Hello Amar,

The experimental works of Mandl, Logan, and their respective co-workers (among other groups) have shown that as shear strain increases in a shear zone, R shears progressively become more and more parallel to the shear zone boundary, ultimately becoming parallel to the Y shears, which eventually take up most of the displacement, together with P shears.

The reason for this may be a progressive rotation from a progessive rotation of the principal stresses within the shear zone as the differential stress increases. Another important factor may be the rigidity of the fault blocks, which restrict the amount of displacement that can be accommodated through R-shear extension.

That may be the reason you don't find any R shears, specially if strain is localised within a narrow zone, which would increase the shear strain for a given displacement.

If you'll allow me, I'd reccommend you specially the following reference, where the development of these structures is widely discussed. I am sorry I can't provide any natural examples.

Logan, J. M., Dengo, C. A., Higgs, N. G., & Wang, Z. Z. (1992). Fabrics of experimental fault zones: their development and relationships to mechanical behavior. Fault Mechanics and Transport Properties of Rocks. https://doi.org/http://dx.doi.org/10.1016/S0074-6142(08)62814-4

Good luck with your research!

Kind regards,

Manuel

Hello, I agree with the comment posted by Manuel. As experimental models and earthquake surface ruptures show, R shears are sort of precursors of fault propagation through a poorly/non-deformed cover( i.e. a Quaternary layer). Depending on the material, they tend to dissapear or to blend with more complexes anastomosed patterns.

So, perhaps adding info on how much cumulated strain encompasses the central Mexico fault (age of deformed layers, fault activity rate etc) could help to explain the mentioned features.

Best regards

c.

Some landslides arise within pre-existing fault plans. Therefore, these faults are not necessarily active.

Amar Agarwal and colleagues.

Let me share two alternative explanation, perhaps at local scale instead regional ones, as I understand your question put focus.

1) The influence that shear waves are sensitive to fluid type in fractured media. Unlike matrix porosity where gas lowers the P-velocity, the introduction of gas in fractures decreases the shear velocity while the P-wave remains largely unaffected. This conclusion has direct implications regarding the exploration for fractured reservoirs and suggests that shear data, rather than the more conventional P ones, may be used as direct hydrocarbon indicators.

2) The influence of velocity anisotropy; this condition is consistent with, but not required by, the existence of fractures in the basement rock having orientations subparallel to the latest episode of local faulting.

Both properties are used in seismic elastic inversion research to detect fractures zones, not only strike-slip, but also fractures and fault zones in general.

You can read about this kind of phenomena:

Seismic Shear Wave Anisotropy Used to Locate Fracture Zones in Limestone. Benjamin H. Richard, Stephanie A. Clasen, and Paul J. Wolfe. Symposium on the Application of Geophysics to Engineering and Environmental Problems 1991 Sep 30, 2008. https://doi.org/10.4133/1.2921927.

Azimuthal response and shear wave splitting of 9-component shear waves for the fracture zones in a tight sand reservoir. Jiwei Cheng, Feng Zhang, Peng Wang, Xiang Yang Li, and Lin Wang. SEG Technical Program Expanded Abstracts 2019 Aug 10, 2019. https://doi.org/10.1190/segam2019-3206101.1.

Anisotropy and fracture zones about a geothermal well from P‐wave velocity profiles. P. C. Leary and T. L. Henyey. GEOPHYSICS article Volume 50, Issue 1 Jan 1, 1985. https://doi.org/10.1190/1.1441833.

A physical model study of shear‐wave splitting and fracture intensity. Authors: Robert H. Tatham, Martin D. Matthews, K. K. Sekharan, Christopher J. Wade, and Louis M. Liro. GEOPHYSICS Volume 57, Issue 4. https://doi.org/10.1190/1.1443278

Best regards, Mario E. Sigismondi