Seismic Studies

1. SOME SEISMOLOGICALCHARACTERISTICS
OF MAY 12, 2008 WENCHUAN EARTHQUAKE
Mahdi, Syed Kazim
Director, WAPDA Directorate of Seismic Studies, Tarbela Dam, Pakistan
sspkazim@hotmail.com
ABSTRACT
A devastating and shallow focused (Mw 8.3) rocked the China’s western province of Sichuan on
May 12, 2008, causing very huge casualties and mammoth damages to the infrastructures, whose
rehabilitation will cost billions of dollars. The extent of the earthquake and after shock-affected
areas lies north-east, along the Longmen Shan fault. Its epicenter and focal-mechanism are
consistent with it having occurred as the result of movement on the Longmenshan fault or a
tectonically related fault. The mega event reflects tectonic stresses resulting from the convergence
of crustal material slowly moving from the high Tibetan Plateau, to the west, against strong crust
underlying the Sichuan Basin and southeastern China. The epicentre was in the mountains of the
Eastern Margin of Qing-Tibet Plateau at the northwest margin of the Sichuan Basin China
frequently suffers large and deadly earthquakes. As many as fifty quake lakes were formed from
the result of landslidings after the mega event. Some substantial cracks were observed at many of
the dams, however, at the gigantic Three Geroges Dams no abnormality was noticed. Failures
were also recorded at the Zipingpu Dam, Sichuan province.
Key Words: Wenchuan Earthquake, Longmenshan Fault, Mw = 8.3, Zipingpu Dam.
INTRODUCTION
The May 12, 2008 Wenchuan earthquake which measured at 8.0 Ms and 8.3 Mw according to
China Seismological Bureau, and 7.9 Mw according to USGS, occurred at 06:28:01.42 UTC in
the Sichuan province of China. The epicenter was 80 kilometres (50 mi) west-northwest of
Chengdu, the capital of Sichuan, with a depth of 19 kilometres (12 mi). The earthquake was felt
as far away as Beijing (1,500 km away) and Shanghai (1,700 km away), where office buildings
swayed with the tremor. The earthquake was also felt in neighbouring countries like Pakistan,
India and Bangladesh. Official figures (as of June 11, 2008) state that 69,146 are confirmed dead,
including 68,620 in Sichuan province, and 374,131 injured, with 17,516 listed as missing. The
earthquake left about 4.8 million people homeless, though the number could be as high as
11 million. It is the deadliest and strongest earthquake to hit China since the 1976 Tangshan
earthquake, which killed at least 240,000 people. Approximately 15 million people lived in the
affected area. On May 25, a major aftershock of 6.0 Mw hit northeast of the original earthquake's
epicenter, in Qingchuan County, causing twelve deaths, around 1015 injuries, and destroying
thousands of buildings. On May 27, 2008 two more major aftershocks, a 5.2 Mw in Qingchuan
County and a 5.7 Mw in Ningqiang (Shaanxi Province), led to the collapse of more than 420,000
homes and injured 143 people.

2. TECTONICS
The Sichuan earthquake of May 12, 2008, occurred as the result of motion on a northeast striking
reverse fault or thrust fault on the northwestern margin of the Sichuan Basin.. The seismicity of
central and eastern Asia is caused by the northward movement of the India plate at a rate of
5 cm/year and its collision with Eurasia, resulting in the uplift of the Himalaya and Tibetan
plateau and associated earthquake activity. This deformation also results in the extrusion of
crustal material from the high Tibetan Plateau in the west towards the Sichuan Basin and
southeastern China. On a continental scale, the seismicity of central and eastern Asia is a result of
northward convergence of the India plate against the Eurasia plate with a velocity of about 50
mm/yr. The convergence of the two plates is broadly accommodated by the uplift of the Asian
highlands and by the motion of crustal material to the east away from the uplifted Tibetan
Plateau. The northwestern margin of the Sichuan Basin has previously experienced destructive
earthquakes. The magnitude 7.5 earthquake of August 25, 1933, killed more than 9,300 people.
Earthquakes of this size have the potential to cause extensive damage and loss of life.
Figure-1. Epicentral location of Wenchuan Earthquake.
Preliminary rupture models of the earthquake indicated displacement of up to 9 meters along a
fault approximately 240 km long by 20 km deep. The earthquake generated deformations of the
surface greater than three meters and increased the stress (and probability of occurrence of future
events) at the northeastern and southwestern ends of the fault. Japanese seismologist Yuji Yagi
said that the earthquake occurred in two stages: The 155-mile Longmenshan Fault tore in two
sections, the first one ripping about seven yards, followed by a second one that sheared four
yards. His data also showed that the earthquake lasted about two minutes and released 30 times
the energy of the Great Hanshin earthquake of 1995 in Japan, which killed over 6,000 people. He
pointed out that the shallowness of the epicenter and the density of population greatly increased

3. the severity of the earthquake. Teruyuki Kato, a seismologist at the University of Tokyo, said that
the seismic waves of the quake traveled a long distance without losing their power because of the
firmness of the terrain in central China. According to reports from Chengdu, the capital of
Sichuan province, the earthquake tremors lasted for about three minutes. The earthquake has a
strike direction of NE-SW (strike; 230deg), and focal type is reverse fault with some strike slip
components (Figure-2).
Figure-2 Main Shock Finite fault parameters. Strike: 230 degree,Dip : 32 degree (dipping to the
NW), Seismic Moment=1.02e21 Nm, Mw 8.3 by CSB, Maxmum slip: 13 m

4. THE QUAKE LAKES
As the result of the Mw 8.3 earthquake and number of strong aftershocks, many rivers became
blocked by large landslides, which resulted in the formation of "quake lakes"; massive amounts
of water pooling up at a very high rate behind the landslide dams eventually crumbled under the
weight of the ever-increasing water mass, potentially endangering the lives of millions of people
if the water is to build up, and then break downstream. As of 27 May 2008, 34 lakes had formed
in nine earthquake-affected counties due to earthquake debris blocking and damming rivers, and
it is estimated that 28 of them are still of potential danger to the local people. Entire villages had
to be evacuated because of the resultant flooding. These "quake lakes" also pose additional
hazards as the natural dams forming them are breached, causing secondary flooding. The most
precarious of these quake-lakes is the one located in the extremely difficult terrain at Tangjiashan
mountain, accessible only by foot or air, in which an Mi-26T heavy lift helicopter belonging to
the China Flying Dragon Special Aviation Company was used to bring heavy earthmoving
tractors to the affected location.
One of the worst-damaged cities was Dujiangyan, site of multiple dams and weirs that irrigate
about 3 million hectares in the fertile Sichuan plain. The Dujiangyan irrigation works date from
the 3rd century BC, when engineers split the Min river where it spills from the mountains, and
diverted it to irrigation channels along the plain. 'Upstream on the Min river is an important
reservoir called Tulong which is already imperiled. The quake caused the 760-megawatt
hydropower generating unit at Zipingpu, nine kilometers upstream of Dujiangyan, to collapse, the
provincial government said. It began operations in 2006, as part of China's program to develop its
poorer western regions. Water was released at 50 percent more than average levels, to lower
water levels in the Zipingpu reservoir and relieve pressure on the cracked dam.
CRACKS
Cracks on the famous Yuzui or 'Fish mouth' levee further downstream, the crux of the
Dujiangyan system, were not serious, the ministry said. Two teams were dispatched to Sichuan,
Chongqing, Yunnan, Gansu and Shaanxi regions to prevent dams that were damaged by the
earthquake from bursting and endangering the lives of residents. The massive Three Gorges Dam,
hundreds of kilometres (miles) downriver from the epicenter, was not affected by the earthquake.
The flow of the Jialing River was effectively blocked in Huixian county, in southeastern Gansu's
Longnan region, by landslides triggered by the devastating earthquake. Rubble created a 6-meter
tall, 30-meter wide and 100-meter long dam, holding back 600,000 cubic meters of water.
Huixian was the site of a collapsed tunnel, which caused a train carrying gasoline to catch fire
along the Chengdu-Baoji line. Reporters from Chengdu said that they saw cracks on walls of
some residential buildings in the downtown areas, but no building collapsed. Many Beijing office
towers were evacuated, including the building housing the media offices for the organizers of the
2008 Summer Olympics. None of the Olympic venues were damaged. However, a cargo train
carrying 13 petrol tanks derailed in Huixian County, Gansu Province, and caught on fire after the
rail was distorted.
STRESS VARIATIONS & VERTICAL DEFORMATIONS
Thanks to the finite fault model provided by the USGS the author computed the Stress variations
(Figure 3) and the vertical deformation at the surface (Figure 4) promoted by the fault rupture.
The equations of Okada (1992), which describe the deformation in a homogeneous elastic half-

5. space due to a rectangular dislocation, and Hooke's law were used to compute the stress tensor.
The Coulomb Failure Stress variation (DCFS) was calculated on fault planes equal to the one of
the main shock, at a depth of 10 km, that could rupture in the future and is defined as DCFS=T-
m'(Sn), where T and Sn are the shear and normal stresses acting on the dislocation respectively,
and m' is the assumed apparent friction coefficient of the fault plane (in this case 0.5).
Figure 4: Stress variations promoted by the fault rupture + aftershocks distribution from the main
shock occurrence until 19/05/2008 13:00 UTC.
The raw gravity record (figure 6) was obtained at the IGN Gravity Laboratory facilities in
Madrid, (40.445 north latitude and 3.71 west longitude, 671 m. M.S.L. height). The sample rate is
one sample every two minutes (test sampling position). The raw gravity data was recorded by
means of the Lacoste & Romberg Graviton EG-1183, which has a sensibility of one microGal
(10-8 ms-2).The maximum peak to peak amplitude registered is 458 microGal. The free
oscillation of the Earth observed in the record is about 58 minutes, corresponding to the 0S2
spheroidal mode.

6. Figure 5: Vertical deformation at the surface + aftershocks distribution from the main shock
occurrence until 19/05/2008 13:00 UTC.
ZIPINGPU DAM
The Zipingpu dam is located 10km east of the earthquake epicenter and after the earthquake of
7.9R, the following failures were recorded:
• Subsidence of the crown in the central part of the dam, of the order of 50cm in relation to
the side survey control points.
• Deformation of the lower face of the dam, an area of approximately 1000. m
2
.
• Deviations and deformations of the construction elements throughout the face of the dam
• Widening of construction joints (approximately 15 cm on the upper face)

7. • Extended massive landslides throughout the reservoir
• Landslides on both left and right abutments of the dam causing further damages to
secondary constructions.
After the evaluation of the dam damages, the discharge of the reservoir was ordered through the
emergency spillway in order to minimize the risk of a potential disaster for the nearby towns and
especially Dujiangyan.
Figure-6 View of Zipingpu dam and its secondary constructions from the side of the outlet
channel. The hydroelectricity plant and the emergency spillway can be seen.
Figure-7 Landslide view on the slopes of the reservoir 200 meters from the crest of the dam

8. REFRENCES
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