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Interaction between tectonics, slow-slip processes and earthquake mechanisms across the North Andean sliver (Ecuador)


IPGP - Îlot Cuvier


Soutenances de thèses


Sandro Vaca

Sismologie (SIS)

In the last ten years, the quality of the seismic and geodetic instrumentation in Ecuador has been greatly improved. Both at the country-scale and in several interesting areas along the margin or in the Andes, permanent GPS, broadband and accelerometric stations allow us to capture the deformation processes in a large amplitude and frequency range. I first use this new information for an updated characterization of the deformation processes throughout the country. Based on a seismic waveform inversion technique accounting for the diversity of the seismic velocity structure, I have determined reliable source parameters (depth, focal mechanism and seismic moment) for 282 earthquakes of the 2009-2015 period. The solutions are consistent with global estimates of the source parameters for the largest events and increases the number of waveform-based focal mechanism solutions by a factor larger than two. Combined with GPS-derived strain rates, these solutions put a better control on the deformations to be expected along and around the CCPP (Cosanga-Chingual-Pallatanga-Puná) fault system, separating the North Andean Sliver from the stable South America Plate. In the Ecuador subduction zone, the focal mechanisms reflect the interseismic coupling derived from GPS: thrust interface mechanisms characterize the coupled interface in Northern Ecuador, while the low-to-moderate coupling in Central and Southern Ecuador results in variable fault plane orientations, confirming that seismicity is not only driven by subduction interface activation. Moving to a local and peculiar area of the Ecuador subduction zone, this study then focus on the so-called Punta Galera-Mompiche zone (PGMZ). This ~50km diameter low-coupled zone is located in the northern part of the margin (latitude ~0.7N) and separates two zones of high coupling and earthquake potential. The exhaustive analysis of the seismicity over a 15 year-long time period reveals seismic swarms occurring with a two-years repeat time. Swarm occurrence appears to be the response of the medium of the stress increments generated by slow slip on the subduction interface. This finding is strongly supported by two sequences in 2007-2008 and 2013-2014 for which GPS data confirm the contemporaneous occurrence of slow slip events (SSE) with equivalent magnitudes larger than 6.3. For the 2013-2014 sequence, the joint geodetic and seismic data analysis shows a close interplay between slow slip and seismic activation during the first weeks of the two-month long SSE. Seismicity occurred close to the borders of the developing slow-slip, supporting the view that a slow-slip zone surrounded by earthquake prone areas responses to stress increments. The later development of the SSE at shallower depths generates much less seismicity, possibly due to frictional properties in the vicinity of the trench impeding seismic rupture. This study shows that seismic swarms are an efficient tool to track the existence of an SSE, but may not be directly used to quantify its extent. Besides these local characteristics, the PGMZ is also understood as playing a major role in the large earthquakes segmentation across the margin. This fact could be anticipated from the distribution of the large earthquakes during the 20th century, but has been even more suggested by the spatial distribution of the Mw 7.8 2016 Pedernales earthquake, which did not break this area although nucleating very close from it. This barrier character of the PGMZ is understood as a joint effect of its low coupling together with the regular occurrence of SSEs, which reduces or even cancels the slow stress accumulation. In such a configuration, only very rare great events, like the 1906 earthquake, can cross such an unfavorable area.