Comprehensive detection of very low frequency earthquakes off the Hokkaido and Tohoku Pacific coasts, northeastern Japan

Satoru Baba1, Akiko Takeo1, Kazushige Obara1, Takanori Matsuzawa2, and Takuto Maeda3

  1. Earthquake Research Institute, The University of Tokyo
  2. National Research Institute for Earth Science and Disaster Resilience
  3. Graduate School of Science and Technology, Hirosaki University

The activity of very low frequency earthquakes (VLFEs), classified as one type of slow earthquake, can reflect the spatiotemporal distribution of interplate slip. We investigated VLFE activity off the Hokkaido and Tohoku Pacific coasts from January 2003 to July 2018. VLFEs were activated by afterslips of large earthquakes, that is, the 2003 Tokachi-Oki earthquake (Mw 8.0) and the 2011 Tohoku earthquake (Mw 9.0). Inside a large coseismic slip area of the 2011 Tohoku earthquake, the VLFE activity was low thereafter. Episodic bursts that occurred every three months to a year and were observed in large coseismic slip areas of very large earthquakes may indicate small slips during an interseismic period between very large earthquakes.

Fig.1. Distribution of the number of detected events at each virtual source. Regions surrounded by polygons denote six groups: off‐Tokachi (brown), off‐Aomori (green), off‐Iwate (magenta), off‐Miyagi (purple), off‐Fukushima (blue), and off‐Ibaraki (red). Inverted triangles, black arrows, yellow stars, colored circles, and dashed contours are the same as in Figure 1. Black circles are the Tohoku earthquake aftershocks that activated very low frequency earthquakes in the off‐Iwate region.
Fig.2. A schematic illustration showing the comparison between VLFE activity and real interplate slip in the coseismic slip area of a very large earthquake and the afterslip area. VLFE, very low frequency earthquake.