The Shear Deformation Zone and the Smoothing of Faults With Displacement Clement Perrin, Felix Waldhauser, Christopher Scholz To cite this version: Clement Perrin, Felix Waldhauser, Christopher Scholz. The Shear Deformation Zone and the Smooth- ing of Faults With Displacement. Journal of Geophysical Research : Solid Earth, American Geophys- ical Union, 2021, 126 (5), pp.e2020JB020447. 10.1029/2020JB020447. hal-03284736 HAL Id: hal-03284736 https://hal.archives-ouvertes.fr/hal-03284736 Submitted on 12 Jul 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. 1 The Shear Deformation Zone and the Smoothing of Faults with Displacement 2 3 Clément Perrin1,2,*, Felix Waldhauser1 and Christopher H. Scholz1 4 5 1 Lamont Doherty Earth Observatory at Columbia University, New York, USA 6 2 Present address : Laboratoire de Planétologie et Géodynamique, UMR6112, 7 Observatoire des Sciences de l’Univers de Nantes Atlantique, UMS3281, Université de 8 Nantes, Université d’Angers, CNRS, 2 rue de la Houssinière - BP 92208, 44322 Nantes 9 Cedex 3, France 10 * Corresponding author: [email protected] 11 12 Keypoints: 13 Across strike distributions of aftershocks of large earthquakes describe the width of the 14 shear deformation zone around large faults. 15 16 The zone of active shear deformation scales with fault roughness and narrows as a 17 power law with fault displacement. 18 19 Earthquake stress drops decrease with fault displacement and hence fault roughness or 20 displacement rate. 21 22 Keywords: 23 Fault maturity; shear deformation zone; large earthquakes; aftershock distributions; 24 scaling laws. 25 1 26 Plain Language Summary 27 Active fault zones worldwide are 3D features made of a parent fault and secondary 28 faults and fractures that damaged the surrounding medium. During and soon after a 29 large earthquake, these structures are reactivated, highlighted by numerous smaller 30 events –aftershocks. Their distribution allows us to characterize the zone of shear 31 deformation around the fault plane. In this study, we show that the width of the shear 32 deformation zone is narrower around mature faults than around immature faults. It 33 decreases as a power law with cumulative fault displacement which we infer to be the 34 result of the smoothing of the fault with wear through geological times. We also find that 35 the stress-drop of mainshocks decrease with fault smoothness or slip rate, both of which 36 correlate with maturity. Our study provides some relations to better understand and 37 anticipate the size of off-fault deformation reactivated during and after an earthquake, 38 based on geological fault parameters. 39 40 Abstract 41 We use high-resolution earthquake locations to characterize the three-dimensional 42 structure of active faults in California and how it evolves with fault structural maturity. 43 We investigate the distribution of aftershocks of several recent large earthquakes that 44 occurred on continental strike slip faults of various structural maturity (i.e., various 45 cumulative fault displacement, length, initiation age and slip rate). Aftershocks define a 46 tabular zone of shear deformation surrounding the mainshock rupture plane. 47 Comparing this to geological observations, we conclude that this results from the re- 48 activation of secondary faults. We observe a rapid fall off of the number of aftershocks at 49 a distance range of 0.06–0.22 km from the main fault surface of mature faults, and 0.6-1 50 km from the fault surface of immature faults. The total width of the active shear 2 51 deformation zone surrounding the main fault plane reaches 1-2.5 km and 6-9 km for 52 mature and immature faults, respectively. We find that the width of the shear 53 deformation zone decreases as a power law with cumulative fault displacement. 54 Comparing with a dynamic rough fault model, we infer that the narrowing of the shear 55 deformation zone agrees quantitatively with earlier estimates of the smoothing of faults 56 with displacement, both of which are aspects of fault wear. We find that earthquake 57 stress drop decreases with fault displacement and hence with increased smoothness 58 and/or slip rate. This may result from fault healing or the effect of roughness on friction. 59 60 1. Introduction 61 A fault zone is a complex brittle-frictional system that wears as slip occurs on it. It is 62 formed of three main features, that evolve with fault growth (Fig. 1): (i) the cataclastic 63 core contains the cataclastic detritus of wear of the slipping surfaces of the fault. Its 64 width (WC in Fig. 1) increases linearly with fault displacement at a rate that depends on 65 the strength of the wall rock (Scholz, 1987, 2019, pp 132). For displacements greater 66 than a few hundred meters, growth of the fault core levels off at a thickness of a few tens 67 of meters (Scholz, 2019, pp 132); (ii) Beyond the fault core lies a region of pervasive 68 tensile fracturing which defines the “dilatant damage zone” (WD, Fig. 1; e.g. Faulkner et 69 al., 2011; Savage & Brodsky, 2011; Vermilye & Scholz, 1998). The fracture density in this 70 zone dies off as a power law with distance from the fault (e.g., Ostermeijer et al., 2020 71 and references therein). The dilatant damage zone width increases linearly with fault 72 displacement, and typically levels out at several hundred meters for fault displacements 73 exceeding several hundred meters (Savage & Brodsky, 2011); (iii) Including and 74 extending beyond the dilatant damage zone is what we call the “shear deformation 75 zone” (WS; Fig. 1) which is defined by a region of enhanced seismicity, first pointed out 3 76 by Powers & Jordan (2010). This zone shows a region of high seismic activity near the 77 fault with a power law fall-off beyond a corner at WS1 to a full half-width of WS2 (Fig. 1). 78 79 The above definitions allow us to distinguish two types of damage zones: the “dilatant 80 damage zone” dominated by volumetric strains, and the “shear deformation zone” 81 dominated by shear strains. The tensile (Mode I) cracks in the dilatant damage zone are 82 dilatant cracks that align parallel to the maximum compression direction and 83 perpendicular to the minimum principal stress. Hence the orientation of cracks provides 84 evidence for the several different mechanisms responsible for their formation (Wilson et 85 al., 2003). The shear deformation zone is characterized by secondary faults (Mode II and 86 III cracks) and hence are oriented parallel to the maximum Coulomb stress. For example, 87 in the case of a strike-slip fault, this zone is defined by a conjugate set of secondary faults 88 (Little, 1995), in which one set is parallel to the primary fault. 89 90 The evolution of these three zones in figure 1 defines what is called fault maturity. The 91 three zones can be viewed as regions controlled by wear processes, and the fault 92 structural maturity can hence be measured by its degree of wear, which depends 93 primarily on the net fault displacement, assuming a constant slip-vector throughout the 94 fault’s history. However, previous studies have shown that, in the absence of data on net 95 fault displacement, several other fault parameters such as the fault initiation age and the 96 geological slip rate can be also used as a proxy of net displacement in evaluating the 97 overall maturity of the fault (e.g. Choy et al., 2006; Choy & Kirby, 2004; Dolan & 98 Haravitch, 2014; Hecker et al., 2010; Ikari et al., 2011; Manighetti et al., 2007; Niemeijer 99 et al., 2010; Perrin, Manighetti, Ampuero, et al., 2016; Stirling et al., 1996; Wesnousky, 100 1988). As these parameters increase, the fault grows and becomes more “mature”. Prior 4 101 studies have suggested that the structural maturity may have a strong impact on 102 earthquake behavior, such as magnitude, stress drop, distribution of slip, rupture 103 velocity, ground motion amplitude, and number of ruptured segments (e.g., Cao & Aki, 104 1986; Dolan & Haravitch, 2014; Hecker et al., 2010; Malagnini et al., 2010; Manighetti et 105 al., 2007; Perrin, Manighetti, Ampuero, et al., 2016; Radiguet et al., 2009; Stirling et al., 106 1996; Wesnousky, 1988). 107 108 The widths of the fault core and dilatant damage zones saturate at fault lengths 109 comparable to the seismogenic thickness, due to the switch from crack like to pulse like 110 earthquake propagation (Ampuero & Mao, 2017). For larger faults, the evolution of fault 111 maturity involves only changes in the shear deformation zone. In this paper we are 112 concerned with the scaling of large faults (i.e., which breach the brittle seismogenic 113 width) and their associated large earthquakes, so we are only concerned with the shear 114 deformation zone. There is evidence that indicates that large faults become smoother 115 with net displacement (Stirling et al., 1996; Wesnousky, 1988). This smoothing is 116 probably the prime attribute of fault maturity. Here we show that the width of the shear 117 deformation zone of large faults decreases with fault displacement, as a consequence of 118 this smoothing. 119 120 Precise earthquake locations can be used to image the internal structure of fault and the 121 zone of brittle deformation, often at a resolution similar to field observations (e.g.