Abstract
SUMMARYThe 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.
Highlights
The Mw 7.8 Pedernales earthquake, which occurred on 16 April 2016, caused significant damage and brought increased attention to the controls on the seismogenic behaviour of the megathrust along the Ecuadorian margin
Given the terranes position in the forearc and depth of the subducting oceanic crust (Slab2 from Hayes et al 2018), we suggest that the forearc accreted material likely includes some mantle lithosphere associated with the accreted terrane and is in contact with the subducting oceanic crust
We present adaptive common conversion point receiver function stacks and a 3-D shear wave velocity model derived from the joint inversion of receiver functions and surface wave dispersion for the upper 50 km of the Ecuadorian forearc
Summary
The Mw 7.8 Pedernales earthquake, which occurred on 16 April 2016, caused significant damage and brought increased attention to the controls on the seismogenic behaviour of the megathrust along the Ecuadorian margin. Following the Mw 7.8 Pedernales earthquake, an international collaboration arranged by the Instituto Geofisico at the Escuela Politecnica Nacional (IG-EPN) mobilized a rapid deployment of 55 broad-band, intermediate, and short-period seismometers and 10 ocean-bottom seismometers in order to capture the aftershock sequence for ∼1 yr (Meltzer et al 2019) This data provides an unprecedented opportunity to image Ecuadorian forearc structure and study how it relates to the seismogenic zone and the range of slip behaviours observed at the plate boundary. The final model fits the dispersion curve data well across the study area, while RF fits are degraded in regions with complicated shallow structure, such as around the Manabı and Borbon basins (Fig. 5). In S4 this fast anomaly reaches shear wave velocities up to 4.5 km s–1 at depths as shallow as ∼10 km in both cross-sections A–A and B–B (Fig. 4). The fast velocities in S3 and S4 are not as prominent in the ANT-only model along these sections, only reaching velocities of ∼4.3 km s–1
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