Scaled energy of deep low-frequency tremor in southwest Japan

Kazuki Horino, Kanazawa University, Kanazawa, Japan, Yoshihiro Hiramatsu, Kanazawa University, Graduate School of Natural Science and Technology, Kanazawa, Japan, Tomoyuki Mizukami, Kanazawa University, Kanagawa, Japan, Kazushige Obara, The University of Tokyo, Earthquake Research Institute, Tokyo, Japan and Takanori Matsuzawa, NIED National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan
Abstract:
In southwestern Japan, deep low-frequency tremor occurs along the subduction zone of Philippine Sea plate. Sources of the tremor show a belt-like distribution of about 30-40 km in depth [Obara, 2002], where the transition from unstable to stable slip occurs at the plate interface. The ratio of radiated energy and seismic moment is called the scaled energy, and it has been often used for comparing the dynamic characteristics of small and large earthquakes. The scaled energy of slow earthquake is in the order of 10-11-10-9 [e.g. Ide et al., 2008; Maeda and Obara, 2009]. This value is about 5 orders smaller than regular earthquakes, indicating that the source process of slow earthquakes is different from that of regular earthquakes. We investigate here the scaled energy of tremor in southwestern Japan to understand the source process of the tremor.

We analyze tremor recorded by NIED Hi-net, with Hybrid catalog [Maeda and Obara, 2009] and Hybrid Clustering catalog [Obara et al., 2010], from January 2001 to June 2011 in southwest Japan. We follow the procedure of Maeda and Obara [2009] to estimate the radiated energy of tremor.

The values of the scaled energy of tremor are in the range of 10-8-10-6 and are smaller than those of regular earthquakes by order of 2. Multiplying the scaled energy by the rigidity provides that the apparent stress drop of tremor is lower than that of regular earthquakes. This result indicates that tremor occurs in the condition of lower-stress and strain than regular earthquake. Furthermore, the obtained value is 2 orders larger than the scaled energy of very low-frequency earthquake [Maeda and Obara, 2009]. These comparisons imply that each slip phenomenon of slow earthquake family has different dynamic characteristics.

We recognize heterogeneous distribution of the scaled energy along the strike and the dip directions of the plate. Shallower events show larger value of the scaled energy. This indicates that the source process of the tremor reflects a spatial variation in frictional property at the transition zone on the plate interface.

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