Interplay between seismic and aseismic processes along the Pta. Galera-Mompiche zone (northern Ecuador subduction zone)

Abstract:

In the northern Ecuadorian margin, where Nazca Plate subducts with a velocity of 48 mm/yr beneath the North Andean Sliver, high and shallow seismic coupling has been geodetically observed. This is consistent with the occurrence of four strong earthquakes (Mw > 7.7) in the past century. We concentrate here on a subarea of this region, hereafter referred as the Punta Galera-Mompiche Zone (PGMZ). At the end of 2007 and 2013, weeks-to-months-long episodes of reversal of the GPS trends (~1 cm westward motion at the closest coastal stations), associated with seismic swarms (maximal magnitudes up to 5.1 in 2007) were observed in the PGMZ. Geodetic and seismic networks, permanent and temporary, allow us to characterize the spatio-temporal evolution of the 2013 Slow Slip Event (SSE) sequence. The associated seismicity is organized in several clusters forming families, located on the subduction interface. Each of these clusters is activated during a few days, and the total seismic moment release of the sequence is ~2.5E15 N.m (Mw ~4.2). This seismicity is mainly triggered during the first stage of the SSE, and becomes sparse in the next weeks. The GPS displacements show that the reversal starts on 2013/12/07, 3 days before the activation of the seismic clusters. During the next 10 days, the rotation of the GPS vectors and the location of the seismicity allow us to estimate a propagation velocity of ~3.2 km/day. We estimate a global geodetic moment equivalent to Mo ~3.0E18 N.m. (Mw ~6.3) with maximal displacements of ~50 mm (figure). For the 2007 sequence, the only GPS station close to PGMZ shows that the geodetic moment is more important than during the 2013 SSE. Seismically, we show by seismic waveforms correlation that this sequence shares some strong similarities with the 2013 one, in particular with the presence of common seismic families. This indicates that this zone is prone to similar seismic-aseismic sequences, which likely play a role in the earthquake potential and seismic cycle of the area.