Ridge subduction and afterslip control aftershock distribution of the 2016 Mw 7.8 Ecuador earthquake

Abstract

11We characterise the aftershock sequence following the 2016 Mw=7.8 Pedernales earthquake. 12More than 10,000 events were detected and located, with magnitudes up to 6.9. Most of the 13 aftershock seismicity results from interplate thrust faulting, but we also observe a few normal 14 and strike-slip mechanisms. Seismicity extends for more than 300 km along strike, and is 15 constrained between the trench and the maximum depth of the coseismic rupture. The most 16 striking feature is the presence of three seismicity bands, perpendicular to the trench, which 17 are also observed during the interseismic period. Additionally, we observe a linear 18 dependency between the temporal evolution of afterslip and aftershocks. We also find a 19 temporal semi-logarithmic expansion of aftershock seismicity along strike and dip directions, 20 further indicating that their occurrence is modulated by afterslip. Lastly, we observe that the 21 spatial distribution of seismic and aseismic slip processes is correlated to the distribution of 22 bathymetric anomalies associated with the northern flank of the Carnegie Ridge, suggesting 23 that slip in the area could be influenced by the relief of the subducting seafloor. To explain 24 our observations, we propose a conceptual model in which the Ecuadorian margin is subject 25 to a bimodal slip mode, with distributed seismic and aseismic slip mechanically controlled by 26 the subduction of a rough oceanic relief. Our study sheds new light on the mechanics of 27 subduction, relevant for convergent margins with a complex and heterogeneous structure 28 such as the Ecuadorian margin. 29

Structural Control on Megathrust Rupture and Slip Behavior: Insights From the 2016 Mw 7.8 Pedernales Ecuador Earthquake

“…A uniform earthquake catalog produced for seismic hazard assessment in Ecuador provides a basis for placing the Pedernales seismic sequence in the context of instrumental and historic seismicity from 1587 to 2009 (Beauval et al, ; Yepes et al, ). Similar patterns of focused clusters of seismicity are observed in both the background (interseismic) seismicity and postseismic Pedernales sequence (Figures a and b; Meltzer et al, , Agurto‐Detzel et al, ). The extent to which seismicity is more focused or diffuse is in part related to station density recording the postseismic sequence and in part due to the presence or absence of earthquake swarms during the period of observation (Figure ).…”

“…Earthquakes, Aseismic Slip, and Structure 5.4.1. Relationship to Interseismic Seismicity and Historical Earthquakes Seismicity in the Pedernales sequence matches patterns observed during the interseismic phase of the earthquake cycle, as well as maximum moment release and aftershock patterns of historic earthquakes (Figure 14; Rolandone et al, 2018Agurto-Detzel et al, 2019;León-Ríos et al, 2019). A uniform earthquake catalog produced for seismic hazard assessment in Ecuador provides a basis for placing the Pedernales seismic sequence in the context of instrumental and historic seismicity from 1587 to 2009 (Beauval et al, 2013;Yepes et al, 2016).…”

“…The Pedernales seismic sequence is dominated by pronounced spatial clustering and a striking downdip limit in seismic activity. Most of the events locate outside the rupture area of the main shock (Figure ; Meltzer et al, ; Agurto‐Detzel et al, ; León‐Ríos et al, ). Three apparent bands of seismicity run perpendicular to the strike of the trench: the Jama band updip from the southern end of the rupture, the Cojimies‐Atacames Seamount band approximately coincident with the northern end of the rupture, and the Galera band approximately 20 km north of the rupture (Figure ).…”

“…The relationship between interseismic coupling and seismicity varies along strike (Figure 14c; Chlieh et al, 2014;Nocquet et al, 2017;Rolandone et al, 2018;Agurto-Detzel et al, 2019). The Pedernales earthquake ruptured a moderately to highly coupled portion of the subduction zone.…”

“…The seismicity clusters at Manta-Puerto Lopez, Jama, and along the Atacames Seamount chain correlate with seafloor topography on the subducting Carnegie Ridge and Atacames Seamounts. These topographic features have been imaged by active source seismic (Collot et al, 2017;Marcaillou et al, 2016) and appear in residual bathymetry (Bassett & Watts, 2015;Agurto-Detzel et al, 2019. ) The composite clusters at Galera and northeast of the Pedernales rupture also correlate with residual bathymetry and gravity anomalies (see Figure 9; Bassett & Watts, 2015), suggesting that focused seismicity in these areas may be controlled by structure and/or deformation (imbricate faults) in the downgoing plate (Angiboust et al, 2014;Menant et al, 2018).…”