10th Asian Regional Conference of IAEG (2015)

Classification of Landslides Induced by China Wenchuan 2008 Earthquake andMechanisms of their Sudden Departure
Xiaoyan Zhao(1), Houtian Hu(2), Liang Liu(2) and Dengfeng Li(2)(1) Faulty of earth science and environmental engineering, Southwest Jiaotong University, China
Email: xyzhao2@swjtu.cn(2) Faulty of earth science and environmental engineering, Southwest Jiaotong University, China
Abstract
Over 300 large landslides were triggered by 2008 Wenchuan earthquake and many ofthem caused the felt ground tremor. The surface tremor, different from aftershocks, wasinduced by the sudden release of elastic strain energy stored in the locked landslide
segments in the form of sudden shearing rupture of the locked segments. So that, thisground tremor is named by landslide-quake in this paper. Our study clearly indicates thefollowing two main characteristics of a landslide-quake: (1) landslide earthquake occurs
prior to the sudden departure of the landslide and (2) its epicenter is located between thepotential sliding mass and the bed of the locked segment, and directly contributes to thedeparture mode and the acceleration of the sliding mass. Based on the above characteristics,
we believe that landslide-earthquake is the essential reason for the sudden departure of mostlarge high speed landslides. Consequently, according to the occurring time of alandslide-quake related to the Wenchuan 2008 earthquake, the large landslides triggered by
the earthquake are classified by three categories: (1) main shock landslide, for which there
is no landslide-quake and the main contributing forces are main shock force of Whenchuan
2008 earthquake and the gravity of sliding mass; (2) co-seismic landslide, for which alandslide-quake develops before the end of main shock of the earthquake and the main
contributing force are landslide-quake force, earthquake force and the gravity of sliding
mass; and (3) after-main-shock landslide, for which a landslide-quake happens after themain shock of the earthquake and the main contributing forces are landslide-quake force
and the gravity of sliding mass. The processes of sudden departures of these three kinds oflandslides were analyzed, by which landslide-quake may be a new explanation for thesudden departure of a large scale high speed landslide.
Keywords: landslide-quake, classification of earthquake-induced landslide, mechanism forsudden departure
1. Introduction“2008.5.12” Chinese Wenchuan earthquake wascharacterized by high magnitude of Ms 8.0, shallow
focus of 14km, long duration of about 120s andstrong destruction. More than 15000 landslides wasinduced by this catastrophic earthquake resulted inserious lives and properties lost. According tostatistics, over 300 landslides were characterized bylarge volumes, of which 112 landslides have a sliding
area of more than 50,000 m2. (Teng, Bai, Yang, et al,2008; Burchiel, Royden, Vander, 2008; Meju,Unsworth, 2008; Xu, Huang, 2008; Huang, Pei, Li,2008; Xu, Pei, Huang, et al, 2009; Yin, 2009; Xu, Dai,2010). About 87,000 people died due to theearthquake among which over 20,000 people losttheir lives because of landslides.
Aftershocks continued after the main shock. In fact,12139 aftershocks were detected by the seismicstations in 33 days after the earthquake, of which 208
aftershocks had a magnitude of about Ms 4.0, 28
aftershocks had a magnitude of about Ms 5.0, 6aftershocks had a magnitude of about Ms 6.0.
However, some local ground tremors were notdetected by seismic stations. These tremors weretriggered by large landslides but not the earthquake or
its aftershocks. They generated due to the sudden
release of elastic strain energy stored in the locked
segments of the landslides because of the sudden1

10th Asian Regional Conference of IAEG (2015)

geotectonic stress with a direction of about 40°. The40° and 130° stress compounded a 70°-85° stresswhich was most the same as the gravity stress and theslope dip direction.
Fig. 2 is the sketch of the longitudinal section of
Wangjiayan landslide, in which AB and is a tectonic
joint with an attitude of 70°-85°∠60° in the landslide.
Due to the intense and repeated vibration, the joint
had a splitting loose but not linked up to the point Din Fig. 2. The CD section was the locked segment,
and strain energy was concentrated in the segment.
The increasing strain energy finally resulted in asudden broken of the rock mass along CD, and theenergy
releasedsuddenlyresultedina
landslide-quake. This imply that not only the gravity
but also the tectonic stress were responsible for theforming of the landslide. It was the coupling of thegravity and the tectonic stress that resulted in thelandslide. So this kind of landslide is not only agravity phenomena but also a tectonic geologic
phenomena.
Thus,wecantaketheearthquake-induced landslides and landslide-quakes
as faults of the rock on the ground surface.

shearing rupture of the locked segments. To
emphasize the forming, we call this kind of ground
tremor landslide-quake here in this paper, which havenot been noticed by landslide researchers. Basing on
characteristics of landslide-quake, the classificationof the landslides induced by Wenchuan earthquake
are carried out and the mechanism for rapid departureof different type of landslides is studied.
Many large landslides triggered landslide-quake,
which can be proved by the phenomenon whenlandslides occurred. For example, many survivedpeople heard a big bang bang prior to the occurrenceof Daguangbao landslide and Laoyingyan landslide in
Anxian county, Wangjiayan landslide in Beichuancounty, Donghekou landslide, Woqian landslide,
Dayanqiao landslide, Dongjia landslide in Qingchuancounty, Wenjiagou landslide in Mianzhu city,
Pingxichun landslide in Pingwu county and alandslide at K24 of the expressway from Yingxiu toWolong. They described the sound as “an explosion”,“a thunderous noise”, “a huge roar”, “a deafeningnoise”. And they were sure that the sound was heard
before the rock masses rushed out. So, the sound canbe considered as a prove of landslide-quake.

Altitude(m)Altitude(m)2.

Description of Wangjiayan landslide and theformation of a landslide-quake
Wangjiayan landslide occurred 10 min after the main
shock of Wenchuan earthquake (refer to Huang, etc,
2008 for details). Fig. 1 shows the ground stress in
the rock mass after the earthquake.
Fig.2 Sketch for the longitudinal section of Wangjiayan
landslide after Wenchuan earthquake (revised from Huang,2008)

From the investigation of the large landslides
triggered by Wenchuan earthquake, landslide-quakes
have two obvious characteristics as below.
(1) A landslide-quake occurs prior to the sudden
departure of the landslide. It is because the sudden
releasing of the concentrated strain energy in thelocked segment that a landslide-quake formed. And
the landslide takes place under the coupling of its
gravity and stress due to the landslide-quake. So thelandslide-quake is the reason the landslide but not theresult of the landslide. There is a short time difference
between their two. Again taking the former described

Fig.1 Sketch for Stress of Wangjiayan landslide

From Fig.1, the dip direction of its gravity stress
was the same as the slope dip direction, about 76°.
The tectonic stress in the landslide had a strike ofabout 130° due to the oblique impaction of its
embedded fault wall because the landslide located in
the upper wall of the reverse fault, Yingxiu-Beichuan
fault. At the same time, the fault was a strike-slip
fault despite a reverse one and the dextral strike slip
co-seismic displacement resulted in a co-seismic210th Asian Regional Conference of IAEG (2015)

landslide-quake force and the gravity, while others
were formed by the landslide-quake force and thegravity because there occurred after the main shock.
By this, the landslides can be classified into three
categories as shown in Tab. 1.

Wangjiayan landslide, it was 10 min after the main
shock of Wenchuan earthquake that a big bang bang
was heard followed by the dust billowing and debris
launching due to the occurrence of the landslide. The
similar process occurred in another landslide,
Laoyingyan landslide. According to the survivor
testimony, after a short time of the main shock ofWenchuan earthquake, a big noise like a huge roar
was heard before the landslide occurring with a big
sound like water rushing out of Jiangbagou ditch.
These two landslides told that the high-speed
landslides occurred after the big noises due tolandslide-quake were heard.
(2) The focus of a landslide-quake is just on thesliding surface, so its effect on the behavior of thesliding body is very directly without distance. The
retain part of a landslide could be consideredinfinitely-great, while the sliding part iscomparatively very small, always a tens depth. So,
the strain energy releases just like an explosion.
Despite of a comparative smaller magnitude toan earthquake, a landslide-quake can induce a very
high speed of a landslide by its coupling with gravity
because of its above characteristics.
Calculation of the magnitude and its releasingenergy of a landslide-quake
As explained earlier, the sudden shearing rupture of
the locked segment of a earthquake-induced landslide
is similar to the process of the shearing broken of aseismogenic active fault. The locked segment can betaken as the focus. Thus, the magnitude of alandslide-quake can be calculated using the method
for an earthquake introduced by Wyss in 1979, in
which the shearing broken area, A in km2, was used
to calculate a magnitude, M, as shown in (1) and thereleasing energy of a landslide-quake, E in Erg., canbe calculated by the use of M, as shown in (2).
M=logA+4.15(1)logE=11.8+1.5M(2)
In Wangjiayan landslide , according to theinvestigation,
A=350m×350m=0.1225km2,
so
ML=3.2 can be obtained by using (1), and thenE=3.98×1016 Erg.=3.98×109 J.

Tab. 1 Classification of large landslides in Wenchuan earthquake
Classification

Forces for

Noises of main shock and(example)

un-stabilizing

landslide-quake

Main-shock

Main-shock

The noise is smaller thanlandslide

force, gravity

the main shock rumbling(Beichuan

when landslide takes

middle school

place.

landslide)Co-seismic
landslide

Main-shock

There is a very bigforce,

bangbang when landslide(Niumiangou

landslide-quake occurs before main shocklandslide)force, gravity

ends.

After-main-shoc landslide-quake

After the main shock,force, gravity

there is a very big bangk landslide(Wangjiayanbang prior the occurrence

landslide)of the landslide.

In this classification, the landslides refer to thosethat form directly under the contribution of intense
earthquakes excluding the landslides triggered by rain
or other factors after earthquakes.
4 Mechanism for sudden departure of large
earthquake-induced landslides
Main-shock landslides
According to “acceleration records without correctionof Wenchuan Ms 8.0 earthquake”, the PGA by nearby
stations are smaller than 1 g with the smallest one of0.2g, and the PGA times are shorter than 60 s withthe shortest of 31.49 s. That is to say, it takes morethan 30 s to reach the PGA of no more than 1 g.
However, most of the landslides triggered inWenchuan earthquake were near the seismogenic
fault, only several km or even 100s m, so thetraveling time of the seismic waves from the focus to
the landslide location is shorter than 30 s, and thenthe PGA was not reached. Additionally, the distancefrom any landslide location to the focus was morethan 14 km, given the focal depth of 14 km. And theenergy spreads with the form of elastic waves to all
the directions from the focus. The energy decreases
with the increasing of its travelling distance. So, it isless possible for the energy from the main shock toeject the rock mass with the volume up to hundreds ofthousands, millions, even hundreds of millions m3

3 Classification of earthquake-induced landslides
From the investigation of the landslides triggered by
Wenchuan earthquake, some landslides were formed
under the main shock force and the gravity, and somewere formed under the main shock force,3

10th Asian Regional Conference of IAEG (2015)

sandstone formation. The stratum counter dippedwith an attitude of 130°∠17°. There were there setsof joints with attitudes 305~310°∠70~80°, 50~60°∠
70~80°, and 300~330°∠40~65°, of which the last
one dipped out of the slope. These discontinuities
including the joints and the bedding surface cut therock mass into parts with various sizes. The dip angleof the middle and rear sections of the sliding surface
was 40-65°, with and average of 45°. Given thefriction angle of the sliding surface was 45° undermain shock, the friction applying on the sliding bodywas f=mgcos45°·tan28°=0.376 mg. The join force ofmain shock force and the gravity on the sliding bodywas shown in Fig.3, that is F=0.82 mg>f =0.376 mg.
Thus, the loose rock mass would slide down with anacceleration of 0.44 g (4.31m/s2).
After-main-shock landslides
The main character of this kind of landslides is thatthe landslide-quake occurs after the main shock stops.
So the main shock force has no direct contribution on
the landslide occurring. It is the coupling oflandslide-quake and gravity that ignites a landslide.
The initial velocity can be calculated by the use ofreleasing energy by the landslide-quake. Still taking
Wangjiayan landslide as an example, the releasingstrain energy was 3.98×109 J as calculated earlier inthis paper. The strain energy, E, changed to thekinetic energy of the rock mass in the locked segment,
the CD part as shown in Fig. 2 and resulted in theinitial velocity, V. According investigation, thevolume of this landslide was 14 million m3 with unit
weight of 25 kN/m3. By the use of the expression,
E=0.5mV2, the initial velocity V=1.49 m/s with aperpendicular direction to the sliding surface. Fromthe velocity, the PGA at Wangjiayan landslide mustbe larger than 1 g, about 1.20 g. With thisacceleration, the rock mass flied at high speed for 400
m and finally buried the old Beichuan county.
By the earlier analyses and the calculations, themechanism of after-main-shock landslides can beconcluded as below:(1)Parts of sliding surface are broken by the main
shock, and then more and more strain energy
concentrated in the locked segment. The rock mass in
the locked segment starts a progressive failure.
(2)Once the locked segment is completely broken,
the highly concentrated strain energy releases
suddenly and triggers a landslide-quake. Under theout of the slope like an explosive. One may concern
what contribute to the high-speed debris of
main-shock landslides. As described in table 1, thelandslides are made up of rock mass involving richjoints and even loose soil, which have not any lockedsegment. The materials are on high and steep slopes
with the dip angle of 40-50°, resulting in a trend offluidization under the intense and repeated shock. In
the coupling of the main shock force and the gravity,
the main shock force may completely offset thegravity resulting in a great reduction of the effective
stress between the particles, even to zero. The rockmass or the soil can not remain the original form andtransform to fluid. Under the repeated shock, every
particle has a trend of traveling down slope. This
trend may be illustrated by Fig. 3, in which thevertical and horizontal components of the main shockforce are all described with acceleration, and they areall given a value of 0.8 g. The join force of the main
shock force and the gravity is OP=0.82 g with adown-slope direction. If the join force is equal to thefriction, the landslide will departure, and its speed
will increase with the increasing of the difference.

Fig.3 Force analysis for normal-earthquake landslides

Beichuan middle school landslide is here taken
as an example to give a further explanation of themechanism for main-shock landslide. Fig. 4 is thelongitudinal section of this landslide which located inthe footwall of Yingxiu-Beichuan fault.

Fig.4 Sketch for the longitudinal section of Beichuan
middle-school landslide before sliding in m. (from Xu,2009)

The rock stratum was C1 gray limestone
formation with interlayer of purple and brown4

10th Asian Regional Conference of IAEG (2015)

combination of the main shock force and the gravity.
It is the coupling of landslide-quake and gravity that
ignites an after-main-shock landslide. For co-seismic
landslides, a landslide-quake is triggered by the main
shock force, tectonic force and gravity before themain shock ends. So, main shock force,landslide-quake force and gravity are all importantfactors contributing on the sudden departure of alandslide.
Our idea is still in the stage of working hypothesis.
More investigations and studies should be carried out
to show evidences for the idea.

coupling of the landslide-quake force and the gravity,
there will be sudden departure of a landslide which
may be the essential reason of the high-speed
landslide.
Co-seismic landslides
For this kind of landslides, a landslide-quake istriggered by the main shock force, tectonic force andgravity before the main shock ends. So, main shockforce, landslide-quake force and gravity are allimportant factors contributing on the sudden
departure of a landslide.
As shown in Tab.2, the PGA in the epicentral area
(Wolong and Mianzhu) is around 1.0 g. We here takethe vertical and horizontal PGA as 0.8 g and take thelandslide-quake PGA as 1.2 g, the same as what has
been explained earlier in this paper. The join
acceleration induced by main shock force,landslide-quake force and gravity will be 2.24 g, asshown in Fig. 5. The rock mass of the landslide will
be projectile upward out of the slope with aperpendicular direction to the sliding surface at thelocked segment. Niumiangou landslide was an
example of this kind of landslides, which had asudden departure by high-speed flying and thentravelled 3.2 km after 4 direction changing due tohitting the hills. The PGA at Niumiangou landslide
was 1.5 g (Yin, 2009).

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2.0g

P=2.24g
2.0g

1.0g

Fig.5 Force analysis for during-earthquake
landslides
5 Conclusions
Many large landslides in Wenchuan earthquake
have triggered landslide-quake which may be theessential reason for the sudden departure of alandslide. Landslide-quake, as well as the main shock
has important effect on the characteristics and thesudden departure of a large landslide, basing on
which large earthquake-induced landslides areclassified into three categories including main-shocklandslides,
co-seismic
landslides,
andafter-main-shock landslides. Main-shock landslides
have not any locked segment and triggered by theYin Y P.(2009): Introduction to Wenchuan
earthquake
geologyandlandslides.Beijing: Geology Press, 2009.

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