Journal Article

Strong ULVZ and Slab Interaction at the Northeastern Edge of the Pacific LLSVP Favors Plume Generation

Dobrosavljevic, Vasilije V., Sun, Daoyuan, Gurnis, Michael, Wu, Wenbo, Jackson, Jennifer M., Lai, Voon Hui, Helmberger, Don V.

2022

Overview

Strong waveform complexities, including multipathing of the S diffracted phase and rapid changes in differential ScS‐S times, are observed for multiple deep Fiji earthquakes recorded at the USArray. The complexities occur at the northeastern edge of the Pacific Large Low Shear Velocity Province (LLSVP), about 12 degrees southeast of present‐day Hawaiʻi. Waveform modeling of the multipathing provides good constraints on an ultra‐low velocity zone (ULVZ) with a width of 5 degree located near the inner edge of the LLSVP. Based on the mineralogical‐modeling of the ULVZ as a solid iron‐rich magnesiowüstite‐bearing assemblage with compatible morphology predicted from geodynamical simulations, a ULVZ model with a thickness of 30 km and a shear wave velocity reduction of 18% is preferred. The rapid change in differential ScS‐S travel time is best explained by having both the aforementioned ULVZ and an adjacent high velocity structure near the LLSVP. Furthermore, a low‐velocity plume‐like structure could potentially explain the observed S travel time delay independent of ScS. These seismic features are proposed to be a ULVZ driven toward the edge of the LLSVP while potentially pushed by a subducted slab. This configuration may trigger plume generation due to strong thermal instabilities and is in the same vicinity where mantle flow models place the present‐day Hawaiian plume source. Multiple ScS can potentially be used to verify vertical plume structure in tomographic models but the accuracy of upper mantle structure, which is a key reflection point, needs to be considered. Plain Language Summary Seismic waves from earthquakes in Fiji recorded by seismometers in the United States travel close to the core‐mantle boundary (CMB) and can be used to image fine‐scale structures along the edge of a previously known large province near the CMB with low seismic wave velocity, namely the Pacific LLSVP. The edges of the Pacific LLSVP are of interest because they contain many structural anomalies, thought to be correlated with hotspots on Earth's surface, including Hawaiʻi, and can host the plume sources for these hotspots. In this study, we observed two phenomena: (a) seismic waves with an additional unexpected pulse due to the presence of a very low velocity structure and (b) a rapid change in the travel time behavior of two seismic phases which can be explained by the same low velocity structure adjacent to a high velocity structure. We constrained these two structures to be at the edge of the LLSVP, a configuration favorable for generating long‐lasting plume, that is, the source for the Hawaiian hotspot. The location of these structures is in agreement with the hypothesized source location from recent geodynamical studies. We also showed that this low velocity structure could be composed of a solid iron‐rich material. Key Points Seismic waves sampling the northeastern edge of the Pacific Large Low Shear Velocity Province (LLSVP) show strong waveform complexity and rapid change in differential time Waveform and mineralogical modeling suggest a magnesiowüstite‐bearing ultra‐low velocity zone adjacent to slab at edge of LLSVP, conducive to plume formation The structural anomalies and proposed plume are located ∼12° southeast of present‐day Hawaiʻi, in agreement with recent mantle flow models

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: body waves / Earth surface / Earthquakes / Hot spots / Iron / lowermost mantle / magnesiowüstite / Modelling / P-waves / Pacific LLSVP / Plumes / Seismic activity / Seismic velocities / Seismic wave velocities / Seismic waves / Seismometers / Shear wave velocities / Structures / Travel time / ULVZ / Upper mantle / Velocity / Wave velocity / waveform modeling