tr><tr><td rowspan="3">Event 2 (single event)</td><td>Xorxoli section</td><td>200</td><td>2<sup>c</sup></td><td>6.6</td><td><strong>7.88</strong></td></tr><tr><td>Annanba section</td><td>70</td><td>5.6<sup>d</sup></td><td>2.59</td><td></td></tr><tr><td>Aksay section</td><td>95</td><td>5.1<sup>e</sup></td><td>3.2</td><td></td></tr><tr><td rowspan="3">Event 2 & 2' (doublet)</td><td>Aksay section</td><td>95</td><td>5.1<sup>e</sup></td><td>3.2</td><td><strong>7.6</strong></td></tr><tr><td>Xorxoli section</td><td>200</td><td>2<sup>c</sup></td><td>6.6</td><td><strong>7.75</strong></td></tr><tr><td>Annanba section</td><td>70</td><td>5.6<sup>d</sup></td><td>2.59</td><td></td></tr><tr><td rowspan="2">Event 3</td><td>Aksay section</td><td>165</td><td>—</td><td>—</td><td><strong>7.44</strong></td></tr><tr><td>Annanba section</td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="4">Event 4</td><td>Tashi section</td><td>465</td><td>—</td><td>—</td><td><strong>8.24</strong></td></tr><tr><td>Aksay section</td><td></td><td></td><td></td><td></td></tr><tr><td>Annanba section</td><td></td><td></td><td></td><td></td></tr><tr><td>Xorxoli section</td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Event 5</td><td>Xorxoli section</td><td>245</td><td>—</td><td>—</td><td><strong>7.62</strong></td></tr><tr><td>Annanba section (west side)</td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Event 6</td><td>Xorxoli section</td><td>245</td><td>—</td><td>—</td><td><strong>7.62</strong></td></tr><tr><td>Annanba section (west side)</td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Event 7</td><td>Tashi section</td><td>195</td><td>—</td><td>—</td><td><strong>7.51</strong></td></tr><tr><td>Aksay section</td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Event 8</td><td>Xorxoli section</td><td>245</td><td>—</td><td>—</td><td><strong>7.62</strong></td></tr><tr><td>Annanba section (west side)</td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Event 9</td><td>Tashi section</td><td>195</td><td>—</td><td>—</td><td><strong>7.51</strong></td></tr><tr><td>Aksay section</td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="2">Even 10</td><td>Aksay section</td><td>165</td><td>—</td><td>—</td><td><strong>7.44</strong></td></tr><tr><td>Annanba section</td><td></td><td></td><td></td><td></td></tr></tbody></table>

Note. This values corresponds to the estimated moment magnitudes (Mw). <sup>a</sup>Rupture length and average coseismic slip of the MRE along the Xorxoli section. <sup>b</sup>Rupture length and average coseismic slip of the MRE along the Xorxoli section. <sup>c</sup>Average coseismic slip of the penultimate event along the Xorxoli section (Han et al., 2018). <sup>d</sup>Average coseismic slip of the MRE along the Annanba section (Shao et al., 2018). <sup>e</sup>Average coseismic slip of the MRE along the Aksay section (Figure 7).

### 4.3. Temporal Organization

The integrated earthquake time series visually exhibits a distinct grouping pattern of the events that occurred along the northern and southern strands of the ATF (Figure 10). Three paleoevents form the youngest group spread over about a millennium AD. Within this time frame, we have historical time constraints about earthquake activity in the vicinity of the eastern ATF. According to the *Records of the Grand Historian* (Sima, 1993), Dunhuang City, located ~80 km northeast of the Aksay section (Figure 1), has had human settlements since 2000 BC. Later, by 9 AD there were around 11,200 households in the area (Bonavia & Baumer, 2004). Based on our correlative framework over the different sites, we found that the MRE (event 1) might have ruptured along the Xorxoli section between 1500 AD and 1785 AD. Given that there is no evidence of a contemporaneous rupture in the Aksay section, it is very likely the Aksay bend (Annanba section) prevented the rupture propagation of event 1 to the northern strand of the ATF (Figure 8). Although this paleoearthquake is the most strongly expressed at the Copper Mine site on the Xorxoli section (Yuan et al., 2018), there is no historical record related to it, likely due to the epicentral distance to the Dunhuang city.

The penultimate event (event 2) is constrained to be between 915 AD and 1300 AD (Figure 9). During this interval, a significant event near Dunhuang damaged the wall paintings in Cave-465 in Mogao Grottoes (Giles
L., 1915). This event was reported in 1110 AD, which is consistent and strongly supports the weighted mean age of event 2 (~1096 AD). Event 2 appears to have ruptured along the entire Aksay section, as it is clearly observed at Banguoba, Banguoba river, and the West old-Aksay sites (referred to as event E3). But there is no evidence of event 2 at the Mobaer site (northern strand), nor to the east along the Tashi section (Figure 8). As mentioned in the previous section, one possibility is to consider event 2 and event 2' as two separate events. Alternatively, although event 2 is not formally identified at the Mobaer site, the obliteration of evidence by erosion cannot be excluded. In such a case, event 2 could have breached the Aksay bend and propagated farther west along the Xorxoli section (Copper Mine site and Bitter Sea site). Given that event 2 and 2' are quite contemporaneous (overlapping PDFs and mean dates), we decided to include just one event in the earthquake time series of central-eastern ATF. This hypothesis is supported by the historical record, although it is possible that chronicles may not have included the second event due to lower intensity or epicentral distance.

The oldest earthquake of the first cluster, event 3, occurred between 414 BC and 1240 AD (Figure 9). According to historical narratives, this paleoearthquake could be associated with one ancient earthquake documented in 360 AD by Dunhuang habitants (Giles, 1915). Although there is a possibility that this event might have broken the barrier, our preferred interpretation is that event 3 ruptured through the Aksay section and the northern strand of the ATF, where it might have propagated all the way to the Mobaer site. However, no evidence of event 3 was identified at the Annanba site, along the southern strand of the ATF, and none of the cracks associated with C2 at the Camel site display significant vertical deformation. Thus, we suggest that event 3 did not rupture through the Aksay restraining bend, and cracks observed at the Camel site might be merely triggered cracks due to the strong ground shaking related to event 3.

Event 4, constrained between 1269 BC and 639 BC, is potentially the largest earthquake that occurred along the central-eastern Alyn Tagh Fault and it is preceded by a ~1407-year-long lull period. According to the PDFs from all the trench sites (Figure 8), this event ruptured the Tashi section, the Aksay section, the Aksay bend (both southern and northern strands), and the Xorxoli basin. Nevertheless, given the uncertainty range of this event, we do not totally rule out the possibility of multiple events across the seven sites over a span of several decades to a few centuries.

The second group of events (cluster 2; Figure 10) occurred over a period of 1 kyr BC. The first earthquake of the second cluster was constrained at 3505–3150 BC (event 8) and it was documented along the Xorxoli section and west side of the Annanba section. Subsequently, a large earthquake (event 7) occurred between 3000 BC and 2509 BC, rupturing through the northern strand of the ATF. It appears that event 7 might have contributed to the seismic reactivation of the Xorxoli section, possibly promoting the two more recent earthquakes of cluster 2. These two events are constrained to 2509–2179 BC (event 6) and 2204–1964 BC (event 5). Remarkably, none of the four paleoevents of the second cluster (cluster 2) ruptured through the restraining bend (Annanba section).

The second cluster was preceded by ~1,668 years of seismic quiescence, at least on the Aksay and Tashi sections. In the Xorxoli section, this period could have been similar or even longer, as the stratigraphic record predating event CM9 spans an interval of 1,529–1,726 years with no evidence of ground deformation. Before this lull period, during the mid-Holocene, two older surface ruptures occurred at 5504–4109 BC (event 9) and 7364–4939 BC (event 10; Figure 9). These two events are characterized by strong vertical deformations at the T1 and T3 sites (previously described). Particularly event 9 represents a contemporaneous large rupture along the Tashi and Aksay sections. This event seemingly did not propagate through the Aksay restraining bend (Annanba section), as no evidence of deformation was observed in contemporaneous units at Mobaer. The partial overlap of event 9 with the upper limit of event GN is likely because of a widely constrained age due to the lack of datable material at the bottom of the Mobaer site. Therefore, Event 10 results from the overlap between events E9 and GN, suggesting that it likely ruptured through the Aksay section and propagated along the northern branch of the Aksay bend (Annanba section). Since we cannot ascertain if the southern strand was seismically active during this period, due to the lack of exposure at the Annanba site and Xorxoli sites, events 9 and 10 are not considered as a cluster.

**5. Insights About the Rupture Behavior and Seismic Hazard**

The coefficient of variation (COV) for our integrated earthquake time series is about 0.6, within the range associated with a quasi-periodic event organization (Goes & Ward, 1994), which is not what we observe in Figure 10. Goh & Barabási. (2008) proposed the burstiness measure (B), as an alternative way to evaluate the
<figure><img src="image_12.png" /><figcaption>Figure 9. Site-to-site correlation of overlapping Probability Density Functions (PDFs) from the rupture pool (see Figure 8). PDFs from the Bitter Sea site were not generated, as conventional radiocarbon ages (C14 years BP) are unavailable. Red shaded PDFs correspond to the mean date for a given earthquake, and red bars are their respective 95% confidence intervals of age. The PDFs that result from the correlation are numbered from 1 to 10 (as in Figure 8). Dotted, dashed, and solid black lines represent individual PDFs from a specific trench site. Black solid dots, squares, and triangles represent the mean age for individual PDFs.</figcaption></figure>

variability among time intervals in time series, which is basically a normalized COV, ranging (−1, 1) and defined as:

$$B = (\sigma - \mu)/(\sigma + \mu)$$

Where $\sigma$ and $\mu$ are the standard deviation and the mean interevent time, respectively. $B = -1$ corresponds to a perfectly periodic sequence, and $B \sim 0$ implies a random distribution (Poisson model). The estimation of $B = -0.29$ falls within the “strongly aperiodic” nomenclature for $-0.33 < B < 0$ ($0.5 < \text{COV} < 1$) proposed by Salditch et al. (2020). Commonly, time series with $\text{COV} > 1$ or $B > 0$ are associated with cluster behavior. However, some sequences with $\text{COV} < 1$ ($B < 0$) can be quite clustered (Salditch et al., 2020). There are two main reasons why $\text{COV}$ and $B$ imperfectly describe the degree of clustering in some time series: (a) A $\text{COV} > 1$ necessarily requires a strong differentiation between long interevent periods vs. shorter periods, such that uncertainty is significantly greater than the average. (b) Both $\text{COV}$ and $B$ ignore the order in which events occur (Chen et al., 2020), that is, these metrics do not provide information on whether shorter interevent times tend to happen one after another, forming clusters, or if they are mixed with longer intervals. Thus, in order to highlight these differences Goh and Barabási (2008) defined the memory coefficient $M$, as the coefficient of correlation of consecutive interevent time values ($\tau_i, \tau_{i+1}$) over a sequence of size $n_\tau$:

$$M = \frac{1}{n_\tau - 1} \sum_{i=1}^{n_\tau - 1} \frac{(\tau_i - \mu_1)(\tau_{i+1} - \mu_2)}{\sigma_1 \sigma_2}$$

$\mu_1$ ($\mu_2$) and $\sigma_1$ ($\sigma_2$) are sample mean and sample standard deviation of $\tau_i$'s ($\tau_{i+1}$'s), respectively ($i = 1, \dots, n_\tau - 1$). The memory coefficient ranges from −1 to 1. Positive values of $M$ imply that short (long) interevent times tend to be followed by short (long) intervals (periodic array), while negative values indicate that
<figure><img src="image_13.png" /><figcaption>**Figure 10.** Probability distributions, weighted mean ages and estimated magnitudes of large events along the central-eastern ATF. These long ruptures potentially involved one or multiple fault sections along the northern (NATF) and southern (SATF) strands of the Altyn Tagh Fault. Two distinct sequences of large earthquakes are identified as Cluster 1 and Cluster 2. Event 4 is a major event that ruptures through the Aksay double restraining bend. The red stars represent historical records of large earthquakes. *The two possible scenarios for event 2 are discussed in Section 4.3.</figcaption></figure>

short and long interevent times alternate in time (cluster fashion). For *M* = 0 there is no correlation between successive interevent times. Some faults with an extensive paleoseismic record exhibit a very conspicuous cluster (or “supercycle”) organization pattern (Griffin et al., 2020), like the North Anatolian Fault (M = −0.24) or the middle-south San Andreas fault (M = −0.3). In our case, the memory coefficient of the central-eastern ATF system (Figure 10) is around −0.07 (slightly negative). Such a value could be related to the small differentiation between seismic quiescence periods and interevent time in clusters. Nevertheless, similar memory coefficient estimations were obtained by Griffin et al. (2020) along the Jordan Valley of the Dead Sea Fault (M = −0.06), the southern San Andreas Fault (M = −0.08), and the Hope Fault (M = −0.03). In these cases, *M* is close to but less than 0, implying that the earthquake time series shows a clustering signature, rather than a sequence produced by periodic events or ideal Poisson processes (random paleoevents).

Our paleoevent time series built from the integration of local-scale observations offers a comprehensive way to explore the rupture behavior of the ATF system. Interestingly, we identified outburst phases of large earthquakes with inter-event periods of about 474 ± 114 years, which are preceded by periods of relative quiescence of 1.1–1.6 kyr. Such clustering behavior at a regional scale suggests that fault sections along the central-eastern ATF system do not rupture independently, but rather, it might indicate periods of seismic synchronicity due to long-term stress interactions between the northern and southern sub-parallel strands of the ATF. In fact, this study, as well as previously published paleoseismic investigations at the local scale reveal quasi-periodic behaviors along the Aksay, Annanba, and Xorxoli sections. However, this behavior is accompanied by high uncertainties as evident in the Aksay section, or notably in the Xorxoli section where despite its long and simple geometry, the variability can be more than half of the average return period. We suggest that such irregularities in the timing of large earthquakes are due to the complexity of the ATF at a regional scale, which allows for interacting fault sections, and thus relatively closely timed large ruptures.

Following the regional scale rupture pattern (Figure 10), we propose two possible scenarios: (a) the central-eastern side of the ATF is currently in the early stage of a long-lull period so that it will take about a millennium until the next large earthquake happens; (b) it may be at the late stage of a cluster cycle leaving the possibility for one or two subsequent large earthquakes within the next century before starting a seismic quiescence period. From these scenarios, the second one seems to be more likely, given that the current open intervals on the Xorxoli, Aksay, and Annaba sections are each relatively proximate to their mean recurrence interval. Particularly the Xorxoli fault section could be a potential candidate to generate the next large earthquake, for three main reasons. First, geodetically constrained models of interseismic fault coupling along the central-eastern ATF reveal a strongly locked patch down to 15 km and extending along more than 100 km of the Xorxoli section (Li et al., 2018; Liu et al., 2018). Second, the elapsed time in Xorxoli considerably surpasses its minimum recurrence interval. Third, if we consider an average slip rate ~10 mm/yr from geodetic measurements, currently the slip deficit is around ~4 m since the MRE, which is greater than the average slip of the most recent and penultimate events documented in Xorxoli (see table 3). Under the assumption that the cumulative slip due to tectonic loading
was totally accommodated by the MRE (event 1), and if an upcoming earthquake is expected in the very short
term, the current accumulated slip is compatible with a M<sub>w</sub>7.6 earthquake rupturing through the Xorxoli section
(Kanamori, 1977).

6. Conclusions

We have investigated the fault rupture behavior at the local (fault section) and regional (multiple sections) scale along the central-eastern ATF system based on paleoseismic observations. Contemporaneous high-ranked event indicators, documented in three trench sites in the Aksay section, reveal six earthquakes that likely ruptured the entire fault section. The coseismic slip distribution unveils that the most recent of these events might have generated an average displacement of 5.1 ± 1.4 m along the Aksay section. Although we identified five paleo-seismic events with a characteristic amplitude of 5.3 m, a larger number of measurements are necessary to fully support a characteristic slip model.

Our results show that the average recurrence interval in the Aksay section is 1,329 ± 588 years with a COV of
~0.44, which statistically indicates a quasi-periodic behavior. Such behavior with high variability is consistent
with previously published research along the Tashi, Annanba, and Xorxoli sections. We suggest that much of the
irregularity in the timing of large earthquakes, observed at the local scale, is due to long-term fault interaction
between the northern and southern strands of the ATF, allowing for synchronized large ruptures over the last
8 kyr. Notably, we found that the spatial distribution of large earthquakes at the regional scale seems to cluster
during the middle (5.4–4.4 kyr) and late (1.6 kyr–present) Holocene. These clusters present interevent times of
474 ± 114 years and are separated by long-lull periods of 1.1–1.6 kyr. The number of earthquakes within each
cluster, as well as their rupture lengths, are determined by the temporal overlap of the event ages. Therefore, some
of the identified paleoevents may have been seismic sequences rather than single earthquakes, suggesting that the
seismic activity in the central-eastern ATF might be more elevated than depicted in our study.

The spatiotemporal clustering is not unique to the central-eastern ATF system; it has also been unambiguously evidenced along other major strike-slip faults such as the southern San Andreas Fault, in United States (Dolan et al., 2007), and the Hope Fault, in New Zealand (Langridge et al., 2013). In all these cases the clustering is controlled by large-scale and long-term (decades to centuries) fault interactions. As a consequence, efforts to assess the seismic hazard at the local-scale may be hampered by large variations in the earthquake cycle. Therefore, when it comes to complex fault systems, a regional scale panorama of the fault behavior might offer a key to anticipate more precisely the time and size of large earthquakes, and thus improve seismic hazard assessments.

Finally, our earthquake time series indicates that the central-eastern ATF might be either at the early stage of a seismic quiescence period or at the late stage of a cluster period. The latter scenario seems to be more likely as the Xorxoli fault section is quite prone to break. Considering the elapsed time since the MRE in Xorxoli and the ~4 m slip-deficit, a forthcoming earthquake might occur within the next century and is compatible with a minimum magnitude of Mw 7.6.

Data Availability Statement

The data files from the Aksay fault section (trench photomosaics–individual radiocarbon ages) used in this paper are available in the Supporting Information S1. The Pleiades images were provided by the CEOS Seismic Hazards Pilot from ESA (http://ceos.org/ourwork/workinggroups/disasters/earthquakes) and the ISIS program from the CNES (https://dinamis.data-terra.org), and are covered by an Institutional User License. Supplementary figures providing details on the methodology on the offset meaurements are available in Supporting Information S2. The data files from Annanba, Xorxoli and Tashi Fault sections (individual radiocarbon ages) are available at (Luo et al., 2019; Shao et al., 2018; Washburn et al., 2001, 2003; Yuan et al., 2018). Earthquake ages were modeled with OxCal version 4.4 (Bronk-Ramsey, 2008).

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Acknowledgments

Part of this work was funded through
program ANR-005-CATT-0006. Pleiades
satellite images used in this work were
acquired through the ISIS CNES program.
We thank Arthur Delorme for his support
in the preparation and processing of the
satellite imagery. We thank Simon
Blockley and Martin Hinz for the
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their constructive comments, and specially
to an anonymous reviewer whose detailed
review significantly contributed to
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