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

Description

The Ecuadorian forearc is a complex region of accreted terranes with a history of largemegathrust earthquakes. Most recently, aMw7.8 megathrust earthquake ruptured the plateboundary offshore of Pedernales, Ecuador on 16 April 2016. Following this event, an inter-national collaboration arranged by the Instituto Geofisico at the Escuela Polit ́ecnica Nacionalmobilized a rapid deployment of 65 seismic instruments along the Ecuadorian forearc. Wecombine this new seismic data set with 14 permanent stations from the Ecuadorian nationalnetwork to better understand how variations in crustal structure relate to regional seismichazards along the margin. Here, we present receiver function adaptive common conversionpoint stacks and a shear velocity model derived from the joint inversion of receiver functionsand surface wave dispersion data obtained through ambient noise cross-correlations for theupper 50 km of the forearc. Beneath the forearc crust, we observe an eastward dipping slowvelocity anomaly we interpret as subducting oceanic crust, which shallows near the projectedcentre of the subducting Carnegie Ridge. We also observe a strong shallow positive conversionin 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 alsoobserve significant north–south changes in shear wave velocity. The velocity changes indicatevariations in the accreted terranes and may indicate an increased amount of hydration beneaththe Manab ́ı Basin. This change in structure also correlates geographically with the southernrupture limit of multiple high magnitude megathrust earthquakes. The earthquake record alongthe Ecuadorian trench shows that no event with aMw>7.4 has ruptured south of∼0.5◦Sinsouthern Ecuador or northern Peru. Our observations, along with previous studies, suggestthat variations in the forearc crustal structure and subducting oceanic crust may influance theoccurrence and spatial distribution of high magnitude seismicity in the region.

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