Shunta Noda

Improvement of back-azimuth estimation in real-time by using a single station record

We propose a new approach to improve the accuracy of the back-azimuth estimation in real-time by using a single station record. Compared with the conventional approach in which the length of the time window for the analysis is fixed for all data, the accuracy and speed of the estimation are drastically improved by introducing a variable-length time window which is determined by the first half cycle of the wavelength of the initial P -wave, because this window tends to reduce the influences of trailing scattered waves. The analysis, using the K-NET dataset, shows that the estimation, using this new approach, is improved by 28% and 0.25 s in accuracy and speed, respectively.

Relationship Between the Detailed Hypocenter Distribution and the Velocity Structure in the Western Nagano Prefecture, Central Japan, Derived from Travel Time Tomography Using Dense Array Data

We conducted three-dimensional travel time tomography in and around the source region of the 1984 Western Nagano Prefecture Earthquake to investigate the generation process of the mainshock and swarm activity near the source. As many as about 250,000 travel time data of good quality (1 ms error) from a dense network were compiled and we obtained hypocentral distribution and three-dimensional P wave velocity structure with high accuracy and resolution. Most of the estimated hypocenters were aligned in lines or planes, not in the form of masses. We found the hypocenter distribution corresponding to the mainshock fault plane, and it was sandwiched by high-velocity regions on both horizontal sides. A low-velocity region spreading horizontally at the bottom of this hypocenter alignment was also seen. In the northeastern side of the mainshock fault, where there is swarm activity, we detected several other alignments of hypocenters, most of which may be located at the boundaries of geological strata. A low-velocity region was also found at the bottom of some of these alignments. This low-velocity region may be due to fluids from below the seismogenic zone. The generation of both the mainshock and the swarm activity might be related to fluid intrusions from the lower side of the hypocenter alignments.