海洋プレート内地震の発生頻度と海底年代の関係およびその発生メカニズムの考察

Abstract

For stable oceanic plates, we investigate the relationship between seismicity and ocean floor age by using IRIS (Incorporated Research Institutions for Seismology) and ocean floor age data. For each age range, the number of earthquakes are counted, and then normalized by the area. As a result, seismicity in stable oceanic plates decreases with the seafloor age until 50 Ma; after that, seismicity is low with some fluctuation. This result suggests that not the absolute value of thermal stress but the change rate of it mainly controls the seismicity. We also investigate the difference of seismicity in areas (the Pacific, Indian, and Atlantic oceans). For young ocean floors, the seismicity is the highest in the Atlantic, where the spreading rate is the slowest, while the seismicity is the lowest in the Pacific, where the spreading rate is the fastest. The difference in seismicity is considered to be ascribed to the difference in the spatial derivative of thermal stress.