EGU General Assembly 2020
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the Creative Commons Attribution 4.0 License.

What controls b-value variations: insights from a physics based numerical model

Pierre Dublanchet
Pierre Dublanchet
  • PSL Research University, MINES ParisTech, Geosciences, France (

The magnitudes of earthquakes are known to follow a power-law distribution, where the frequency of earthquake occurrence decreases with the magnitude. This decay is usually characterized by the power exponent, the so-called b-value. Typical observations report b-values in the range 0.5-2. The origin of b-value variations is however still debated. Seismological observations of natural seismicity indicate a dependence of the b-value with depth, and with faulting style, which could be interpreted as a signature of a stress dependence. Within creeping regions of major tectonic faults, the b-value of microseismicity increases with creep rate. Stress dependent b-value of acoustic emissions is also commonly reported during rock failure experiments in the laboratory. Natural and laboratory observations all support a decrease of b-value with increasing differential stress. I report here on the origin of b-value variations obtained in a fault model consisting in a planar 2D rate-and-state frictional fault embedded between 3D elastic slabs. This model assumes heterogeneous frictional properties in the form of overlapping asperities with size-dependent critical slip distance distributed on a creeping segment. This allows to get complex sequences of earthquakes characterized by realistic b-values. The role of frictional heterogeneity, normal stress, shear stress, and creep rate on the b-value variations is systematically explored. It is shown that the size distribution of asperities is not the only feature controlling the b-value, which indicates an important contribution from partial ruptures, and cascading events. In this model cascades of events (and thus b-value) is strongly influenced by frictional heterogeneity and normal stress through fracture energy distribution. If the decrease of b-value with differential stress is reproduced in these simulations, it is also shown that part of the b-value fluctuations could be attributed to changes of nucleation length and stress drop with normal stress. A slight increase of b-value with slip rate exists but remains an order of magnitude smaller than the observations.

How to cite: Dublanchet, P.: What controls b-value variations: insights from a physics based numerical model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11079,, 2020

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Presentation version 1 – uploaded on 03 May 2020
  • CC1: Comment on EGU2020-11079, Paul Selvadurai, 03 May 2020

    Can you comment on how 'secondary' off-fault seismicity could affect the b-value estimates? Could your model be extended to also model complex near-fault (possibly) oriented, secondary structures? How would affect your results?  

    • AC1: Reply to CC1, Pierre Dublanchet, 04 May 2020

      Hi Paul. The off fault seismicity issue regarding b value estimation in real datasets is commented in Spada et al., GRL 2013. The model I am using is not able to study the case of multiple faults with different orientations (only one planar, possibly heterogeneous fault). I agree b values in such fault networks need to be investigated.

      • CC3: Reply to AC1, Paul Selvadurai, 04 May 2020

        Thank you for the answer, Pierre. 

        Very nice study and slides, good luck with the submission.

  • CC2: Comment on EGU2020-11079, Giuseppe Petrillo, 04 May 2020

    Good morning and congratulations for the presentation. I would like, if possible, to have more information on how interact the planar 2D frictional fault and the 3D elastic slab. In particular, how are they arranged geometrically? I don't think I understand geometry in general. Thank you in advance!

    • AC2: Reply to CC2, Pierre Dublanchet, 04 May 2020

      Good morning, and thank you for your comment. The model consists of two elastic plates (that I call slabs) with a finite thickness (between z=+/- H) and in contact in the (x,y) plane (frictional interface). There is a schematic diagram of the model in the slides I uploaded. I hope this answers your question.