Foreshock (and slow slip?) triggering of the 1 April 2014 Mw 8.2 Iquique, Chile earthquake

Abstract:

In the two weeks leading up to the 1 April 2014 Mw 8.2 (USGS moment magnitude) earthquake offshore of Iquique, Chile, a sequence of foreshocks occurred south of the 1 April main shock epicenter and up-dip of the main shock moment release region. Starting on 16 March, this sequence included four Mw 6.0+ events, 17 Mw 5-6 events, and numerous smaller events. Using multiple-event relocated hypocenters and source parameters (focal planes and magnitudes) derived from regional and teleseismic data, we determine the evolution of Coulomb stress change (ΔCS) from these foreshocks on the subduction plate boundary. We assess if each event successively triggered the next, ultimately leading to the 1 April main shock rupture. The three largest foreshocks following the initial Mw 6.7 event (Mw 6.4, 6.2 and 6.3) occurred in regions of significant positive ΔCS (0.01 MPa and larger), indicating that these locations were brought closer to failure by the preceding seismicity. In addition, the net ΔCS at the main shock hypocenter accumulated during the foreshock sequence was large and positive (~0.05 MPa), implicating the foreshock sequence in aiding the initial rupture of the main shock. This initial hypocentral rupture, which preceded the rupture in the region of primary moment release by ~25 seconds, had the effect of increasing the ΔCS in that region by ~0.01 MPa.

We also examine the role aseismic slip may have had in the loading of the plate boundary before the main shock. Horizontal GPS displacements observed along the northern Chile coast prior to the 1 April main shock cannot be accounted for by displacements generated from the foreshock seismicity alone. Assuming aseismic slip on the subduction interface is responsible for these excess displacements, we perform a grid search for the location and magnitude of plate interface slip that can account for this difference. We find that a region with slip corresponding to a magnitude ~Mw 6.8 earthquake co-located with or up-dip of the foreshock seismicity best fits the difference. This slow slip patch adds to the positive loading in the main shock hypocentral area, and importantly in main region of moment release down-dip of the foreshocks.