庄内平野下で観測された奇妙な超低周波微動活動 西田究(地震研)・汐見勝彦(防災科研) Kiwamu Nishida (ERI), Katsuhiko Shiomi (NIED) Enigmatic very low frequency tremor beneath the Shonai plain

Introduction: very low frequency tremor Very low frequency tremor (0.05-0.1 Hz) Volcanic tremor: e.g. Aso Non-volcanic tremor: a family of slow earthquakes Systematic event search Enigmatic very low frequency tremor

Data Hi-net tiltmeters: horizontal components 26 stations: around the Shonai plain 2004/6 - 2006/6

Enigmatic very low frequency tremor Propagation up to 500 km Dominance of Love wave Monochromatic 0.085 Hz (12 s) Duration several days

Why is Love wave dominant? Excitation source in the sedimentary layer Weak excitation of Rayleigh wave Rayleigh wave coupled with sedimentary P wave Love wave coupled with sedimentary S wave

Source location of the tremor Grid search algorithm Fitting synthetics to transverse components A double couple source Mrq, Mrf Parameters: a source time function, direction Maximum of VR>85% Bootstrap error < 2 km

Temporal variations High activity in winter 3-5 times/month, duration ~ 2-4 days Coincidence with ocean-swell activity Time delay ~ 1-2 days

A possible mechanism Fluid reservoir on the basement of the sedimentary layer Sub-vertical CLVD Dynamic triggering by microseisms CLVD: compensated linear vector dipole

Conclusions Very low frequency tremor Love wave, monochromatic (0.085 Hz) Duration: 2~4 days in winter Location: estuarine region of Mogami river A sub-vertical CLVD on the basement of the sedimentary layer Dynamic triggering by microseisms Hydrologic origin