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Topographic Rossby waves (TRWs) are oscillations generated on sloping topography when water columns travel across isobaths under potential vorticity conservation. From our large-scale observations from 2016 to 2019, near-65-day TRWs were first observed in the deep basin of the South China Sea (SCS). The TRWs propagated westward with a larger wavelength (235 km) and phase speed (3.6 km day−1) in the north of the array and a smaller wavelength (80 km) and phase speed (1.2 km day−1) toward the southwest of the array. The ray-tracing model was used to identify the energy source and propagation features of the TRWs. The paths of the near-65-day TRWs mainly followed the isobaths with a slightly downslope propagation. The possible energy source of the TRWs was the variance of surface eddies southwest of Taiwan. The near-65-day energy propagated from the southwest of Taiwan to the northeast and southwest of the array over ~100–120 and ~105 days, respectively, corresponding to a group velocity of 4.2–5.0 and 10.5 km day−1, respectively. This suggests that TRWs play an important role in deep-ocean dynamics and deep current variation, and upper-ocean variance may adjust the intraseasonal variability in the deep SCS. Significance Statement: Topographic Rossby waves (TRWs) play an important role in deep-ocean dynamics and deep current variation, and they have potential impacts on sediment transport and abyssal biogeochemical processes. From large-scale observations of abyssal currents in the deep basin of the South China Sea (SCS), energetic near-65-day TRWs with a westward phase speed of 1.2–3.6 km day−1 were first observed. The ray-tracing model was used to identify the energy source and propagation features of the TRWs. The possible energy source was the variation of mesoscale eddies southwest of Taiwan. The study has implications for adjustments to the energy and intraseasonal variability generation in the deep basin of the SCS. [ABSTRACT FROM AUTHOR]

Ambient noise analysis in Northern Taiwan revealed obvious lateral variations related to major geological units. The empirical Green’s functions extracted from interstation ambient noise were regarded as Rayleigh waves, from which we analyzed the group velocities for period from 3 to 6 s. According to geological features, we divided Northern Taiwan into seven subregions, for which regionalized group velocities were derived by using the pure-path method. On average, the group velocities in mountain areas were higher than those in the plain areas. We subsequently inverted the S -wave velocity structure for each subregion down to 6 km in depth. Following the analysis, we proposed the first models of geology-dependent shallow S- wave structures in Northern Taiwan. Overall, the velocity increased substantially from west to east; specifically, the mountain areas, composed of metamorphic rocks, exhibited higher velocities than did the coastal plain and basin, which consist of soft sediment. At a shallow depth, the Western Coastal Plain, Taipei Basin, and Ilan Plain displayed a larger velocity gradient than did other regions. At the top 3 km of the model, the average velocity gradient was 0.39 km/s per km for the Western Coastal Plain and 0.15 km/s per km for the Central Range. These S -wave velocity models with large velocity gradients caused the seismic waves to become trapped easily in strata and, thus, the ground motion was amplified. The regionalized S -wave velocity models derived from ambient noises can provide useful information regarding seismic wave propagation and for assessing seismic hazards in Northern Taiwan. [ABSTRACT FROM AUTHOR]

The lapse-time dependence of the quality factor Qc measured from the S-wave codaof local earthquakes at frequencies above 1 Hz has long been proven effective toinvestigate separately the scattering and absorption properties of the crust. On the otherhand, recent studies demonstrate that the Green’s function retrieved from the noisecross-correlation (NCF) between two stations contains a tail of multiply-scattered waves afterballistic waves like the coda of earthquakes, the decay rate of which can thereforebe also exploited for mapping of subsurface attenuation structures. In this study,we estimate frequency-dependent Qc of the coda in the dominant period range ofsecondary microseisms (3-9 s) derived from the yearly-stacked NCFs between allthe available station pairs separated by less than 80 km apart in Taiwan, which isknown as one of the most seismically hazardous and geologically complex regions inthe world. To ensure the Qc measured by the coda at sufficiently long lapse timeto be representative of the physically meaningful intrinsic quality factor (Qi) assuggested by Calvet and Margerin (2013), we choose different onset times (tw)of the coda at 10 or 20 s after the direct surface wave arrivals calculated by theinter-station distance divided by the average group velocity and measure Qc as afunction of the length of the coda window (Lw). The Qc values at periods of 3-6 sand 4.5-9 s determined with different choices of tw and the fixed Lw reveal noobvious trend with the inter-station distance. However, similar to the Qc observationsfrom the earthquake coda, they generally follow a transient increase with Lw atrelatively short lapse times and then approach a stabilized plateau value at Lw ∼75-100 s considered as an optimal approximation of Qi. We preliminarily map thelateral variations of intrinsic attenuation by simply distributing the determined Qcuniformly along the corresponding inter-station path and taking the mean values ofQc within individual 0.2∘× 0.2∘ cells. The results show that the Qc variation atthe shorter periods of 3-6 s is more pronounced and the lowest (∼50), implyingthe highest intrinsic attenuation, in the Coastal Plain of SW Taiwan covered withthick sedimentary deposits. It gradually increases toward the Western Foothillsfold and thrust belt in south central Taiwan and reaches the highest (∼100) in theHsuehshan Range in north central Taiwan, which seems to be correlated with theprogressive increase of the metamorphic grade in the Taiwan orogen as a result of thesouthward-propagating arc-continent collision. Further attenuation tomographyusing the proper sensitivity kernels will be conducted to reveal both lateral anddepth variations of the absorption properties across diverse litho-geological units inTaiwan. [ABSTRACT FROM AUTHOR]