Detailed seismic velocity of the incoming subducting sediments in the 2004 great Sumatra earthquake rupture zone from full waveform inversion of long offset seismic data

Abstract

The nature of incoming sediments defines the locking mechanism on the megathrust, and the development and evolution of the accretionary wedge. Here we present results from seismic full waveform inversion of 12 km long offset seismic reflection data within the trench in the 2004 Sumatra earthquake rupture zone area that provide detailed quantitative information on the incoming oceanic sediments and the trench‐fill sediments. The thickness of sediments in this area is 3–4 km, and P wave velocity is as much as ~4.5 km/s just above the oceanic crust, suggesting the presence of silica‐rich highly compacted and lithified sediments leading to a strong coupling up to the subduction front. We also find an ~70–80 m thick low‐velocity layer, capped by a high‐velocity layer, at 0.8 km above the subducting plate. This low‐velocity layer, previously identified as high‐amplitude negative polarity reflection, could have porosity of up to 30% containing overpressured fluids, which could act as a protodécollement seaward from the accretionary prism and décollement beneath the forearc. This weak protodécollement combined with the high‐velocity indurated sediments above the basement possibly facilitated the rupture propagating up to the front during the 2004 earthquake and enhancing the tsunami. We also find another low‐velocity layer within the sediments that may act as a secondary décollement observed offshore central Sumatra, forming bivergent pop‐up structures and acting as a conveyer belt in preserving these pop‐up structures in the forearc region.

Stratigraphic Control of Frontal Décollement Level and Structural Vergence and Implications for Tsunamigenic Earthquake Hazard in Sumatra, Indonesia

“…We interpret the top of Unit I as the top of the igneous oceanic crust. The reflectivity features of Unit I are typical of basaltic crust formed by oceanic spreading within the latest Cretaceous Wharton Basin (Bécel, ; Qin & Singh, ). The Unit I upper reflector is notably featureless and smooth on southern profiles but becomes more rugged in the vicinity of the Investigator Fracture Zone.…”

“…We interpret Unit II as heterolithic pelagic sediments deposited onto oceanic crust. High‐amplitude negative polarity (HANP) reflectors such as the one found locally at the top of Unit II have previously been imaged at the top of the basal pelagic sequence along the northern Sumatran subduction system (Dean et al, ; Qin & Singh, ) and are thought to reflect overpressurization of porous rocks by fresh water released during clay diagenesis (Hupers et al, ; Qin & Singh, ). The average thickness of Unit II is similar to the thickness of pelagic sediments penetrated by Integrated Ocean Drilling Program core U1480 (McNeill, Dugan, Backman, et al, ) and imaged on seismic lines within the Wharton Basin (Carton et al, ; Dean et al, ).…”

Full-waveform inversion of short-offset, band-limited seismic data in the Alboran Basin (SE Iberia)

“…While some studies use waveform modelling in downward continued data (Qin and Singh, 2018), here only the traveltime information is used for inversion. The wave field during the back-propagation is altered due to several factors.…”

GO_3D_OBS – The Nankai Trough-inspired benchmark geomodel for seismic imaging methods assessment and next generation 3D surveys design (version 1.0)

“…from complementary OBS data) and the deep reflection are recorded with an acceptable signal-to-noise ratio. Higher-resolution velocity models can also be built from streamer data by FWI (Shipp and Singh, 2002;Qin and Singh, 2017) if the information carried out by diving waves is made usable after the re-datuming of the data on the seabed (Gras et al, 2019). As an alternative to classical FWI, velocity model can be built by Reflection Waveform Inversion (RWI), a reformulation of FWI where the reflectivity estimated by least-squares RTM is used as a secondary buried source to update the velocities along the reflection paths connecting the reflectors to the sources and receivers (Xu et al, 2012;Brossier et al, 2015;Zhou et al, 2015;Wu and Alkhalifah, 2015).…”