Reflection seismic images and amplitude ratio modelling of the Chilean subduction zone at 38.25°S

Abstract

Active source near-vertical reflection (NVR) data from the interdisciplinary project TIPTEQ were used to image and identify structural and petrophysical properties within the Chilean subduction zone at 38.25 • S, where in 1960 the largest earthquake ever recorded (M w 9.5) occurred. Reflection seismic images of the subduction zone were obtained using the post-stack depth migration technique to process the three components of the NVR data, allowing to present P-and S-stacked time sections and depth-migrated seismic reflection images. Next, the reflectivity method allowed to model traveltimes and amplitude ratios of pairs of reflections for two 1D profiles along the studied transect. The 1D seismic velocities that produced the synthetic seismograms with amplitudes and traveltimes that fit the observed ones were used to infer the rock composition of the different layers in each 1D profile. Finally, an image of the subduction zone is given. The Chilean subduction zone at 38.25 • S underlies a continental crust with highly reflective horizontal, as well as dipping events. Among them, the Lanalhue Fault Zone (LFZ), interpreted to be east-dipping, is imaged to very shallow depths for the first time. In terms of seismic velocities, the inferred composition 1 of the continental crust is in agreement with field geology observations at the surface along the profile. Furthermore, no measurable amounts of fluids above the plate interface in the continental crust in this part of the Chilean subduction zone are necessary to explain the results. A largescale anisotropy in the continental crust and upper mantle is qualitatively proposed. However, quantitative studies on this topic in the continental crust of the Chilean subduction zone at 38.25 • S do not exist to date.

Transient stripping of subducting slabs controls periodic forearc uplift

“…The map shows ocean-continent subduction zones around the Circum-Pacific region with emphasis on segments where tectonic underplating is suspected, based on geophysical observations. (1) Hikurangi margin [64][65][66] , (2) Nankai margin 67 , (3) Sagami trough 23 , (4) Alaska margin 68,69 , (5) Cascadia margin [70][71][72] , (6) Costa Rica margin 73 , (7) North Chilean margin 74,75 and (8) Central Chilean margin 74,[76][77][78][79] . Black frames locate regions where trench-perpendicular topographic profiles have been compiled.…”

Deep Structure of the Continental Plate in the South‐Central Chilean Margin: Metamorphic Wedge and Implications for Megathrust Earthquakes

“…(2006) and Ramos et al. (2018), which is the only deep seismic section generated in the onshore forearc of the study zone and is located along the track of profile P2. Krawczyk et al.…”

“…10.1029/2021JB021879 Segmented blue lines are the main reflectors identified in the image behind, which corresponds to the onshore reflection seismic section presented by Krawczyk et al (2006). Black dots are the upper plate seismicity located by Haberland et al (2006) and segmented red line is the geometry of Lanalhue Faut Zone (LFZ) inferred by Ramos et al (2018) To complement a regional structural analysis, in Figure 8a, we project all segments of deep reflectors according to their position with respect to the deformation front. Because of the large distance between seismic profiles, the objective of this analysis is to study the structural style of the continental wedge at large scale, rather than the north-south continuity of specific reflectors.…”

Forearc density structure of the overriding plate in the northern area of the giant 1960 Valdivia earthquake

“…Gravity modelling techniques does not define a downward limit of WS/ES (without independent deep constraints). Nevertheless, interpretations of seismic reflection data at ~38.25ºS (Krawczyk et al, 2006;Ramos et al, 2018) showed the downward prolongation of WS/ES reaching deep levels near continental Moho interface (~30km depth). As previously mentioned, the seaward limit of WS/ES is not defined by direct lithological observations; their presence beneath the shelf basin is confirmed by 405 exploration boreholes (González, 1989).…”

“…At the same time, according to Menant et al (2019), the deformation style of basal accrationary complexes (typically an antiformal stack of duplexes) favoured upward fluid fluxes from the interplate boundary, generating dewatering and the increase of µb* in some adjacent regions of the subduction channel (mainly downward from basal accretionary complex). Several authors have siggested the presence of this deformation style in the deep zone of WS unit (Krawczyk et al, 2006;Ramos et al, 2018, Moreno et al 2018Maksymowicz et al, submitted 495 for publication). Under this interpretation, the widening of MWU and CC domains to the north of ~42ºS could favoured high friction in the deep region of interplate boundary (bellow CC domain) and a relatively low friction in the seaward portion of MWU.…”