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Short and long term analysis of upper plate deformation in northern Chile subduction zone with GPS and tiltmeter data

Abstract : Subduction zones involve the recycling of oceanic lithosphere in the convective mantle to form continental crust by upward migration of the melted part of the slab. In this long-term process, the upper plate accumulate stresses that are instantaneously released in the form of earthquakes, repeating over a cyclic process: the subduction earthquake cycle (SEC). Nevertheless, fast slip on faults is not the only process contributing to the deformation during the SEC. For instance, the low frequency process of aseismic slip is observed in the form of steady fault creeping and transient slow slip. Examples of the latest are the phenomenon of afterslip, and slow slip earthquakes (SSEs) with much longer durations than ''normal'' earthquakes. Of the same importance is the effect of stress induced by an earthquake in the mantle. Because it behaves like a viscoelastic body, the mantle will relax this stress during a slow, long-lasting (up to decades) and large-scale (up to thousands of km) deformation process. All these phenomena impact the upper plate deformation and have to be considered in slip budget analysis over the SEC in order to obtain the real estimation of the seismic hazard of a region. Following this premise, the principal motivation of this work is to contr ibute to the determination of the actual locking state of the north seismic gap of the Chilean subduction zone. The main part of this study was conducted along two directions: analyzing and interpreting both short-term (days, weeks) and long-term (several years) transient deformations detected by permanent GPS stations deployed in north Chile. First, with an exhaustive search for short-term transient signals in 15-years long geodetic time series, we aim at contributing to the current debate of whether slow slip events do occur in the Chilean subduction zone or not. Up to now, only a single and debated case of slow slip associated to the nucleation phase of Iquique 2014 has been reported. Our results, obtained after thorough filtering and analysis of the times series devoted to reduce their inherent noise, indicate the existence of only 3 events that are likely to correspond to small episodes of aseismic slip. They occur between 2009 and 2011, with durations of few weeks and amplitudes that do not exceed 4 mm. Additionally, the occurrence of the larger aseismic slip episode simultaneous to the foreshock activity of Iquique 2014 earthquake is confirmed, and the analysis is pushed further with the help of long-base tiltmeter records. Thanks to this refined analysis and to the sensitivity of this instrument, 4 distinct slow slip events can be identified in the larger region of slow slip revealed by GPS data, occurring during the 3 months previous to the mainshock with magnitudes ranging between Mw 5.8 and 6.2. Finally, a longterm analysis of the same cGPS time series was conducted in order to identify small but significant changes of trends over long durations. Available data indicate a long-term decrease of the upper plate deformation after the intermediate depth earthquake of Tarapaca 2005 (Mw 7.7). We test the viscoelastic effect of the asthenosphere on surface deformation triggered by deep failure (~ 100 km depth). For this, we build a realistic 3D viscoelastic finite element model and tried Maxwell and Burgers viscoelastic rheologies. We find a very good correspondence between the observed postseismic signal associated to this event and the modeled postseismic deformation using a Burgers rheology with a long-term viscosity of 1.9e+18 Pa s, challenging the hypothesis of a decrease of interseismic coupling eventually leading to the megathrust failure of Iquique.
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Gianina Meneses-Provoste. Short and long term analysis of upper plate deformation in northern Chile subduction zone with GPS and tiltmeter data. Earth Sciences. Université Paris sciences et lettres, 2018. English. ⟨NNT : 2018PSLEE022⟩. ⟨tel-02088340⟩

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