The size distributions of fractures and earthquakes: implications for orogen-internal seismogenic deformation

Abstract. Pre-existing geological discontinuities such as faults and fractures represent structural and mechanical discontinuities in rocks which influence earthquake processes. As earthquakes occur in the subsurface, seismogenic reactivation of pre-existing fracture networks is difficult to investigate in natural settings. However, it is well-known that there exists a physical link between both fractures and earthquakes since an earthquake's magnitude is related to the ruptured fracture area and therefore fracture length. Furthermore, fractures and earthquakes exhibit similar statistical properties, as their size distributions follow power laws. In this study, we exploit the relation between the size (or length) distributions of pre-existing fractures and earthquakes to decipher the seismic deformation processes within the exhumation-related orogen-internal setting of the southwestern Swiss Alps, which due to its well-monitored seismic activity and the excellent outcrop conditions provides an ideal study site. Characterizing exhumed fracture networks from different tectonic units based on multi-scale drone-based mapping, we find that power law exponents of 3D fracture size distributions generally range between 3 and 3.6. Comparing these values with the depth-dependent exponents of estimated earthquake rupture lengths, we observe significantly larger values of 5 to 8 for earthquake ruptures at shallow depths (< 3 km below sea level (b.s.l.)). At intermediate crustal depths (∼ 3 to 9 km b.s.l.), the power law exponents of fractures and earthquakes appear to be similar. These findings imply depth-dependent differences in the seismogenic reactivation of pre-existing fractures in the study region: while partial rupturing is the prevailing deformation mechanism at shallow depths, fractures are more likely to rupture along their entire length at intermediate crustal depths. Therefore, the present-day near-surface differential stresses are likely insufficient to rupture entire pre-existing fractures seismogenically. Our findings have direct implications for seismic hazard considerations, as earthquakes that rupture along entire fractures appear to become less likely with decreasing depth.