Recurrent slow slip events as a barrier to the northward rupture propagation of the 2016 Pedernales earthquake (Central Ecuador)

Abstract

The northern Ecuador segment of the Nazca/South America subduction zone shows spatially heterogeneous interseismic coupling. Two highly coupled zones (0.4° S–0.35° N and 0.8° N–4.0° N) are separated by a low coupled area, hereafter referred to as the Punta Galera-Mompiche Zone (PGMZ). Large interplate earthquakes repeatedly occurred within the coupled zones in 1958 (Mw 7.7) and 1979 (Mw 8.1) for the northern patch and in 1942 (Mw 7.8) and 2016 (Mw 7.8) for the southern patch, while the whole segment is thought to have rupture during the 1906 Mw 8.4–8.8 great earthquake. We find that during the last decade, the PGMZ has experienced regular and frequent seismic swarms. For the best documented sequence (December 2013–January 2014), a joint seismological and geodetic analysis reveals a six-week-long Slow Slip Event (SSE) associated with a seismic swarm. During this period, the microseismicity is organized into families of similar earthquakes spatially and temporally correlated with the evolution of the aseismic slip. The moment release (3.4×1018Nm, Mw 6.3), over a ~60×40km area, is considerably larger than the moment released by earthquakes (5.8×1015Nm, Mw 4.4) during the same time period. In 2007–2008, a similar seismic-aseismic episode occurred, with higher magnitudes both for the seismic and aseismic processes. Cross-correlation analyses of the seismic waveforms over a 15years-long period further suggest a 2-year repeat time for seismic swarms, which also implies that SSEs recurrently affect this area. Such SSEs contribute to release the accumulated stress, likely explaining why the 2016 Pedernales earthquake did not propagate northward into the PGMZ.