Location of Mid-Crustal Reflectors by a Reflection Method Using Aftershock Waveform Data in the Focal Area of the 1984 Western Nagano Prefecture Earthquake

A reflection method using waveform data of natural earthquakes is successfully applied for imaging a mid-crustal structure below the focal zone of the 1984 Western Nagano Prefecture Earthquake. We use digital waveform data of about 300 aftershocks at 27 telemetering stations operated by the 1986 Joint Seismological Research in Western Nagano Prefecture. Seismograms observed at each station from many events are plotted in the order of their hypocenter locations, and the AAC (automatic amplitude control) and the NMO (normal moveout) corrections are made for these seismograms. The NMO correction is made by using parameters of hypocenter locations which are very precisely determined by arrival times of the 27 telemetering stations distributed above the focal area. In the seismic section thus obtained, we found two branches of very clear S-wave reflection from the mid-crust. Estimated reflection coefficients for the reflected waves suggest the two reflectors to be the upper surfaces of unusually low rigidity materials, such as a magma body, underlying the focal zone of the 1984 event. The estimated deeper reflector after migration dips toward east-northeast with an angle of about 40 degrees and the other reflector (shallower one) becomes deep toward west-southwest with an angle of about 20 degrees. It was found that the upward extension of the deeper reflector coincides with the western end of the focal area of the 1984 event. The shallower reflector is nearly parallel to and about 2 km apart from the cut-off depth for aftershocks of the 1984 event. Close geometrical relationship between the reflectors and the focal area suggests that the anomalous materials below the focal area play an important role for the stress accumulation within the seismogenic zone above them.