Upper-plate structure in Ecuador coincident with the subduction of the Carnegie Ridge and the southern extent of large mega-thrust earthquakes

Abstract

SUMMARY The Ecuadorian convergent margin has experienced many large mega-thrust earthquakes in the past century, beginning with a 1906 event that propagated along as much as 500 km of the plate interface. Many subsections of the 1906 rupture area have subsequently produced Mw ≥ 7.7 events, culminating in the 16 April 2016, Mw 7.8 Pedernales earthquake. Interestingly, no large historic events Mw ≥ 7.7 appear to have propagated southward of ∼1°S, which coincides with the subduction of the Carnegie Ridge. We combine data from temporary seismic stations deployed following the Pedernales earthquake with data recorded by the permanent stations of the Ecuadorian national seismic network to discern the velocity structure of the Ecuadorian forearc and Cordillera using ambient noise tomography. Ambient noise tomography extracts Vsv information from the ambient noise wavefield and provides detailed constraints on velocity structures in the crust and upper mantle. In the upper 10 km of the Ecuadorian forearc, we see evidence of the deepest portions of the sedimentary basins in the region, the Progreso and Manabí basins. At depths below 30 km, we observe a sharp delineation between accreted fast forearc terranes and the thick crust of the Ecuadorian Andes. At depths ∼20 km, we see a strong fast velocity anomaly that coincides with the subducting Carnegie Ridge as well as the southern boundary of large mega-thrust earthquakes. Our observations raise the possibility that upper-plate structure, in addition to the subducting Carnegie Ridge, plays a role in the large event segmentation seen along the Ecuadorian margin.

Forearc Crustal Structure of Ecuador Revealed by Gravity and Aeromagnetic Anomalies and Their Geodynamic Implications

“…The second scenario ( Figure 7) involves a similar autochthonous polygon with a paleohydrated mantle wedge necessary to match the high positive magnetic anomaly. This last scenario is the preferred one, together with the presence of continental crust, as supported by the velocity model presented by Lynner et al [56]. Indeed, partial serpentinization of the lower crust or a remnant hydrated mantle wedge may be the source of the positive magnetic and negative gravity anomaly pair (see next section).…”

“…Indeed, partial serpentinization of the lower crust or a remnant hydrated mantle wedge may be the source of the positive magnetic and negative gravity anomaly pair (see next section). The magnetic anomaly to the west in profile B-B' (between 80 and 110 km, Figure 7) is located near to a high to low shear velocity zone according to the model proposed by Lynner et al [56] (Figure 4). Considering the gravity decrease in this area, the presence of magnetic underplated material or a hydrated mantle wedge seems plausible models.…”

“…Indeed, a three-dimensional velocity model covering the same block [61] supports a low-velocity zone that may be related to altered and hydrated mafic and ultramafic rocks, commonly observed along margins consisting of oceanic or island arcs accreted terranes. Furthermore, recent tomography-based shear velocity inversion models reveal low-velocity crustal bodies possibly associated with the subduction of the Carnegie Ridge, north of 1°S (Figure 3) [55,56]. East of the low-density zone (Figures 6(b) and 6(c)), a high-amplitude magnetic anomaly shows decreasing values towards the Borbón basin.…”

“…The densities for basement rocks are derived from the velocity analysis of wide-angle seismic data [47,49,56], using empirical velocity-density relationships for igneous rocks [60]. For the sedimentary cover, densities were estimated from industrial borehole logs, located in the Gulf of Guayaquil and the Progreso basins.…”

“…Overlying sedimentary packages were roughly averaged with a seismic velocity of 2.3 km/s, to depth convert the top of the acoustic basement. The results of the seismic interpretation of deep and conventional seismic profiles ( [46,49,56,61,62]; and this work) were used to constrain the forearc basement structural configuration as input geometry for the 2D forward models.…”