Downdip variations in seismic reflection character: Implications for fault structure and seismogenic behavior in the Alaska subduction zone

Abstract

Seismic reflection data collected offshore of Alaska Peninsula across the western edge of the Semidi segment show distinctive variations in reflection characteristics of the megathrust fault with depth, suggesting changes in structure that may relate to seismic behavior. From the trench to ~40 km landward, two parallel reflections are observed, which we interpret as the top and bottom of the subducted sediment section. From ~50 to 95 km from the trench, the plate interface appears as a thin (<400 ms) reflection band. Deeper and farther landward, the plate interface transitions to a thicker (1–1.5 s) package of reflections, where it appears to intersect the fore‐arc mantle wedge based on our preferred interpretation of the continental Moho. Synthetic waveform modeling suggests that the thin reflection band is best explained by a single ~100 to 250 m thick low‐velocity zone, whereas the thick reflection band requires a 3 to 5 km thick zone of thin layers. The thin reflection band is located at the center of the 1938 Mw 8.2 Semidi earthquake rupture zone that now experiences little interplate seismicity. The thick reflection band starts at the downdip edge of the rupture zone, correlates with a dipping band of seismicity, and projects to the location of tremor at greater depth. We interpret the thin reflection band as a compacted sediment layer and/or localized shear zone. The thick reflection band could be caused by a wide deformation zone with branching faults and/or fluid‐rich layers, representing a broad transition from stick‐slip sliding to slow slip and tremor.

A fluid‐rich layer along the Nankai trough megathrust fault off the Kii Peninsula inferred from receiver function inversion

“…Similar thin LVZs have been revealed through active source seismic surveys at seismically locked zones of other subduction zones based on intense P ‐to‐ P reflection phases from the plate interface [ Nedimović et al ., ; Mochizuki et al ., ; Bell et al ., ; Li et al ., ], suggesting that subducting fluid‐rich sediment layers are a ubiquitous feature of subduction zones. Nedimović et al .…”

“…[] and Li et al . [] have estimated the thicknesses of the LVZs at Cascadia and Alaska subduction zones to be <2 km and 100–250 m, respectively, roughly consistent with our results. They have also revealed abrupt change in the thickness along the dip direction: the LVZs become thicker (> 2 km) at greater depth, downdip of seismogenic plate interfaces.…”

“…Most RF studies using on‐land data have obtained thicker LVZs which are interpreted as the hydrated oceanic crust [ Bostock , , and references therein]. The discrepancy in the LVZ thickness reported by our study and those on‐land‐based studies may reflect the depth‐dependent properties of the megathrusts [ Nedimović et al ., ; Li et al ., ]. Otherwise, analyzing RFs at high frequency using on‐land stations might help detect thinner low‐velocity sediment layers.…”

Along‐Trench Structural Variations of the Subducting Juan de Fuca Plate From Multichannel Seismic Reflection Imaging

“…Although the incoming sediment sequence at Cascadia is 2–3‐km thick, existing seismic data suggest that there are regions, such as offshore Washington and Vancouver Island, with very little sediment subduction (e.g., Davis & Hyndman, ; Han et al, ). In these regions, the plate interface properties at propagator wakes with abundant underthrust sediments may be significantly different from the adjacent zones where the oceanic crust is in direct contact with the upper plate.At the depths where ETS is observed, the plate interface is inferred to be a thick shear zone from seismic reflection imaging (Li et al, ; Nedimović et al, ). Observations from exhumed subduction fault rock records provide evidence for viscous deformation of the matrix for which sediment is a major contributing component (relatively weak material) and brittle deformation within the rock blocks (relatively strong material; Bebout & Penniston‐Dorland, ) and suggest that ETS may arise from a combination of frictional and viscous behaviors (Fagereng & den Hartog, ; Hayman & Lavier, ).…”

Tuning, interference and false shallow gas signatures in geohazard interpretations: beyond the rule

“…Avseth, Mukerji and Mavko ; Chopra and Marfurt ; Ashcroft ; Li et al . ) regardless of the match between the genuine subsurface reflectivity and the simplified Widess () model.…”