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Strong long‐period seismic signals at periods around 25 and 18 s appear in the ambient noise cross‐correlation functions recorded by an array of ocean bottom seismometers (OBSs) in the western‐central Pacific. The signal amplitude varies from time to time, and the apparent travel times of the signals are typically smaller than those expected for the Rayleigh waves propagating along the great circle connecting the station pairs. From the cross‐correlation functions, the signal sources are located in the Vanuatu Arc. Local data analysis suggests the signals originate from two different sources possibly located at depths of ~0–1 km below the sea level beneath the active cones of Ambrym volcano. Plain Language Summary: Spatially localized sources that persistently generate long‐period seismic waves at periods longer than 10 s are reported worldwide. For some sources, the excitation mechanisms have been well‐investigated. A volcano in Japan generates such waves that travel more than thousands of kilometers. For others, however, the excitation origins are still poorly understood. These include the one observed in the Vanuatu Arc. A temporal observation network of seismometers installed on the seafloor in the western‐central Pacific observed persistent waves peaking at around two different periods. From the data analysis, we find the signals originate from an active volcano, Ambrym volcano, in the Vanuatu Arc. Besides, we find that those seismic waves are most likely originated from two different sources possibly located at shallow depths beneath the volcano. Key Points: Persistent long‐period (25 and 18 s) seismic signals appear in ambient noise cross‐correlations of OBS data in the western‐central PacificThe signals are originated from Ambrym volcano in the Vanuatu ArcSource locations of the two signals may be different, but both are possibly located at shallow depths right beneath the active cones [ABSTRACT FROM AUTHOR]

The vibration of a fluid-filled crack is considered to be one of the most plausible source mechanisms for the long-period events and volcanic tremors occurring around volcanoes. As a tool for the quantitative interpretation of source process of such volcanic seismic signals, we propose a method to numerically simulate the dynamic response of a fluid-filled crack. In this method, we formulate the motions of the fluid inside and the elastic solid outside of the crack, using boundary integrals in the frequency domain and solve the dynamic interactions between the fluid and the elastic solid using the point collocation method. The present method is more efficient compared with the time-domain finite difference method, which has been used in simulations of a fluid-filled crack and enables us to study the dynamics of a fluid-filled crack over a wide range of physical parameters. The method also allows us direct calculation of the attenuation quality factor of the crack resonance, which is an indispensable parameter for estimating the properties of the fluid inside the crack. The method is also designed to be flexible to many applications, which may be encountered in volcano seismology, and thus, extensions of the method to more complicated problems are promising. [ABSTRACT FROM AUTHOR]