Enigmatic very low frequency tremors beneath the Shonai Plain in northeastern Japan
K. Nishida and K. Shiomi (NIED)
Recently, dense and sensitive modern seismic networks have revealed tectonic and volcanic tremors. Although most studies of seismic tremors focused on these two types, other types of tremor activities also exist. To detect such tremor activities, we analyzed data from the Hi-net high-sensitivity accelerometers (tiltmeters) between June 2004 and June 2006. The results elucidate very low frequency (VLF) Love wave tremors with a typical frequency of 0.085 Hz beneath the Shonai Plain in northeastern Japan. The tremor activity lasted for several days and occurred several times per month in winter. The activity was triggered by secondary microseisms, which provide a proxy for local ocean swell activity. A possible source is a subhorizontal crack coupled with a fluid reservoir at the bottom of the sedimentary layer. All the observed features suggest that hydrologic phenomena are potential sources of VLF tremors. Because similar hydrologic phenomena can be expected even in tectonically and volcanically inactive regions, modern array observations by broadband seismometers may reveal similar hydrologic tremors in such regions.