Earthquake Science

, Volume 22, Issue 6, pp 589–594 | Cite as

Depth and region dependence of b-value for micro-aftershocks of the May 12th, 2008 Wenchuan earthquake and its tectonic implications

  • Mei FengEmail author
  • Meijian An
  • Lin Zhao
  • Jizhong Zhang


Micro-aftershocks with magnitude range of 1.5–4 around the Wenchuan earthquake epicenter, the southern part of the Longmenshan fault zone, exhibit good frequency-magnitude linear relationships, thus enabling b-value analysis. The average b-value for micro-aftershocks of M1.5–4 from July to December of 2008 in our local study region is about 0.88, similar to the b-value for all aftershocks of M3.0–5.5 from May, 2008 to May, 2009 along the whole Longmenshan fault zone. The similarity between the local and regional b-values possibly indicates that the southern part of the Longmenshan fault zone has similar seismogenic environment to the whole Longmenshan fault zone. Alternatively, it may also imply that b-values derived from all events without consideration of structural variation can not discriminate local-scale tectonic information. The present study shows that the b-value for the Wenchuan earthquake micro-aftershocks varies with different regions. The b-value in southwest of the Yingxiu town is higher than that in the northeast of the Yingxiu town. The high b-value in the southwest part where the Wenchuan earthquake main shock hypocenter located indicates that the current stress around the hypocenter region is much lower than its surrounding area. The b-values are also dependent on depth. At shallow depths of <5 km, the b-values are very small (∼0.4), possibly being related to strong wave attenuation or strong heterogeneity in shallow layers with high content of porosity and fractures. At depths of ∼5–11 km, where most aftershocks concentrated, the b-values become as high as ∼0.9–1.0. At the depth below ∼11 km, the b-values decrease with the depth increasing, being consistent with increasing tectonic homogeneity and increasing stress with depth.

Key words

micro-aftershocks frequency-magnitude relationship tectonic implication Wenchuan earthquake 

CLC number



  1. An M, Feng M, Dong S, Long C, Zhao Y, Yang N, Zhao W and Zhang J (2009a). Seismogenic structure around the epicenter of the May 12, 2008 Wenchuan earthquake from micro-seismic tomography. Acta Geologica Sinica (English Edition) 83(4): ): 724–732.CrossRefGoogle Scholar
  2. An M, Feng M and Long C (2009b). Deep ruptures around the hypocenter of the 2008 Wenchuan earthquake implied from aftershock observations. Tectonophysics (submitted).Google Scholar
  3. Chen X Z, Lu X J and Wang H M (2001). b value of the seismically active and quiescent periods and research of seismicty tendency in China’s continent. Earthquake21(1): 59–62 (in Chinese with English abstract).Google Scholar
  4. Christensen N I (1996). Poisson’s ratio and crustal seismology. J Geophys Res101(B2): 3 139–3 156.CrossRefGoogle Scholar
  5. Gutenberg B and Richter C F (1944). Frequency of earthquakes in California. Bull Seism Soc Amer34(4): 185–188.Google Scholar
  6. Hu X M (2007). Seismicity background around Zipingpu Dam before impounding. Earthquake Research in Sichuan (123): 18–23 (in Chinese with English abstract).Google Scholar
  7. Lee W H K and Lahr J C (1972). HYPO71: A computer program for determining hypocenter, magnitude, and first motion pattern of local earthquakes. U.S. Geol Surv Open-File Rep, 100.Google Scholar
  8. Mori J and Abercrombie R E (1997). Depth dependence of earthquake frequency-magnitude distributions in California: Implications for rupture initiation. J Geophys Res102(B7): 15 081–15 090.CrossRefGoogle Scholar
  9. Richter C F (1958). Elementry Seismology. W H Freeman & Co, San Francisco, California, 768.Google Scholar
  10. Scholz C H (1968). The frequency-magnitude relation of microfracturing in rock and its relation to earthquakes. Bull Seism Soc Amer58(1): 399–415.Google Scholar
  11. Scholz C H (2002). The Mechanics of Earthquakes and Faulting: 2nd edition. Cambridge University Press, New York, 496.CrossRefGoogle Scholar
  12. Tsapanos T M (1990). b-values of two tectonic parts in the circum-pacific belt. Pure Appl Geophys134(2): 229–242.CrossRefGoogle Scholar
  13. Turcotte D L and Schubert G (2001). Geodynamics: 2nd edition. Cambridge University Press, Cambridge, UK, 528.Google Scholar
  14. Vakov A V (1996). Relationships between earthquake magnitude, source geometry and slip mechanism. Tectonophysics261(1–3): 97–113.CrossRefGoogle Scholar
  15. Wells D L and Coppersmith K J (1994). New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement. Bull Seism Soc Amer84(4): 974–1 002.Google Scholar

Copyright information

© Seismological Society of China and Springer Berlin Heidelberg 2009

Authors and Affiliations

  1. 1.Institute of GeomechanicsChinese Academy of Geological SciencesBeijingChina

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