We applied a polarization analysis of InSight seismic data to estimate the temporal variation and frequency dependence of the Martian ambient noise field. Low‐frequency (<1 Hz) P waves show a diurnal variation in their dominant back azimuths that are apparently related to wind and the direction of sunlight in a distant area. Low‐frequency Rayleigh waves (0.25–1 Hz) show diurnal variations and a dominant back azimuth related to the wind direction in a nearby area. Low‐frequency signals that are derived mainly from wind may be sensitive to subsurface structure deeper than the lithological boundary derived from an autocorrelation analysis. On the other hand, dominant back azimuths of high‐frequency (>1 Hz) waves point toward the InSight lander, especially in daytime, indicating that wind‐induced lander noise is dominant at high frequencies. These results point to the presence of several ambient noise sources as well as geologic structure at the landing site.

We derived a three-dimensional S-wave velocity model for the San-in area of southwest Japan to examine heterogeneous structures such as tectonic faults. Many earthquakes occur in this area, but much of the activity has been relatively recent, so the fault distribution has yet to be fully clarified. Here, we used continuous ambient noise data from a dense seismic network, deployed from November 2009 to extract Rayleigh and Love wave dispersion data between station pairs, and then applied a direct surface wave inversion to the phase velocities of each station pair to determine a three-dimensional S-wave velocity model. In the resulting model, faults and a previously unrecognized tectonic boundary appeared as low-velocity anomalies or velocity boundaries, and the velocity anomalies were also associated with many past earthquake hypocenters. These results contribute to our understanding of heterogeneous structures caused by recent tectonic motion and of possible future tectonic activity, such as intraplate earthquakes. Surface wave tomography using ambient noise recorded in dense seismic networks could also be applied in other parts of the world to reveal new heterogeneous geological structures (i.e., unrevealed tectonic faults) and could contribute to disaster mitigation.

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The 2016 Kumamoto earthquake (Mw 7.0) caused hot springs in the Uchinomaki area of Aso caldera to become dormant. Geodetic and borehole observations have previously demonstrated that the area around the hot springs slid horizontally ~ 2 m to the northwest during the earthquake. However, the subsurface structure in the area has not been investigated and the mechanism of sliding is unclear. To reveal geological structures in and around the hot spring area, we conducted a seismic microtremor survey at 60 sites and used the Extended Spatial Auto Correlation (ESPAC) method to determine surface-wave dispersion curves from the microtremor data. We then derived S-wave velocity profiles by inversion of the dispersion curves and constructed from them a 3D S-wave velocity model to ~ 100 m depth over the hot springs and surrounding areas. New surface fissures (indicative of extension) that opened during the 2016 earthquake correspond to a boundary in the southeast of the study area between modeled lower velocities (to the northwest) and higher velocities (to the southeast). In the central area of the hot springs, where the largest displacement occurred, the 3D model shows a plume-like high-velocity anomaly, indicative of morecompetent sediments there. The lowest S-wave velocities (less-competent rocks) are in paddy fields north of the hot spring area. We interpret the above aspects of the 3D velocity model to indicate that during the 2016 earthquake the relatively competent (higher S-wave velocity) sediments in the central area of the hot springs slid northwestward, causing compressional deformation of the less-competent (lower S-wave velocity) sediments in the northern paddy fields and extensional deformation (fissures) southeast of the sliding block. A distinct increase in S-wave velocity at ~ 50 m depth coincides with the depth of a pumice layer in drillcore from the central hot spring area. Shaking during the 2016 earthquake could have caused a sudden increase in pore pressure in this widely distributed porous layer, thus providing a slip plane for the observed horizontal sliding to the northwest.

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