Complex Nucleation Process of Large North Chile Earthquakes, Implications for Early Warning Systems

We studied the nucleation process of Northern Chile events that included the large earthquakes of Tocopilla 2007 Mw 7.8 and Iquique 2014 Mw 8.1, as well as the background seismicity recorded from 2011 to 2013 by the ILN temporary network and the IPOC and CSN permanent networks. We built our catalogue of 393 events starting from the CSN catalogue, which has a completeness of magnitude Mw > 3.0 in Northern Chile. We re-located and computed moment magnitude for each event. We also computed Early Warning (EW) parameters - Pd, Pv, τc and IV2 - for each event including 13 earthquakes of Mw>6.0 that occurred between 2007-2012. We also included part of the seismicity from March-April 2014 period. We find that Pd, Pv and IV2 are good estimators of magnitude for interplate thrust and intraplate intermediate depth events with Mw between 4.0 and 6.0. However, the larger magnitude events show a saturation of the EW parameters. The Tocopilla 2007 and Iquique 2014 earthquake sequences were studied in detail. Almost all events with Mw>6.0 present precursory signals so that the largest amplitudes occur several seconds after the first P wave arrival. The recent Mw 8.1 Iquique 2014 earthquake was preceded by low amplitude P waves for 20 s before the main asperity was broken. The magnitude estimation can improve if we consider longer P wave windows in the estimation of EW parameters. There was, however, a practical limit during the Iquique earthquake because the first S waves arrived before the arrival of the P waves from the main rupture. The 4 s P-wave Pd parameter estimated Mw 7.1 for the Mw 8.1 Iquique 2014 earthquake and Mw 7.5 for the Mw 7.8 Tocopilla 2007 earthquake.