ABSTRACT
A time domain attenuation deconvolution method is applied for velocity seismograms recorded in the Western Nagano region, Central Japan in order to investigate rupture initiation of earthquakes. For this aim, we estimate intrinsic Q by using spectral analysis. Using spectral analysis, we find that path-averaged Q p is 250 for earthquakes with hypocentral distances, r, of about 7 to 8 km from the 800 m depth borehole seismometer. By using this Q-value, we deconvolve a causal Q operator from 15 borehole velocity seismograms in the time domain with the cross validation method. For all eight events (M < 3; 5 < r < 14 km) and one event (M > 3; r = 7.5 km), we find that velocity seismograms rise in a simple linear manner. However, for six events (M > 3; 4 < r < 14 km), we observe a slow initial phase. These six events begin with a simple linear phase that is comparable in strength to the simple, smaller events. The six events depart from this linear trend after 1 to 3 msec into the rupture time. For three M > 3 events (4 < r < 9 km), we observe very complex velocity seismograms. Linearly rising velocity can be explained with a simple circular crack model. However, the slow initial phases seen after deconvolution of velocity seismograms are caused not by path effects but by source effects. It means that the time domain deconvolution with cross validation is applicable not only to deconvolve empirical Green’s function but also Q.
