1D-velocity structure and seismotectonics of the Ecuadorian margin inferred from the 2016 Mw7.8 Pedernales aftershock sequence

Abstract

On April 16 th 2016 a Mw 7.8 earthquake ruptured the central coastal segment of the Ecuadorian subduction zone. Shortly after the earthquake, the Instituto Geofisico de la Escuela Politecnica Nacional of Ecuador, together with several international institutions deployed a dense, temporary seismic network to accurately categorize the post-seismic aftershock sequence. Instrumentation included short-period and broadband sensors, along with Ocean Bottom Seismometers. This deployment complemented the permanent Ecuadorian seismic network and recorded the developing aftershock sequence for a period of one year following the main-shock. A subset of 345 events with M L > 3.5, were manually picked in the period of May to August 2016, providing highly accurate P-and S-onset times. From this catalogue, a high-quality dataset of 227 events, with an azimuthal gap < 200°, are simultaneously inverted for, obtaining the minimum 1D velocity model for the rupture region, along with hypocentral locations and station corrections. We observe an average Vp/Vs of 1.82 throughout the study region, with relatively higher Vp/Vs values of 1.95 and 2.18 observed for the shallowest layers down to 7.5 km. The high relative Vp/Vs ratio (1.93) of the deeper section, between 30 km and 40 km, is attributed to dehydration and serpentinization processes. For the relocated seismicity distribution, clusters of events align perpendicular to the trench, and crustal seismicity is also evidenced, along with earthquakes located close to the trench axis. We also compute Regional Moment Tensors to analyze the different sources of seismicity after the mainshock. Aside from thrust events related to the subduction process, normal and strike-slip mechanisms are detected. We suggest that the presence of subducting seamounts coming from the Carnegie Ridge act as erosional agents, helping to create a scenario which promotes locking and allows seismicity to extend up to the trench, along zones of weakness activated after large earthquakes.

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

“…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 ; Rolandone et al, , Meltzer et al, ; Agurto‐Detzel et al, ; León‐Ríos et al, ). 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, ).…”

“…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 ).…”

3D local earthquake tomography of the Ecuadorian margin in the source area of the 2016 Mw 7.8 Pedernales earthquake

“…For our inversion, we defined a grid of 200 × 250 × 60 km 3 (see Figure S1) with nodes organized in profiles perpendicular to the trench axis. The 1D velocity model (León-Ríos et al, 2019; see Figure S2) represents the structure of the margin below a reference station located at the coast, approximately where northern limit of the CR subducts. To ensure, this does not bias our 3D results and because of the margin segmentation (Gailler et al, 2017), we followed a graded approach (e.g., Eberhart-Phillips, 1986;Husen et al, 2000;Roecker, 1993;Thurber et al, 1995, among others) and invert for a coarse 2D velocity model (2DVM) and precise earthquake localization in two segments located north and south of the equator.…”

“…Quaternary sediments cover these as well as other small formations, forming several basins along the coast such as the Borbon, Manta-Jama, and Manabi basin (see (1), (2), and (3) in Figure 1b). The local tectonic and seismic activity is controlled by major faults that bound these basins (Agurto-Detzel et al, 2019;Font et al, 2013;León-Ríos et al, 2019;Luzieux et al, 2006;Reyes & Michaud, 2012;Reynaud et al, 1999;Soto-Cordero et al, 2020). Reyes and Michaud (2012) updated the coastal geological map for Ecuador extending several faults observed at the surface.…”

“…To obtain the 2DVM, we extended our minimum 1D model (León-Ríos et al, 2019) along a 2D grid with a west-east orientation perpendicular to the trench. In depth, the distance between nodes follows the layering of the minimum 1D model and varies from 2.5 km at shallower layers to 10 km at greater depths (see Figures S1 and S2).…”