Geodetic exploration of strain along the El Pilar Fault in northeastern Venezuela

Abstract

We use Global Navigation Satellite Systems observations in northeastern Venezuela to constrain the El Pilar Fault (EPF) kinematics and to explore the effects of the variable elastic properties of the surrounding medium and of the fault geometry on inferred slip rates and locking depth. The velocity field exhibits an asymmetric velocity gradient on either side of the EPF. We use five different approaches to explore possible models to explain this asymmetry. First, we infer a 1.6 km locking depth using a classic elastic half-space dislocation model. Second, we infer a 1.5 km locking depth and a 0.33 asymmetry coefficient using a heterogeneous asymmetric model, including contrasting material properties on either side of a vertical fault, suggesting that the igneous-metamorphic terranes on the northern side are~2 times more rigid than the sedimentary southern side. Third, we use a three-dimensional elastostatic model to evaluate the presence of a compliant zone, suggesting a 30% reduction of rigidity in the upper 3 km at the depth of a 1 to 5 km wide fault zone. Fourth, we evaluate the distribution of fault slip, revealing a widespread partial creep pattern in the eastern upper segment, while the upper western segment exhibits a partially locked area, which coincides with the rupture surface of the 1797 and 1929 earthquakes. To supplement these models, we upgrade the previously published displacement simulation method using nonvertical dislocations with data acquired between 2003 and 2013. The localized aseismic displacement pattern associated with creeping or partially creeping fault segments could explain the low level of historic seismicity.

Spatial and temporal variations in creep rate along the El Pilar fault at the Caribbean‐South American plate boundary (Venezuela), from InSAR

“…To compare GNSS and InSAR signals, all horizontal GNSS velocities (in fault‐parallel component) from Reinoza et al . [] (plotted in Figure ) were projected onto the profile. On this profile plotted in Figure , the jump across the fault is slightly higher in 2007–2011 InSAR velocities than in 2003–2005–2013 GNSS velocities.…”

“…Variations of elastic properties would also be expected across this tectonic boundary, as suggested by Reinoza et al . []. The El Pilar fault is composed of several structural segments, defined by neotectonic analyses [ Beltran et al ., ; Audemard et al ., ; Van Daele et al ., ].…”

“…(b) GPS and INSAR velocity field profile across the El Pilar fault (A–A ′ ). Dots correspond to GNSS velocities for the stations shown in Figure [ Reinoza et al ., ] projected along the A–A ′ profile (we keep GNSS stations situated up to 100 km of the A–A ′ profile). Dot color scale represents the distance between the GNSS stations and the profile.…”

“…The E‐W dextral strike‐slip El Pilar fault, which accommodates almost all of the 20 mm/yr relative displacement between the Caribbean and South American plates (Figure ) [ Weber et al ., ; Jouanne et al ., ], is an interesting case study for analyzing the relationship between aseismic and seismic slip modes. Seismicity, paleoseismology, and geodesy indicate that this fault exhibits seismic as well as aseismic behavior [ Audemard , , ; Reinoza et al ., ]. For example, the El Pilar fault produced seismic events in 1684, 1797, 1853, 1929, and 9 July 1997 [ Audemard , , , , ; Altez and Audemard , ; Aguilar et al ., ].…”

“…Despite the low resolution in space and time, Reinoza et al . [] showed that, in the seismogenic layer (0–12 km depth), between 40 and 50% of the fault area is locked, and that the aseismic slip is certainly not spatially uniform.…”