On May 4th 2022, the seismometer on Mars observed the largest marsquake (S1222a) during its operation. One of the most specific features of S1222a is the long event duration lasting more than 8 hours from the occurrence, in addition to the clear appearance of body and surface waves. As demonstrated on Earth, by modeling a long-lasting and scattered surface wave with the radiative transfer theory, we estimated the scattering and intrinsic quality factors of Mars (Qs and Qi). This study especially focused on the frequency range between 0.05 - 0.09 Hz, where Qs and Qi have not been constrained yet. Our results revealed that Qi = 1000 - 1500 and Qs = 30 - 500. By summarizing the Martian Qi and Qs estimated so far and by comparing them with those of other celestial bodies, we found that, overall, the Martian scattering and absorption properties showed Earth-like values.

Ocean swell activities excite body-wave microseisms that contain information on the Earth's internal structure. Although seismic interferometry is feasible for exploring structures, it faces the problem of spurious phases stemming from an inhomogeneous source distribution. This paper proposes a new method for inferring seismic discontinuity structures beneath receivers using body-wave microseisms. This method considers the excitation sources of body-wave microseisms to be spatially localized and persistent over time. To detect the P-s conversion beneath the receivers, we generalize the receiver function analysis for earthquakes to body-wave microseisms. The resultant receiver functions migrate to the depth section. The detected 410- and 660-km mantle discontinuities are consistent with the results obtained using earthquakes, thereby demonstrating the feasibility of our method for exploring deep-earth interiors. This study is the first step toward body-wave exploration while considering the sources of P-wave microseisms to be isolated events.

Microcracks in geomaterials cause variations in the elastic moduli under applied strain, thereby creating seismic wave velocity variations. These are crucial for understanding the dynamic processes of the crust, such as fault-zone damage, healing, and volcanic activities. Solid earth tides have been used to detect seismic velocity changes responding to crustal-scale deformations. However, no prior research has explored the characteristics of the seismic velocity variations caused by large-scale tidal deformation. To systematically evaluate the tidal response to velocity variations, we developed a new method that utilized the flexibility of a state-space model. The tidal response was derived from hourly stacked noise autocorrelations using a seismic interferometry method throughout Japan. In particular, large tide-induced seismic velocities were observed in the low S-wave velocity region of the shallow crust. Overall, the tidal responses to velocity variations can provide new insights into the response mechanisms of the shallow crust to applied strain.

Convective vortices (whirlwinds) and dust devils (dust-loaded vortices) are one of the most common phenomena on Mars, observable on a daily basis. They reflect the local thermodynamical structure of the atmosphere and are the driving force of the dust cycle. Additionally, they cause an elastic ground deformation, which is useful for retrieving the subsurface rigidity. Therefore, investigating Martian convective vortices with the right instrumentation can lead to a better understanding of the Martian atmospheric structures as well as the subsurface physical properties. In this study, we quantitatively characterized the convective vortices detected by NASA’s InSight mission (~13,000 events) using both meteorological (e.g., pressure, wind speed, temperature) and seismic data. The evaluated parameters, such as the signal-to-noise ratio, event duration, asymmetricity of pressure drop profiles, and cross-correlation coefficient between seismic and pressure signals, are compiled as a catalog. To demonstrate how our catalog can contribute to scientific investigations, we show two analysis results about (i) asymmetrical features seen in the vortex-related pressure drops and (ii) atmospheric-ground interaction to retrieve the subsurface physical properties.

Seismic interferometry is a powerful tool to monitor the seismic velocity change associated with volcanic eruptions. For the monitoring, changes in seismic velocity with environmental origins (such as precipitation) are problematic. In order to model the environmental effects, we propose a new technique based on a state-space model. An extended Kalman filter estimates seismic velocity changes as state variables, with a first-order approximation of the stretching method. We apply this technique to three-component seismic records in order to detect the seismic velocity change associated with the Shinmoe-dake eruptions in 2011 and 2018. First, ambient noise cross-correlations were calculated from May 2010 to April 2018. We also modeled seismic velocity changes resulting from precipitation and the 2016 Kumamoto earthquake, with exponential type responses. Most of the results show no significant changes associated with the eruptions, although gradual inflation of the magma reservoir preceded the 2011 eruption by one year. The observed low sensitivity to static stress changes suggests that the fraction of geofluid and crack density at about 1 km depth is small, and the crack shapes could be circular. Only one station pair west of the crater shows the significant drop associated with the eruption in 2011. The gradual drop of seismic velocity up to 0.05% preceded the eruption by one month. When the gradual drop began, volcanic tremors were activated at about 2 km depth. These observations suggest that the drop could be caused by damage accumulation due to vertical magma migration beneath the summit.

We derive the 3-D S-wave velocity structures of sediments and upper crust in the region off Ibaraki by applying ambient noise tomography to a dense array of short-period ocean bottom seismometers (OBSs). The cross-spectra were calculated using 27- or 142-day continuous seismic data, and the phase velocities of the fundamental and the first-higher Rayleigh wave modes are obtained in the frequency ranges of 0.1–0.25 Hz and 0.17–0.3 Hz, respectively. Our 1-D S-wave velocity inversion based on the trans-dimensional Markov chain Monte Carlo method revealed multiple sedimentary layers above the acoustic basement and the upper crustal structure. The 1-D structure was then used as a reference model to conduct ambient noise tomography and non-linear inversion of the 3-D S-wave velocity structure by collecting data of the local 1-D S-wave velocity structure. Our 3-D S-wave velocity structure revealed three main points: (1) The acoustic basement is situated at a depth of ~4 km depth; (2) the crustal structure is more complex than the that of the sedimentary layers; and (3) the southern region has a complex crustal structure in which subducting seamounts were identified by previous P-wave velocity tomographies.

A centroid location catalog of P-wave microseisms is crucial for understanding the origins of microseisms. Although a back-projection method is feasible for locating the centroids, the computational cost is still expensive for making a global catalog over ten years. Contrary, although the computational cost of beamforming is low, it cannot distinguish P from PP waves. To combine the advantages of both methods, we develop the auto-focusing method as a natural extension of beamforming. In the first step of this method, we estimated the slowness vector based on conventional beamforming and the epicentral distance inferred from the wavefront curvature by maximizing the beam power. In the second step, we iteratively update the values based on the perturbation theory. In the third step, based on the classified phase according to the estimated epicentral distance and the slowness, we infer the source location from the slowness vector with corrections for a global 3-D P-wave velocity structure. We also infer the centroid-single-force (CSF) from the beam power. We applied this method to the vertical components of seismic records at approximately 780 Hi-net stations in Japan from 2004 to 2020. We also compare the CSF catalog with a synthetic CSF catalog based on a numerical ocean wave model: WAVEWATCH III. Both catalogs generally show similar temporal-spatial patterns of centroids. The amplitudes of CSF are consistent with each other, although the seismic signal-to-noise ratio limits the detected events. Exceptionally, significant activities in the Gulf of Carpentaria cannot be explained by the ocean wave model.

Seismic waves can be significantly amplified by soft sediment layers. Large dynamic strains can trigger a nonlinear response of shallow soils having low strength, which is characterized by a shift of the resonance frequencies, ground motion deamplification, and in some cases, soil liquefaction. We investigate the response of marine sediments during earthquake ground motions recorded along a fiber-optic cable offshore the Tohoku region, Japan, with Distributed Acoustic Sensing (DAS). We compute AutoCorrelation Functions (ACFs) of the ground motions from 103 earthquakes in different frequency bands. We detect time delays in the ACF waveforms that are converted to relative velocity changes (dv/v). dv/v drops, which are characteristic of soil nonlinearity, are observed during the strongest ground motions. Moreover, the dv/v values show a strong variability along the cable. This study demonstrates that DAS can be used to infer the dynamic properties of the shallow Earth with an unprecedented spatial resolution.