The 2014 Pisagua-Iquique (Chile) earthquake sequence : geodetic constraints on space-time slip behaviour of a megathrust segment

Abstract:

The April 1, 2014 Pisagua earthquake (Mw 8.1) can be considered as the paroxysm of a long sequence of unusually high seismic activity within the Northern Chile megathrust system. The sequence started in March 2014, with two weeks of intense foreshock activity, suggesting that the mainshock may have been trigerred by a slow slip event (Ruiz et al., 2014). The April 1, 2014 mainshock broke a portion of the subduction interface that had been previously identified as highly coupled, and seems to have been limited along strike by two zones of lower coupling. A significant earthquake occurred some 100 km to the south of, and approx. 24 hour after the mainshock (April 3, 2014 Iquique Mw 7.6 earthquake). This sequence illustrates the importance of short-term, short-range earthquake interaction mechanisms in controlling the slip behaviour of a megathrust interface, with important implications on associated hazards.

Static surface displacements during each phase of the sequence are determined using a combination of continuous GPS, InSAR (TerraSAR-X ScanSAR) and tide gauge records. Using a single elastic inversion procedure, we invert for the slip distribution associated with the sub-events, as well as interseismic coupling using continuous GPS, campaign GPS and InSAR measurements acquired in the years preceding the sequence. This methodology allows for consistently determining the spatial relationship between the different slip patches (pre-seismic, co-seismic, post-seismic and inter-seismic).

The bulk of the moment during the April 1, 2014 earthquake (Mw 8.1) was released 50 km to the south of the hypocenter, 50 km offshore from Pisagua town. In contrast, the April 3, 2014 earthquake (Mw 7.6) occurred beneath the coastal city of Iquique. Coseismic slip during the two main events depicts a complex pattern, broadly complementary with interseismic coupling, preshock / aftershock distribution and post-seismic afterslip. A detailed comparison of the successive sub-events aims at providing a first-order interpretation of the heterogeneity of frictional properties of the megathrust segment, alongside with the initial stress conditions that prevailed before the main rupture. The existence of major structural / geometric complexities within the megathrust system may also be inferred.