Interactions Between Background Ground Vibration and Earthquakes Before and After the 2011 Mw 9 East Japan Earthquake

We have been investigating the background ground vibration (BGV) that is imperceptible to humans. In this study, we examined how this BGV interacts with earthquakes and successfully elucidated the dynamics of the ground before and after the Mw9 Great East Japan Earthquake (GEJE) as several consecutive dynamic events. Stress-related parameters (stress) and geometry-related parameters (fractal dimension representing geometric complexity) were extracted from the BGV data to characterize the BGV. Insights into the interaction between the BGV and earthquakes before and after the GEJE were obtained as follows: Approximately 4 to 3 years before the GEJE, the stress increased significantly and remained at that level until the occurrence of the GEJE. During this period of high stress, a mid-scale earthquake occurred. However, the decrease in stress due to the mid-scale earthquake was significantly smaller compared to the background stress, and its impact on the BGV was minor. However, the remaining stress suggests the possibility of larger earthquakes occurring after the mid-scale earthquake. From about 1 year before the GEJE to the time of the GEJE, the fractal dimension indicating subtle geometric damage increased significantly, leading to the occurrence of the GEJE. Approximately 1 to 3 years after the GEJE, the stress remained high, but the fractal dimension gradually decreased and stabilized. Subsequently, a mid-scale earthquake occurred, and although there was no change in the fractal dimension, the stress decreased significantly. This can be interpreted as a transition induced by the mid-scale earthquake to a more stable state of the BGV.