The cryptic seismic potential of the Pichilemu blind fault in Chile revealed by off-fault geomorphology

Abstract

The first step towards assessing hazards in seismically active regions involves mapping capable faults and estimating their recurrence times. While the mapping of active faults is commonly based on distinct geologic and geomorphic features evident at the surface, mapping blind seismogenic faults is complicated by the absence of on-fault diagnostic features. Here we investigated the Pichilemu Fault in coastal Chile, unknown until it generated a Mw 7.0 earthquake in 2010. The lack of evident surface faulting suggests activity along a partly-hidden blind fault. We used off-fault deformed marine terraces to estimate a fault-slip rate of 0.52 ± 0.04 m/ka, which, when integrated with satellite geodesy suggests a 2.12 ± 0.2 ka recurrence time for Mw~7.0 normal-faulting earthquakes. We propose that extension in the Pichilemu region is associated with stress changes during megathrust earthquakes and accommodated by sporadic slip during upper-plate earthquakes, which has implications for assessing the seismic potential of cryptic faults along convergent margins and elsewhere.

Pore-pressure diffusion controls upper-plate aftershocks of the 2014 Iquique earthquake

“…5), and because the poroelastic ∆CFS values are much larger than a critical triggering value of +0.1 bar 2,7 in the region where most of the upperplate aftershocks occur. This contrasts the widely used ∆CFS estimation to investigate aftershock sequences based on coseismic stress changes derived from purely elastic models in all tectonic settings 2,5,7,8,41 , where stress changes are generally much smaller and which fail to explain the time-dependency. This time dependency could be explained by the exponential decay of afterslip 12,42 or non-linear viscoelastic relaxation 43 , but the estimation of ∆CFS resulting from these processes is highly sensitive to the used fault orientation and the assumed style of faulting, respectively.…”

“…Here, positive ∆CFS values indicate that the stress state has been brought closer to failure, while negative ∆CFS values further away from failure 7 . However, even when coseismic ∆CFS may explain the spatial distribution of some events 2,3,8 , they fail to explain the time dependency. A plausible candidate to explain the time dependency may be the postseismic crustal deformation exhibiting similar exponential time decay.…”

Uplifted Pleistocene Marine Terraces at Active Margins: Modeling Reveals the Effects of Sea Reoccupation and Coseismic Uplift on Uplift Rate Calculation

“…In order to investigate the effect of earthquake‐driven versus steady uplift, we consider periodical earthquakes with RI of 100, 200, 500, 600, 700, 800, 900, 1,000, 3,000, and 5,000 years; the magnitude of the D is adjusted so that all the models with different RI yield equal average uplift (hereafter $$\overline{U}$$). The chosen RIs encompass both splay fault earthquakes associated to megathrust events as well as upper plate earthquakes sensu stricto (Jara‐Muñoz et al., 2022; Litchfield et al., 2020; Mouslopoulou et al., 2015; Ota et al., 2000; Philibosian & Meltzner, 2020; Tiberti et al., 2014; Williams et al., 2019).…”

Causes of permanent vertical deformation at subduction margins: Evidence from late Pleistocene marine terraces of the southern Hikurangi margin, Aotearoa New Zealand

“…We model deformation using the elastic dislocation method of Okada (1985). Strictly, our use of this method models terrace uplift as the elastic response of a homogeneous solid to hundreds of metres of slip on a fault, as many previous studies have done (e.g., Anderson and Menking, 1994;Jara-Muñoz et al, 2019;Jara-Muñoz et al, 2022;Nicol et al, 2022). Since surface displacement scales linearly with fault slip in this formulation, the calculated uplift response is equivalent to the cumulative sum of the metre-scale individual elastic response to each earthquake.…”

“…The magnitude of the difference varies depending on model parameters such as regional uplift rate, but the required slip rate is typically ~15-25% higher if T e is 40 km than if T e is 100 km. This effect is important because flexural-isostatic effects are sometimes neglected in studies that use terrace uplift to constrain fault slip rates (e.g., Jara-Muñoz et al, 2022;Nicol et al, 2022). We acknowledge that it is often difficult to constrain T e , but recommend that future studies consider the possible impact of flexural-isostatic compensation on their estimated slip rates and adjust their uncertainties where appropriate.…”