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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.
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Sedimentary and Crustal Structure of the Western United States From Joint Inversion of Multiple Passive Seismic Datasets
Abstract Accurate seismic images of the crust are essential for assessing seismic hazards and elucidating tectonic processes that shape surface landforms. Although California and Nevada have been studied extensively using various seismic datasets and tomographic methods, the region lacks a seismic model that can accurately define both the shallow (<8 km) and deeper crust. We take the advantage of recent increases in seismic data coverage to build a new 3D shear wave speed model by jointly inverting Rayleigh wave ellipticity, phase velocity, and teleseismic P waveforms. In the Great Valley, the new model reveals an asymmetric basement, steeply dipping in the west and gently dipping in the east. Beneath its western margin, in the Coast Ranges, we resolve a wedge‐shaped, low‐velocity zone in the upper‐middle crust, interpreted as Franciscan Complex. Our images confirm that uplift of the western Great Valley and an eastward shift of its depositional center are caused by wedging and underthrusting of the complex during subduction. Across the Basin and Range, the resolved crust has an average thickness of 38 km in the southern half of the northern Basin and Range, about 5 km thicker than neighboring regions. The thickened crust overlaps with major volcanic centers of the mid‐Cenozoic ignimbrite flare‐up. This spatial correlation may suggest magmatic intrusions and underplating contributed to crustal growth and thickening prior to Miocene Basin and Range extension. Overall, the new model is consistent with active source studies in the region but provides a more comprehensive view of shallow and deep structures across this large and tectonically complex region.
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- Award ID(s):
- 1942431
- PAR ID:
- 10367788
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 127
- Issue:
- 2
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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