Ultra-thin clay layers facilitate seismic slip in carbonate faults

Luca Smeraglia,Andrea Billi,Elena Spagnuolo,Giulio Di Toro,Federico Zorzi,Eugenio Carminati,Andrea Cavallo

doi:10.1038/s41598-017-00717-4

Abstract

Many earthquakes propagate up to the Earth’s surface producing surface ruptures. Seismic slip propagation is facilitated by along-fault low dynamic frictional resistance, which is controlled by a number of physico-chemical lubrication mechanisms. In particular, rotary shear experiments conducted at seismic slip rates (1 ms−1) show that phyllosilicates can facilitate co-seismic slip along faults during earthquakes. This evidence is crucial for hazard assessment along oceanic subduction zones, where pelagic clays participate in seismic slip propagation. Conversely, the reason why, in continental domains, co-seismic slip along faults can propagate up to the Earth’s surface is still poorly understood. We document the occurrence of micrometer-thick phyllosilicate-bearing layers along a carbonate-hosted seismogenic extensional fault in the central Apennines, Italy. Using friction experiments, we demonstrate that, at seismic slip rates (1 ms−1), similar calcite gouges with pre-existing phyllosilicate-bearing (clay content ≤3 wt.%) micro-layers weaken faster than calcite gouges or mixed calcite-phyllosilicate gouges. We thus propose that, within calcite gouge, ultra-low clay content (≤3 wt.%) localized along micrometer-thick layers can facilitate seismic slip propagation during earthquakes in continental domains, possibly enhancing surface displacement.

Highlights

Many earthquakes propagate up to the Earth’s surface producing surface ruptures, possibly associated with tsunami generation, infrastructure damage, and fatalities[1,2,3,4]
X-ray Powder diffraction (XRPD) and energy dispersive spectroscopy (EDS) analyses revealed that the cataclasite consists solely of calcite, whereas the ultracataclasite consists of calcite, phyllosilicates (~1.5 wt.%; mainly smectite, illite, and kaolinite; see Supplementary Fig. S1 and Supplementary Table S2), detrital micas, quartz, K-feldspar, and plagioclase (
We conclude that calcite gouges with an illite/smectite content as low as 3% localized along pre-existing micrometer layers (Fig. 3a) weaken faster than calcite and mixed calcite-phyllosilicate gouges (Fig. 3c,d, and e) at seismic slip rates (1 ms−1) both in dry and in wet conditions

Summary

Introduction

Many earthquakes propagate up to the Earth’s surface producing surface ruptures, possibly associated with tsunami generation, infrastructure damage, and fatalities[1,2,3,4]. Experiments have revealed the weakening behavior of phyllosilicates, especially in water-saturated conditions, due to their low steady-state dynamic friction and to the absence of peak friction[19,20,21,22,23] For this reason, previous studies[24,25,26,27] suggested that pelagic clay commonly present along offshore subduction zones participate to co-seismic fault lubrication, possibly facilitating and enhancing upward co-seismic slip propagation, sometimes associated with tsunamigenic seafloor displacement. During post-orogenic extension (Pliocene to present time), the fold-thrust belt has been dissected by a system of extensional faults, which bound several intramountain basins filled by Plio-Quaternary continental deposits[39, 40] These faults have generated historical and instrumental seismicity up to Mw 7.041 (Fig. 1a; e.g., Avezzano, 1915, Mw 7.0 earthquake; L’Aquila, 2009, Mw 6.1 earthquake; Amatrice-Norcia, 2016, Mw 6.0 and 6.5 earthquakes). Geological and mechanical studies of faults exhumed from shallow depths, such as the Tre Monti Fault, are significant as the small-scale compositional heterogeneity of shallow fault zones can promote or inhibit seismic slip propagation up to the Earth’s surface

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Conclusion