Enigmatic very low frequency tremor beneath the Shonai plain in northern Japan

In ten years, a new generation of dense and sensitive seismic networks has revealed non-volcanic tremor. Now that is recognized as a member of a family of slow earthquakes as shear slip in subduction zones. Another type of low-frequency tremor is excited by volcanic fluid systems. Recent observation by broadband seismometers reveals a physical picture of the fluid system. In this study, we show a new evidence of non-volcanic and non-tectonic very low frequency tremor beneath the Shonai plain in northern Japan. We analyzed records of Hi-net tiltmeters at 26 stations within a 100-km radius of an assumed hypocenter from 2004/6 to 2006/6. For example, the records on 12/6/2004 exhibit very low frequency tremor with typical frequency of about 0.085 Hz. Surprisingly the tremor was dominant in transverse components: Love wave amplitudes were much larger than those of Rayleigh wave. We determined source locations of the tremor using a grid search method as follows: With an assumption of a local 1-D structure, we calculated synthetic Love wave excited at an assumed hypocenter. For each time segment of 1024 s, we calculated variance reduction (VR) between the synthetic Love waves and transverse components of observed records. Then we searched the minimum of VR. We define detected tremor when VR is lager than 0.85, and bootstrap error of hypocenter location is estimate to be smaller than 2 km. The tremor activities with duration of about 2 days occurred about 3/month in winter months. In summer months, we detected few of events. They were also coincident with activities of secondary microseisms, which are a proxy for the local ocean wave height. A possible source is a shear crack on the bottom of the sedimentary layer with a fluid reservoir. Fluid migration triggered by the secondary microseisms may induce oscillation of the crack.