An experimental approach to test parameters controlling the fault structure for large continental strike-slip fault | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS

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  An experimental approach to test parameters controlling the fault structure for large continental strike-slip fault

A/ Mapview of surface rupture for a continental earthquake (Landers, California, 1992, Mw=7.2). The surface rupture is not linear, but is formed of a succession of fault segments. B/ Sketch of the fault growth for a strike-slip fault. Initially Riedel shears, oblique to the basal shear direction, appear at the base of the brittle crust (15 to 20 km), that later grow upward. In a second stage, the deformation coalesces into a localized fault zone that cross cuts the Riedel shears. C/ Mapview of a sand-box experiment where one can see the trace of the Riedel shears cross cut by the latter shear zone. The initial Riedel shears continue to influence the geometry of the strike-slip fault, despite the fact that they do not accommodate any more deformation.

Seismic hazard assessment for large continental strike-slip faults (vertical faults allowing for tectonic plates to move horizontally past each others) such as the San Andreas fault in USA, or the Dead Sea fault in Middle East, involves combining past and present seismic activity along that fault, rate of tectonic loading and the geometry of the fault. These data are used to build synthetic catalogues over long periods that are then used to consider and weight different scenarios in hazard assessment probabilistic models.

 

In this framework, beside uncertainties related to measurements, both the seismic activity and the tectonic loading rate are solid observation with only limited ground for discussion. Conversely, the fault geometry, and more specifically the way strike-slip faults are laterally structured, is still a matter of fierce debates. Following different approaches that would consider either continuous faults or segmented faults, associated to a set of rules defining how a seismic rupture propagates from one segment to the next one, seismic scenario might vary drastically with strong implication for hazard models.

 

A research group from the IPGP, CY Cergy Paris Université and Sorbone Université, has focused on the fault segmentation issue, aiming at testing if one could evidence intrinsic parameters controlling lateral strike-slip fault geometry. Thus, they have conducted a series of analogue sand-box experiments to reproduce shear at the base of the brittle crust. One of the properties of the sand is that it is allowing for localization of deformation, behaving as a brittle material while it accommodates deformation, although it does not store elastic energy. During experiments, photos have been acquired at regular time steps to image the top surface of the experiments. Then, using the image correlation tool MicMac, it has been possible to measure details of the surface deformation, including the formation of fault segments.

 

Shear experiment in sand box. Here surface deformation is measured by image correlation of successive photos. The three illustrations show different stages of deformation localization. 1/ Riedel shears oblique to the direction of the basal shear. The successive Riedel shears are disconnected ones from each other. 2/ When the cumulative deformation start to be large (indicated in the upper left corner of each image), the deformation localizes along a strike-slip fault zone that takes over the accommodation of the deformation. Riedel shear are progressively abandoned and dismantled by the strike-slip fault. 3/ The deformation is only accommodated by the strike-slip fault. The Riedel shears do not accommodate any deformation. However, the geometry of the strike-slip fault is still controlled by the initial spatial distribution of the Riedel shear. Those experiments show how the initial pattern of deformation associated will Riedel shears keep influencing the final geometry of the strike-slip fault, although the Riedel shear are not accommodating deformation any more.

Several parameters have been systematically tested during these experiments, such as the internal friction of the material (which type of sand and how sand is initially packed in the box), basal friction (how does the sand interact with material at the bottom of the experimental setting), and thickness of the sand pack. This last parameter appears to be critical in controlling the size of the segments, with a ratio between the thickness of the sand and the length of the successive segments, close to 1. The other parameters have only second order impact on the segment geometry.

 

The comparison between experimental results and data derived from actual observations of surface ruptures for large continental earthquakes shows that a similar value of ~1 can be derived from the ratio between the length of rupture segments, as observed in the field, and the thickness of the brittle crust. Hence, it suggests that both in the experimental and in nature the physical processes involved are the same.

 

Thus, this work shows that for continental strike-slip faults, fault segmentation exists that spatial scale is controlled by the thickness of the brittle crust, in a similar fashion to the effect of the sand thickness in the experiments. Because the thickness of the brittle crust does not vary significantly in continents, it explains why the length of rupture segments, as observed during large earthquakes, is always the same at first order in any continental setting. This result allows reconsidering the concept of fault segmentation, which was mostly limited to a descriptive perspective so far, into the framework of physical processes involved during earthquake ruptures. It should lead to a better integration of the fault geometry in the seismic hazard models, with an emphasis on specific spatial scales that are relevant for earthquake rupture propagation.

 

More information :

M. Lefevre, P. Souloumiac, N. Cubas, Y. Klinger; Experimental evidence for crustal control over seismic fault segmentation. (2020) Geology ; 48 (8): 844–848. doi: 10.1130/G47115.1

 

Date de publication : 
17 September 2020