SCIENCE CHINA Recurrence of paleoearthquakes on the

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SCIENCE CHINA Recurrence of paleoearthquakes on the
SCIENCE CHINA
Earth Sciences
• RESEARCH PAPER •
January
2012 Vol.55 No.1: 1–8
doi: 10.1007/s11430-012-4540-y
doi: 10.1007/s11430-012-4540-y
Recurrence of paleoearthquakes on the southeastern segment of
the Ganzi-Yushu Fault, central Tibetan Plateau
LI An, SHI Feng, YANG XiaoPing* & XU XiWei
National Center for Active Fault Studies, Institute of Geology, China Earthquake Administration, Beijing 100029, China
Received March 15, 2012; accepted August 22, 2012
Although there are many earthquake relics preserved in the southeast segment of the Ganzi-Yushu Fault in the central Tibetan
Plateau, the recurrence regularity of paleoearthquakes is not yet clear. This work studies paleoearthquakes on this fault segment since the Holocene through geomorphic investigation and trench excavation. The results show that sinistral dislocation of
the T3/T2 terrace boundary is up to 80 m at the Cuoa Township. A 1.5 m-high fault scarp extends 3 km near the Renguo
Township. A number of paleoearthquakes are exposed in trenches at two places, respectively. In combination with historical
records, our work has identified 5 or 6 paleoearthquakes on this fault segment since last 5600 years. The occurrence times and
recurrence intervals of these paleoearthquakes are estimated by 14C dating on strata in the trenches. Our analysis shows that
these paleoearthquakes do not exhibit evident periodicity, but instead show a clustering characteristic. From 5600 a to present,
seismicity of the southeastern segment of the Ganzi-Yushu Fault has two active periods and one quiet period, and the present-day time is just in the second active epoch. The recurrence intervals of each active epoch are different: 1000–1300 a in the
first one, 534 a in the second one.
Ganzi-Yushu Fault, paleoearthquake, recurrence, clustering
Citation:
Li A, Shi F, Yang X P, et al. Recurrence of paleoearthquakes on the southeastern segment of the Ganzi-Yushu Fault, central Tibetan Plateau. Sci
China Earth Sci, 2012, doi: 10.1007/s11430-012-4540-y
On 14 April 2010, an Ms7.1 earthquake struck the Yushu
County, Qinghai Province, China. It is another disastrous
event in western mainland China after the Wenchuan great
earthquake of 2008, which killed more than 2200 people,
and destroyed a large number of houses in the epicenter
area.
The Yushu Ms7.1 earthquake produced a 31 km-long
surface rupture zone with about 1.8m maximum horizontal
offset along the Ganzi-Yushu Fault [1]. Tectonically, it took
place on the boundary of the Bayan Har Block where a series of big earthquake had already occurred in recent 10
years, such as the Mani Ms7.5 in 1997, the Kunlun Ms8.1 in
2001 and the Wenchuan Ms8.0 in 2008, probably implying
*Corresponding author (email: [email protected])
© Science China Press and Springer-Verlag Berlin Heidelberg 2012
an apparent association with the activity of this tectonic
block. So it is important to study the faults that did not yet
rupture on the boundary of the block recently. History records an M71/2 earthquake ruptured the Luoxu-Ganzi section
of the southeastern Ganzi-Yushu Fault in 1854 and another
major event occurred near Manigango in 1320, both of
which lie on the aforementioned block boundary. But more
early events cannot be further traced forward. The geomorphic dislocation observed indicates a 12±2 mm/a sinistral slip rate on the Ganzi-Yushu Fault since 50 ka and the
coseismic sinistral dislocation of the 1854 earthquake
reaches 2.5–5.3 m [2]. The earthquake recurrence interval in
the southeast segment of the Ganzi-Yushu Fault may be
200–600 years by a simple calculation using coseismic dislocation and slip rate. Obviously, this estimation is not well
constrained by sufficient data. In this paper, we attempt to
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further study the earthquake recurrence regularity of this
fault segment by searching for paleoearthquakes in trenches
and analyzing of geomorphic deformation.
1 Tectonic setting
The Ganzi-Yushu Fault is the southwest boundary of the
Bayan Har Block in the Tibet Plateau. The south side of this
fault is the Qiangtang Block. It starts from the Ganzi County in Sichuan Province, extending northwestward through
the Yushu County, and ends at the Zhiduo County of Qinghai Province, nearly 500 km long, The fault mainly dips
northeast with an angle of 70°–85°, and produced a 50–100
m wide fracture zone in the Triassic strata [3]. The geological mapping indicates the Ganzi-Yushu Fault began to slide
since 8–6 Ma [4, 5] at an average sinistral slip rate about 10
mm/a [6–9]. The GPS data also reveal the slip rate of 9±2
mm/a along the Ganzi-Yushu Fault same as the Xianshuihe
Fault [8, 9] which connects the southeastern segment of the
Ganzi-Yushu Fault, also has a sinistral slip rate of 10±2
mm/a [3,10,11], and experienced a series of M7 earthquakes
with an interval of 20 years. The Ganzi-Yushu Fault can
be divided into three segments (Figure 1) [12]: the GanziLuoxu segment in the southeast, the Luoxu-Yushu segment
in the middle, and the Yushu segment in the northwest. The
Yushu Ms7.1 earthquake of 2010 occurred in the middle
segment. Every segment has at least one historical earth-
January (2012) Vol.55 No.1
quake. For example, the west Yushu M61/2 earthquake ruptured the northwest segment in 1738, the Dengke M7
earthquake shook the middle segment in 1896 [2], the
Zhuqin-Ria M8 earthquake in 1320±65 a and the M71/2
earthquake in 1854 shook the southeast segment of the
Ganzi-Yushu Fault [13]. In other words, this fault is characterized by high slip rates and frequent earthquakes.
2 Relics of paleoearthquakes
There are many narrow small pull-apart basins along the
southeastern segment of the Ganzi-Yushu Fault, where various kinds of paleoearthquake relics were observed, including seismic bulges, gully dislocations and fault scarps.
2.1 Geomorphologic deformation at the Cuoa Township
There are three levels of river terraces at the Cuoa Township, of which T3 and T2 are entirely preserved on the west
side of the river. The fault scarp is 8m high on T3, 2–3 m
high on T2, respectively, and the sinistral dislocation is
about 80m on the T3/T2 boundary. Because terrace T1 has
been eroded, the horizontal dislocation is not preserved on
the T2/T0 boundary (Figure 2(a)). The fault plane appears
under the fault scarp of T2 terrace. The Triassic slate and
metamorphic sandstone thrust up onto the Quaternary sand
Figure 1 Map showing seismotectonics and the epicenter distribution of around the Ganzi-Yushu Fault. (a) Black line shows the position of (c). (b) Position of faults comes from the map of active tectonics in China. Focal mechanisms of the Yushu earthquake come from CDSN, USGS and Harvard, and black
frame expresses the position of (c). (c) Red frames are sites of the trench.
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January (2012) Vol.55 No.1
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Figure 2 Geomorphology, the fault profile and the trench site at the Cuoa Township. (a) General geomorphology (northwest view); (b) seismic bulges on
T2 terrace (northwest view); (c) exposed fault (northwest view); (d) sinistral dislocation of the T3/T2 boundary and trench position.
and gravel sediment. The fault dips to 305° with an angle of
68° (Figure 2(c)). The latest earthquake created long bulges
or residual scarps with a height of 10–20 cm in front of the
big fault scarp of T2 terrace. Individual bulges with a length
of 1.2–1.5 m and a width of 30–40 cm extend about 20 m
long on the strike direction. A trench (CATC1) was excavated perpendicular to the strike direction of the fault (Figure 2(d)).
2.2
Geomorphologic deformation at the Renguo
Township
On aerial photographs, a remnant earthquake scarp with a
height of 0.5–1.5 m and a length of 3 km, trending in 300°,
was recognized on the gentle piedmont zone near the Renguo Township (Figure 3(a), (b)). The fault scarp extends
southeast to Ezhong village and dislocates a country road,
which was built in an intermittence gully. The east boundary
of the country road was rebuilt in a straight line but the west
boundary of the country road still keeps the sinistral dislocation of 3 m same as the dislocation of the gully (Figure
3(c)). The fault scarp extends northwest to the Kagong
Township and produces a 200m long sag-pond (Figure 3(d)).
According to the presentation of the leader of the Renguo
village, there was a village in this region in the past, which
disappeared about 100 years ago. This fact is consistent
with the old cultivating layer revealed by the trench at
Renguo. And the disappearing time of the village is in accordance with the Ms7 earthquake in 1854 as documented
by history, of which the macro epicenter is located near the
Renguo Township [13]. It can be inferred that the surface
rupture and the strong ground motion caused by the 1854
event resulted in the disappearance of the village.
3 Trench descriptions and analysis of paleoearthquakes
3.1
Methods
In the paleoearthquake trench method, parameters of a
paleoearthquake event (time, magnitude and displacement,
etc) are estimated by the cover-cut relationship, displacements of strata and normal sediment process in the trench.
To clarify relationship between earthquake deformations
and stratum units is the key of trench method. In other
words, the occurrence time of a paleoearthquake can be
limited by dating the dislocated stratum and overlying strata
(or colluvial wedge); and the displacement of the earthquake was presumably recorded by the dislocated stratum
[14].
3.2
Trench at the Cuoa Township
The CATC1 trench was set in the T2 terrace of the Cuoa
Township (Figure 2(d)). The height of the fault scarp is
about 2m on the site of the trench.
Strata revealed by the CATC1 trench are as follows
(Figure 4):
U1: Cyan upper Triassic metamorphic sandstone.
U2: Brown coarse gravel layer. The stratum contains clay.
The major gravel diameter is about 10–20 cm but the grain
size of some small gravel is 2–5 cm.
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Figure 3 Geomorphologies of several sites at the Renguo Township. (a) Fault scarp at the Renguo Township (southwest view); (b) measured fault scarp
(black frame is trench, trench section RGTC1 is perpendicular to the fault, section RGTC2 is parallel to the fault); (c) dislocation of the country road
at Ezhong village (view is southwest); (d) sag-pond at the Kagong Township (west view).
Figure 4
Cross sections of CATC1 trench at the Cuoa Township. (a) West wall; (b) east wall.
U3: Black clay layer. The stratum contains some small
gravel and mica fragments. The grain size of gravel is 2–3
cm; a few gravel diameters reach 10 cm. The 14C sample
dating shows this stratum age is 5600–5470 a (upper) and
8050–7940 a (middle). All 14C samples were measured at
the America Beta Analytic Laboratory.
Wedge A: Light black clay wedge. The wedge contains
some small gravel with a diameter about 2–3 cm. Materials
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of the wedge come from U3 and ancient surface. The dating
of this wedge is 4730–4510 a.
U4: Brown clay layer. It contains some gravel with a
diameter of 1–2 cm. The age result is 4160–3970 a.
Wedge B: Light brown clay and small gravel wedge. The
grain size of small gravel is 3–5 cm, Materials ingredients
are almost in accord to U5; the dating of the wedge is 3570–
3390 a.
U5: Brown clay layer (surface layer). It contains small
gravel. The diameter of the gravel is about 2–5 cm. The
layer is disordered near the fault position. The 14C dating is
800–680 a.
The fault expresses a positive flower structure and five
fault planes can be identified in the CATC1 trench. The
hanging wall is the bedrock in the southwest side of the
trench. The fault plane dips to southwest. The bedrock
hanging wall is hard to collapse by the dislocation of the
earthquake and easy to form a wedge space between sediment stratum and bedrock. The ancient surface soil and
sediment clay near the fault scarp filled fast in the wedge
space when the earthquake happened and the surface clay
filled in the wedge with sheet flow scours after earthquake.
The occurrence time of paleoearthquake can be ascertained
between the wedge (upper limit time) and dislocated stratum (lower limit time).
Two paleoearthquake events are recorded in the CATC1
Figure 5
Interpretation of paleoearthquakes in the CATC1 trench.
January (2012) Vol.55 No.1
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trench. At the beginning, U1, U2 and U3 were deposited in
order before paleoearthquake events (Figure 5(a)). The first
event broke F1 and F2, and formed wedge A between U1
and U3. Materials of the wedge came from U3 and the ancient surface (Figure 5(b)). The 14C dating of the upper U3
and wedge A respectively ascertain the lower limit time of
5600–5470 a and the upper limit time of 4730–4510 a (Figure 4). U4 was deposited after the first event and then the
second paleoearthquake event occurred (Figure 5(c)). Expressions of the second event are different on two walls of
the CATC1 trench. A suite of F3 dislocates wedge A and F2
resumed. The active F2 resulted in wedge A slipping down
between F2 and F4 on the east wall. Covering layer U4 collapsed and the place was filled with wedge B between F4
and F5. Differently, F3 did not break on the west wall of the
trench. F4 dislocated U4 and wedge B was formed between
F4 and U4 of the hanging wall (Figure 5(d)). The upper
limit and lower limit time are respectively determined by
wedge B dating of 3570–3390 a and U4 dating of 4160–
3970 a (Figure 4). The surface layer U5 deposited after the
second event, and 14C dating of U5 was 800–600 a (Figure
5(e)).
The epicenter of the Zhuqin-Ria M8 earthquake in
AD1320 is only 20km away from the CATC1 trench. Seismic bulges might be produced at Cuoa by this historical
earthquake (Figure 2(b)) which occurred after U5 age and
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F5 has some directional gravel in U5 (Figure 4).
3.3
Trench at the Renguo Township
The trench of the Renguo Township was set on a rangefront fault scarp southwest of the Renguo 4th village (Figure 3(a)). Micro-geography shows that a NW trending hillock is present at north of the footwall in the trench, where a
narrow expanse of depression is formed. A similar physiographic feature is also seen between the hanging wall of the
trench and a gully in the west (Figure 3(b)).
From lower to upper, strata exposed in the Renguo trench
are as follows (Figure 6):
U1-1: Grey-yellow coarse gravel layer, dominated by
gravel with a diameter about 5–20 cm, and some over 20 cm.
The gravel has a poor sorting and is slightly rounded. In
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partial areas the stratum is largely black and becomes
grey-white after sunning dry. The 14C dating is 16040–
15750 a and 9460–9260 a. The 14C dating of this stratum in
the RGTC2 trench is 3880–3690 a and 2920–2770 a (top
layer). So, the deposition process of this stratum continues
for a long time.
U1-2: Light black gravel layer. The stratum has a high
content of clay and fine soil. The gravel diameter is about
5cm and some over 10 cm. This stratum only appears on the
hanging wall, in a gradual transition relation to U1-1, with a
14
C dating age 6290–6000 a indicative of a coeval deposit as
U1-1.
U2: Black small gravel layer with high clay content and
darker color than U1-2. The gravel diameter is 1–3 cm. This
stratum only appears on the west wall of the RGTC1 trench
as local deposit in a depression. The 14C dating is 5050–
Figure 6 Cross sections of the Renguo trench. (a) West wall of RGTC1 trench; (b) east wall of RGTC1 trench; (c) south wall of RGTC2 trench; (d) north
wall of RGTC2 trench.
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4850 a.
U3: Light black or brown small gravel layer with grain
size about 5 cm. Its color on the west wall of the RGTC1
trench is slightly darker than that on the east wall which
appears only on the hanging wall of the trench. The stratum
thins out toward east in the RGTC2 trench, where sinistral
offset of 8 m is measured relative to the east wall of the
RGTC1 trench. The 14C dating is 2730–2470 a. It is noted
the top of U1 underlying the northern and southern walls of
the RGTC2 trench is 3880–3690 and 2890–2760 a, respectively. Thus the strata exposed in two trenches RGTC1 and
RGTC2 should be the same strata.
U4-1: Yellow clay layer (old cultivating layer) with grain
size 3 cm, poorly rounded, since the farmers have selected
big gravel and put them down under the fault scarp and left
fine clay for cultivating crops. In this layer, there is much
charcoal from burned straw, which is distributed horizontally. The 14C dating is 1180–1050, 940–800 and 970–900 a.
U4-2: Brown sandy clay layer which is surface sward
layer and has gravel with grain size of 1–3 cm.
Wedge A: It overlies U1-1.
Wedge B: The 14C dating of 1060–930 and 980–910a is
roughly the same as U4-1. The villagers put gravel of
U4-1down near the wedge B.
Four fault planes have been recognized in the trench,
which are described as follows.
F1: The top of the fault plane terminates in U1-1. No
wedge or flower structure is found in the upper portion.
F2: Wedge A and B overlie the top of F2. On the east
wall of the RGTC1trench, wedge A is displaced (Figure
6(b)), and F2 plane can be seen in A on the west wall of this
trench (Figure 6(a)).
F3: It is 10–15 cm wide, where the overlying bottom of
U2 is disturbed with an uneven surface.
F4: It dislocates U3 and is intruded by a ball of small
gravel which has the 14C dating age 2890–2760 a.
Two paleoearthquake events have been determined by
using the cut-cover relationship between the fault plane and
the strata. The first event had occurred before U1 deposition
ended, which produced faults F2, F3 and wedge A. In the
top of F3, U2 was disturbed and undulation occurred in its
bottom. There are two possibilities for the occurrence time
of this event. One is that U2 did not begin to deposit when
the seismic event took place, but the surface of that time
(top of U1) was deformed. After that U2 was deposited. The
other is that U2 had begun to deposit when the earthquake
occurred and continued to deposit afterward. In both cases,
the bottom of U2 overlying F2 would be deformed while its
top remained intact and flat. The 14C dating of the sample
collected from the bottom of U2, disturbed by the fault, is
5050–4850 a. In the first possibility, the age of U2 is the
upper limit time and in the second possibility, it is the lower
limit time. But whatever this age can be considered to be or
approximate the occurrence time of the first paleoearthquake event. The second event occurred after U3 had been
January (2012) Vol.55 No.1
7
deposited when F4 dislocated U3 and was squeezed by
some gravel of size like that in U3 on west wall. F4 is not
clear on the east wall; no evident deformation was seen in
overlying U3 which does not appear in the footwall. The 14C
dating of squeezed gravel is 2890–2760 a and the 14C dating
of U3 is 2730–2470 a, respectively. So the occurrence time
of the second paleoearthquake is approximately 2890–2760
or 2730–2470 a. U3 thins out to east in the RGTC2 trench,
where an 8m left offset is measured with respect to the east
wall of the RGTC1 trench. At least, this offset contains displacements of two seismic events: the second paleoearthquake event aforementioned and the historical earthquake in
1854 that produced a 3m displacement on the Ezhong country road.
4 Occurrence times of paleoearthquakes and
intervals
Shimazaki and Nakata [15] proposed the simple timepredictable recurrence model for large earthquakes. Afterwards, Savage and Cockerham [16] suggested the quasiperiodic occurrence model which means earthquakes have
periodicity in time and their magnitudes have a finite range
of variation based the research of earthquakes in 1978–
1986 Bishop-Mammoth Lakes sequence. Sieh [17] discussed the clustering feature of earthquakes based on precise chronology of earthquakes produced by the San Andreas Fault.
This work demonstrates that totally 3–4 paleoearthquake
events are recorded in two trenches (Figure 7). They respectively occurred dbetween 5600–5470 a and 4730–4510 a,
before 5050–4850 a (maybe a same earthquake as the former), between 3570–3390a and 4160–3970 a, between
2890–2760 a and 2730–2470 a. Two documented historical
earthquakes respectively occurred in 1320 AD and 1854 AD.
Because the strata deposited in the trench are continuous,
the paleoearthquake event is impossible to miss. Three or
four paleoearthquakes events occurred during about 3000
years from 5600–5470 a to 2730–2470 a, and afterwards no
earthquake was recorded during about 2000 years from
2730–2470 a to 630 a (1320 AD), and finally two historical
earthquakes happened in the past 630 years (Figure 7). It
shows a conspicuous clustering characteristic of seismicity
on the southeast segment of the Ganzi-Yushu Fault. The
period of 5600–5470 a to 2730–2470 a is the first active
epoch, in which the recurrence interval is 1000–1300 a. The
subsequent period of 2730–2470 a to 630 a (1320 AD) is a
calm stage. Then the second active stage comes from 630a
(1320 AD) to present. By estimation using the intermediate
value between the upper and lower limit times, the recurrence interval for the first active epoch is 1000–1300a and
that for the second active epoch, during which only two
events took place, is 534 years, respectively. It seems that
there is no evident periodicity of earthquakes, and instead
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January (2012) Vol.55 No.1
2
3
4
Figure 7 Occurrence times of paleoearthquake events and active periods
of the southeastern segment of the Ganzi-Yushu Fault.
they likely have a clustering feature.
5 Conclusions
5
6
7
The earthquake recurrence on the southeastern segment of
the Ganzi-Yushu Fault exhibits a clustering character. From
5600 a to present, this fault section has two active periods
and one quiet period. And recurrence intervals are different
in different active stages. The earthquake recurrence interval is 1000–1300 a in the first active stage. The interval
between the two historic earthquakes is 534 years in the
second active period which continues to present time.
Previous studies suggest that the Ganzi-Yushu Fault has
the average of sinistral slip rate of 7–10 mm/a [2, 6, 8]. The
southeastern segment of this fault has the maximum coseismic displacement of 5.3 m and the average coseismic
displacement of 3–5 m [2]. By geomorphology analysis, the
recurrence interval was estimated to be about 500 years [2],
not well consistent with the result of this article by the
paleoearthquake method.
8
9
10
11
12
13
14
15
This study was supported by the Yushu Earthquake Science Investigation of
China Earthquake Administration. Thanks are due to Prof. Gao Xianglin
and anonymous reviewers for their valuable comments.
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