Seasonal Crustal Seismic Velocity Changes Throughout Japan

Abstract

Noise‐based crustal seismic velocity changes are known to be affected by environmental perturbations, such as rainfall, atmospheric pressure loading, and temperature changes. Similar to geodetic observations, these external perturbations can mask the effects of tectonic and volcanic processes. In this study, we benefit from the dense Hi‐net short‐period seismic network that covers the entire Japan to measure continuous changes in seismic velocities over a few years, using noise‐based seismic monitoring. Some strong seasonal seismic velocity changes are observed in both southern Japan (Kyushu Island) and northern Japan (Hokkaido Island). Decreasing of seismic velocities in summer in southern Japan can be clearly explained by a model of increased crustal fluid pore pressure associated with high rainfall. In northern Japan, it is necessary to adopt a more complex model to explain the observed seismic velocity variations, which takes into account precipitation, snow depth, and sea level changes. Moreover, western and eastern Hokkaido Island show very different responses to these different external perturbations. The models developed are used to remove the seasonal components of the seismic velocity changes. The minimum remaining detectable seismic velocity change reduces to 10−5, which allows detection of crustal responses to small earthquakes that are previously hidden in the strong seasonal perturbations.

Volcanic, Coseismic, and Seasonal Changes Detected at White Island (Whakaari) Volcano, New Zealand, Using Seismic Ambient Noise

“…Recent studies have tried to apply corrections for changes induced by rainfall (e.g., Budi-Santoso & Lesage, 2016;Lecocq et al, 2017;Rivet et al, 2015;Wang et al, 2017) and thermoelastic strain (e.g., Lecocq et al, 2017). Recent studies have tried to apply corrections for changes induced by rainfall (e.g., Budi-Santoso & Lesage, 2016;Lecocq et al, 2017;Rivet et al, 2015;Wang et al, 2017) and thermoelastic strain (e.g., Lecocq et al, 2017).…”

Evidence of Changes of Seismic Properties in the Entire Crust Beneath Japan After the Mw 9.0, 2011 Tohoku‐oki Earthquake

“…The continuous monitoring of seismic velocities offers a new tool to capture the physical processes that take place after large earthquakes. This can provide insights not only into the tectonic and volcanic processes (Brenguier, Campillo, et al, ; Brenguier, Shapiro, et al, ; Brenguier et al, ; Chen et al, ; Froment et al, ; Obermann et al, ; Taira & Brenguier, ; Wegler et al, ) but also into some transient fluctuations that are derived from external environmental perturbations (Sens‐Schönfelder & Wegler, ; Meier et al, ; Hillers et al, ; Hillers, Ben‐Zion, et al, ; Wang et al, ). The characteristic depth at which such changes can be monitored varies from meters (Hillers, Retailleau, et al, ; Mao et al, ; Sens‐Schönfelder & Wegler, ) down to dozens of kilometers into the crust (Froment et al, ; Obermann et al, ; Rivet et al, ), through measurements at various periods.…”

Sensitivity of Seismic Velocity Changes to the Tidal Strain at Different Lapse Times: Data Analyses of a Small Seismic Array at Izu‐Oshima Volcano

“…Such seismic velocity reductions are expected to be shallow and to correlate with the strength of shaking, which is clarified by analyzing seismic data recorded at the top and bottom of a borehole (Nakata & Snieder, ; Sawazaki et al, ; Takagi et al, ). Water level variation due to precipitation leads to pore pressure changes, and seismic velocity changes thereby are also sensitive to precipitation (Meier et al, ; Rivet et al, ; Sens‐Schönfelder & Wegler, ; Wang et al, ). Strain changes cause preexisting cracks to close and open, and consequently the seismic velocities decrease and increase in regions of dilatation and contraction of the structure, respectively (e.g., Nur, ; Walsh, ).…”