TOWARDS A PRECISE MODELLING OF THE EL SALVADOR FAULT ZONE USING GEODETIC TECHNIQUES

The El Salvador Fault Zone (ESFZ) comprises a set of a strike-slip faults, extending through the Central American VolcanicArc within El Salvador, where the Cocos plate subducts under the Caribbean plate. These structures act as a boundarybetween the forearc sliver and the western margin of the Chortís block, accommodating the relative movement betweenthem. The ESFZ has been responsible for several shallow, destructive earthquakes in El Salvador, thus posing a seriousthreat for millions of inhabitants. Understanding its seismic potential and the behaviour of its different segments results ofgreat importance for the assessment and mitigation of seismic risk in the region. Geodetic techniques, such as GNSS andInSAR, are useful tools for measuring surface deformation related to tectonic activity. We are in the process of updatingand densifying the existing GNSS velocity field in El Salvador, aiming to characterise the individual faults in the region bydetermining their slip rates and locking depth. Additionally, we will process InSAR data, trying to obtain a continuousmeasurement of the interseismic deformation. The combination of this information with other data (e.g. seismological andgeological) through kinematic models will allow us to better understand the factors controlling the seismogenic behaviourof the ESFZ faults, evaluate their seismic potential and improve the seismic hazard assessment.

Preliminary results on the characterisation of the Guaycume fault (El Salvador) from geodetic and seismological data

<p>The Guaycume fault is a right-lateral strike-slip structure located in Western El Salvador, within the El Salvador Fault Zone (ESFZ). The ESFZ consists of a strike-slip fault system extending through the Central American Volcanic Arc, on the western margin of the Chortís block, where the Cocos plate subducts under the Caribbean plate.</p><p>The Guaycume fault has been proposed as a possible source for the Mw 6.4 1917 El Salvador destructive earthquake, presenting high seismic potential in close proximity to San Salvador (Alonso-Henar et al., 2018). Its geomorphological expression has been clearly identified (Martinez-Diaz et al., 2016); however, few specific studies are currently published, and its behaviour and kinematics remain widely unknown. Notably, there is a lack of precise information about the amount of deformation that this fault currently absorbs of the westward movement (relative to the Chortís block) of the forearc sliver.</p><p>We process GNSS data in the area from 2007 to 2020 in order to retrieve the GNSS velocity field surrounding the Guaycume fault. We use these data to perform a thorough kinematic study, updating the previously existing slip rates (Staller et al., 2016). Combined with seismological data, this information allows us to understand the seismic cycle of the fault to a better extent, thus leading to a better comprehension of its seismic potential.</p>

Comment on “Crustal faults in the Chilean Andes: geological constraints and seismic potential” by Santibáñez et al. (2019), Andean Geology 46 (1): 32-65

Understanding the location and nature of Quaternary active crustal faults is critical to reduce both the impact of fault rupture and strong ground motions hazards (when these faults rupture causing earthquakes). It is also important for understanding how and where deformation related to plate tectonics is accommodated along geological structures (oftentimes faults and folds). In Chile, work on active tectonics in the upper crust (neotectonics or earthquake geology) is relatively new, in particular regarding fault-focused studies. Therefore, any effort to further progress in our understanding of active fault systems for the benefit of the public, and for aiding local and regional governments and the earthquake engineering and scientific community with mitigation strategies should be applauded. Demonstrating where active faults are located through careful mapping, and to determine how fast they accommodate tectonic deformation and their seismic and fault rupture hazards are key questions in neotectonics. Recently Santibáñez et al. (2019) explore active fault systems in the Chilean Andes. In their paper they outline active and potentially seismogenic (i.e., earthquake producing) fault systems in the Chilean Andes through a review of the literature, seismicity, case studies (earthquakes), and modeling data and then they define potential tectonic domains for subdivision of Chile. These domains were suggested to allow “a first-order approach for seismic potential assessment” (Santibáñez et al., 2019). The three subdivisions they suggest, i.e., domains are the External Forearc, Inner Forearc and Volcanic Arc, were proposed based on several fault parameters (e.g., fault length), case studies, the morphotectonic setting and seismicity. Their paper generates a great foundation to build upon for both the active tectonics and geological hazards community, in addition to being useful for potential end users such as the Chilean local and national government from a planning perspective. Although the Santibáñez et al. (2019) paper takes steps in the right direction, and should be considered an important contribution to the scientific community, this comment addresses three potential issues with their analysis and conclusions that should be reflected upon by the seismic hazard and active tectonics community. These ideas are summarized below and expanded on in detail thereafter.