Wu
Wenbo
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Wenbo
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ArticleOn the origin of small‐scale seismic scatters at 660‐km depth(American Geophysical Union, 2022-11-19) Mao, Wei ; Gurnis, Michael ; Wu, WenboStrong small‐scale seismic scatters (<10 km) have been recently observed at 660 km depth, but their origin remains uncertain. We systematically conduct both high‐resolution 2‐D geodynamic computations that include realistic thermodynamic properties, synthetic seismic waveforms, and insight from shallow seismic observations to explore their origin. We demonstrate that neither short‐term subduction, nor long‐term mechanical mantle mixing processes can produce sufficiently strong heterogeneities to explain the origin of such small‐scale seismic scatters. Instead, the intrinsic heterogeneities inside the oceanic lithosphere which subducts into the mantle transition zone and the uppermost lower mantle can explain the observed short‐wavelength scatter waves.Plain Language Summary
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ArticleVertical‐slice ocean tomography with seismic waves(American Geophysical Union, 2023-04-15) Callies, Jörn ; Wu, Wenbo ; Peng, Shirui ; Zhan, ZhongwenSeismically generated sound waves that propagate through the ocean are used to infer temperature anomalies and their vertical structure in the deep East Indian Ocean. These T waves are generated by earthquakes off Sumatra and received by hydrophone stations off Diego Garcia and Cape Leeuwin. Between repeating earthquakes, a T wave's travel time changes in response to temperature anomalies along the wave's path. What part of the water column the travel time is sensitive to depends on the frequency of the wave, so measuring travel time changes at a few low frequencies constrains the vertical structure of the inferred temperature anomalies. These measurements reveal anomalies due to equatorial waves, mesoscale eddies, and decadal warming trends. By providing direct constraints on basin‐scale averages with dense sampling in time, these data complement previous point measurements that alias local and transient temperature anomalies.
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ArticleAutomatic determination of focal depth with the optimal period of Rayleigh wave amplitude spectra at local distances(Oxford University Press, 2023-08-16) He, Xiaohui ; Zhang, Peizhen ; Ni, Sidao ; Chu, Risheng ; Wu, Wenbo ; Zheng, KaiyueFocal depth of earthquakes is essential for studies of seismogenic processes and seismic hazards. In regions with dense seismic networks, focal depth can be resolved precisely based on the traveltime of P and S, which is less feasible in case of sparse networks. Instead, surface waves are usually the strongest seismic phases at local and regional distances, and its excitation is sensitive to source depth, thus theoretically important for estimating focal depth even with a limited number of seismic stations. In this study, short-period (0.5–20 s) Rayleigh waves are explored to constrain focal depths. We observe that the optimal period (the period corresponding to the maximum amplitude) of Rayleigh waves at local distances (≤200 km) shows an almost linear correlation with focal depth. Based on this finding, we propose an automated method for resolving the focal depth of local earthquakes using the linear regression relation between the optimal period of Rayleigh wave amplitude spectra and focal depth. Synthetic tests indicate the robustness of this method against source parameters (focal mechanism, source duration and non-double-couple component) and crustal velocity structure. Although the attenuation (Q factor) of shallow crust can introduce complexities in determining focal depth, it can be simultaneously estimated if a sufficient number of stations are available. The proposed method is applied to tens of small-to-moderate earthquakes (Mw 3.5–5.0) in diverse tectonic settings, including locations in the United States (Oklahoma, South Carolina, California, Utah, etc.) and China (Sichuan, Shandong). Results demonstrate that reliable focal depth, with uncertainty of 1–2 km, can be determined even with one or a few seismic stations. This highlights the applicability of the method in scenarios characterized by sparse network coverage or historical events.
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ArticleSmall-scale layered structures at the inner core boundary(Nature Research, 2023-10-11) Zhang, Baolong ; Ni, Sidao ; Wu, Wenbo ; Shen, Zhichao ; Wang, Wenzhong ; Sun, Daoyuan ; Wu, ZhongqingThe fine-scale seismic features near the inner core boundary (ICB) provide critical insights into the thermal, chemical, and geodynamical interactions between liquid and solid cores, and may shed light on the evolution mechanism of the Earth’s core. Here, we utilize a dataset of pre-critical PKiKP waveforms to constrain the fine structure at the ICB, considering the influence of various factors such as source complexity, structural anomalies in the mantle, and properties at the ICB. Our modeling suggests a sharp ICB beneath Mongolia and most of Northeast Asia, but a locally laminated ICB structure beneath Central Asia, Siberia, and part of Northeast Asia. The complex ICB structure might be explained by either the existence of a kilometer-scale thickness of mushy zone, or the localized coexistence of bcc and hcp iron phase at the ICB. We infer that there may be considerable lateral variations in the dendrites growing process at ICB, probably due to the complicated thermochemical and geodynamical interaction between the outer and inner core.
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ArticleSeismic ocean thermometry using CTBTO hydrophones(American Geophysical Union, 2023-09-08) Wu, Wenbo ; Shen, Zhichao ; Peng, Shirui ; Zhan, Zhongwen ; Callies, JornDue to limited observational coverage, monitoring the warming of the global ocean, especially the deep ocean, remains a challenging sampling problem. Seismic ocean thermometry (SOT) complements existing point measurements by inferring large-scale averaged ocean temperature changes using the sound waves generated by submarine earthquakes, called T waves. We demonstrate here that Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) hydrophones can record T waves with a higher signal-to-noise ratio compared to a previously used land-based T-wave station. This allows us to use small earthquakes (magnitude <4.0), which occur much more frequently than large events, dramatically improving the resulting temporal resolution of SOT. We also find that the travel time changes of T waves at the land-based T-wave station and the CTBTO hydrophone show small but systematic differences, although the two stations are only about 20 km apart. We attribute this feature to their different acoustic mode components sampling different parts of the ocean. Applying SOT to two CTBTO hydrophones in the East Indian Ocean reveals signals from decadal warming, seasonal variations, and mesoscale eddies, some of which are missing or underestimated in previously available temperature reconstructions. This application demonstrates the great advantage of hydrophone stations for global SOT, especially in regions with a low seismicity level.