1 Effects of internal structure and local stresses on 1 fracture propagation, deflection, and arrest in fault zones 2 3 Agust Gudmundsson a , Trine H Simmenes b , Belinda Larsen b , Sonja. L. Philipp c 4 5 a Department of Earth Sciences, Royal Holloway, University of London, 6 Egham TW20 0EX, UK (a.gudmundsson@gl.rhul.ac.uk) 7 8 b Department of Earth Science, University of Bergen, Norway 9 10 c Geoscience Centre, University of Göttingen, Germany 11 12 Abstract 13 14 The way that faults transport crustal fluids is important in many fields of earth sciences such 15 as petroleum geology, geothermal research, volcanology, seismology, and hydrogeology. For 16 understanding the permeability evolution and maintenance in a fault zone, its internal 17 structure and associated local stresses and mechanical properties much be known. This 18 follows because the permeability is primarily related to fracture propagation and their linking 19 up into interconnected clusters in the fault zone. Here we show that a fault zone can be 20 regarded as an elastic inclusion with mechanical properties that differ from those of the host 21 rock. As a consequence, the fault zone develops its own local stresses which differ from the 22 associated regional stresses. The local stresses, together with fault-rock heterogeneities and 23 interfaces (discontinuities; fractures, contacts), determine fracture propagation, deflection 24 (along discontinuities/interfaces), and arrest in the fault zone and, thereby, its permeability 25 development. We provide new data on the internal structure of fault zones, in particular the 26 fracture frequency in the damage zone as a function of distance from the fault core. New 27 numerical models show that the local stress field inside a fault zone, modelled as an 28 inclusion, differ significantly from those of the host rock, both as regards the magnitude and 29 the directions of the principal stresses. Also, when the mechanical layering of the damage 30 zone, due to its variation in fracture frequency, is considered, the numerical models show 31 Manuscript Click here to view linked References