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Mw 7.9 Wenchuan earthquake, Eastern Sichuan, China on May 12th, 2008 at 06:27 UTC
- An event of magnitude Mw 7.9 occurred on 12/05/2008 at 06:27:59.0 UTC in Wenchuan county, Easter Sichuan, China.
- Report the main shock
- Send us pictures or videos of the main shock effects
- China Earthquake Administration (in Chinese)
- USGS/NEIC Denver, USA
- Preliminary finite fault model by KazutoHikima, Earthquake Research Institute, Univ. of Tokyo
- Slip distribution by Agalos A., University of Athens, Faculty of Geology
Distances to cities:
94 km NW Chengdu (pop 3,950,437 ; local time 14:27 2008-05-12)
39 km W Guangkou (pop 65,379 ; local time 14:27 2008-05-12)
Main shockEpicenter location(click to enlarge) |
Moment tensors solutions(click to enlarge) |
| More information on the main shock |
Since the main shock occurrence and until 23/05/2008 07:00 UTC, 110 aftershocks of magnitude larger than 4.5 have been recorded. All have a magnitude lower than 6.0, 39 of them have a magnitude larger than 5.0 (Figure 1 and 3).
The aftershocks are distributed along an approx. 300 km long North-East direction which is consistent with the mechanisms reported by Harvard and the USGS as well as with the regional topography (Figure 1 and 3).
As the main shock epicentre is located at the South-West end of the aftershock distribution, the main shock rupture propagated towards the North-East. Moreover, since May 15th the seismicity seems to migrate Westward.
The regional seismicity over the last 2 years before the main shock does not show any significant earthquake with 2 events of magnitude lower than 5.2 (Figure 2).
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Aftershocks distribution |
Regional seismicity before the main shock |
| Figure 1: Aftershocks distribution | Figure 2: Regional seismicity in the 2 last year before the main shock |
| Figure 3: Aftershocks distribution since main shock occurrence until 10/06/2008 07:00. The purple circle represents the main shock. |
Coulomb Failure Stress variation & Vertical displacement
by José A. Álvarez Gómez (Instituto Geografico Nacional, Madrid, Spain)
Thanks to the finite fault model provided by the USGS (http://earthquake.usgs.gov/eqcenter/eqinthenews/2008/us2008ryan/finite_fault.php), the author computed the Stress variations (Figure 3) and the vertical deformation at the surface (Figure 4%29 promoted by the fault rupture. The equations of Okada (1992), which describe the deformation in a homogeneous elastic half-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). |
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| Figure 4: Stress variations promoted by the fault rupture + aftershocks distribution from the main shock occurrence until 19/05/2008 13:00 UTC. | Figure 5: Vertical deformation at the surface + aftershocks distribution from the main shock occurrence until 19/05/2008 13:00 UTC. |
Gravity record of the Madrid gravimeter
by E. Rodriguez Pujol (Instituto Geografico Nacional, Madrid, Spain)
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. |
| Figure 6: Gravity record displayed trough "TSoft" software (Royal Observatory of Belgium). |
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