Revisiting the North Chile seismic gap segmentation using GPS-derived interseismic coupling

Abstract

No major earthquake occurred in North Chile since the 1877 M w 8.6 subduction earthquake that produced a huge tsunami. However, geodetic measurements conducted over the last decade in this area show that the upper plate is actually deforming, which reveals some degree of locking on the subduction interface. This accumulation of elastic deformation is likely to be released in a future earthquake. Because of the long elapsed time since 1877 and the rapid accumulation of deformation (thought to be 6-7 cm yr −1 ), many consider this area is a mature seismic gap where a major earthquake is due and seismic hazard is high. We present a new Global Positioning System (GPS) velocity field, acquired between 2008 and 2012, that describes in some detail the interseismic deformation between 18 • S and 24 • S. We invert for coupling distribution on the Nazca-South America subduction interface using elastic modelling. Our measurements require that, at these latitudes, 10 to 12 mm yr −1 (i.e. 15 per cent of the whole convergence rate) are accommodated by the clockwise rotation of an Andean block bounded to the East by the subandean fold-and-thrust belt. This reduces the accumulation rate on the subduction interface to 56 mm yr −1 in this area. Coupling variations on the subduction interface both along-strike and along-dip are described. We find that the North Chile seismic gap is segmented in at least two highly locked segments bounded by narrow areas of weak coupling. This coupling segmentation is consistent with our knowledge of the historical ruptures and of the instrumental seismicity of the region. Intersegment zones (Iquique, Mejillones) correlate with high background seismic rate and local tectonic complexities on the upper or downgoing plates. The rupture of either the Paranal or the Loa segment alone could easily produce a M w 8.0-8.3 rupture, and we propose that the Loa segment (from 22.5 • S to 20.8 • S) may be the one that ruptured in 1877.

Gradual unlocking of plate boundary controlled initiation of the 2014 Iquique earthquake

“…1); the largest until now was the Mw 7.7 Tocopilla earthquake in 2007, which broke the southern rim of this segment beneath and north of Mejillones Peninsula along a total length of 150 km. Only the downdip end of the locked zone slipped in this event, and the total slip in the rupture area was less than 2.6 m 6,7 leaving most of the past slip deficit of c. 8-9 m accumulated since 1877 unaffected 8 . On April 1st, 2014, the Mw 8.1 Iquique earthquake north of Iquique struck the central portion of the gap.…”

“…While models of interseismic coupling for this region exist 2,8,13 it is in the interest of consistency to produce a locking model on the same geometry as used for the coseismic slip modelling. We took the ITRF08 GPS velocities data of Métois et al 8 and performed our own corrections to fix velocities to a stable South American reference frame, and to correct for overlapping tectonic signals of shortening and sliver motion.…”

“…We took the ITRF08 GPS velocities data of Métois et al 8 and performed our own corrections to fix velocities to a stable South American reference frame, and to correct for overlapping tectonic signals of shortening and sliver motion.…”

“…The Iquique rupture affected an area shown from geodetic analysis to be a zone of intermediate interseismic coupling at 18.5-21°S interrupting the otherwise fully locked Northern Chile Southern Peru gap 8,13 (Fig. 3a).…”

On the Origin of Mega-thrust Earthquakes

“…Iquique ridge entering into the subduction zone in the border area between northern Chile and southern Peru is a good example for this (e.g. Pritchard and Simons 2006;Contreras-Reyes and Carrizo 2011;Métois et al 2013). The strong coupling along this zone has previously been modelled (e.g.…”