Spatiotemporal variations of the components of seismic hazard in the Chilean subduction zone




Herrera, Carlos

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This dissertation presents an analysis of the spatiotemporal variations of strong ground shaking and style of faulting at regional scales in the Chilean subduction zone. These properties are part of the two main components of seismic hazard: ground motions and seismic source characterization. Results are interpreted to be related to the tectonic dynamics and heterogeneities in the region. The ground motion component was evaluated using residual analysis between observed and predicted earthquake strong ground motions. By analyzing strong motions of interplate earthquakes along the Chilean subduction zone (from the northern border to the tectonic triple junction in the south), we corroborate the better predictive performance of locally-derived ground motion models over globally-derived models. The results presented here show a stronger short-period radiation generated by deeper interplate earthquakes in north and central Chile, which is not currently considered in local ground motion models. We interpret this depth-dependent radiation pattern as a result of frictional variations on the plate interface. Additionally, it is shown that not every aftershock sequence following a large interplate earthquake exhibits predominantly weaker short-period radiation, which could be dependent on whether there was precursory activity before the mainshock. This work also outlines the need of developing local ground motion models for crustal earthquakes, since ground motion observations from the Mw 5.7 Pica earthquake (a crustal reverse-oblique event in northern Chile with a large stress drop) are significantly larger than predictions from current global models, particularly at short periods. The seismic source component was assessed for crustal earthquakes in northern Chile. Clear regionalization is found in the spatial patterns of style of faulting and the tectonic stress field. The coastal region exhibits a clear margin-parallel compressional regime shown by the mostly reverse and strike-slip earthquakes in this area, while the analyzed portion of the Andean Precordillera shows a strike-slip regime with a compressional direction nearly parallel to the plate convergence direction. We interpret these two tectonic regimes as a result of the concave shape of the subduction margin and the effect of local topography, respectively. Our results show that these regimes do not show any temporal change during the ∼10 year analysis period, and likely have remained stationary for the last 10 Ma. Although the earthquake recurrence relations presented in this work show that crustal earthquakes happen less frequently and at smaller magnitudes than interplate and intraplate earthquakes in northern Chile, crustal earthquakes still pose an important hazard, with the possibility of occurrence of more earthquakes with damaging-level ground accelerations, such as the Pica earthquake.



Earthquakes, Seismology, Chile