[S09] PM-1

[S09-24] Searching for Very Low Frequency Earthquake (VLFE) around Western Kuril Trench

*Novia Anggraini¹, Hiroaki Takahashi¹ (1. Institute of Seismology and Volcanology, Hokkaido University)

A Very Low-Frequency Earthquake (VLFE) is a type of slow earthquake that occurs with a frequency of 0.02 to 0.05 Hz. In many studies, VLFE is observed to obtain a hint at the occurrence of fast earthquakes by examining their relation through the spatiotemporal distribution of VLFE activity. For instance, as revealed in a study by Baba et al. (2020), growing and decreasing VLFE activity were detected in some areas around the asperity of the 2011 Tohoku earthquake either before or after the rupture. Around Kuril Trench, VLFE was first recognized by Asano et al. (2008), which occurred in the southwesternmost part of the trench, referred to as the Tokachi region. By using F-net broadband and Hi-net tiltmeter data from July 2003 to December 2007, around 8,500 VLFEs were detected. This study also demonstrated the possibility of VLFE activity in the area with an underdeveloped accretionary prism. Another study is from Baba et al. (2020), which aims to detect VLFE activity around Hokkaido and the Tohoku Pacific coast. The Hokkaido area includes the Tokachi and Nemuro regions. Using F-net data from January 2003 to July 2018, more than 20,000 VLFEs were detected around the Tokachi region. Meanwhile, the Nemuro region was dominated by false detections. This led to the exclusion of this area from their analysis. In this work, we attempt to identify the activity of VLFE around the Western Kuril Trench by reviewing a detection catalog from the previous study, as there is a possibility that VLFE occurs in this area. We make use of a catalog from the study of Baba et al. (2020), which was discerned between March 2007 and July 2018. Here, there have been around 886 detections. The waveforms under examination are produced by F-net stations located on the Pacific coast of Japan and five additional distant stations from IRIS. Then, resampled one sample per second, and filtered by a bandpass filter with a frequency of 0.02-0.05 Hz. The waveforms are examined visually, and the event of VLFE is selected based on the arrival time and amplitude. In addition, local and distant events are removed from the list of selected events. The epicenter location is estimated using a plot of particle motion. Initially, there were six candidates for VLFE. However, after rechecking the probability of these events being surface waves of distant events, no VLFE is detected. The difficulty of VLFE detection is indicative of the low activity of VLFE in this area. It might be due to the plate interface being strongly coupled. Comparing this condition to those of the Japan trench region before the Tohoku earthquake 2011, it suggests that the off Nemuro region probably will be part of major rupture area of potential large event. In addition, the absence of this shallow slow slip in this trench could also notify the possibility of another disaster which is a tsunami. Lindsey et al. (2021), in their study on slip rate deficit and earthquake potential, used geodetic data to infer the high slip rate deficit in the up-dip of the asperity of the Tohoku earthquake before the rupture which later excite a large tsunami. The same case also occurred around Hokkaido in the 17th century (Ioki and Tanioka, 2015) in which a large tsunami was triggered by a very large slip (25 m) in the shallow part of Tokachi and Nemuro segments. For further study, we intend to examine another catalog that is from NIED which recorded probable VLFE from June 2003 to June 2023. Acknowledgment: Dr. Baba of JAMSTEC and Dr. Asano of NIED for providing us with the VLFE catalog, Dr. Yamashita of DPRI Kyoto University, and Dr. Yoshida of GRI for discussion and suggestions.