A document by Hikaru Imazato. Click on the document to view its contents.

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.

Research interest in the Kinki region, southwestern Japan, has been aroused by the frequent occurrence of microearthquake activity that do not always coincide with documented active fault locations. Previous studies in the Kinki region focused mainly on deep, large-scale structures and could not efficiently resolve fine-scale (~10 km) shallow crustal structures. Hence, characterization of the upper crustal structure of this region at an improved spatial resolution is required. From the cross-correlation of the vertical components of the ambient seismic noise data recorded by a densely-distributed seismic array, we estimated Rayleigh wave phase velocities using a frequency domain method. Then, we applied a direct surface wave tomographic method for the measured phase velocity dispersion data to obtain the 3D S-wave velocity model of the Kinki region. The estimated velocity model reveals a NE-SW trending low-velocity structure coinciding with the Niigata-Kobe Tectonic Zone (NKTZ) and the active Biwako-seigan Fault Zone (BSFZ). Also, we identified fine-scale low-velocity structures coinciding with known active faults on the eastern side of the NKTZ, as well as sets of low-velocity structures across the Tanba region, that may be attributable to the weathering effects or activity of unidentified concealed fault zones. Furthermore, sedimentary basins manifest as low-velocity zones extending to depths ranging from ~1.5 to 2 km, correlating with those reported in previous studies. Our results therefore contribute towards fundamental understanding of earthquake faulting as well as tectonic boundary and will be useful for hazard assessment and disaster mitigation.

In an effort to reveal the subsurface hydraulic changes in fractures by seismic monitoring, aperture-related velocity changes need to be investigated. We developed a numerical approach for calculating changes in elastic wave velocity with fracture aperture opening by determining the internal energy of a digitized fracture model based on natural rough surfaces. The simulated local elastic energy revealed that the interaction energy converged within 1.5 mm of the mean fracture position, and was insignificant unless the fractures intersected. This energetic approach clarified the aperture–velocity relationship and reproduced the experimental results. Further calculations using digital fractures with various sizes and density demonstrated that the velocity can be accounted for by the superposition of a linear function of fracture density and quadratic function of aperture, and is insensitive to the fracture size. Although the relationship between fracture permeability and elastic wave velocity (i.e., the k-V relationship) depends on the fracture density, the offset-normalized k-V relationship shows clear linearity with the fracture density. The proposed k-V relationship as a function of the aperture and fracture density indicates that laboratory-scale fracture properties of a single fracture can be applied to multiple fractures on a larger scale. Our findings can be used to interpret temporal changes in seismic observations and to monitor fluid flow in fractures.