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Repetitive volcanoseismic signals, including very‐long‐period signals (VLP) and long‐period signals (LP), provide a unique probe of fluid transport processes inside magmatic‐plumbing system. While syneruptive signals are often detected and analyzed with regional or/and global seismic networks to retrieve eruption location and mechanism, repetitive noneruptive volcanoseismic signals are generally small, and they are typically detected with in situ stations near the volcanic edifices. Here, we show that repetitive VLP and synchronous deformation events in Aso volcano, Japan, can be detected in the high (15–30 s) and low (50–100 s) VLP bands, respectively, at seismic stations located ∼30–1,000 km away from their sources. Changes in the polarities, phases, and amplitudes of VLP and synchronous deformation events observed at the in situ stations can be verified by the seismic waves in the two VLP bands, respectively, at distant stations up to 150 km. Forward modeling of the amplitude decay in the two VLP bands against epicentral distance corroborates the source locations previously determined by the in situ data, whereas the joint data analysis of in situ and distant stations at high VLP band suggests the presence of single‐force component (i.e., force/moment ratio of 10−4 m−1) in the source of VLPs. We advocate that not only can systematic data mining against established global and regional seismic networks potentially expand the detection capability of repetitive volcanoseismic signals backward in time when in situ observations were unavailable, but it could also substantially improve the detection and monitoring capacity in otherwise uninstrumented volcanoes, complementary to remote sensing of ground deformation. Plain Language Summary: Seismic signals emitted from volcanic regions carry invaluable information on the transport of magma or/and hydrothermal fluid beneath the volcanic edifice. During explosive eruptions, seismic signal can be detected and analyzed using regional or global seismic networks. On the other hand, during the intereruption interval, repetitive volcanoseismic signals directly linked to fluid transport in the magma plumbing system rely on in situ seismic stations near the volcano edifice. Using repetitive seismic signals from one of the most active volcanoes in Aso as an example, we demonstrate that synchronous seismic signals emitted from a shallow crack‐like conduit and a deep chamber roof can be detected at seismic stations even hundreds of kilometers away from their sources. The inclusion of data at distant stations from established global and regional seismic networks improves source characterization, expands the detection capability of repetitive volcanoseismic signals backward in time and increases the monitoring capacity of uninstrumented volcanoes. Key Points: Repetitive noneruptive volcanoseismic signals are often monitored by in situ seismic networks but rarely detected in distant stationsSeismic data at distant stations validate source properties of very‐long‐period signal and synchronous deformation events in Aso volcanoRemote monitoring using established regional seismic networks helps provide insights into in otherwise uninstrumented volcanoes [ABSTRACT FROM AUTHOR]

The seismic discontinuity at 410?km depth in the Earth's mantle is generally attributed to the phase transition of (Mg,Fe)2SiO4 (refs 1, 2) from the olivine to wadsleyite structure. Variation in the depth of this discontinuity is often taken as a proxy for mantle temperature owing to its response to thermal perturbations. For example, a cold anomaly would elevate the 410-km discontinuity, because of its positive Clapeyron slope, whereas a warm anomaly would depress the discontinuity. But trade-offs between seismic wave-speed heterogeneity and discontinuity topography often inhibit detailed analysis of these discontinuities, and structure often appears very complicated. Here we simultaneously model seismic refracted waves and scattered waves from the 410-km discontinuity in the western United States to constrain structure in the region. We find a low-velocity zone, with a shear-wave velocity drop of 5%, on top of the 410-km discontinuity beneath the northwestern United States, extending from southwestern Oregon to the northern Basin and Range province. This low-velocity zone has a thickness that varies from 20 to 90?km with rapid lateral variations. Its spatial extent coincides with both an anomalous composition of overlying volcanism and seismic ‘receiver-function’ observations observed above the region. We interpret the low-velocity zone as a compositional anomaly, possibly due to a dense partial-melt layer, which may be linked to prior subduction of the Farallon plate and back-arc extension. The existence of such a layer could be indicative of high water content in the Earth's transition zone. [ABSTRACT FROM AUTHOR]

High-frequency seismic waves (> 1Hz) contain valuable information indicating the existence of random heterogeneity in the lithosphere. These waves, namely Po and So coda, are typically characterized by their dominance over long distance range (up to 3000 km), high-frequency content and low amplitude decay. Their generation in the oceanic lithosphere is thought to be a result of multiple scattering of P and S waves linked to small-scale heterogeneities within the plate.Previous efforts focused on plates of moderate (15-60 Ma) to old age (> 100 Ma) but no observation and modelling has been done aiming at very young plates (e.g., < 10 Ma).In order to contribute to new insights on the nature of these waves in very young oceanic plates, we analysed P and S-coda waves recorded by Ocean Bottom Seismometers (OBSs) of Cascadia Initiative deployed in Juan de Fuca and Gorda plate. This effort will provide new constrain on the presence or absence of small-scale heterogeneities in a very young plate (< 10 Ma). We analyse apparent P wave velocity, radiation patterns of P wave, and amplitude decay of P and S-coda waves in three component seismogram envelopes over the frequency band of 1 to 16 Hz. The observations not only show changes in the P wave apparent velocity varying between 7.3 to 8.2 km/s, but also reveal considerable spatial variation in the amplitude decay of the P and S-coda. To obtain direct evidence of the azimuthal variation of the amplitude distribution, we analyse a series of envelopes in vertical component over a limited range of azimuth. The apparent radiation pattern obtained for P waves, shows important distortion of the pattern. Together, these results, suggest a distortion and a change in the velocity as frequency increases and they could potentially be explained by the presence of small-scale heterogeneities. [ABSTRACT FROM AUTHOR]