Creep on the Argentine Precordillera Décollement Following the 2015 Illapel, Chile, Earthquake: Implications for Andean Seismotectonics

• Published in: Geophysical research letters 2024-10, Vol.51 (19), p.n/a
• Main Authors: Figueroa, M. A., Sobrero, F. S., Gómez, D. D., Smalley, R., Bevis, M. G., Griffith, W. A., Caccamise, D. J., Kendrick, E. C.
• Format: Article
• Language: English
• Subjects: Andean seismotectonics; coseismic deformation; Deformation; Earthquakes; Elastic deformation; Geological hazards; Global navigation satellite system; GNSS; Hazard assessment; Illapel earthquake; Mountains; Navigation; Navigation satellites; Navigation systems; Navigational satellites; Orogeny; Plates; postsesimic deformation; Precordillera décollement; Satellite observation; Satellites; Seismic activity; Seismic hazard; Slip; Solifluction; Velocity; Velocity distribution
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2024GL110945

The Central and South‐Central Andes form a “two‐sided” mountain belt bounded by distinct zones of convergence in the forearc and backarc flanks. Previous geodetic interseismic deformation studies found that the forearc to backarc velocity field is better explained when elastic models allow reverse aseismic slip on the Andes eastern‐flank décollement faults. Here, we extend the earlier interpretation of interseismic motion and argue that normal aseismic creep of the Precordillera décollement is required to explain backarc Global Navigation Satellite System displacements during the co‐ and early postseismic phases of the 2015 Illapel, Chile, earthquake. This model significantly reduces the previously reported overlap between coseismic slip and afterslip on the megathrust of this earthquake, consistent with the expectation that these slip modes are spatially partitioned. These findings have direct implications for estimating recurrence interval and slip rate, and for probabilistic seismic hazard analysis on both sides of the orogen. Plain Language Summary Between consecutive earthquakes in the Central and South‐Central Andes, during what is known as the interseismic phase, the traditional Nazca‐South America two‐plate model underpredicts the surface Global Navigation Satellite System (GNSS) velocities on the east side of the mountain belt. Previous studies show that adding an “Andean microplate,” thus forming a three‐plate model, kinematically explains the observed velocity field on both sides of the Andes. We analyzed the GNSS coseismic displacements caused by the Mw 8.3 2015 Illapel, Chile, earthquake, and found that the traditional two‐plate model also underpredicts GNSS observations in the eastern side of the Andes. We show that adding the Andean microplate to the model, in the same manner as for the interseismic phase, and allowing aseismic slip on the detachment interface beneath it significantly reduces the observed surface displacements misfit. We also show that this holds for the displacements observed 50 days after the main shock, evidencing that the detachment geometry slips before, during, and after a large magnitude earthquake. Using the three‐plate model produces fault slip distributions that decrease overlap between local maxima in coseismic and aseismic slip, consistent with expectations from rate‐ and state‐dependent friction laws. Key Points Revised co‐ and postseismic slip models account for forearc‐backarc coupling by allowing slip on the