A comprehensive analysis of the Illapel 2015 Mw8.3 earthquake from GPS and InSAR data

- A uniquely dense coseismic dataset derived from measurements at cGPS + 52 sGPS sites.
- Snapshot of deformation over different timescales: from seconds to days and weeks.
- Slip rake aligned with plates convergence direction, implying no strike-slip deficit.
- A fine analysis of the interplay between interseismic and coseismic deformation.

The September 16, 2015 Mw8.3 Illapel Earthquake occurred on a locked segment of the South American subduction in Chile. This segment ruptured during comparable size earthquakes in the past, in 1880 and 1943, suggesting a somehow regular pattern of characteristic Mw8+ earthquakes occurring every 60 to 80 yr. This recurrence is in agreement with the accumulation of elastic deformation in the upper plate due to the Nazca-South America subduction at a constant rate of 6.5 cm/yr, leading to a deficit of ∼4.5 m of slip to be released every 70 yr. Previous studies consistently imaged the distribution of co-seismic slip along the fault based on geodetic, seismological and far field tsunami data and all described a significant amount of shallow slip resulting in a large tsunami. In addition, some models highlighted an apparent mismatch between the modeled rake of slip and the direction of plate convergence, suggesting the buildup of large strike-slip deficit. Some of these important questions remain open. Is shallow slip really well resolved and substantiated? Is the apparent principal direction of slip during the earthquake really required by the geodetic data?Here, using a comprehensive analysis of continuous GPS sites (including high rate and static displacements) and new survey data from acquired over more than 50 pre-existing sites, complemented with InSAR data, we show that the 2015 rupture overlaps very well the 1943 rupture, with the absence of significant slip south of 32°S and north of 30.2°S (peninsula Lingua de Vaca). Despite the wealth of geodetic data, the shallowest part of the subduction interface remains poorly resolved. We also show that the rake of the earthquake is fully compatible with the oblique plate convergence direction (rather than perpendicular to the trench), meaning that no subsequent trench-parallel motion is required by the data. We propose that the large Low Coupling Zone (LCZ) at the latitude of La Serena revealed by present day coupling distribution is stable over at least two seismic cycles. Inside the coupled area, peak coseismic slip is located precisely offshore the highest coastal topography and elevated terraces, adding weight to a potential correlation between the seismic cycle and long term permanent deformation. Finally, we show that early post-seismic after-slip occurs mostly down-dip of co-seismic asperity(ies), extending north and south of the 2015 rupture area.