Focal Mechanisms of Volcanic Deep Low Frequency Earthquakes in Northeast Japan

Deep low-frequency earthquakes (LFEs) are small events (M<2) with dominant frequencies of 2-8 Hz. LFEs in Northeast Japan mainly occur at depths of 20-40km around active volcanoes. Many previous studies obtained various focal mechanisms including both double-couple and non-double-couple components (e.g. Aso and Ide, 2014). Therefore, physical models related to magma such as cooling magma have been suggested (e.g Aso and Tsai, 2014). However, we need further case studies to constrain their physical mechanisms. Thus, we determine the focal mechanisms of volcanic LFEs beneath Zao, Iwatesan, and Hijiori and examine the temporal variation of focal mechanisms in each region in relationship to the seismicity and the 2011 Tohoku earthquake.

To analyze the mechanisms of LFEs, we use the amplitude ratios of S-wave to P-wave.First, the site effect at each station is corrected using ordinary earthquakes with known mechanisms. Next, assuming four candidate models (double-couple, CLVD, tensile-crack, and single-force), we carry out a grid search to estimate the optimal orientation of each model which minimize the residuals between observed and theoretical S/P ratios. Finally, we determine the best solution for each event based on the AIC.

We successfully determined focal mechanisms of 29 events in Zao, 24 events in Iwatesan, and 21 events in Hijiori. The majorities are double couple LFEs in all region, including the ones similar to the solution of Nakamichi et al. (2003) in Iwatesan. The mechanisms have changed 2-3 years after the Tohoku earthquake in Zao and Hijiori, whereas there is no significant change in Iwatesan. The transition in Zao and Hijiori is interpreted to be caused by the viscoelastic response to the stress change induced by the Tohoku earthquake and possible fluid diffusion thereafter.