Structure of the Ecuadorian forearc from the joint inversion of receiver functions and ambient noise surface waves

Abstract

SUMMARY The Ecuadorian forearc is a complex region of accreted terranes with a history of large megathrust earthquakes. Most recently, a Mw 7.8 megathrust earthquake ruptured the plate boundary offshore of Pedernales, Ecuador on 16 April 2016. Following this event, an international collaboration arranged by the Instituto Geofisico at the Escuela Politécnica Nacional mobilized a rapid deployment of 65 seismic instruments along the Ecuadorian forearc. We combine this new seismic data set with 14 permanent stations from the Ecuadorian national network to better understand how variations in crustal structure relate to regional seismic hazards along the margin. Here, we present receiver function adaptive common conversion point stacks and a shear velocity model derived from the joint inversion of receiver functions and surface wave dispersion data obtained through ambient noise cross-correlations for the upper 50 km of the forearc. Beneath the forearc crust, we observe an eastward dipping slow velocity anomaly we interpret as subducting oceanic crust, which shallows near the projected centre of the subducting Carnegie Ridge. We also observe a strong shallow positive conversion in the Ecuadorian forearc near the Borbon Basin indicating a major discontinuity at a depth of ∼7 km. This conversion is not ubiquitous and may be the top of the accreted terranes. We also observe significant north–south changes in shear wave velocity. The velocity changes indicate variations in the accreted terranes and may indicate an increased amount of hydration beneath the Manabí Basin. This change in structure also correlates geographically with the southern rupture limit of multiple high magnitude megathrust earthquakes. The earthquake record along the Ecuadorian trench shows that no event with a Mw >7.4 has ruptured south of ∼0.5°S in southern Ecuador or northern Peru. Our observations, along with previous studies, suggest that variations in the forearc crustal structure and subducting oceanic crust may influance the occurrence and spatial distribution of high magnitude seismicity in the region.

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

“…This correlation suggests that the termination in seismicity is due to the rheological contrast between sediments and basalt at depth (Figure 14f and Segovia et al, 2018). This observation is consistent with findings of shear-wave velocity structure from ambient noise tomography and joint inversion of receiver functions and surface-wave dispersion measurements (Koch et al, 2018;Lynner et al, 2020). The region of deep aseismic slip correlates with the Coastal Range at the surface (Figure 14d).…”

3D Local Earthquake Tomography of the Ecuadorian Margin in the Source Area of the 2016 Mw 7.8 Pedernales Earthquake

“…Southernmost profiles P9 and P10 image reduced ratios covering a large area in the overriding plate. These anomalies can be associated with the San Lorenzo formation (SL) mapped by Reyes and Michaud (2012-Figure 6) and also observed by Lynner et al (2020) and Koch et al (2020).…”

“…(2020) and Koch et al. (2020) used ambient noise and joint ambient noise and receiver function methods respectively to image the coastal forearc but were unable to image the marine forearc together with the P wave velocity in the region.…”

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

“…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.…”

“…This work. Aizprua et al[4]; Bethoux et al[50]; Calahorrano et al[46]; Feininger and Seguin[15], Font et al[51]; Gutscher et al[52];Graindorge et al[49]; Gailler et al[53]; Hernández et al[45]; Jaillard et al[54]; Koch et al[55]; Luzieux et al[10]; Lynner et al[56]; Michaud et al[57]; Vallejo et al[8]; Witt and Bourgois[58]…”