To systematically investigate seismic azimuthal anisotropy in the Sumatra subduction zone and probe mantle dynamics associated with the subduction of the Australian Plate beneath the Sunda Plate, a total of 169 pairs of teleseismic XKS (including PKS, SKKS, SKS) and 115 pairs of local
This study represents the first campaign‐style teleseismic shear wave splitting (SWS) investigation of central Myanmar, an area that is tectonically controlled by the oblique subduction of the Indian Plate underneath the Eurasian Plate. The resulting 678 well‐defined and 247 null SWS measurements obtained from recently deployed 71 broadband seismic stations show that the Indo‐Burma Ranges (IBR) possess mostly N‐S fast orientations that are parallel to the trend of the depth contours of the subducted slab. Relative to the global average of 1.0 s, extremely large splitting times with station‐averaged values ranging from 1.28 to 2.79 s and an area‐averaged value of 2.09 ± 0.55 s are observed in the IBR. In contrast, the Central Basin (CB) and the Shan Plateau (SP) are characterized by slightly larger than normal splitting times. The fast orientations observed in the CB are mostly NE‐SW in the northern part of the study area, N‐S in the central part, and NW‐SE in the southern part. The fast orientations change from nearly N‐S along the N‐S oriented Sagaing Fault, to NW‐SE in the central and eastern portions of the SP. These observations, together with SWS measurements using local S events, crustal anisotropy measurements using P‐to‐S receiver functions, and the estimated depth of the source of anisotropy using the spatial coherency of the splitting parameters, suggest the presence of a trench‐parallel sub‐slab flow system driven by slab rollback, a trench‐perpendicular corner flow, and a trench‐parallel flow possibly entering the mantle wedge through a slab window or gap.
more » « less- NSF-PAR ID:
- 10371982
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 126
- Issue:
- 10
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract S splitting parameters are obtained using broadband seismic data recorded at ~70 stations. Additionally, crustal anisotropy in the overriding Sunda Plate is measured by analyzing the moveout ofP ‐to‐S conversions from the Moho using a sinusoidal function. Comparison between the three sets of anisotropy measurements obtained using shear waves with different depths of origin suggests that (1) the crust of the Sunda Plate is anisotropic with mostly trench‐parallel fast orientations and a mean splitting time of 0.28 ± 0.05 s; (2) the mantle wedge is azimuthally anisotropic with dominantly trench‐parallel fast orientations and splitting times ranging from 0.22 to 0.81 s, which generally increase with the focal depth; and (3) subslab anisotropy is mostly trench‐normal beneath the fore‐arc region with an averaged splitting time of 1.48 ± 0.06 s, and becomes trench‐parallel beneath the arc and back‐arc areas with a mean splitting time of 0.33 ± 0.04 s. The resulting lateral and vertical distributions of anisotropy obtained using splitting of three types of shear waves advocate the presence of an entrained subslab flow that is deflected by the mantle transition zone. The flow enters the mantle wedge through a slab window and flows horizontally parallel to the trench. -
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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
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Abstract Seismic azimuthal anisotropy characterized by shear wave splitting analyses using teleseismic
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