We present first constraints from tectonic geomorphology and
paleoseismology along the newly discovered Sharkhai fault near the capital
city of Mongolia. Detailed observations from high-resolution Pleiades
satellite images and field investigations allowed us to map the fault in
detail, describe its geometry and segmentation, characterize its kinematics,
and document its recent activity and seismic behavior (cumulative
displacements and paleoseismicity). The Sharkhai fault displays a surface
length of ∼ 40 km with a slightly arcuate geometry, and a
strike ranging from N42 to N72∘. It affects
numerous drainages that show left-lateral cumulative displacements reaching
94 m. Paleoseismic investigations document faulting and
depositional/erosional events for the last ∼ 3000 years and
reveal that the most recent event occurred between 775 and 1778 CE and
the penultimate earthquake occurred between 1605 and 835 BCE. The
resulting time interval of 2496 ± 887 years is the first constraint for
the Sharkhai fault for large earthquakes. On the basis of our mapping of the
surface rupture and the resulting segmentation analysis, we propose two
possible scenarios for large earthquakes with likely magnitudes of 6.7 ± 0.2 or 7.1 ± 0.7. Furthermore, we apply scaling laws to infer
coseismic slip values and derive preliminary estimates of long-term slip
rates. Finally, these data help build a comprehensive model of active faults
in that region and should be considered in the seismic hazard assessment for
the city of Ulaanbaatar.
Introduction and context
The tectonics of Mongolia are characterized by the transition between the
compressive structures associated with the India–Asia collision to the
south and the vast extensive structures of the Baikal Rift to the north.
(Fig. 1). This induces important complexity and variability expressed by
dominantly strike-slip structures with minor thrust and normal faults
(Khilko et al., 1985; Cunningham, 2001; Ritz et al., 2003; Cunningham, 2007;
Walker et al., 2008; Parfeevets and Sankov, 2012). In Central Mongolia, the
Hangay dome is surrounded by right- and left-lateral faults (Cuningham et
al., 1996; Schlupp, 1996; Bayasgalan, 1999; Bayasgalan et al., 1999;
Etchebes, 2011). Western Mongolia is dominated by NW–SE-striking
right-lateral and thrust faults distributed across the Mongolian Altai
ranges, while southern Mongolia shows E–W left-lateral and thrust faults
that produce the Gobi Altay restraining-bend topography. Finally, to the
north the E–W Bolnay left-lateral strike-slip fault begins the transition
with the Baikal rift system. The rate of deformation along faults in western
and central Mongolia are relatively low with 1.5 ± 0.26 to 3.8 ± 0.2 mm yr-1 based on geological observations (Ritz et al., 2006; Etchebes,
2011; Rizza et al., 2015) and 2 ± 1.2 to 2.6 ± 0.5 mm yr-1 based
on geodetic data (Calais et al., 2003). Presently, the historical seismicity
record in the region is short and poorly constrained (Khilko et al., 1985).
Since 1905, seismicity has been highlighted by four great earthquakes with
Mw ranging from 7.9 to 8.3–8.5 (9 and 23 July 1905, 11 August 1931, and 4 December 1957), which occurred along the strike-slip faults of western and
southwestern Mongolia (Fig. 1) with moderate background activity.
Tectonic map of Mongolia (modified from Rizza et al., 2015). The
four great earthquakes of magnitude 8+ that occurred since 1905 are
labeled 1 to 4. The inset map shows active deformations in Asia with
Mongolia between the India–Asia collision to the south and extensive
structures of the Baikal Rift to the north. “UB” is Ulaanbaatar, capital
of Mongolia, and the rectangle shows the location of Fig. 2.
The region of Ulaanbaatar (capital of Mongolia) is situated in a folded
system composed of Lower to Middle Carboniferous and Quaternary deposits
(Tomurtogoo et al., 1998; Manandhar et al., 2016) (Fig. 2). The Carboniferous
formations are sandstone, mudstone, alternating beds of sandstone and
mudstone with limited outcrops of conglomerate, siliceous mudstone, chert,
felsic tuff, and basalt (Takeuchi et al., 2013). Compared to western and
southwestern Mongolia, the Ulaanbaatar region displays a different
seismotectonic situation. Firstly, although several tectonic faults are
clearly documented in the geological map (Fig. 2), their potential
Quaternary activity remains unknown. Secondly, the level of recorded
seismicity is significantly lower, in terms of both event frequency and
magnitude (One century of seismicity in Mongolia map, 2000; Dugarmaa and
Schlupp, 2000). The historical seismicity is poorly known, and since 1957,
when the instrumental period started, the activity has been limited to
moderate earthquakes with magnitude less than 4.5 (Adiya, 2016).
Nevertheless, several earthquakes were largely felt in Ulaanbaatar during
the last century (MSK intensity up to VI) without significant damage (Khilko
et al., 1985). Regional deformation characterized by geodesy indicates 2–4 mm yr-1 of ESE horizontal displacement with respect to Eurasia
(Miroshnichenko et al., 2018).
Geological and seismo-tectonic context of the Ulaanbaatar region.
Red dots are earthquakes recorded between 1994 and 2011 (Institute of
Astronomy and Geophysics, Mongolian Academy of Sciences, National Data
Center). Black lines represent the active faults (HF: Hustai fault, EF:
Emeelt fault, SF: Sharkhai fault, AF: Avdar fault, UBF: Ulaanbaatar fault,
GF: Gunj fault). UB: Ulaanbaatar city, GA: Ghingis Khan old international
airport, NA: new international airport. The background DEM is from SRTM1
data (see data and resources). Geological map is an extract from the geologic map of Mongolia (scale 1:1 M) (Tomurtogoo et al., 1998).
Between 2005 and 2019, more than 10 swarm episodes of moderate earthquakes
(M≤4.5) have been recorded and accurately relocated ∼ 10 km west of the capital (Adiya, 2016). Tectonic geomorphology investigations
focused on the swarm area revealed evidence of Quaternary activity along the
Emeelt fault (Ferry et al, 2010; Schlupp et al, 2010a; Ferry et al, 2012;
Schlupp et al., 2012; Dujardin et al, 2014). This structure is located near
the eastern end of the Hustai fault, strikes N140 (Fig. 2) and
displays dominantly right-lateral kinematics with a reverse component.
Recent studies suggest that it could produce earthquakes of Mw 6–7 (Schlupp
et al., 2012). Located ∼ 30 km west of Ulaanbaatar, the Hustai
(alternative spelling Khustai) fault exhibits a remarkable morphology that
displays recent markers affected by left-lateral and normal faulting and is
composed of several segments with a total length of 212 km. It is considered
capable of producing earthquakes of Mx 6.5–7.5 (Ferry et al., 2010; Schlupp
et al., 2010b; Fleury et al., 2011; Ferry et al., 2012). To the northeast of
Ulaanbaatar at ∼ 15 km from the city center, the surface
expression of the Gunj Fault is visible along ∼ 20 km; it is
oriented N45 and is evidenced by right-lateral displacements
affecting gullies and reaching 25 m (Demberel et al., 2011), vertical scarps
and flower structures (Imaev et al., 2012). Finally, the Ulaanbaatar Fault
has been recently described by Suzuki et al. (2020): it displays scarps,
pressure ridges and deformed Pleistocene deposits over a length of
∼ 50 km. Preliminary results suggest the fault could produce
earthquakes with Mw ranging from 6.5–7.1 depending on the rupture
scenario (surface rupture length from 20 to 50 km).
The most recent addition to the ongoing effort to document active faults
within the intensely developing greater Ulaanbaatar region was carried out
to the south of the city, where the new international airport is built.
There, we combined the analysis of high-resolution satellite images and
field investigations and discovered two active faults hereafter called
“Sharkhai fault”, located ∼ 35 km south of the capital and
only 10 km south of the new airport, and “Avdar fault” (Fig. 2)
(Al-Ashkar, 2015). In this study, we present a detailed characterization of
the Sharkhai fault based on remote sensing analysis, geomorphological
observations and paleoseismological investigations and propose the first
results pertaining to its Holocene activity and associated characteristics
(segmentation, kinematics and paleoseismicity).
Considering the well-expressed geology (Carboniferous age) combined with
slow active deformation rates, and low erosion and sedimentation rates
(continental steppe context), our strategy consisted of mapping faults at
high spatial resolution and characterizing their subtle cumulative
expression within Quaternary deposits. To identify and quantify horizontal
and vertical deformation we based our analysis on very high-resolution
orthorectified Pleiades satellites images (multispectral RGB-NIR at 2 m
resolution and panchromatic at 0.5 m resolution, hereafter referred to as HR
images) and high-resolution digital elevation models SRTM 1′′ at 30 m
resolution and TanDEM-X at 12 m resolution (hereafter referred to as DEM).
Additional images from Google Earth acquired at different seasons provided
complementary information. Remote sensing analysis was supplemented by field
campaigns to verify, correct and complement these observations; perform
detailed geomorphological mapping; and excavate a paleoseismological trench.
Overview
Our observations show that the main trace of the Sharkhai fault, striking
ENE–WSW, extends along 40 km (from A1 to A7 in Fig. 3). Along most of its
length, the surface rupture corresponds to a documented geological structure
(Fig. 2) that was not characterized as active in previous studies
(Tomurtogoo et al., 1998). The main geomorphological features observed along
the Sharkhai fault are offset drainages connected by faint lineaments that
can be followed on HR images. In the field, they are locally expressed as
smoothed scarps (less than 50 cm high) and breaks in slope and mark the
eroded fault trace. Near the middle of the fault trace, a well-developed
1.4 km wide extensional jog (Fig. 3, between points A3 and A4) accommodates
a right step, which suggests that the fault can be segmented into two major
sections: the southern section (strike N42 to N55) and the northern section
(strike N55 to N72) (Figs. 3 and 5). Below we describe the fault surface
trace from the southwest to the northeast and detail the various features
documenting recent activity and segmentation.
(a) SRTM1 DEM (see data and resources) with arrows showing the
location of the Sharkhai active fault. (b) Simplified map of the
Sharkhai active fault about 40 km long and strikes from N42 at south to N72
at north. Letters A1–A7, B and C indicate the location of sites described in
the text. Letters P1 to P7 (see details in Figs. 6–9 and figures in
supplementary materials) indicate the locations of documented offset
drainages. Note the left step-over which divides the fault into two sections
between points A3 and A4. Coordinates are in UTM zone 48N.
Despite a generally weak morphological expression due to long-term erosion
and, locally, recent stream deposits, the surface trace can be followed on
HR images and confirmed by field observations (Fig. 4). The southern section
runs for ∼ 22 km from points A1 to A3 (Fig. 3) where the fault
trace dies out at a large extensional step-over. The main geometric features
that we detail hereafter are strike changes and step-overs.
At its southern extremity between points A1 and A2 the fault strikes N42 on
average (Figs. 3, 4 and 5). North of A2, the average direction turns from
about N42 to N50, which is the largest strike
variation along the southern section. In detail, we observe several small
step-overs (3, 7 and 70 m width) and locally several changes in strike over
short distances (a few hundred meters). Between A2 and B, the fault trace
cuts through a Carboniferous hill (1450 to 1645 m elevation) and the top of
two successive hills that are oriented N5 and N330 (Figs. 3 and 4). The fault
displays an en echelon geometry between (B) and (C) (Fig. 6) with secondary
branches parallel or oblique to the main trace. Their lengths range from 190 m to 1.6 km and strike between N58 and N74. Beyond, the fault continues
through a valley floor covered with Quaternary alluvial deposits where the
trace disappears. Along an 8 km long section where the trace cuts hills and
valleys, we identified six cumulative left-lateral offsets (Figs. 3 and 6). The first is drainage shifted by 53 ± 6 m (P1 in Figs. 3 and 7). It corresponds to the maximum offset identified along the southern
section of the Sharkhai fault (Table 1). The minimum offsets observed are
6.25 ± 1.65 m (P2 in Figs. 3 and 8) and 6.5 ± 1.5 m (P5 in
Figs. 3 and 9). The three other cumulative offsets are 36 ± 5 m
(P3 in Figs. 3 and S1 in the Supplement), 30 ± 5 m (P4 in Figs. 3 and S2) and 36 ± 2 m (P6 in Figs. 3 and S3). It should be noted that half of the documented offsets display
similar values (30 to 36 m), which suggests the offset features may have a
common climatic origin and thus age (i.e., a Late Pleistocene humid period).
Map of the fault trace and major strike changes (according to the
average direction of every segment). Dashed rectangle is the left step-over
which divides the fault into two main segments, southern and northern segments,
with a local 5∘ clockwise strike change (from N50 to
N55). The average strike change between the southern and
northern segments is larger, with 13∘ clockwise (N50
to N63). Secondary branches parallel or oblique to the fault
with direction varying between N56 and N83. Coordinates
are in UTM zone 48N.
The northern section runs for ∼ 22 km from point A4 to point A7 (Fig. 3). It has a slightly arched shape geometry; its strike turns from
N55 to N63 and N72 (Fig. 5). In contrast with the southern section, it shows
less in-strike segmentation (no clear step-overs) and more off-fault
deformation (10 m long to 1 km long sub-parallel or oblique secondary
branches). Locally we also observe changes in the main fault strike over a
few hundred meters. This section affects mostly Quaternary deposits
(Tomurtogoo et al., 1998) as the trace runs through an area of lower
elevation (mainly <1500 m), and the trace frequently disappears,
which may suggest limited deformation or high rates of sedimentation. At the
northern part of the section we measured 94 ± 3 m of left-lateral
horizontal offset affecting a stream (P7 in Figs. 3 and S4), the only one identified along the northern
Sharkhai section and the largest along the entire fault. The drainage
pattern along the northern section is less complex than that along the
southern section but also less developed or preserved, which limits the
possible records of displacement. As it reaches the SE part of the Khoshigt
Khondii basin where the new international airport of Ulaanbaatar is built
(point A7 in Fig. 3), the trace of the Sharkhai fault cannot be observed
anymore, neither on remote sensing data nor in the field. It terminates into
fluvial plains covered by Quaternary sediments. Hence, the total surface
rupture length of the Sharkhai fault could be underestimated by a few
kilometers.
Summary of cumulative left-lateral offsets measured on the
Sharkhai fault.
There is only limited information about historical seismicity in the region
(catalog duration limited to about three centuries) with the maximum known
event reaching magnitude 5 to 5.5. However, possible rupture scenarios and
associated magnitudes along the Sharkhai fault are key parameters for
estimating seismic hazard levels of the city of Ulaanbaatar and the new
airport. Hence, characterizing large earthquakes possibly associated with
mapped faults requires applying empirical relationships (Wells and
Coppersmith, 1994; Leonard, 2014).
We use the identified discontinuities along the fault to discuss whether the
fault could be divided into several segments (Fig. 5) that could break
independently or not. Step-overs, secondary branches and fault strike
changes can play an important role in the propagation of a rupture
(nucleation and barrier) and consequently in the size of expected
earthquakes (Poliakov et al., 2002; Wesnousky, 2006; Klinger, 2010; Finzi
and Langer, 2012; Biasi and Wesnousky, 2016, 2017).
Usually, only kilometer-scale discontinuities are considered for the
segmentation (Crone and Hailer, 1991; De Polo et al., 1991; Harris et al.,
1991; Wesnousky, 2006, 2008; Carpenter et al., 2012). Therefore,
only the central step-over appears wide enough to separate the fault into
two potential segments, the southern and the northern. The width of the
other step-overs is much more limited, between 3 and 173 m, and is not
clearly expressed in the geomorphology. Thus, we do not consider them as
potential segment boundaries. Similarly, it has been proposed that changes
in strike of more than 5∘ could also play a role in fault
segmentation (Lettis et al., 2002; Harris et al., 1991; Wesnousky, 2006;
Finzi and Langer, 2012). Nevertheless, recent large earthquakes in Mongolia
have shown that even larger changes in orientation had no impact on the
fault segmentation (5 January 1967 Mogod earthquake, Mw 7.1, Bayasgalan and Jackson,
1999; 4 December 1957 Bogd earthquake, Mw 8, Rizza et al., 2011). Along the Sharkhai
fault, the changes in the orientation are either very local or not exceeding
9∘. Thus, they are not considered as likely segment boundaries.
In conclusion, we propose two possible scenarios for large earthquakes on
the Sharkhai fault depending on the role that the central step-over may play
in the propagation of the rupture. The first scenario is that the entire
fault (40 km) breaks during one earthquake. The second scenario is that the
southern segment and the northern segment (22 km each) break independently.
Paleoseismic investigations
To retrieve the chronology of surface-rupturing paleoearthquakes, we
conducted the first paleoseismological study along the Sharkhai Fault at a
site called Muka (Figs. 3 and 10). This site was selected based on
geomorphological observations performed from high-resolution Pleiades
satellite images, high-resolution TanDEM-X DEM and field surveys.
Considering an a priori slow rate of deformation, our strategy was to avoid
apparently recent deposits found in wide alluvial valleys, as well as
associated erosion processes, that could cover the recent deformation in the
last 3–4 m or erode the last event records and rather target relatively
slow deposition processes such as colluvium on gentle slopes and abandoned
or intermittent drainages. The subtle geomorphological expression of the
Sharkhai fault combined with high elevation along most of its trace yielded
only a few favorable sites where the fault is well expressed and potentially
datable deposits are expected. The Muka site is located near the Zuunmod–Buren road and ∼ 10 km SW of the new airport. There, the trace
of the fault is clear, enhanced by a small scarp (about 30 cm high) (Fig. 10c) and a striking difference in vegetation type and color, often indicative
of a local contrast in lithology and/or hydrology in the shallow
sub-surface. This small scarp suggests surface deformation with an apparent
vertical offset that could be induced by horizontal slip along slopes. The
fault affects here surface colluvium deposited along the flank of a small
valley. Local gullies are intermittent and probably only active during
important rainfall (Fig. 10a and b). Hence, we consider this site favorable
to the accumulation of deposits, the preservation of the fault's
paleoseismic history and the determination of paleoearthquake chronology
by radiocarbon and/or OSL approaches.
Offset reconstruction for drainage P2. (a) Field photograph of P2:
the black dashed line indicates the fault trace. The north direction in the
photograph is approximate. (b) Differential GPS measurements used to build
the digital topographic map. (c) Digital topographic map based on GPS
measurements. (d) Present-day situation: the offset is measured on images by
projecting the average upstream and the downstream to the fault trace. We
consider for the upstream a “wide zone” giving an uncertainty on its
piercing position for the back-slip reconstruction. (e) Minimum back-slip
reconstruction of 5.6 m. (f) Maximum back-slip reconstruction of 6.9 m.
Hence, the left-lateral offset is estimated at 6.25 ± 1.65 m. White
arrows: water flow direction. For location, see Fig. 3.
The Muka site is located at 628 253 m E/5 268 367 m N along a straight section
of the fault where deformation at the surface appears well-localized (Fig. 10). There, the fault marks a break in slope with a ∼ 30 cm high scarp and is crossed by short (100–500 m in length) shallow
gullies. We excavated two trenches called Muka-K and Muka-L (Fig. 10b),
∼ 150 m apart. Both trenches were ∼ 20 m long, 1 m wide and up to 3 m deep as limited by the local permafrost. Heavy rainfall
and thawing of the exposed permafrost destabilized overnight the fine
deposits (silt and sand) found in Muka-K. Wide sections of the trench
collapsed, and it was considered unsafe. Stable substratum crops out at the
bottom of Muka-L, which stabilized the whole section and gave time to
reinforce the walls with wooden shores. In the following, we present the
Muka-L exposure only.
Trench stratigraphy
Both trench walls were cleaned, gridded, photographed and logged in detail.
The Photomosaic of the trench (west and east wall), 15 m long and 3m deep,
is built using 210 photographs. Since both walls yield similar information
in terms of paleoseismicity, we only present the west wall in detail along
with close-ups of the east wall for illustration (Fig. 11). In the
following, we describe the stratigraphy, provide age constraints on the
basis of radiocarbon-dated sediment samples and analyze abutting
relationships to decipher the chronology of surface-rupturing earthquakes at
this site.
The base unit visible along the whole trench is composed of massive
Carboniferous bedrock (U70). The U70 exhibits widespread fracturation and
localized shear zones with thin gouge development (<2 cm). The
uppermost 10–50 cm of U70 are composed of deeply weathered, well-sorted
unstratified fine clasts (<3 cm) that we interpret as the product
of gelifraction. Numerous thin shear zones marked by whitish-to-yellowish
clay cut through the whole unit and stop at its top surface. They generally
exhibit a relatively steep dip to the south and produce duplexing features
within the weathered part of U70. The top surface is very rough with deep
troughs and systematically truncates reverse-geometry shear zones; it is
interpreted as a well-developed erosion surface. Although the bottom of the
trench was still frozen during the excavation done in summer, we did not find
clear indication of gelifraction of the erosional surface at the top of U70.
Muka-L trench exposure. (a–e) Close-ups showing deformation
features (step-like geometry, geometry resembling flower structures,
apparent offsets). (f) General orthophoto mosaic of the west wall, originally
rendered at 1 mm resolution. (g) Detailed paleoseismic log of the west wall.
The ruptures associated with the last two events are in red. Event horizons
are shown for the most recent event (MRE) and the penultimate event (PE).
See text for details.
Over the northern section of the trench, U70 is overlain with a
∼ 1 m thick unit of massive clast-supported coarse gravels and
pebbles (U60). Clasts present the same lithology as U70 are very angular
and well stratified, which suggests they have been transported by water but
only over a very short distance. U60 contains a few lenses of dark brown to
black fine sand. Combining with the geometry of the lower erosion surface,
we interpret U60 as a channel fill. Sample W3-S03 (Fig. 11g and Table 2) was
collected within this unit and yields a radiocarbon calibrated age of 1515 ± 90 BCE (3220 ± 30 BP).
Radiocarbon dating of bulk-sediment samples collected in the Muka-L
trench and dated by the Poznań Radiocarbon Laboratory. The software
OxCal V2.4 (Ramsey, 2013) with 2-sigma error was used to obtain the
calendric ages with an Intcal13 calibration curve (Reimer et al., 2013).
In the central part of the trench, U70 is overlain by a ∼ 8 m wide, 50 cm thick unit that pinches out at both tips (U50). This lens
contains similar clasts to those in U60 with a much smaller matrix fraction
(clast-supported to openwork). It exhibits well-defined sub-horizontal
stratigraphy and is interpreted as a low-energy channel.
The southern half of U50 is itself overlain by a 5–10 cm thick well-sorted
fine sand unit (U40) that changes laterally to massive amounts of clay, locally grey
but with widespread secondary oxidation. It fills a small basin bounded by
U70 at the southernmost end of the trench. There, U40 displays growth strata
and contains massive clay with rare scattered angular gravels (Fig. 11a).
This marks a change in the depositional environment: a small pond in a
rather dry climate with occasional clasts from the surrounding slope.
A higher well-developed layer (U30) crops out over the whole length of the
trench. Unit 30 is composed of massive red clay and coarse sand with
abundant scattered gravel and some well-sorted grey sand lenses (Fig. 11a).
The clay fraction is dominant within the small depression (between x=0 and
x=4 m) to the south and diminishes to the north where sand lenses are
thicker (5–8 cm) and more continuous. There, the matrix contains numerous
pockets of secondary white clay (Fig. 11b). Overall, the stratigraphic
facies of U30 resemble red clay formations generally associated with a warm
and humid climate (Feng et al., 2007).
Estimation of the magnitude and average co-seismic slip using Wells
and Coppersmith (1994) and Leonard (2014) regressions. The fault length is
determined from the segmentation scenarios.
Between x=9 and x=12 m, three blocks with well-defined edges make
up unit U20, composed of well-stratified sand and angular fine gravel with
very little matrix-resembling channel fill unit U50. It is considered
allochthonous with respect to the rest of the stratigraphic section and
interpreted as a small channel that flowed oblique to the fault and was
dragged along it. A modern equivalent could be seen in the shallow
intermittent stream that flows across the site next to trench Muka-K (Fig. 11b). We collected two samples from the top of U20: W2-S04 yielded a
calibrated date 945 ± 110 BCE (2745 ± 30 BP) and W2-S05 a
calibrated date 45 ± 80 CE (1950 ± 30 BP). Sample W2-S05
sits very close to a rupture and exhibits dense live rootlets that could
have been a guide for contamination. Hence, we interpret W2-S05 as
contaminated and rejuvenated with respect to its stratigraphic position and
discard it from our analysis.
Finally, the uppermost unit called U11 is a 0.8–1.5 m thick massive fine
sand and silt layer. It is overall grey in color, is darker near its base and
displays discontinuous brown to black lenses throughout the section. At the
southern end of the trench, it contains clasts of U30, which indicates the
base of U11 is an erosion surface. Above this local transition, no internal
stratigraphy could be observed. Its top is dominated by weak present-day
soil development (U10), which is only visible within the first 8–10 cm from
the ground surface. We collected two sediment samples from U11 within dark
lenses: one at the bottom (sample W4-S02) yielded a calibrated date 450 ± 70 BCE (2360 ± 30 BP) and one in the mid-section (sample
W2-S06) with a calibrated date 860 ± 85 CE (1180 ± 25 BP).
This is the youngest age constraint found in the Muka-L trench.
Surface faulting events at the Muka-L site
Trench Muka-L revealed numerous deformation features (Fig. 11a–e):
interrupted and offset layers displaying step-like geometry (Fig. 11a),
splay structures (Fig. 11b) and grabens (between 6 and 7 m in Fig. 11g),
among others.
The Carboniferous bedrock (U70) is intensely deformed by widespread
fractures and numerous shear zones dipping 30–50∘ to
the south and infiltrated by white to yellow clay. This unit is brittle
enough for groundhogs to be able to dig through it (see the large burrow at
x=8 m in Fig. 11f–g). This deformation is inconsistent with ruptures
observed in upper units and is limited to U70; it is therefore considered
representative of an ancient tectonic regime and will not be described any
further here.
The sedimentary section (units U60 to U10) is affected by ruptures
exhibiting generally near-vertical dips with some dipping slightly to the
south and a few to the north. Splays with geometries resembling flower and
double flower structures (Fig. 11b–e and g at x=4.5 m) are the
cross-section expressions of horizontal movement along en echelon fissures
and indicate a strike-slip component. This is confirmed by significant
variations in unit thickness across faults as displayed by U60 between x=9
and 12 m. Furthermore, numerous extensional features such as stepping
ruptures at the edge of the pond, a graben at x=6–6.5 m, and the collapse
of the completely sedimentary section between x=10.5 and 12 m suggest
transtensional deformation. The detailed trench log (Fig. 11g) reveals that
apparent normal geometry ruptures are dominant south of x=8 m (main
burrow) and expressed as distributed minor vertical individual offsets of
5–15 cm (with a possible contribution from strike-slip displacement).
Dominantly strike-slip deformation appears to be limited to a narrow band
between x=9 and 12 m. There, large vertical apparent displacements
(>50 cm) and allochthonous blocks suggest significant horizontal
deformation.
Logged ruptures display terminations at different levels. Between x=5 and
7.5 m all ruptures terminate at the top of U30 and are truncated by the
upper erosion surface. A few more ruptures between x=8 and 9 m appear to
display a similar geometry, though extensive burrowing hinders proper
observations. These ruptures would have affected the stratigraphy posterior
to the deposition of U30 and prior to the erosion of its top surface; i.e., between 1605 BCE (upper bound of Muka-L-W3-S03) and 835 BCE (lower bound of
Muka-L-W3-S0). A second generation of ruptures cuts through the whole
section and affects U11 and possibly U10 (soil development renders our
observations inconclusive): between x=3 and 5 m, at x=7 m, and between
x=9 and 12 m. The event occurred posterior to the deposition of the
youngest unit (U11); i.e., it should be noted that a few isolated ruptures
located at around x=3 and x=6 m affect the upper erosion surface (top
of U30) but do not appear to propagate further upward. Although they could
be associated with an intermediate event, we propose they are associated
with the most recent one, and their upward continuation could not be observed
due to the lack of clear stratigraphy within U11. Furthermore, small
vertical offsets affect the top of U30 between x=3 and 5 m with an
apparent component (the bottom and top of U30 do not display the same
offsets).
In summary, the Muka-L trench documents the erosion and deposition record
for the last ∼ 3000 years with varying environments. Abutting
relationships reveal at least two deformation events. (1) The first is a most recent
event (MRE) after 775 CE (lower bound of W2-S06). Considering that
Ulaanbaatar was installed in 1778 (e.g., Majer and Teleki, 2006), a large
earthquake after this date along this fault would have been reported in the
historical documentation, which is not the case. Thus, the MRE occurred
anytime between 775 and 1778 CE. (2) The second is a penultimate event (PE) occurred
between 1605 BCE (upper bound of Muka-L-W3-S03) and 775 BCE (lower bound of
Muka-L-W3-S6).
Minimum and maximum inter-event time and slip rate for the Sharkhai
fault (WC94: Wells and Coppersmith, 1994; L14: Leonard, 2014).
Synthesis of geological or geodesic slip rates and recurrence time
for large events published for large faults in western Mongolia.
FaultGeological slip rateRecurrence timeGeodesic slip rate(mm yr-1)(mm yr-1)Fu-Yun3.8 ± 0.23–4 kyr2.6 ± 0.5(EQ M8+ in 1931)(Etchebes, 2011)(Etchebes, 2011)(Calais et al., 2003)Bolnay3.1 ± 1.7 52.43–3.1 kyr2.6 ± 1(EQ M8+ in 1905)(Rizza et al., 2015)(Rizza, 2010)(Calais et al., 2003)Bogd1.5 ± 0.263.6–3.5 kyr2 ± 1.2(EQ M8+ in 1957)(Ritz et al., 2006)(Rizza, 2010)(Calais et al., 2003)Discussion and conclusionsSurface trace geometry and inter-event time
From our morphotectonic analysis based on field observations and HR remote
sensing data, we mapped the Sharkhai fault, oriented N57 (N57 ± 15), over a length of ∼ 40 km (Fig. 5). The tips
of the surface rupture terminate into wide fluvial plains (a few kilometers wide)
where they are covered by sediments. Hence, the total surface rupture length
of the Sharkhai fault could be underestimated by a few kilometers. The
surface expression of the fault is divided into two main segments displaying
a slightly arcuate shape and separated by a large extensional step-over of
1.4 km in width. Both segments are of similar length (∼ 22 km)
with a lateral overlap of ∼ 4 km. We also describe internal
geometric discontinuities that are typical for large strike-slip faults:
strike changes of 5 to 9∘, local step-overs of 3 to
173 m in width, and secondary branches of 10 m to 1.6 km in length (Figs. 5 and
6). Generally, these discontinuities are too small to play an important role
in the rupture propagation and total length and related earthquake size
(Poliakov et al., 2002). Conversely, the width of the main extensional
step-over corresponds to features that may equally stop or promote the
propagation of the rupture in similar settings (Wesnousky, 2006).
Along the strike, we documented seven streams affected by left-lateral cumulative
offsets ranging from 6.25 to 94 m with two of about 6 m (Figs. 8 and 9) and
three of 30–36 m (Figs. S1, S2 and S3 in Supplement, and Table 1). We did not observe systematic vertical deformation, the local vertical
displacements being easily explained as apparent and induced by horizontal
slip along slopes.
Our work is the first paleoseismological study along the Sharkhai Fault. The
Muka trench site is located near the end of the mapped rupture (Fig. 3),
which is not the standard strategy for such a study since deformation may be
weakly expressed and the resulting record may be less legible and possibly
incomplete. However, potential sites are scarce along the Sharkhai Fault, and
this site was selected on the basis of remote sensing and field observations
for its relatively high sedimentary potential. It delivered well-expressed
surface deformation and adequate deposits for age determinations. The Muka-L
trench analysis reveals two paleoearthquakes along the Sharkhai fault: the
most recent event (MRE) occurred between 775 and 1778 CE and the
penultimate earthquake (PE) occurred between 1605 and 775 BCE, which
yields an inter-event time of 2496 ± 887 years (between 3383 years and
1610 years). This is the first inter-event time constraint for the Sharkhai
fault, and it is comparable to values derived for major active faults
elsewhere in Mongolia (e.g., Prentice et al., 2002; Rizza et al., 2015).
Magnitude, co-seismic displacement and slip rates
The data collected on the Sharkhai fault, although preliminary, allow us to
make some considerations on the seismic potential of this fault. Based on
the fault geometry and internal organization we may consider two rupture
scenarios: (i) the entire fault ruptures into a single event over a length of
40 km and (ii) the two segments rupture independently into two distinct
events over lengths of 20 km (Table 3). In the absence of coseismic slip
observed along the fault, we used the scaling laws of Wells and Coppersmith (1994) and more recent work done by Leonard (2014) to associate magnitudes and
co-seismic slip values to each scenario based on the length of the activated
segments. We used the regression to estimate magnitude (M) according to
surface rupture length (SRL), and the regression between co-seismic slip or
average displacement (AD) according to surface rupture length (SRL).
M=a+b⋅log(SRL).
Wells and Coppersmith (1994) give for strike-slip faults a=5.16± 0.13 and b=1.12± 0.08. Leonard (2014) gives for strike-slip faults a=4.17 (3.77 to 5.55) and b=1.667.
Log(AD)=a+b⋅log(SRL).
Wells and Coppersmith (1994) give for strike-slip faults a=-1.70± 0.23 and b=1.04± 0.13. Leonard (2014) gives for strike-slip faults with SRL 3.4 to 40 km a=-3.844 (-4.30 to -3.40) and b=0.833.
The deduced magnitudes Mw are 6.7 ± 0.2 and 7.1 ± 0.7 for the
two segments and entire fault scenarios respectively (Table 3). It is
important to note that we did not observe a single co-seismic offset in
the field. Therefore, the co-seismic slip values are estimates based on the
length of the rupture, considering the two scenarios, and Wells and
Coppersmith (1994) or Leonard (2014) relations (see relations above). The
deduced co-seismic slip estimates vary between 0.65 ± 0.5 m and 1.3 ± 0.9 m (Table 3).
For the scenario when the entire fault breaks in one event, the slip rate
would be between 0.4 ± 0.3 and 0.8 ± 0.6 mm yr-1, and for the
scenario when the two segments break separately, it is between 0.2 ± 0.1 and 0.5 ± 0.2 mm yr-1 (Table 4).
The timing of the last event (between 775 and 1778 CE), the inter-event
time (between 1610 and 3383 years) and the slip rate (between 0.2 ± 0.1
and 0.8 ± 0.6 mm yr-1) are consistent with the weakly expressed
morphology of the fault. Notice that considering the uncertainties, the
lowest slip rate value could be as low as ≈ 0.1 mm yr-1 with the
scenario of an event breaking only one segment of the Sharkhai fault every
3383 years on average. The upper bound (0.8 ± 0.6 mm yr-1) appears
unrealistically high for a single structure concerning region-wide values.
The first results from a local GPS network deployed in the Ulaanbaatar area
since 2010 (Miroshnichenko et al., 2018) show local complexities and a high heterogeneity in
direction and velocity. However, most GPS stations
moved 3 ± 1 mm yr-1 to ESE, horizontal displacement with respect to
Eurasia (Miroshnichenko et al., 2018). However preliminary, this is
consistent with our observations and previous studies that the region
absorbs part of the deformation along various active faults.
Several slip rates and recurrence times have been estimated and published in
western Mongolia (Calais et al., 2003; Ritz et al., 2006; Etchebes, 2011;
Rizza et al., 2015), focused on faults where large earthquakes (M8+)
occurred (1905, 1931, 1957) and associated with hundreds of kilometers of
surface ruptures (Table 5). Their estimated slip rate values, 1.5 to 3.8 mm yr-1 for geological slip rates and 2 to 2.6 mm yr-1 for geodetic slip rates,
are about 2 to 10 times faster than those we estimate for the Sharkhai fault.
The recurrence times estimated over that region (2.43 to 4 kyr) are of the same
order as the inter-time estimated for Sharkhai (1.6 to 3.4 kyr), but the
magnitudes considered in western Mongolia are about 8 and more while they are
about 7 for the Sharkhai fault. The deformation along the Ulaanbaatar
region's active faults is much lower than in western Mongolia.
Our results are therefore consistent with other observations in the region.
However, our preliminary findings do not favor a specific rupture scenario
and associated magnitude for the Sharkhai fault.
Implications for the seismic hazard model
Ulaanbaatar is the commercial and industrial center of Mongolia with a
concentration of nearly half of the country's total population (about 3.2
million), according to the national statistics office of Mongolia (2018). The
growth of the capital in the last two decades has been significant, the
population in 1998 being lower than 0.7 million. In terms of seismic risk,
the population is spread in buildings with various vulnerability qualities.
The majority of structures in Ulaanbaatar are masonry (62 %) then steel
structures (18 %), wooden structures and gers (also called yurts) (2 %). Masonry buildings
(usually apartments) are considered seismically safe, but the first floor is
generally modified without safety considerations to transform them to shops or
restaurants, making the building weaker in terms of seismic resistance (Dorjpalam et
al., 2004). The stakes and their location are also modified. In the city,
new tall buildings have been erected. As the international airport in use
since 1957 is too short and too close to the city, a new airport has been
constructed 30 km to the south of Ulaanbaatar and has been in operation since
mid-2021.
In this work, we identified and mapped the Sharkhai active fault that has to
be included as an earthquake scenario affecting Ulaanbaatar and its region
and be used in the seismotectonic model for seismic hazard assessment of the
region of Ulaanbaatar and especially in the area of the new airport that
will be a place of new construction. We suggest considering both
scenarios, with the entire fault breaking in one event and the two segments
breaking independently. Our results are the first estimates on this fault
of magnitude of a large event (6.7 ± 0.2 and 7.1 ± 0.7) depending
on the scenario considered, for their inter-event time (2496 ± 88 years), and an attempt for the estimation of the rate of deformation (between
0.2 ± 0.1 and 0.8 ± 0.6 mm yr-1). Although the uncertainties are
still substantial, the estimates are consistent with the regional knowledge.
Our work contributes to the construction of the seismotectonic model, the
first step of any seismic hazard assessment. But the model still faces
several unknowns. This fault is a part of a larger system with several
parallel structures, such as the Hustai and Avdar active faults. The question that
arises is whether these faults break independently or in a short time sequence
followed by a long period of quiescence. Other active faults in the area
have been identified as Emeel, Gunj and Ulaanbaatar faults. Are there still
other unknown active faults in this area? Are the deformation rates or
inter-event time on all these faults consistent with GPS regional
deformation that also need to be improved with longer
measurements? Another challenge is to confirm, by complementary works, all
the estimates recently published, including this work, of some of the active
faults in the Ulaanbaatar region. Despite their uncertainties, all these
works already strongly improve the knowledge of active faults in the region and
the seismic hazard assessment, and they contribute to the seismic risk
mitigation.
For a complete seismic hazard assessment, in addition to the seismotectonic
model, propagation and sites effects (which amplify the ground motion during
earthquakes) are also essential, especially for Ulaanbaatar, located at the
Tuul River Valley on a sedimentary basin of alluvial deposits with a
thickness up to 120 m (Odonbaatar, 2011; Tumurbaatar et al., 2019). To answer
such questions, future complementary works in the area are still necessary,
which may improve our ability to assess seismic hazard in the region.
Data availability
Pleiades high-resolution (2 m Multispectral, 0.5 m Panchromatic) data were
acquired by Pleiades satellites and broadcast by Astrium.
High-resolution (12 m) topography was taken from DLR's TerraSAR-X/TanDEM-X
satellite and
Shuttle Radar Topography Mission 1 arcsec Global (10.5066/F7PR7TFT, Earth Resources Observation and Science (EROS) Center, 2018), last accessed June 2019.
Google Earth views were taken from http://www.google.com/earth, last access: July 2017.
Radiocarbon dating was carried out in the Poznań Radiocarbon Laboratory-Poland, and the date
of the sample acquisition is 16 July 2013.
The supplement related to this article is available online at: https://doi.org/10.5194/se-13-761-2022-supplement.
Author contributions
AAA, AS and MF carried out fieldwork, collected and prepared the data. All authors contributed to analyzing the data and writing the manuscript.
Competing interests
The contact author has declared that neither they nor their co-authors have any competing interests.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
Access to Pleiades images was
supported by CNES (Centre National d'Etudes Spatiales). TanDEM-X data were
kindly provided by DLR (Deutsches Zentrum für Luft- und Raumfahrt)
through the TanDEM-X Science program (project DEM_OTHER1719).
The authors would like to thank the many colleagues and students from IAG
for their help in the field. We thank Bayarsaikhan Enkhee of IAG for the GPS
acquisition used for some 3D reconstructions. The authors thank Michel
Granet for his support of this work, especially during the PhD work of Abeer Al-Ashkar. We are thankful to the reviewers Daniela Pantosti and Laurent Bollinger for their thoughtful, constructive and invaluable remarks which
improved our paper.
Financial support
This research has been supported by the EOST (Ecole et Observatoire des Sciences de la Terre) and IPGS (Institut de Physique de Globe de Strasbourg, now ITES – Institut
Terre et Environnement de Strasbourg), University of Strasbourg-CNRS, and in Mongolia by IAG (Institute of Astronomy and Geophysics, Academy of Sciences of Mongolia).
Review statement
This paper was edited by Federico Rossetti and reviewed by Daniela Pantosti and Laurent Bollinger.
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