The April 20, 2013, Lushan, , , earthquake: seismogenic model by multidisciplinary ‘evidence chain’

Zhongliang Wu, and the Team of the Field Investigation of the Lushan Earthquake Institute of Geophysics, China Earthquake Administration, 100081 Beijing, China [email protected] • The April 20, 2013, Lushan, Sichuan, China, MS7.0 earthquake is one of the significant earthquakes in China in the 21st century. • The seismo-tectonic picture of this earthquake is clear at the scales of lithosphere plate and crustal block, but remains puzzle at the scale of seismogenic faults.

• Chen, L.C., Wang, H., Ran, Y.K., et al., 2014, The 2013 Lushan Ms7.0 earthquake: varied seismogenic structure from the 2008 Wenchuan earthquake. Seismol. Res. Lett, 85(1): 34-39, doi: 10.1785/0220130109. • Dai, J. W., Qu, Z., Zhang, C. X., et al., 2013. Preliminary investigation of seismic damage to two steel space structures during the 2013 Lushan earthquake. Earthquake Engineering and Engineering Vibration, 12(3): 495-500. • Fang, L. H., Wu, J. P., Wang, W. L., et al., 2013. Relocation of the mainshock and aftershock sequences of Ms7.0 Sichuan Lushan earthquake. Chin. Sci. Bull., 58: 3451- 3459. • Jia, K., Zhou, S. Y., Zhuang, J. C., et al., 2014. Possibility of the independence between the 2013 Lushan earthquake and the 2008 Wenchuan earthquake on Longmen Shan fault, Sichuan, China, Seismol. Res. Lett., 85(1): 60-67. • Liu, M., Luo, G. and Wang, H., 2014. The 2013 Lushan Earthquake in China Tests Hazard Assessments. Seismol. Res. Lett., 85(1): 40-43. • Zhang, Y., Wang, R. J., Chen, Y. T., et al., 2014. Kinetmatic ruture model and hypocenter relocation of the 2013 Mw6.6 Lushan earthquake constrained by strong-motion and teleseismic data. Seismol. Res. Lett., 85(1): 15-22.

• Wu, Z. L., Jiang, C. S., Li, X. J., et al., 2014. Earthquake Phenomenology from the Field: the April 20, 2013, Lushan Earthquake. Singapore: Springer. Wenchuan-Maowen Fault

Beichuan-Yingxiu Fault

Jiangyou-Guanxian (Shuangshi-Dachuan) Fault

Dayi Fault • In the field, there was no surface fault observed. This conclusion is based on the ‘evidence chain’ from systematic geological survey which includes fault mapping, trench detection, and shallow profiles of seismic sounding, to the inversion of seismic and strong ground motion data for the slip distribution, and further to the inversion of ground deformation for the earthquake fault model, as well as the distribution of aftershocks by double-difference (DD) location. • In the field, there was no surface fault observed. This conclusion is based on the ‘evidence chain’ from systematic geological survey which includes fault mapping, trench detection, and shallow profiles of seismic sounding, to the inversion of seismic and strong ground motion data for the slip distribution, and further to the inversion of ground deformation for the earthquake fault model, as well as the distribution of aftershocks by double-difference (DD) location. • In the field, there was no surface fault observed. This conclusion is based on the ‘evidence chain’ from systematic geological survey which includes fault mapping, trench detection, and shallow profiles of seismic sounding, to the inversion of seismic and strong ground motion data for the slip distribution, and further to the inversion of ground deformation for the earthquake fault model, as well as the distribution of aftershocks by double-difference (DD) location. • In the field, there was no surface fault observed. This conclusion is based on the ‘evidence chain’ from systematic geological survey which includes fault mapping, trench detection, and shallow profiles of seismic sounding, to the inversion of seismic and strong ground motion data for the slip distribution, and further to the inversion of ground deformation for the earthquake fault model, as well as the distribution of aftershocks by double-difference (DD) location. • In the field, there was no surface fault observed. This conclusion is based on the ‘evidence chain’ from systematic geological survey which includes fault mapping, trench detection, and shallow profiles of seismic sounding, to the inversion of seismic and strong ground motion data for the slip distribution, and further to the inversion of ground deformation for the earthquake fault model, as well as the distribution of aftershocks by double-difference (DD) location. • In the field, there was no surface fault observed. This conclusion is based on the ‘evidence chain’ from systematic geological survey which includes fault mapping, trench detection, and shallow profiles of seismic sounding, to the inversion of seismic and strong ground motion data for the slip distribution, and further to the inversion of ground deformation for the earthquake fault model, as well as the distribution of aftershocks by double-difference (DD) location. • In the field, there was no surface fault observed. This conclusion is based on the ‘evidence chain’ from systematic geological survey which includes fault mapping, trench detection, and shallow profiles of seismic sounding, to the inversion of seismic and strong ground motion data for the slip distribution, and further to the inversion of ground deformation for the earthquake fault model, as well as the distribution of aftershocks by double-difference (DD) location.

14 • Associate of the seismic source with a known or unknown fault is another key issue, which needs the accurate location of the earthquake and the precise mapping of the active faults. The ‘evidence chain’ for this associate is from the back-projection of seismic wave field which obtained the accurate location of the mainshock, to reflection seismology which retrieved the seismic velocity profile crossing the epicenter area, further to the pattern of ground deformation and the relocated aftershock distribution pattern. • Associate of the seismic source with a known or unknown fault is another key issue, which needs the accurate location of the earthquake and the precise mapping of the active faults. The ‘evidence chain’ for this associate is from the back-projection of seismic wave field which obtained the accurate location of the mainshock, to reflection seismology which retrieved the seismic velocity profile crossing the epicenter area, further to the pattern of ground deformation and the relocated aftershock distribution pattern. • Associate of the seismic source with a known or unknown fault is another key issue, which needs the accurate location of the earthquake and the precise mapping of the active faults. The ‘evidence chain’ for this associate is from the back-projection of seismic wave field which obtained the accurate location of the mainshock, to reflection seismology which retrieved the seismic velocity profile crossing the epicenter area, further to the pattern of ground deformation and the relocated aftershock distribution pattern.

Wide-angle reflection profile 450km length Inter-station distance 20km 8 shots, 0.8~2.9 tons

Deep reflection profile 50km length Inter-station distance 30m 256 shots

• Associate of the seismic source with a known or unknown fault is another key issue, which needs the accurate location of the earthquake and the precise mapping of the active faults. The ‘evidence chain’ for this associate is from the back-projection of seismic wave field which obtained the accurate location of the mainshock, to reflection seismology which retrieved the seismic velocity profile crossing the epicenter area, further to the pattern of ground deformation and the relocated aftershock distribution pattern. • Associate of the seismic source with a known or unknown fault is another key issue, which needs the accurate location of the earthquake and the precise mapping of the active faults. The ‘evidence chain’ for this associate is from the back-projection of seismic wave field which obtained the accurate location of the mainshock, to reflection seismology which retrieved the seismic velocity profile crossing the epicenter area, further to the pattern of ground deformation and the relocated aftershock distribution pattern. TK TK TK TK J TJ TJ T TJ TJ F6 TJ TT TT TT TT TT TT 1 T T1 T1 T1 F1 T1 F4 T1 TG TG TG TG F2 F3 F7 F5

TC TC

Moho Moho TK TK TK TK J TJ TJ T TJ TJ F6 TJ TT TT TT TT TT TT 1 T T1 T1 T1 F1 T1 F4 T1 TG TG TG TG F2 F3 F7 F5

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Moho Moho • The multi-disciplinary ‘evidence chain’ indicates that the Lushan earthquake was produced by a blind thrust fault to the east of the Dachuan-Shuangshi fault (a branch of the southern ) which was thought to be the seismogenic fault in the early stage of earthquake emergency. Version 1.0 Version 2.0 Version 3.0 Version 4.0 In conclusion

• What is NOT the case: there is no primary surface faults, from the ‘evidence chain’ of geology, seismology, geodesy, … • What might BE the case: a blind thrust fault, from the ‘evidence chain’ of geology, seismology, geodesy, deep seismic sounding, … • What has been done: an overview of the multi-disciplinary investigation… • What has not been done: details of the analysis of each evidence… • Positive aspect: it has been possible to carry out multi-disciplinary investigation after a moderate to major earthquake… • Negative aspect: For the long-term seismic hazard assessment, how to consider that historical/paleo earthquakes which did not leave any ‘footprint’… Thank you

Zhongliang Wu, and the Team of the Field Investigation of the Lushan Earthquake Institute of Geophysics, China Earthquake Administration, 100081 Beijing, China [email protected]