Structural features and shear‐velocity structure of the “Pacific Anomaly”

Abstract

1] We constrain structural features and shear-velocity structure of a low-velocity anomaly in the lower mantle beneath the Pacific (we term it the ''Pacific Anomaly'') on the basis of forward travel time and waveform modeling of the observed direct S, Sdiff, ScS, SKS, and SKKS phases sampling a great arc across the anomaly from eastern Eurasia to southern South America. After correction for the effects of earthquake mislocation and the seismic heterogeneities outside the Pacific Anomaly, seismic observations suggest that the Pacific Anomaly along the great arc consists of at least two separated portions with a 740-km-wide gap between them. The western portion of the anomaly is about 1050 km wide, extends at least 740 km above the core-mantle boundary (CMB), and exhibits a trapezoidal shape with lateral dimensions increasing slightly with depth. The velocity structure of the western portion varies from À3.0% at the top (740 km above the CMB) to À3.5% at 100 km above the CMB and an average shear-velocity reduction of À5% in the bottom 100 km of the mantle. The eastern portion of the anomaly reaches at least 340 km above the CMB beneath the mid-Pacific with an 1800-km-wide base and has a uniform velocity reduction of À3%. Waveform modeling further suggests a very low velocity layer with a shear-velocity reduction of À10% located at the edge of the western portion of the anomaly. Combining the latest results from others, we present a general picture of structural and velocity structures of the Pacific Anomaly. The structural and velocity features suggest that the anomaly represents a cluster of metastable thermo-chemical piles.Citation: He, Y., and L. Wen (2009), Structural features and shear-velocity structure of the ''Pacific Anomaly,''

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Iron-spin transition controls structure and stability of LLSVPs in the lower mantle

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Morphology of seismically slow lower-mantle structures

“…Using the waveforms and traveltimes of a suite of shear wave phases sampling the lowermost mantle, He & Wen (2009) infer that the West Pacific anomaly reaches 740 km above the CMB and has steep sides. In He & Wen (2012), they confirm the presence of a separate anomaly to the east, 450 km high and bounded by shallowly sloped sides.…”