Ban‐Yuan Kuo

Typhoons generate powerful ocean gravity waves that interact with the seafloor, producing microseisms detectable by remote seismometers. This study analyzed microseism generation associated with two consecutive super typhoons, Trami and Kong-Rey, which traversed the Ryukyu arc in 2018. Using continuous seismic and pressure records from ocean-bottom seismometers (OBSs) and nearby land stations combined with ERA5 reanalysis and WAVEWATCH III wave model outputs, we investigated the spatiotemporal evolution of typhoon-driven microseism sources across the subduction margin. We identified distinct short-period (2–6 s) and long-period (6–10 s) double-frequency microseisms. As the typhoons approached the OBS array, microseism power surged, the spectral bandwidth broadened, and the dominant period shifted to longer periods, exhibiting strong correlations with decreasing sea surface pressure and increasing significant wave height. Source tracking reveals two dominant excitation mechanisms. The first involves coastal reflections of storm-induced waves along the Ryukyu island arc (class II), generating opposing swells that interact with incident waves. The second corresponds to single-storm opposing-swell interactions (class IIIa), in which a rapidly moving storm overtakes its previously generated swell, creating new opposing waves that interact with the pre-existing swell. Our results confirm the role of the Ryukyu subduction system in modulating the transfer of ocean wave energy into seismic ground motion and demonstrate the effectiveness of OBSs in tracking typhoon-induced microseism sources.

Shear‐wave splitting measurements have the potential to constrain multiple layers of anisotropy and thereby enhance depth resolution. Using the formulation of Silver and Savage (1994, https://doi.org/10.1111/j.1365‐246x.1994.tb04027.x ), previous studies have employed deterministic grid‐search approaches to identify best‐fitting two‐layer models. In this study, we adopt a Bayesian inversion that incorporates a factor accounting for uncertainties in shear‐wave polarization orientations to resolve two‐layer anisotropic structures. We demonstrate this inversion using source‐side S and SKS splitting data from the Cocos subduction zone and then apply it to the South American and Hikurangi subduction zones. In the Cocos system, despite strong parameter tradeoff, our model resolves two layers without relying on a priori tectonic arguments, in contrast to previous deterministic inversions. The fast direction of the upper layer is perpendicular to the trench or parallel to the Cocos plate motion predicted by the no‐net‐rotation, rather than the hotspot, reference frame. The lower‐layer anisotropy is trench‐oblique, consistent with previous suggestions. In the other two subduction zones, our approach tightly constrains the upper‐layer anisotropy as trench‐subparallel and the lower‐layer anisotropy as trench‐normal, deviating from previously reported best‐fitting models. A three‐layer inversion suggests a transitional layer between the two layers for all subduction zones, but such models are not warranted by the data. The layering order in the South America and Hikurangi subduction zones is opposite to that in the Cocos and the poloidal–toroidal dual structure in the subslab mantle predicted by dynamic modeling, implying possible contributions from the slab and the overriding plate to the upper layer.

Shear wave splitting is widely used to probe seismic anisotropy, but its depth resolution is limited. Building on the formulation of Silver and Savage (1994), we apply a Bayesian inversion that explicitly accounts for uncertainty in shear-wave polarization orientation (θ) to resolve multilayer anisotropy from splitting parameters. We apply this approach to source-side S and SKS data from the Cocos subduction zone, and to SKS data from station NNA above the South America subduction zone and station SNZO at the southern Hikurangi margin. Subduction in all three regions is shallow to flat, minimizing dip-angle effects on SKS. The inversion yields tightly constrained fast directions for both upper and lower anisotropic layers. Three-layer inversions show progressive rotation with depth consistent with two-layer solutions, but are not required by the data. In the Cocos system, the upper-layer fast direction is unambiguously aligned with Cocos plate motion in the NNR reference frame, consistent with subduction-entrained flow. In contrast, beneath NNA and SNZO, the upper and lower layers exhibit trench-subparallel and trench-normal anisotropy, respectively—opposite in layering sense to the poloidal–toroidal flow structure predicted by dynamic models. If both layers reside in the subslab mantle, the trench-parallel upper layer flow would decouple the deeper mantle from the slab, raising questions about how the apparent subduction-driven flow is maintained at depth. Alternatively, the upper layer may reflect deformation within or above the slab. Possible sources of trench-parallel anisotropy include frozen-in oceanic lithospheric fabrics, trench-parallel mantle-wedge flow, or inherited fabrics related to nearby continental shear zones. These results highlight the complexity of subslab dynamics and demonstrate the value of probabilistic multilayer inversion for interpreting shear-wave splitting.

This study investigates the characteristics and generation mechanisms of ocean microseisms in the Northern Okinawa Trough‐Ryukyu Arc‐Trench System, using continuous recordings from in situ ocean‐bottom seismometers and nearby land stations, integrated with the ERA5 climate and ocean reanalysis data sets. The power spectral densities (PSDs) of double‐frequency microseisms (DFMs) deviate notably from the classical Peterson's model in the 0.8–4 s period range, suggesting regional noise characteristics that may not be fully captured by global reference curves, especially in complex marine environments. Two distinct DFM peaks are identified at 2–6 s (SPDFM) and 6–10 s (LPDFM), both of which intensify during winter monsoons and typhoons as strong seasonal winds generate high‐energy waves that couple efficiently with the seafloor, amplifying microseism activity. SPDFM PSDs correlate strongly with significant wave height (SWH) in marginal seas, such as the East China Sea, where local wave conditions and bathymetric barriers predominantly govern asymmetric energy distributions. In contrast, LPDFM exhibits broader, more symmetrical correlations with SWH, extending into the deeper Philippine Sea and Western Pacific. Single‐frequency microseisms (SFM, 10–20 s) display similar patterns but weaker correlations, with both LPDFM and SFM linked to distant swells from the North Atlantic and equatorial Indian Oceans. Polarization analysis indicates that Love waves dominate SPDFM, with contributions from S waves, while sediment cover and topographic features introduce complexity in wave separation. These findings emphasize the role of local and distant wave sources, atmospheric conditions, and bathymetric undulations in modulating microseism generation and propagation, providing a framework for studying atmosphere‐ocean‐solid earth interactions.

How continental plate boundary faults develop with depth has been under debate. We inverted SKS shear wave splitting data along the San Andreas Fault (SAF) into two layers of anisotropy using a Bayesian inversion. While the two layers are statistically required, the fast polarization directions of the upper layer do not match the strike of the SAF as previously reported. To capture the lithospheric shear zone, we progressively decrease the upper limit of the delay time of the upper layer. In northern California where SAF strikes 140-150, the upper layer fast directions get close to these azimuths when the delay time is reduced to ~0.5 s. In southern California where the SAF strikes 120-130, the upper layer fast directions capture the SAF with delay times of a similar magnitude. For olivine LPO with vertical shear plane, these delay times translate to a anisotropy layer of 40 km thickness, or a depth of 70 km from the surface, assuming the seismogenic zone in the crust is too localized to influence the SKS splitting. This depth coincides with the depth of lithosphere-asthenosphere boundary independently estimated. The lower-layer fast directions are in between the absolute plate motion directions of the American and the Pacific plates, or at least agree with that predicted from surface wave study in northern California. We picture a vertical continental shear zone widening to at least 200 km at the bottom of the lithosphere, transitioning to a horizontal shear regime in the asthenosphere driven by plate motions. This architecture of continental shear zone is consistent with our understanding of the rheology of crust and mantle.

Theories related to the Earth's attenuation effect and earthquake source properties predict a similar spectral falloff of body waves, creating severe trade‐off problems when determining these two processes from waveform spectra. Previous efforts attempted to suppress this trade‐off by either applying an empirical Green's function or conducting linear inversion with a specially designed event–station configuration. However, both approaches encounter difficulties when dealing with large datasets or site effect contamination. In this study, we modify a previously used cluster‐event method (CEM), combining both the spectral amplitude and the spectral ratio of the P‐waves for the clustering of events (CEM2); we then apply CEM2 to the High‐Sensitivity Seismograph Network data in the Tohoku region of Japan. An initial inversion with CEM2 yields the first estimates of the source and path parameters, that is, the corner frequency (f_c) and the quality factor (Q). The residuals of this inversion demonstrate distinct spectral characteristics that are well correlated with vertical‐to‐horizontal (V/H) spectral ratios of the P‐coda waves at each station, suggesting that they represent a form of site response unaccounted for by theoretical predictions of Q and f_c. After removing this particular site response, the second inversion yields an improved determination of Q. We hypothesize that this site effect is dominated by P to SV conversion upon P‐wave arrival and develops and amplifies toward the coda waves. Furthermore, the site effect demonstrates systematic regional characteristics, some of which correlate well with those derived from the V/H spectral ratios of the P‐coda waves.

In this study, we reveal the deformational structure of the crust of the northern part of the Ryukyu Arc and Okinawa Trough using ambient noise tomography. Compared with southern Ryukyu, the northern segment exhibits a wide and shallow basin, a crust without localized thinning, slow extension rates, and highly arc‐oblique, right‐lateral retreat of the Ryukyu Arc. We present both isotropic and azimuthally anisotropic shear‐wave velocity models using data recorded by an ocean‐bottom seismometer array and nearby island stations. The isotropic model demonstrates a monotonic decrease in velocity from the backarc to the forearc, in accord with the accretionary‐prism origin of the latter. The resolved azimuthal anisotropy exhibits arc‐parallel and arc‐perpendicular fast shear‐wave polarization directions in the upper to mid‐crust and the lower crust to uppermost mantle in much of the arc and backarc, respectively. We interpret the arc‐parallel anisotropy as resulting from the anisotropic fabrics aligned by the vertical shearing imposed by the right‐lateral motion of the Ryukyu Arc. The underlying arc‐perpendicular anisotropy may be attributed to horizontal shearing driven by corner flow in the mantle wedge. We found arc‐perpendicular anisotropy in the forearc upper crust, which may reflect crack alignment caused by the collision of the Amami Plateau. The oblique arc retreat and the resolved deformation fabrics in the arc and backarc together attest to the shear‐dominant, transtensional nature of the northern Ryukyu continental rift system. Some of the features in northern Ryukyu may be better explained from the perspective of transtensional rifting.

A lab-fabricated ocean bottom seismometer was modified and deployed terrestrially to detect low-frequency (<10 Hz) ground vibrations produced by debris flows. A frequency–response test of the new seismometer revealed that it can detect seismic signals at frequencies of 0.3–120 Hz. Its seismic ground motion detection ability was investigated by comparing its measurements of seismic signals produced by rockfalls with those of a geophone. Two new seismometers were deployed at the Aiyuzi Stream, Nantou County, Taiwan, in September 2012. Seismic signals produced by two local earthquakes, two teleseisms, and three debris flows detected by the seismometer in 2013 and 2014 were discussed. The seismic signal frequencies of the local earthquakes and teleseisms (both approximately 1800 km apart) were 0.3–30 and <1 Hz, respectively. Moreover, seismometer measurements revealed that seismic signals generated by debris flows can have minimum frequencies as low as 2 Hz. Time-matched CCD camera images revealed that debris flow surge fronts with larger rocks have lower minimum frequencies. Finally, because the seismometer can detect low-frequency seismic waves with low spatial decay rates, it was able to detect one debris flow approximately 3 min and 40 s before it arrived.

Carried by the Philippine Sea Plate converging with the Eurasian Plate, the Luzon Arc is obliquely colliding with and accreting to the Eurasian continental margin to build the Taiwan orogen. Northward along the collision suture, the Luzon Arc extends offshore and is being subducted under the Ryukyu Trench. How the northern segment of the Luzon Arc responds to both collision and subduction remains poorly understood. At present, seismic tomography and conventional stress analyses cannot fully capture images of this arc or discern how it deforms following subduction. We employed a three‐dimensional spatial clustering approach in eastern Taiwan to reveal how stress varies along the collision suture. Our results show that the maximum horizontal compression orientation, S_H, is sub‐parallel to the plate motion at all depths across the lithosphere, except at shallow depths for the northern segment, where S_H exhibits a clockwise rotation from convergence‐parallel by 10°–30°. Accompanying the S_H rotation in the northern crustal segment is a tilt of the maximum compressive P‐axes by approximately 20°. We interpret this systematic variation along the suture and with depth as resulting from the rotation of the stress regime in the crust of the Luzon Arc forced by the oblique subduction, and likely augmented by the bending of the Central Range crustal block. The location of rotation/tilting and its disparity with depth may signify a decoupling of the volcanic arc from the mantle lithosphere during early subduction.

Mapping both isotropic and anisotropic velocity structures of the crust provides insight into the dominant mechanisms that produce the deformation. We performed ambient noise tomography for shear velocity V_s in the Tohoku region, Japan, where plate kinematics remains simple as the Pacific plate is subducting under the Okhotsk plate. Cross‐correlation functions were retrieved from Hi‐net recordings and phase velocities of 3–16 s were measured, followed by a one‐step, wavelet‐based multiscale inversion for both V_s and its azimuthal anisotropy. The isotropic model shows that high V_s anomalies underlie the Cretaceous aged Kitakami and Abukuma Mountains from shallow crust to lower crust. Low V_s is found to track the active volcanoes with the strongest anomalies concentrated at roughly 10 km depth, which we interpret as the shallow magma reservoirs of the Quaternary magmatic system. The resolved azimuthal anisotropy exhibits a switch of anisotropy in the upper crust from island‐parallel to a random or suppressed state in 5–15 km depth interval and back to island‐parallel. The island‐parallel fast direction may map the structural fabrics of the crustal deformation, and the spatial correlation between velocity and anisotropy patterns suggests that continuous volcanic activities and magma ponding disrupt the otherwise consistent fabrics in the upper crust. Below 20 km depth, the anisotropy turns to convergence‐parallel, which may result from shearing either imposed by the return flow in the mantle wedge or frozen‐in from the last stage of extension of the continental margin that opened the Sea of Japan.