ORI GIN AL PA PER

Revisions of the M 8.0 Wenchuan earthquake seismicintensity map based on co-seismic landslide abundance

Chong Xu • Xiwei Xu • Bengang Zhou • Guihua Yu

Received: 11 March 2013 / Accepted: 10 June 2013 / Published online: 18 June 2013� Springer Science+Business Media Dordrecht 2013

Abstract Hundreds of thousands of landslides were triggered by the May 12, 2008, Mw

7.9 Wenchuan earthquake in China. A detailed inventory of landslides triggered by the

earthquake was prepared based on geographic information systems and remote sensing

technology. Visual interpretation of high-resolution satellite images and aerial photos taken

pre- and post-earthquake and selected field investigation showed that at least 197,481

co-seismic landslides related to the earthquake were triggered throughout an area of

approximately 110,000 km2. These co-seismic landslides are delineated as individual solid

polygons. The landslides cover a total area of approximately 1,160 km2. Most of the

landslides are distributed in an approximate ellipse area with a total area of approximately

44,031 km2. In this paper, quantitative co-seismic landslide criteria for seismic intensity

zonation were constructed based on the original ‘‘M 8.0 Wenchuan Earthquake Seismic

Intensity Map’’ (MWESIM) and co-seismic landslide abundance analyses. These quanti-

tative criteria, which correspond to landslide area percentages (LAP), indicate the per-

centage of the area affected by landslides in certain calculation windows and are listed as

follows: Areas of LAP C70 % correspond to the XII intensity district; areas of

20 % B LAP \ 70 % correspond to the XI intensity district; areas of 1 % B LAP \ 5 %

correspond to the IX intensity district; and areas of LAP\1 % correspond to the VIII and

less-than-VIII intensity districts. The VII and greater-than-VII intensity districts of the

original MWESIM, which constituted an area of approximately 123,832 km2, were cor-

rected based on the above quantitative criteria. The degrees of fit of the original and revised

MWESIMs with the co-seismic landslides were 93.8 and 95.8 %, respectively, indicating a

2.0 % improvement. The quantitative criteria of the constructed earthquake-triggered

landslides for seismic intensity zonation can provide a scientific reference, standard, and

C. Xu (&) � X. Xu (&) � B. Zhou � G. YuKey Laboratory of Active Tectonics and Volcano, Institute of Geology, China EarthquakeAdministration, 1# Huayanli, P.O. Box 9803, Chaoyang District, Beijing 100029,People’s Republic of Chinae-mail: xc11111111@126.com

X. Xue-mail: xiweixu@vip.sina.com

123

Nat Hazards (2013) 69:1459–1476DOI 10.1007/s11069-013-0757-0

basis for seismic intensity zonation in other earthquake events, especially those occurring

in mountainous areas.

Keywords Wenchuan earthquake-triggered landslides � Spatial distribution �The M 8.0 Wenchuan earthquake seismic intensity map � GIS � Revision

1 Introduction

Seismic intensity is defined as the strength of seismic ground shaking and its effects

[General Administration of Quality Supervision, Inspection and Quarantine of the People’s

Republic of China (AQSIQ) and Standardization Administration of the People’s Republic

of China (SAC) 2008]. Seismic intensity refers to a level or degree of ground damage or

the effect of earthquakes on the structure of buildings. In areas lacking instrumental

records, seismic intensity is categorized by macroscopic scales mainly based on several

indices, including the feelings of people during the earthquake, the reactions of various

objects after earthquakes, the damage or destruction of the structures of buildings, and the

changes of the earth’s surface. Therefore, seismic intensity zonations mainly rely on

macroscopic investigations and qualitative descriptions of the above indices. Seismic

intensity is affected by five parameters, namely magnitude, focal depth, distance from

epicenter, geologic setting, and buildings and structures. Therefore, seismic intensity is an

important indicator for evaluating earthquake hazards. Objective and accurate seismic

intensity mapping is very important for the intensity fortification of constructions in

earthquake zones.

After the May 12, 2008, Wenchuan earthquake in China, the original ‘‘M 8.0 Wenchuan

earthquake seismic intensity map’’ (MWESIM) was immediately produced by the

‘‘Earthquake Emergency Response and Rescue, China Earthquake Administration’’ based

on seismic disaster emergency field investigations for the earthquake on August 29, 2008

(China Earthquake Administration 2008). The original MWESIM provided important and

essential information for identifying seismic intensities and earthquake hazard zonations.

However, the Wenchuan earthquake occurred in areas with mountains and canyons.

There are only a few towns and villages distributed along the earthquake’s associated fault

zone. The area of the towns and villages within the fault zone is relatively small compared

to the entire area that was actually impacted by the earthquake. Therefore, a seismic

intensity zonation based strictly on field investigations of seismic disasters in these towns

and villages that are within the fault zone is not an objective approach. Steep slopes are the

main type of landscape in the areas affected by the Wenchuan earthquake. Therefore,

landslide abundances could be the most important indicator for evaluating the extent of

earthquake-caused ground damage. It was difficult to construct a detailed and complete

inventory of landslides triggered by the Wenchuan earthquake immediately after the

earthquake because of the large size of the earthquake area and limited availability of pre-

and post-earthquake remote sensing images. The original MWESIM (China Earthquake

Administration 2008) was produced under insufficient conditions considering the available

information for Wenchuan earthquake-triggered landslides, which was mainly based on

field investigations of co-seismic hazards.

Recently, a detailed and complete inventory of landslides triggered by the Wenchuan

earthquake was completed (Xu et al. 2013a, b). This inventory can now be used to revise

1460 Nat Hazards (2013) 69:1459–1476

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the original MWESIM. In this paper, quantitative fitting standards for the landslide area

percentage (LAP) and degree of seismic intensity related to the 2008 Wenchuan earth-

quake are proposed by analyzing the correlations of landslide abundance maps and the

original MWESIM. These standards are as follows: LAP C70 % corresponds to the XII

seismic intensity zone; 20 % B LAP \ 70 % corresponds to the XI seismic intensity zone;

5 % B LAP \ 20 % corresponds to the X seismic intensity zone; 1 % B LAP \ 5 %

corresponds to the IX seismic intensity zone; and LAP\1 % corresponds the VIII and less-

than-VIII seismic intensity zones. A revised MWESIM was produced based on these

standards for the VII and greater-than-VII seismic intensity zones, which had a total area of

approximately 123,832 km2. There were a 93.8 % degree of fit between the original

MWESIM and the co-seismic landslides and a 95.8 % degree of fit for the revised

MWESIM, an increase of 2.0 percentage points. The quantitative correlations between

landslide abundances and seismic intensity zones may provide a useful reference for

seismic intensity zonation of other earthquake events, especially those occurring in

mountains and canyons.

2 Landslides triggered by the 2008 Wenchuan earthquake

2.1 Landslide inventory related to the earthquake

The Ms 8.0 Wenchuan earthquake occurred in the Longmen Mountain area, which has

steep terrains and canyons, on May 12, 2008. The strong earthquake ruptured two main co-

seismic surface fault-ruptures (e.g., Xu et al. 2008, 2009a, b, 2010), creating the Yingxiu-

Beichuan and Guanxian-Jiangyou co-seismic surface ruptures. The Yingxiu-Beichuan co-

seismic surface rupture, along the Yingxiu-Beichuan fault, is 240 km in length and exhibits

thrust and dextral strike-slip moving patterns. The Guanxian-Jiangyou co-seismic surface

rupture, which is 72 km in length, mainly exhibits a thrust moving characteristic. In

addition, another surface rupture, the Xiaoyudong co-seismic surface rupture, occurred and

links the Yingxiu-Beichuan co-seismic surface rupture and the Guanxian-Jiangyou co-

seismic surface rupture. The Xiaoyudong co-seismic surface rupture is approximately

6 km in length and exhibits thrust and sinistral strike-slip moving patterns (Tan et al.

2012). Although there are many towns and villages along the main co-seismic surface

ruptures, the topography of the earthquake-struck area is mainly mountainous. Therefore,

earthquake-triggered landslides are more appropriate identifiers of seismic intensity for the

Wenchuan earthquake, as opposed to building damage.

Detailed and comprehensive landslide inventories are very important for subsequent

scientific research of earthquake-triggered landslides (Keefer 2002; Harp et al. 2011a;

Guzzetti et al. 2012). The landslides triggered by the 2008 Wenchuan earthquake occurred

in large numbers with a high density and had a wide distribution area. After the earthquake

occurred, some inventories of the landslides triggered by the earthquake were produced

(e.g., Xu et al. 2009c, d; Dai et al. 2011; Qi et al. 2010; Gorum et al. 2011; Huang and Li

2009; Parker et al. 2011; Chigira et al. 2010; Yin et al. 2010a). However, all of the

inventories were either incomplete or only located by points and did not meet the landslide

inventory standard of Harp et al. (2011a). We spent more than 3 years after the earthquake

producing a detailed and complete inventory of landslides triggered by the earthquake,

based on visual interpretation of aerial photographs, high-resolution satellite images, and

select field investigations (c.f. Xu 2012; Xu and Xu 2012a; Xu et al. 2013a, b). The final

results of these data indicated that 197,481 landslides, with a total area of 1,160 km2

Nat Hazards (2013) 69:1459–1476 1461

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throughout an area of approximately 110,000 km2, were triggered by the earthquake.

Among these, 196,007 landslides (99.25 % of the total analyzed), with an area of

1,150.622 km2 (99.19 % of the total landslide area), were concentrated in an approximate

oval area of 44,031 km2 (Fig. 1).

Various types of landslides were triggered by the Wenchuan earthquake. The most

common types were rock falls and shallow, disrupted landslides. The spatial distribution

characteristics of the landslides include the following: (1) Most of the landslides occurred

northeast of the epicenter, which is consistent with the single rupture direction of the

Yingxiu-Beichuan fault; (2) most of the landslides were distributed along the Yingxiu-

Beichuan fault, yet landslide abundances showed obvious differences along the fault; (3)

most of the landsides occurred on the hanging wall of the Yingxiu-Beichuan fault,

Fig. 1 Inventory of landslides triggered by the 2008 Wenchuan earthquake. a the Yingxiu-Beichuanco-seismic surface rupture; b the Guanxian-Jiangyou co-seismic surface rupture; c the Xiaoyudong co-seismic surface rupture

1462 Nat Hazards (2013) 69:1459–1476

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especially the southwest segment of the fault (between the town of Yingxiu and Beichuan

County); (4) the number of landslides is also high in the long strip area between the

Yingxiu-Beichuan and the Guanxian-Jiangyou surface ruptures but lower than that of the

hanging wall areas; (5) landslides along the northeast segment of the fault (northeast of

Beichuan County) exhibited a long strip with a beaded spatial distribution; and (6) in areas

on the hanging wall of the southwest segment of the Yingxiu-Beichuan fault, many

landslides occurred far from the fault and mainly along rivers.

2.2 Quantitative thematic maps extracted from the landslide inventory

The inventory of landslides triggered by the earthquake cannot be directly used for seismic

intensity zonation. Therefore, three quantitative thematic maps, including a landslide

number density (LND, the number of landslides per 1 km2 square-shaped areas) map, a

landslide area percentage (LAP, the percentage of the area affected by the landslides) map,

and a landslide erosion thickness (LET, landside erosion material thickness per 1 km2)

map, were derived from the inventory of landslides triggered by the Wenchuan earthquake

for seismic intensity zonation. All the three quantitative thematic maps are in raster format

with 1 km 9 1 km grids.

The landslide volume is calculated based on a scaling relationship to convert the

landslide area to landslide volume, similar to previously described methods (e.g., Larsen

et al. 2010; Parker et al. 2011; Guzzetti et al. 2009; Chaytor et al. 2009):

Vls ¼ a� Acls ð1Þ

where Vls is the volume of a certain individual landslide, Als is the area of the landslide, and

a and c are scaling parameters that vary with different areas. We applied published scaling

parameters for landslides triggered by the 2008 Wenchuan earthquake from Parker et al.

(2011) in this study. The scaling parameters from Parker et al. (2011), with a = 0.106 and

c = 1.388, yield a total volume of approximately 6.12 km3 for 19,007 landslides. The

number of landslides, landslide area, and landslide volume in each grid were calculated to

produce LND (Fig. 2a), LAP (Fig. 2b), and LET (Fig. 2c) maps of the landslide density

area (Fig. 2). The maximum LND, LAP, and LET values are 281 landslides/km2, 100 %,

and 47.9 m, respectively. The maximum LAP and LET are both located at the site of the

Daguangbao landslide (e.g., Yin et al. 2011; Huang et al. 2012). The Daguangbao landslide

covers 6.97 km2 and was the largest landslide triggered by the Wenchuan earthquake. The

landslides cover at least one 1 km 9 1 km square grid. Landslides with scales similar to

the Daguangbao landslide would cause dramatic ground, mountain, and river changes.

Areas where such large landslides occur can often be identified as the XII seismic intensity

zone. The LND, LAP, and LET maps were the preliminary quantitative proxies of the

landslide inventory map (Fig. 1), all of which showed similar distribution features in terms

of landslide abundances (Fig. 2). The three maps can be used to analyze the correlations

between earthquake-triggered landslide abundances and seismic intensity zones.

3 The original M 8.0 Wenchuan earthquake seismic intensity map

3.1 Landslide implications for the Chinese seismic intensity scale (GB/T 17742-2008)

The terminology ‘‘landslides’’ in this paper represents landslides in a general sense and

includes various types of landslides. A landslide is defined as the movement of a mass of

Nat Hazards (2013) 69:1459–1476 1463

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rocks, debris, or earth down from slopes under the influence of gravity and strong ground

shaking (Varnes 1978; Cruden and Varnes 1996; Guzzetti et al. 2012; Keefer 1984, 2002).

Similar to landslides occurring under other triggers, such as rainfall and other earthquake

events (Guzzetti et al. 2012), various types of landslides were triggered by the 2008

Wenchuan earthquake, including flowing, sliding, toppling, falling, and two or more types

of movements. There are four indicators for identifying seismic intensity in the Chinese

seismic intensity scale (GB/T 17742-2008) (AQSIQ and SAC 2008), including the feelings

of people, the damage to buildings, other seismic damage phenomena, and the horizontal

ground motion intensity. In the GB/T 17742-2008, seismic intensity indicator selection

criteria were also listed for different seismic intensity zones based on an investigation of

these indicators. For assessing the I–V seismic intensity zones, the feelings of people on

the ground or in the bottom of housing and other seismic damage phenomena should be the

main indicators; for the VI–X seismic intensity zones, building damage should be the main

indicator, in addition to references to other earthquake damage phenomena; and for the

XI–XII seismic intensity zones, there should be an integration of indicators of building

damage and ground surface seismic damage phenomena. However, there are only a few

descriptions about applying the abundance of earthquake-triggered landslides for seismic

intensity zonation in the Chinese seismic intensity scale (GB/T 17742-2008) from the IX to

XII seismic intensity zones. Descriptions in the Chinese seismic intensity scale (GB/T

17742-2008) are listed as follows:

1. Description of the IX seismic intensity zone in the scale includes the following: many

cracks appeared on hard, dry soil, and some cracks and dislocations appeared in

bedrock, and small landslides and rock falls often occurred;

Fig. 2 Proxies of landslide abundance and their classifications. a LND map, landslide number density map;b LAP map, landslide area percentage map; c LET map, landslide erosion thickness; d LND classificationmap; e LAP classification map; f LET classification map

1464 Nat Hazards (2013) 69:1459–1476

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2. Description of the X seismic intensity zone includes the following: mountain collapses

and co-seismic surface ruptures appeared;

3. For the XI seismic intensity zone, standards of seismic damage include the following:

continuation of co-seismic surface ruptures and a large number of mountain collapses

and landslides occurred; and

4. For the XII seismic intensity zone, standards of seismic damage include the following:

ground surface, mountains, and great river changes occurred.

Considering the descriptions ‘‘small landslides and rock falls often occurred’’ for the IX

seismic intensity zone and ‘‘mountain collapses and co-seismic surface ruptures appeared’’

for the X seismic intensity zone, the definitions of ‘‘small landslides and rock falls’’ and

‘‘mountain collapses’’ are different among the scales because generally more frequent and

larger landslides occur in higher seismic intensity zones.

The description ‘‘Small landslides and rock falls’’ of the IX seismic intensity zone

indicates that small and shallow landslides occur in the lower topographic positions of

loose accumulation layers, causing smaller hazards. By contrast, the description ‘‘mountain

collapses’’ of the X seismic intensity zone indicates larger, deep-seated bedrock landslides

and rock falls with long run out distances that occur at higher slope positions, such as

ridges and slope shoulders, and may cause serious hazards. In the X seismic intensity zone,

‘‘mountain collapses’’ appear. The description ‘‘A large number of mountain collapses and

landslides’’ for the XI seismic intensity zone indicates a frequent occurrence of this type of

large-scale landslide. The description for the XII seismic intensity zone ‘‘dramatic ground

and mountains changes, and great rivers changes’’ indicates that most of the area has been

exposed to such large-scale landslides, and it is estimated that the landslide area percentage

may exceed 70–80 %.

All information and descriptions of landslides for seismic intensity zonation in the

Chinese seismic intensity scale (GB/T 17742-2008) are qualitative. Currently, quantitative

correlations between landslide abundance and seismic intensity zones have not been

defined. Therefore, standards of seismic intensity zonation are mainly based on various

housing and building damages. For the 2008 Wenchuan earthquake, investigations of the

feelings of people and building damage could only be obtained in areas of human settle-

ment. Seismic intensity zonation standards based on peak horizontal ground acceleration

require densely distributed ground motion monitoring stations; otherwise, the error from

the interpolation of the result would be too excessive to be useful. Therefore, landslide

abundance information from ‘‘other seismic damage phenomena’’ is the most important

and useful indicator for seismic intensity zonation of the 2008 Wenchuan earthquake.

3.2 Characteristics of the original MWESIM

After the 2008 Wenchuan earthquake occurred, many earthquake seismologists organized

by the China Earthquake Administration (CEA) carried out earthquake damage field

investigations in the provinces of Sichuan, Gansu, Shaanxi, Yunnan, and Ningxia, the city

of Chongqing, etc. A total of 4,150 field investigation points were completed throughout

500,000 km2, and the original MWESIM (Fig. 3b) was produced based on the results of

these field investigations. Characteristics of the seismic intensity map are listed as follows

(CEA 2008): (1) The highest seismic intensity zone is the XI zone, and two XI zones were

distributed in the town of Yingxiu and Beichuan County; (2) close to the Yingxiu-

Beichuan fault, there is serious damage in the IX and greater-than-IX seismic intensity

zones, showing a long strip along the fault. The boundaries of the X and IX seismic

Nat Hazards (2013) 69:1459–1476 1465

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intensity zones show a convex shape around the Chengdu basin in the cities of Mianzhu,

Shifang, and, to some extent, Dujiangyan, which may be affected by the Guanxian-Ji-

angyou fault; (3) at the edge of the foreland basin, intensity is rapidly attenuated to the east

but decays relatively slowly to the west; (4) and the intensity distribution is northeast–

southwest and asymmetric. The VIII and VII seismic zones were more expanded to

northward than southward. In the provinces of Gansu and Shaanxi, a fault rupture prop-

agates to the northeast. The largest aftershock occurred in the northeast segment of the

fault.

3.3 Limitations of the original MWESIM

Although there are many towns and frequent human activity along the 2008 Wenchuan

earthquake associate fault zone, the main terrain and landform type of the earthquake stuck

area is steep mountainous uninhabited areas. Therefore, with respect to the 2008

Wenchuan earthquake, landslides should be the main indicator for seismic intensity

zonation. The LND, LAP, and LET maps related to the earthquake were overlapped on the

original WMESIM (Fig. 2) to analyze the relationship between landslide abundances and

seismic intensity zones. Based on the overlap of these landslide abundance proxy maps and

the original MWESIM, two main limitations of the original MWESIM emerged: (1) The

hanging wall effect of the thrust segment of the fault was not obvious; and (2) the seismic

intensities were generated by faults mainly with a thrust movement behavior and those

with a strike-slip movement behavior showed only slight differences. In the original

MWESIM, although the XI intensity zone in the town of Yingxiu showed a certain degree

of the hanging wall effect, the effect should actually be much higher because the high-LAP

value-distributed area on the hanging wall was larger than the XI-Yingxiu seismic intensity

zone. More landslides were triggered by the southwest segment than by the northeast

segment of the Yingxiu-Beichuan fault, whereas no similar conclusions existed in the

original MWESIM. The areas of the XI-Yingxiu and the XI-Beichuan intensity zones were

the same in the original MWESIM, whereas co-seismic disasters around the segment of the

fault that mainly exhibited thrusting were much greater than those around the segments

exhibiting strike-slipping.

The correlations between the VII and greater-than-VII seismic intensity zones and LND,

LAP, and LET were calculated (Table 1). The LND, LAP, and LET values in the VII

Fig. 3 Correction of the M 8.0 Wenchuan earthquake seismic intensity map. a Overlap of LAP and theoriginal seismic intensity map; b overlap of the LAP rank map and the original seismic intensity map;c regional classification of the LAP map; d correction of the seismic intensity map based on LAP rank; e theoriginal M 8.0 Wenchuan earthquake seismic intensity map; f the amended M 8.0 Wenchuan earthquakeseismic intensity map

1466 Nat Hazards (2013) 69:1459–1476

123

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me

Nat Hazards (2013) 69:1459–1476 1467

123

intensity zone were 0.04 landslide/km2, 0.014 %, and 0.0005 m, respectively. For the XI

intensity zone, they were 28.5 landslide/km2, 17.85 %, and 0.97 m. From the VII to XI

intensity zone, all the LND, LAP, and LET values increased gradually.

It seems that the original MWESIM is objective because the earthquake-triggered

landslide abundances increased as seismic intensity increased. However, there are obvious

limitations in the original MWESIM, especially with regard to the X and XI seismic

intensity zones. For example, the XI intensity zone contains two separate areas, XI-

Yingxiu and XI-Beichuan, which are located in Beichuan County and in the town of

Yingxiu. Figure 4 shows the correlations of three landslide forms (the three proxies of

landslide abundance are LN%, LA%, and LV%) and seismic intensity zones. All three

proxies show similar tendencies with seismic intensity zones. The values of LA% for all

seismic intensity zones were between LN% and LV% because landslide volume is cal-

culated using a scaling relationship in Eq. (1). Given a determined landslide area in a

certain seismic intensity zone, more small landslides will result in a smaller landslide

volume. LN% is larger than LV% from the VII to the IX seismic intensity zones, whereas

the opposite trend exists in the X and XI seismic intensity zones (Fig. 5). Thus, larger

landslides are more prone to occur in higher seismic intensity zones.

Fig. 4 Comparison of landslide abundance, areas, and volumes of the seismic intensity zones. LN%indicates the percentage of landslides in comparison with the total number of landslides; LA% indicates thepercentage of the landslide area in comparison with the total landslide area; and LV% indicates thepercentage of the landslide volume in comparison with the total landslide volume

Fig. 5 Correlations of proxies of landslide abundance for the indicated seismic intensity zones

1468 Nat Hazards (2013) 69:1459–1476

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There were obvious differences in the three proxies of landslide abundance between the

two XI seismic intensity zones. Only the landslide area was used to analyze the limitations

of the original MWESIM because the three proxies showed similar tendencies with seismic

intensity zones. There were two XI seismic intensity zones, namely the XI-Yingxiu and XI-

Beichuan seismic intensity zones. The LAP value of the XI-Yingxiu seismic intensity zone

was 25.90 %, which is much higher than that of the XI-Beichuan seismic intensity zone

(only 8.870 %). The overall LAP value of the XI zone was 17.85 %, and the LAP value of

the X seismic intensity zone is 11.62 %. It is clearly incorrect that the LAP value of the X

seismic intensity zone is larger than that of the XI-Beichuan seismic intensity zone. In

addition, there were relatively high LAP values in the area of the X seismic intensity zone

between the XI-Yingxiu and the XI-Beichuan seismic intensity zones. However, the area

was only classified as an X seismic intensity zone. In the hanging wall area of the

southwest segment of the Yinxiu-Beichuan surface fault-rupture, even far from the fault,

there is an abundance of landslides, but the area was classified into lower intensity zones

(such as the VIII and IX seismic intensity zones). Therefore, considering these limitations,

the original MWESIM needs to be revised based on landslide abundance information.

4 Revisions of the original MWESIM

In this section, a revision scheme considering the limitations mentioned previously for the

original MWESIM is proposed based on the abundances of Wenchuan earthquake-trig-

gered landslides. The seismic intensity zones, especially the high seismic intensity zones,

were re-delineated based on the functions of seismic intensity zones and three proxies

(LND, LAP, and LET) of earthquake-triggered landslide abundance.

4.1 Functions of seismic intensity zones with three proxies of landslide abundances

The standards of seismic intensity zonation based on the three proxies of landslide

abundance values are as follows (presented in Table 2):

LAP C70 %, LND C 100 landslides/km2, or LET C10 m corresponds to the XII

intensity zone;

20 % B LAP \ 70 %, 30 B LND \ 100 landslides/km2, or 1 B LET \ 10 m corre-

sponds to the XI intensity zone;

5 % B LAP \ 20 %, 10 B LND \ 30 landslides/km2, or 0.3 B LET \ 1 m corre-

sponds to the X intensity zone;

1 % B LAP \ 5 %, 1 B LND \ 10 landslides/km2, or 0.1 B LET \ 0.3 m corre-

sponds to the IX intensity zone;

Table 2 Standards of seismic intensity zonation based on the three proxies for landslide abundance values

Seismic intensity zones LND LAP/% LET/m

VIII and lower than VIII 0–1 0–1 0–0.1

IX 1–10 1–5 0.1–0.3

X 10–30 5–20 0.3–1

XI 30–100 20–70 1–10

XII 100–281 70–100 10–47.9

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LAP\1 %, LND\1 landslides/km2, or LET\0.1 m corresponds to the VIII and less-

than-VIII intensity zones.

These quantitative standards were proposed based on the correlation of the original

MWESIM and the proxies of landslide abundance related to the Wenchuan earthquake. As

shown in Table 1, 52,017 landslides, covering an area of 310.77 km2, were distributed in

the 1,200 km2 XI-Yingxiu seismic intensity zone, and the corresponding LND, LAP, and

LET values were 43.35 landslides/km2, 25.90 %, and 1.35 m, respectively. However, in

the XI-Beichuan seismic intensity zone, with an area of 1,075 km2, 12,817 landslides

occurred covering an area of approximately 95.38 km2, and the corresponding values of

the three proxies were only 11.92 landslides/km2, 8.87 %, and 0.55 m, respectively. There

were obvious differences in the landslide abundances in the two XI seismic intensity zones.

The three proxies of landslide abundance in the X seismic intensity zones were 17.66

landslides/km2, 11.62 %, and 0.72 m, respectively, and were higher than those of the

XI-Beichuan seismic intensity zone. Therefore, the original MWESIM is not appropriate

because the proxy values of landslide abundance of the lower seismic intensity zone are

greater than those of the higher seismic intensity zone. Figure 1 show co-seismic landslide

distribution. The values of the three landslides proxies, LND, LAP, and LET, in the XI-

Yingxiu seismic intensity zone exhibited high intensity and consistency (Fig. 2). The

southwest boundary of the XI-Yingxiu intensity zone exhibited generally sudden changes

in the LND, LAP, and LET values, showing that the southwest boundary of the XI-Yingxiu

intensity zone is objective. In addition, the spatial distributions of LND, LAP, and LET

values were consistent in the XI-Yingxiu seismic intensity zone. Therefore, the LND, LAP,

and LET thresholds of the XI and X intensity zones were set to *30 landslides/km2,

*20 %, and *1 m, respectively; these values are consistent with those of the indicators of

other seismic damage phenomena descriptions for the XI seismic intensity zone, including

the description ‘‘continuation of co-seismic surface ruptures and a large number of

mountain collapses and landslides.’’ The descriptions of the XII intensity zone included

‘‘ground dramatic changing, and mountains and rivers great changing,’’ indicating the

highest landslide abundance; thus, the LND, LAP, and LET thresholds of the XII intensity

zone were set to *100 landslides/km2, *70 %, and *10 m, respectively. Next, the

boundaries of the XI and XII seismic intensity zones were generally remapped based on

these thresholds (Fig. 3a). The LAP values of the X and IX intensity zones of the original

MWESIM were 11.62 and 2.63 %, respectively. Therefore, the LAP value ranges of the X

and IX intensity zones were set to be 5–20 and 1–5 %. Correspondingly, the LND value

ranges of the X and IX intensity zones were set to 10–30 and 1–10 landslides/km2, and the

LET thresholds of the X and IX intensity zones were set to 0.3 and 0.1 m, respectively,

because the LND and LET values of the X and IX intensity zones in the original MWESIM

were 17.66 and 4.88 landslides/km2 and 0.72 and 0.12 m, respectively.

4.2 Revisions of the original MWESIM

Based on the fitting criteria of the three proxies for landslide abundance (Table 2) and

seismic intensity zones, the original MWESIM (Fig. 3b) was revised by manual mapping

(Fig. 3a). For the VIII and VII intensity zones, the values of the three proxies, including

LND, LAP, and LET were 1.23 and 0.04 landslides/km2, 0.57 and 0.014 %, and 0.02 and

0.0005 m, respectively. Therefore, it is not appropriate to produce a seismic intensity map

based on co-seismic landslides because of the relatively low amount of landslides in these

areas. Therefore, the boundaries of the VIII and VII intensity zones of the original

MWESIM were mainly based on field investigation (e.g., feelings of people, building

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damage), and the original boundaries of the VIII and VII intensity zones remained

unchanged. However, landslide abundances were slightly higher southwest of the VIII

intensity zone (Fig. 3a). Therefore, as shown in Fig. 2d, the southwest boundary of the VIII

intensity zone was revised slightly to be more consistent with co-seismic landslides

(Fig. 3a). Considering the macroscopic characteristics of the seismic intensity zonation, the

IX, X, and XI seismic intensity zones were generally remapped as shown in Fig. 3.

Although the LND, LAP, and LET values in some areas were higher than 100 landslides/

km2, 70 %, and 10 m and the phenomenon of ‘‘dramatic ground and mountains changes,

and great rivers changes’’ occurred in the XII intensity zone, these areas only cover

approximately 10 km2. Therefore, these areas were ignored and not classified into the XII

seismic intensity zone. In this way, the revised MWESIM was prepared (Fig. 3c).

Fig. 6 Overlap of the original and revised seismic intensity zones with LND, LAP, and LET maps based ondifferent calculation windows; a 2 km 9 2 km calculation window for LND; b 4 km 9 4 km calculationwindow for LND; c 8 km 9 8 km calculation window for LND; d 2 km 9 2 km calculation window forLAP; e 4 km 9 4 km calculation window for LAP; f 8 km 9 8 km calculation window for LAP;g 2 km 9 2 km calculation window for LET; h 4 km 9 4 km calculation window for LET; i 8 km 9 8 kmcalculation window for LET

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5 Effects of the calculation for window size on landslide abundance

The above revisions for the original MWESIM were based on a 1 km 9 1 km calculation

window for calculating LND, LAP, and LET values. To study the effects of the LAP value

calculation window size, 2 km 9 2 km, 4 km 9 4 km, and 8 km 9 8 km windows were

used for calculating LND, LAP, and LET values, respectively. The original and modified

MWESIM maps were overlapped on the LND, LAP, and LET maps based on the different

values of the calculation windows (Fig. 6). There were good fits between the modified

MWESIM map and the three types of maps with landslide abundance proxies, indicating

that the LND, LAP, and LET value calculation window size had no (or only a slight effect)

effect on seismic intensity zonation.

6 Validation of the revised MWESIM with co-seismic landslides

Landslide abundances from the LAP index (1 km 9 1 km) were used to validate the

revised MWESIM. Validation of the original and revised MWESIMs with landslides

triggered by the Wenchuan earthquake was carried out. First, the total areas, landslide

areas and LAP values for each seismic intensity zone of the original and revised MWE-

SIMs were calculated (Tables 1, 3). The LAP values (Table 3) in each intensity zone of the

revised MWESIM were in accordance with the criteria for calculating LAP values and

intensity zones. Figure 7 shows the calculated results of the areas, landslide areas, and LAP

values for each seismic intensity zone of the new and original MWESIM. There are clear

changes in the areas and landslide areas between the two MWESIMs. Area changes in each

intensity zone of the two MWESIMs were not obvious. In the VIII, IX, and X intensity

zones, the landslide areas decreased, and the LAP values were also slightly lower. In the XI

seismic intensity zone, the landslide area, and LAP value increased considerably, from

406.07 to 602.26 km2 in landslides area and from 17.85 to 23.45 % in LAP.

We then quantitatively compared the fitted correlations between the co-seismic land-

slides and the two MWESIMs. Correlations between the cumulative percentages of area

and cumulative percentages of landslide area under LAP values were established in

descending order (Fig. 8) for the original and revised MWESIMs. The results show that the

degrees of fit for landslides in the original and revised MWESIMs were 93.8 and 95.8 %,

respectively. The degree of fit for landslides in the original MWESIM was high (93.8 %)

because most of the landslides occurred in higher intensity zones, such as the IX and

greater-than-IX intensity zones, and the areas covered by VII and VIII intensity zones are

more larger than those of other higher intensity zones. The 2 % increase in the degree of fit

between for landslides and high-intensity zones is rather significant for this study.

Table 3 Statistical result of landslide in each intensity zone of the new MWESIM

Intensity zones Area/km2 LA/km2 LAP/% LAP value criteria

VII 82,209 4.51 0.005 Landslides occurred rarely

VIII 23,345 36.64 0.16 LAP \1 %

IX 11,137 161.19 1.45 1 % B LAP \ 5 %

X 4,574 346.02 7.57 5 % B LAP \ 20 %

XI 2,568 602.26 23.45 20 % B LAP \ 70 %

LA landslide area, LAP landslide area percentage

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7 Discussion

It is impractical to carry out seismic intensity zonation mainly based on the feelings of

people or the damage to buildings because of the limitations of field surveys in steep-

topography areas. The abundance of earthquake-triggered landslides was used as a main

indicator for seismic intensity zonation in mountainous areas. New criteria for the IX and

Fig. 7 Correlations of seismic intensity zones with the landslide areas; LAP values of the original andrevised MWESIMs. a LA landslide area, b LAP landslide area percentage

Fig. 8 Fitted validation of theoriginal and revised MWESIMswith co-seismic landslides

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greater-than-IX seismic intensity zones in mountainous areas based on LND, LAP, and

LET values were proposed. Limitations were found when overlapping the original

MWESIM on the Wenchuan earthquake-triggered LND, LAP, and LET maps, namely the

hanging wall effect of the section thrust fault was not obvious without considering the

intensity difference generated by the thrust fault and strike-slip fault. Especially in the XI-

Beichuan intensity area, the LAP value was only 8.87 %, which is considerably less than

the LAP value of the X intensity zone (11.62 %). The original MWESIM was revised for

the VII and greater-than-VII intensity zones, which had an area of 123,832 km2.

Based on the original MWESIM and LND, LAP, and LET maps related to the 2008

Wenchuan earthquake, the following quantitative criteria for the fitting correlations

between LND, LAP, and LET values and seismic intensity zonation were proposed:

LND C 100 landslides/km2, LAP C 70 %, or LET C 10 m represents the XII intensity

zone; 30 B LND \ 100 landslides/km2, 20 % B LAP \ 70 %, or 1 m B LET \ 10 m

represents the XI intensity zone; 10 B LND \ 30 landslides/km2, 5 % B LAP \ 20 %, or

0.3 m B LET \ 1 m represents the X intensity zone; 1 B LND \ 10 landslides/km2,

1 % B LAP \ 5 %, or 0.1 m B LET \ 0.3 m represents the IX intensity zone; and LND

\1 landslides/km2, LAP \1 %, or LET \0.1 m represents the VIII and less-than-VIII

seismic intensity zones. Different LND, LAP, and LET maps were prepared based on

1 km 9 1 km, 2 km 9 2 km, 4 km 9 4 km, and 8 km 9 8 km calculation windows for

studying the effect of the window size of co-seismic landslide abundance calculations on

the results of the maps. All the landslide abundance maps (LND, LAP, and LET maps) in

every calculation windows (1 km 9 1 km, 2 km 9 2 km, 4 km 9 4 km, and

8 km 9 8 km) could be well fitted with the revised MWESIM. It can be concluded that the

calculation window size had no effect or only a slight effect on the LND, LAP, and LET

maps. The degrees of fit of the original and revised MWESIMs with the co-seismic

landslides were 93.8 and 95.8 %, respectively, representing a two percent increase in the

degree of match. For the seismic intensity zonation, the effects were more prominent near

the southwest segment of the Yingxiu-Beichuan fault (southwest of the Beichuan County),

which mainly exhibited thrusting characteristics, compared with the northeast segment of

the fault (northeast of the Beichuan County), which mainly exhibited strike-slip features.

Seismic hazards were also more serious on the hanging wall than on the footwall. The

original MWESIM was the basis of the revised MWESIM. Therefore, seismic hazards field

investigations soon after the earthquake occurrence were necessary and indispensable for

seismic intensity zonation. The methods of this study can provide a good supplement for

field investigations.

8 Conclusion

In conclusion, because it is difficult or impossible to obtain indices based on the ‘‘feelings

of people’’ and ‘‘damage to buildings’’ for seismic intensity zonation and due to the relative

accessibility of co-seismic landslide abundances, with the help of remote sensing and

geographic information systems (GIS) technology in large mountainous areas, we devel-

oped new quantitative criteria for seismic intensity zonation based on co-seismic landslide

abundance. We then revised the original MWESIM. The quantitative criteria can serve as a

scientific reference, standard, and basis for seismic intensity zonation, considering the

abundance of landslides related to other earthquake events in mountainous areas, such as

the April 14, 2010, Yushu earthquake (Xu et al. 2012a, 2013c; Yin et al. 2010b; Zhang

et al. 2010), the September 21, 1999, Chi–Chi earthquake (Liao et al. 2002; Wang et al.

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2002, 2003), and the January 12, 2010, Haiti earthquake (Xu et al. 2012b; Xu and Xu

2012b; Harp et al. 2011b; Gorum et al. 2013). However, this study is exploratory research,

and the quantitative criteria require further verification for individual earthquake events.

Acknowledgments This research is supported by the National Science Foundation of China (Grant Nos.41202235, 91214201).

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