Multiscale Imaging of the 2014, Mw 8.2 Pisagua Earthquake Source (northern Chile)

Abstract:

The April 1, 2014 Mw 8.2 Pisagua (northern Chile) earthquake ruptured a ~150 km segment along the megathrust interface between the Nazca and South America plates, within the ~500 km long Iquique seismic gap.

Here we study the main rupture process following a multi-scale approach integrating global and regional seismic observations.

At teleseismic scale, we determine the co-seismic slip distribution from body-wave kinematic inversion, and we image high-frequency radiation sources (0.1 – 2.0 Hz) from backprojection of direct P-waves.

The results show a SE, down-dip rupture propagation, with strong high-frequency emission close to the epicenter and to the coast, and largest slip (~7-8 m) SE of the epicenter. Rupture propagation, seen by HF emissions, is characterized by initial slow rupture speed (~1.5 km/s, during the first 30 s), with a later acceleration (~2.5 km/s). Source duration is ~100 s.

We further study regional accelerometers from the IPOC/CSN and ONEMI networks (Iquique region), using an original detection and backprojection approach, based on high-order statics signal representation. This method allows dealing with the wavefield complexity, at regional scale, related to the extended source duration, and to the local velocity structure. We image coherent radiation sources in the frequency range 0.05 – 0.5 Hz, confirming SE rupture propagation.

We finally study in finer detail the initial seconds of the rupture, from the analysis of a small-scale strong motion array and of P-wave polarization at the closest stations.

The resulting multi-scale image of the Pisagua rupture is examined in light of the resolved space-time distribution of co-seismic high-frequency sources and fault slip, together with the GPS-derived interface coupling map (Métois et al, 2013), and the rupture areas of fore- and aftershocks. A possible mechanical model of earthquake nucleation and propagation along the offshore-Pisagua segment of the megathrust interface is proposed and discussed.