[SCG58-P24] Estimation of the spatial distribution of Qs values beneath the Kii Peninsula, Japan by the twofold spectral ratio method
Keywords:Deep low frequency tremor, Twofold spectrum ratio method, Qs value, Kii Peninsula
We use a dense seismic array (39 three-component velocity sensors, array aperture:1.5 km) installed by AIST at the Kii Peninsula. We also use seismograms recorded at a Hi-net station (N.HYSH ) that is located close to the seismic array. We analyze the data for 2 years from July 2012 to July 2014, referring to the tremor catalog determined by Sagae et al. (2019 JpGU) in which 27,066 events are listed for the 2 years. For tremors recorded at the stations, we calculate velocity amplitude spectra and estimate their slopes at a frequency range of 2.0-4.0 Hz. Outliers are removed by setting a threshold of 4.4 times Median Absolute Deviation of the spectral slopes at each station. Totally, 18,330 events are selected for the following analysis.
We create horizontal blocks of 0.1×0.1° within a region of 136°E to 137°E and 34°N to 35°N. The twofold spectral ratio method is applied when the number of tremor events in a block is more than 100. We calculate twofold spectral ratios for two events locating in the block, when their separations are more than 5 km. Qs value is estimated by the twofold spectral ratio method. In this method, a logarithm of a twofold spectral ratio that is corrected for the geometrical spreading (log TSR) is linearly proportional to a twofold differential travel time (TDTT); The slopes is -πf/Qs, and an intercept is a term about the radiation pattern. However, the intercept is expected to be close to 0 when focal mechanisms of tremors occurring in a block are similar. At each narrow frequency band (2.0-4.0 Hz, 2.8-5.6 Hz, 4.0-8.0 Hz), we assume that Qs value of the structure along the ray paths from hypocenters to the seismic array and the Hi-net station is constant. Then, we estimate the slope (-π/Qs) and the intercept at each frequency band.
Estimated Qs values are summarized as follows; Qs values are in a range of 63-458, 58-896, and 66-2044 at 2.0-4.0 Hz, 2.8-5.6 Hz, and 4-8 Hz, respectively. Qs values estimated at the high frequency band tend to be larger. However, at the high frequency band, the number of blocks for which the Qs value is estimated to be a negative value increases. The intercepts are confirmed to be close to 0 (-0.03 ~ 0.008) in blocks for which Qs values are estimated to be a positive. This suggests that focal mechanisms of tremors occurring within the same block are similar.