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Oregon State University

United States

Project Description

(NOTE: I was not able to include the figures supporting this project description. Please contact me for a PDF version that includes the figures.)
Submitted by Anne Tréhu, Nathan Bangs, Eduardo Contreras-Reyes, and Andrei Maksymowicz
We request travel funds for Research Assistant Sebastian Obando Orrego from Departamento de Geofísica, Universidad de Chile to spend one month in Corvallis OR from mid-September to mid-October 2024. The objective of this visit will be to integrate information from several different datasets acquired during the 2017 cruise MGL1701 (informally called CEVICHE for Crustal Evaluation from Valdivia to Illapel to Characterize Huge Earthquakes) in order to develop a 3D model of the subduction zone beneath the continental shelf and Coast Ranges in the region that spans the segment of the subduction zone from 38°S (where the southern boundary of the 2010 Maule rupture overlaps with the precursor to the 1960 great earthquake) to 39°S, across the northernmost high slip patch in 1960. Various subsets of the CEVICHE data have been presented in several publications (Olsen et al., 2021; Bangs et al.,2021; Maksymowicz et al., 2021) and in an OSU MS thesis (Zhang, 2019).

FIGURE 1 HERE
Figure 1A. Simplified geologic map showing location of 2D transects presented by Maksymowicz et al. (2021). Inverted black triangles are seismic stations deployed during CEVICHE. This project focuses on expanding P2 and P3 into 3D using all shots and local earthquakes recorded on these stations and integrating the results with the MCS reflection data. B. MCS lines shot during CEVICHE. The primary study region is indicated by a box on A and B.

Figure 1A (from Maksymowicz et al., 2021) shows the geologic setting and the slip contours for 1960 and 2010 earthquakes. It also shows transects along which Maksymowicz et al. (2021) developed 2D crustal models based on large aperture data recorded offshore on the 15-km long MCS streamer and onshore on temporary seismic stations (Figure 2). Figure 1B shows the seismic reflection lines acquired during CEVICHE. Zhang (2019) processed MCS data long the strike line closest to shore (shown in orange) and developed a series of 1D models based on onshore recordings of the shots along those lines in order to estimate the depth of deep reflections and depth to Moho. Both studies show that the Slab2.0 model is generally consistent with the CEVICHE results; however, there are multiple zones of reflectivity that imply localized structures along the plate boundary not reflected in the Slab2.0 model. A 3D model of seismic velocities is critical to our understanding of the structure and composition of the plate boundary along the margin, which is needed to establish links between the along strike variability and the behavior of the Chilean seismogenic zone. This model of along strike variability targets the major transition from the large rupture area in the south into the SZ4D study area in the north.

The focus for this study extends from ~38-39°S and will include deep reflections observed on both strike and dip MCS lines as well as recordings of those shots and local earthquakes on the onshore instruments. Although these data are south of the primary SZ4D earthquake study region, we have chosen to focus on this segment because it addresses a fundamental science question of interest to SZ4D (i.e. plate boundary characteristics across a strong gradient in slip) and because deep reflections beneath the continental shelf in this location show strong reflectivity on MCS15 (Figure 3) between 6-9 s two-way time (~15-25 km) as well as on MCS09, MCS14 (not shown).

FIGURE 2 HERE
Figure 2. 2D velocity model from Maksymowicz et al. (2021) based on large aperture data from shots along MC13.

FIGURE 3 HERE
Figure 3 (left) Reflections from the lower crust along MCS15. (right) Shots from MCS08R recorded onshore.

References:
Bangs, N.L., Morgan, J., Tréhu, A.M., Contreras-Reyes, E., Arnulf, A., Han, S., Olsen*, K.M., & Zhang*, E., Basal accretion along the south-central Chile margin and its relationship to great earthquakes, (2020), J. Geophys. Res., v. 125, doi:10.1029/2020JB19861.
Maksymowicz, A., Contreras-Reyes, E., Diaz, D., Comte, D., Bangs, N., Tréhu, A.M., Vera, E., Hervé, F., & Rietbrock, A. (2021) Deep structure of the continental plate in the south-central Chilean margin: metamorphic wedge and implications for megathrust earthquaks, J. Geophys. Res., 10.1029/2021JB021879
Obando-Orrego, S., Contreras-Reyes, E., Tréhu, A.M., & Bialas, J. (2021) Shallow seismic investigations of the accretionary complex offshore central Chile, Marine Geology, v. 434, 17 pp. https://doi.org/10.1016/j.margeo.2021.106437
Olsen, K.M., Bangs, N.L., Tréhu, A.M., Han, S., Arnulf, A., & Contreras-Reyes, E. (2020). Thick, strong sediment subduction along south-central Chile and its role in great earthquakes, Earth Plan. Sci. Lett., v. 538, 9 pp., doi: 10.1016/j.epsl.2020.116195
Zhang, E. (2019) Investigation of deep seismic reflections beneath the forear in the southern central Chile subduction zone, MS Thesis, Oregon State Un., 111 pp.

Proposed Hosting Period

mid September - mid October, 2024

Facilities/Resources

Software, computers, and a desk in a shared office will be provided as well as access to CEOAS/OSU scientific and recreational facilities (e.g. seminars, exercise facilities). OSU is a participant in the recently funded CRESCENT (Cascadia earthquake science center), and the visitor will able to make contacts through that center.

Additional Comments

PI Name & email: 

Anne Tréhu

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