Characteristics of Shallow Low‐Frequency Earthquakes off the Kii Peninsula, Japan, in 2004 Revealed by Ocean Bottom Seismometers

Abstract

Abstract The characteristics of shallow low‐frequency earthquakes (LFEs) are related to stress changes on the shallow plate boundary, which are important for understanding the megathrust earthquake cycle. The 5 September 2004 off the Kii Peninsula earthquakes ( M JMA = 7.1, 7.4) occurred near the Nankai trough subduction zone, off southwest Japan. Ocean bottom seismometer observations from 22 September to 30 November 2004 detected many shallow LFEs among the ordinary aftershocks. During the observation period, the frequency of shallow LFEs steadily decayed with the exception of episodic activities, which were very sensitive to stress changes caused by tides and the 23 October 2004 Niigata earthquake ( M JMA = 6.8). We also confirmed correlations between shallow LFEs and shallow very low frequency earthquakes, which suggest that these slow events represent the same slip phenomenon. These findings will contribute to clarifying the impact of LFEs on megathrust earthquakes.