T waves excited by earthquakes propagate along the SOFAR channel with low transmission loss, and therefore can be recorded on land-based seismic stations and hydrophones located thousands of kilometers away from earthquake epicenters. Early T-wave observations are mostly based on recordings by land-based stations due to the mechanics of the energy conversion of acoustic waves into seismic phases. Recently, T-wave signals have also been detected by ocean-bottom seismometers (OBSs) at deep ocean basin offshore eastern Taiwan, raising the question of how deep ocean environment affects the generation and propagation of T-waves. In this study, to understand how acoustic energy scatters and interacts with different seafloor topography, we apply the acoustic ray theory to simulate acoustic propagation in the presence of realistic seafloor topography and sound speed profile. Our simulations indicate that seafloor topography indeed affects the acoustic propagation pattern, part of which may reach deep ocean regions. We also simulate seismic energy of T-waves by stacking energy coming from a series of potential conversion points within a specific time-window. The stacked energy distribution expresses a pattern similar to the envelope function of T-waves, indicating that the long-lasting waveform may result from a series of seismic-acoustic conversion processes.
Abstract Slow earthquakes play a crucial role in understanding stress accumulation and release along plate interfaces in subduction zones. The northern Ryukyu Trench, where the Philippine Sea Plate subducts northwestward beneath the Eurasian Plate, experienced a major earthquake in 1911 and is currently regarded as a low-seismicity area (LSA). Understanding the seismic activity in this region, particularly the relationship between very-low-frequency earthquakes (VLFEs) and regular seismic events, is crucial for understanding subduction zone dynamics. We investigated the spatial and temporal distribution of VLFE activity in the northern Ryukyu Trench using broadband ocean-bottom seismometers deployed around Amami Island between September 2018 and June 2019. Our analysis, employing the envelope correlation method, revealed that VLFE activity is primarily concentrated northeast of Amami Island, an area characterized by low regular earthquake activity, with the distribution of VLFEs spatially segregated from that of regular earthquakes. Furthermore, we observed earthquake swarm activity at the edges of the LSA in the northern Ryukyu Trench following VLFE activity. In November 2018, intense VLFE activity northeast of Amami Island migrated northeastward, which was followed by a regular earthquake swarm at the edge of this LSA. Following VLFE activity in January 2019, additional seismic activity, including foreshocks, occurred at the edges of this LSA approximately 1 month later. The spatial segregation of VLFEs and regular earthquakes suggests that VLFE activity may be influenced by the migration of high-pressure fluids within the subducted slab. This migration appears to trigger related time-delayed seismic activity, similar to mechanisms observed in other subduction zones such as Hikurangi. Understanding these dynamics is essential for assessing the coupling state of subduction zones and associated fluid behaviors, which play a critical role in evaluating seismic hazards in LSAs. Graphical Abstract
Abstract This study focuses on developing and evaluating the broadband ocean bottom seismometer (Yardbird-BB OBS) in Taiwan. The Yardbird-BB OBS is a crucial instrument for recording seismic signals in deep-sea environments. Rigorous testing ensures optimal performance and data recording capabilities. Following assembly, the Yardbird-BB OBS undergoes a 3–6 month deployment test in the deep sea, capturing seismic signals worldwide. Data from 2016 and 2017 deployments in the Okinawa Trough analyze significant seismic events, including a magnitude 7.8 earthquake in New Zealand and a magnitude 6.3 earthquake from a North Korean nuclear test. Waveform analysis, focusing on tele-seismic events and waveform quality, assesses the OBS’s performance, highlighting successful automatic leveling adjustment. These high-quality recordings benefit research, aiding the study of plate tectonics, crustal age estimation, seafloor ambient noise determination, and earthquake location accuracy improvement. The study also details methods for verifying instrumental self-noise, dynamic range, digitization sensitivity, linearity error, clock drift, and data logger power consumption. Calibration procedures and evaluation methods provide insights into Yardbird-BB OBS performance characteristics, contributing to its understanding and enhancement for effective long-term underwater data recording and valuable scientific research.
Abstract Robust determination of earthquake source parameters over a continuous depth range is central to inferring rupture mechanisms dominant at different depths. We employed a cluster‐event method to constrain the source parameters as well as along‐path attenuation for earthquakes over 0–150 km depths and 4 orders of seismic moments in the Japan subduction zone. We found that corner frequency and stress drop increase with depth, whereas the radiated energy scaled by seismic moment declines with depth slightly. As a result, the radiation efficiency exhibits a notable deficit for events deeper than 60 km. Together these suggest an increased energy dissipation during faulting in ductile deformation regime, consistent with shear heating instability as an important faulting mechanism for intermediate‐depth earthquakes.
Abstract T waves are conventionally defined as seismically generated acoustic energy propagating horizontally over long distances within the minimum sound speed layer in the ocean (SOFAR axis minimum). However, T waves have also been observed by ocean‐bottom seismometers in ocean basins at depths greater than the SOFAR axis minimum. Previously, nongeometrical processes, such as local scattering at rough seafloor and water‐sediment interface coupling, have been proposed as possible mechanisms for deep seafloor detection of T waves. Here we employ a new T wave modeling approach based on hydroacoustic ray theory to demonstrate that seismoacoustic energy can propagate to reach deep seafloor, previously considered as shadow zone of acoustic propagation. Our new hydroacoustic simulations explain well the observations of T waves on ocean‐bottom seismometers at deep ocean basins east of Taiwan and shed new light on the mechanism for deep ocean T wave propagation.
Taiwan is located near the edge of the continental Eurasia plate and the Philippine Sea plate, a quake-prone zone. Averagely speaking, out of 1000 sensible quakes detected per year, more than 50% of the epicenters are located in the surrounding waters. Therefore, there is a great need from our seismology community to deploy ocean bottom seismometer (OBS) to have high fidelity data, and widen the aperture of the observation as well. In this paper, we report the development of a "midweight" OBS called Yardbird which has one year deployment duration. We utilized the newly available ultra-low power MCU (micro control unit) and SD card to design a compact data logger. The response of 4.5-Hz geophones was extended to 3 s and used as the seismic sensors. With the compactness of the sensor module, it enables us to design a compact dual-axis motor-driven stage to level the vertical component to be less than 0.1 degree with respect to the gravity. Currently, the total power consumption of Yardbird OBS is less than 0.2 mW for three channels. A pilot experiment and was conducted in the south western Okinawa trough in 2010. During the three months experiment, an array of five Yardbirds captured and deep quakes from the subducting oceanic plate. The results prove that the performance of Yardbird OBS suffices the basic needs of seismic research.
To explore upper mantle heterogeneity beneath the North Atlantic we have measured 70 SS ‐ S differential travel times using the waveform cross‐correlation method. Both the two‐way preliminary reference earth model (PREM) and J‐B (Jeffreys‐Bullen) residuals exhibit a maximum variation of 17 s, ranging from −10 to 7 and −9 to 8 s, respectively. The lack of correlation between SS ‐ S and S residuals, along with the strong correlation between SS ‐ S and SS residuals, suggests that SS ‐ S differential travel times eliminate source and receiver effects and sample the upper mantle anomalies near the SS bounce points without being strongly affected by lower mantle heterogeneities. We find evidence for a strong local anomaly in the North Atlantic; residuals for bounce points immediately to the north of the Azores‐Gibraltar plate boundary average about 4 s faster than for the bounce points to the south of the boundary. We infer that this velocity contrast exists in the lithosphere and continues below a depth of 200 km. A series of linear multiple‐regression experiments demonstrate that variation in velocity with increasing age of the seafloor and azimuthal anisotropy are both significant contributors to the PREM and J‐B residuals. Assuming that travel time delays are a linear function of the square root of the seafloor age, we find the coefficient for (age)½ to be about −1.0 s/(m.y.)½. This is smaller than but not significantly different from the corresponding slope for the S delays of intraplate earthquakes in the Atlantic reported by J. D. Duschenes and S. C. Solomon (1977) and favors the presence of small amounts of water (e.g., 0.1%) in the upper mantle. In modeling anisotropy we carry out an experiment using synthetic seismograms to examine the shear wave splitting phenomena in the anisotropic medium and investigate three possible forms for SH wave delay as a function of azimuth (θ): sinusoidal variations as a function of 2θ, 4θ, and a combination of 4θ (predominant) and 2θ that mimics the delay expected in olivine crystals when the α axis is aligned horizontally. Our regression models strongly prefer the predominance of a 4θ to a 2θ variation, and the last type of anisotropy explains our data best. From the preferred anisotropy model the olivine α axis, or the upper mantle shearing flow, is inferred to be oriented N‐S. The magnitude of the azimuthal variation suggests that anisotropy extends over a depth range of several hundred kilometers in the upper mantle.
Abstract Many continental orogens feature a pattern of SKS shear wave splitting with fast polarization directions parallel to the mountain fabrics and delay times of 1–2 s, implying that the crust and lithosphere deform consistently. In the Taiwan arc‐continent collision zone, similar pattern of SKS splitting exists, and thereby lithospheric scale deformation due to collision has been assumed. However, recent dynamic modeling demonstrated that the SKS splitting in Taiwan can be generated by the toroidal flow in the asthenosphere induced by the subduction of the Philippine Sea plate and the Eurasian plate. To further evaluate this hypothesis, we analyzed a new data set using a quantitative approach. The results show that models with slab geometries constrained by seismicity explain the observed fast splitting direction to within 25°, whereas the misfit grows to ~50–60° if the toroidal flow is disrupted by the presence of a sizable aseismic slab beneath central Taiwan as often suggested by tomographic imaging. However, small sized aseismic slab or detached slab fragment can potentially reconcile the splitting observations. We estimated the scale of vertical coherence to be 10–40 km in the lithosphere and 100–150 km in the asthenosphere, making the former unfavorable for accumulating large delay times. The low coherence is caused by the subduction of the Eurasian plate that creates complex deformation different from what characterizes the compressional tectonics above the plate. This suggests that the mountain building in Taiwan is a shallow process, rather than lithospheric in scale.
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