We present a new 3‐D seismic structural model of the eastern Indonesian region and its surroundings from full‐waveform inversion (FWI) that exploits seismic data filtered at periods between 15–150 s.
We present the first continental‐scale seismic model of the lithosphere and underlying mantle beneath Southeast Asia obtained from adjoint waveform tomography (often referred to as full‐waveform inversion or FWI), using seismic data filtered at periods from 20 to 150 s. Based on >3,000 hr of analyzed waveform data gathered from ∼13,000 unique source‐receiver pairs, we image isotropic
- NSF-PAR ID:
- 10393590
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 127
- Issue:
- 3
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract SASSY21 —a recent 3‐D FWI tomographic model of Southeast Asia—is used as a starting model, and our study region is characterized by particularly good data coverage, which facilitates a more refined image. We use the spectral‐element solverSalvus to determine the full 3‐D wavefield, accounting for the fluid ocean explicitly by solving a coupled system of acoustic and elastic wave equations. This is computationally more expensive but allows seismic waves within the water layer to be simulated, which becomes important for periods ≤20 s. We investigate path‐dependent effects of surface elevation (topography and bathymetry) and the fluid ocean on synthetic waveforms, and compare our final model to the tomographic result obtained with the frequently used ocean loading approximation. Furthermore, we highlight some of the key features of our final model—SASSIER22 —after 34 L‐BFGS iterations, which reveals detailed anomalies down to the mantle transition zone, including a convergent double‐subduction zone along the southern segment of the Philippine Trench, which was not evident in the starting model. A more detailed illumination of the slab beneath the North Sulawesi Trench reveals a pronounced positive wavespeed anomaly down to 200 km depth, consistent with the maximum depth of seismicity, and a more diffuse but aseismic positive wavespeed anomaly that continues to the 410 km discontinuity. -
SUMMARY The detailed structure near the 410-km discontinuity provides key constraints of the dynamic interactions between the upper mantle and the lower mantle through the mantle transition zone (MTZ) via mass and heat exchange. Meanwhile, the temperature of the subducting slab, which can be derived from its fast wave speed perturbation, is critical for understanding the mantle dynamics in subduction zones where the slab enters the MTZ. Multipathing, i.e. triplicated, body waves that bottom near the MTZ carry rich information of the 410-km discontinuity structure and can be used to constrain the discontinuity depth and radial variations of wave speeds across it. In this study, we systematically analysed the trade-off between model parameters in triplication studies using synthetic examples. Specifically, we illustrated the necessity of using array-normalized amplitude. Two 1-D depth profiles of the wave speed below the Tatar Strait of Russia in the Kuril subduction zone are obtained. We have observed triplications due to both the 410-km discontinuity and the slab upper surface. And, seismic structures for these two interfaces are simultaneously inverted. Our derived 410-km discontinuity depths for the northern and southern regions are at 420$\pm $15 and 425$\pm $15 km, respectively, with no observable uplift. The slab upper surface is inverted to be located about 50–70 km below the 410-km discontinuity. This location is between the depths of the 1 and 2 per cent P-wave speed perturbation contours of a regional 3-D full-waveform inversion (FWI) model, but we found twice the wave speed perturbation amplitude. A wave speed increase of 3.9–4.6 per cent within the slab, compared to 2.0–2.4 per cent from the 3-D FWI model, is necessary to fit the waveforms with the shortest period of 2 s, indicating that high-frequency waves are required to accurately resolve the detailed structures near the MTZ.more » « less
-
Subduction of the very young (<15 Myr old) oceanic lithosphere of the Nazca plate in central to southern Colombia is observationally related to an unusually high and unusually variable amount of intermediate (>50 km) depth seismicity. From 2010 through 2019 89% of central and southern Colombia’s 11,466 intermediate depth events occurred between 3.5°N and 5.5°N, highlighting these unusual characteristics of the young slab. In addition, morphologic complexity and possible tears characterize the Nazca slab in Colombia and complicate mantle flow in the region. Prior SKS-phase shear-wave splitting results indicate sub-slab anisotropy is dominated by plate motion parallel-to-subparallel orientations in the region, suggesting the young slab has entrained a relatively thick portion of the sub-slab mantle. These observations suggest the subduction of young lithosphere has significant effects on both the overlying and underlying asthenosphere in the Colombia subduction zone. Here we use more than 10 years of data to calculate receiver functions for the Red Sismológica Nacional de Colombia’s network of broadband seismometers. These receiver functions allow us to tie these prior observations of the Colombia subduction zone to distinct, structural features of the slab. We find that the region of high seismicity corresponds to a low seismic velocity feature along the top of the subducting plate between 3.5°N and 5.5°N that is not present to the south. Moderately elevated P-wave velocity to S-wave velocity ratios are also observed within the slab in the north. This feature likely represents hydrated slab mantle and/or uneclogitized oceanic crust extending to a deeper depth in the north of the region which may provide fluids to drive slab seismicity. We further find evidence for a thick layer of material along the slab’s lithosphere-asthenosphere boundary characterized by spatially variable anisotropy. This feature likely represents entrained asthenosphere at the base of the plate sheared by both the overlying plate and complex flow related to proposed slab tears just north and south of the study region. These observations highlight how structural observations provide key contextual constraints on short-term (seismogenic) and long-term (anisotropic fabric) dynamic processes in the Colombia subduction zone. Plain-language Summary The Nazca oceanic plate is very young (<15 million years old) where it is pulled or subducted beneath the South America plate in central and southern Colombia. Earthquakes occurring in the subducted Nazca plate at depths greater than 50 km are nearly 9x more common in central Colombia than in southern Colombia. The subducted Nazca plate also has a complex shape in this region and may have been torn both in northern Colombia and to the south near the Colombia-Ecuador border. The slow flow of mantle rock beneath the subducted plate is believed to be affected by this and earlier studies have inferred this flow is mostly in the same direction as the subducting plate's motion. We have used 10+ years of data to calculate receiver functions, which can detect changes in the velocity of seismic waves at the top and bottom of the subducted plate to investigate these features. We found that the Nazca plate is either hydrated or has rocks with lower seismic velocities at its top in the central part of Colombia where earthquakes are common. We also find that a thick layer of mantle rock at the base of the subducted plate has been sheared.more » « less
-
more » « less
Abstract The structure of the mantle transition zone (MTZ) in subduction zones is essential for understanding subduction dynamics in the deep mantle and its surface responses. We constructed the P (
V p ) and SH velocity (V s ) structure images of the MTZ beneath Northeast Asia based on two‐dimensional (2‐D) triplicated waveform modeling. In the upper MTZ, a normalV p but 2.5% lowV s layer compared with IASP91 are required by the triplication data. In the lower MTZ, our results show a relatively higher‐velocity layer (+2%V p and −0.5%V s compared to IASP91) with a thickness of ~140 km and length of ~1,200 km atop the 660‐km discontinuity. Taking this anomaly as the stagnant slab and considering the plate convergence rate of 7–10 cm/year in the western Pacific region during the late Cenozoic, we deduced that the stagnant slab has a subduction age of less than 30 Ma. This suggests that the observed stagnancy of the slab in the MTZ beneath Northeast Asia may have occurred no earlier than the Early Oligocene. From the constraints derived individually onV p andV s structures, highV p /V s ratios are obtained for the entire MTZ beneath Northeast Asia, which may imply a water‐rich and/or carbonated environment. Within the overall higher‐velocity stagnant slab, a low‐velocity anomaly was further detected, with a width of ~150 km,V p andV s reductions of 1% and 3% relative to IASP91. Such a gap may have provided a passage for hot deep mantle materials to penetrate through the thick slab and feed the Changbaishan volcano. -
more » « less
Abstract I present a 3‐D isotropic shear wave velocity model of the crust and uppermost mantle beneath the Alaska‐Aleutian subduction zone offshore of the Alaska Peninsula, based on seismic data recorded by the Alaska Amphibious Community Seismic Experiment (AACSE) array and some other networks. The model derives from Rayleigh wave phase speed measurements extracted from ambient seismic noise. A new three‐station interferometry (Zhang et al., 2020) approach is applied to improve the data coverage of ambient noise surface waves. Based on the ambient noise Rayleigh wave dispersion data, a Bayesian Monte Carlo inversion is performed to produce the shear wave velocity model. There are several prominent structures captured by the model, including: (1) The major sedimentary basins across the study region are identified by model. (2) Crustal thickness estimates are related with the geological structures. (3) The imaged slab edge is consistent with both the Slab 2.0 model (Hayes et al., 2018) and earthquake locations. And lots of geological and tectonic features related to subduction zone are captured, including the serpentinized forearc and partial melting zone beneath the Aleutian arc volcanoes. (4) Near the Shumagin gap, reduction in Vs is observed at the uppermost part of the incoming Pacific plate, consistent with the active source study of Shillington et al. (2015). The Vs reduction reflects hydration of the oceanic plate which could be related to local seismicity variation.
