A dense temporary seismograph network was set up in the aftershock area of the 1984 Western Nagano Prefecture Earthquake of M 6.8 by the group for the 1986 Joint Seismological Research in Western Nagano Prefecture. Seismograms of about 1, 500 events were obtained during the observation period of 52 days from September 1 to October 22, 1986, and 530 events in and around the aftershock area were located precisely. Extent of the aftershock area is about 20 km in the east-west and about 7 km in the north-south. The hypocenter distribution in the present study shows that the depths of the deepest aftershocks become deep toward the west. The maximum depth in the eastern part of the aftershock area is about 6 km and it becomes 10 km at the western end. Seismic activity is high at a belt near the maximum depths of the aftershock area and no events were detected below this seismic belt. It was also found that the size and location of the aftershock area found by the present observation are almost the same as that obtained by a temporary seismic observation in 1984, which was made just after the main shock. Since no events below the inclined lower oundary were found in either of the observations, it is suggested that the deeper region below the boundary cannot accumulate stresses sufficient to generate earthquakes, suggesting the existence of large locality in the lower boundary of seismogenic zone.

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Focal mechanisms around Ontake Volcano, central Japan, were investigated by using the data set obtained by a seismic observation project of the 1986 Joint Seismological Research in Western Nagano Prefecture (JSR'86). The data set included P-wave first motions of aftershocks of the 1984 Western Nagano Prefecture Earthquake (M=6.8) mainly.
A total of 556 events was selected for the focal mechanism analysis. The P-wave data from the surrounding university routine networks were also utilized additionally. Four hundred two events were successfully analyzed. Based on the types of focal mechanisms and on the spatial distribution of the events, we classify these events into the following 4 groups: (1) events of strike-slip fault type with a common nodal plane parallel to the ENE-WSW striking nodal plane in the fracture zone of the 1984 main shock; (2) events of reverse fault type in the area of the preexisting earthquake swarm to the northeast of the main shock, in a possible relation with an earthquake swarm activated by the main shock; (3) events of reverse fault type that occurred near the largest aftershock (M=6.2), but are not the continued aftershock sequence of the largest aftershock; (4) events of strike-slip fault type at depths to the easternmost part where no activity had been observed before the operation of JSR'86.
The plural types of focal mechanisms obtained and the spatial distribution of foci suggest a complex seismo-tectonic structure that consists of at least 2 kinds of fracturings around Ontake Volcano.

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The detailed three-dimensional P and S wave velocity structure in the focal region of the 1984 Western Nagano Prefecture Earthquake occurring close to an active volcano, Mt. Ontake, central Japan, is derived from a tomographic inversion of travel time data obtained by the 1986 Joint Seismological Research in Western Nagano Prefecture. The data set includes 7, 693 P-wave and 6, 070 S-wave arrival times observed at 49 stations from 212 local earthquakes and 2 explosions. The velocities in the shallow portion of upper crust are determined at each grid point with its spacing of 1-2 km and good resolutions are obtained from the Earth's surface to a depth of 4 km.
There exist strong lateral heterogeneities especially from the surface to a depth of 1 km. The seismic velocity map obtained in the height of 1 km above sea level well corresponds to the maps of surface geology and Bouguer gravity anomaly. This correspondence, however, is not so clear in the deeper layers. And, the greater the depth, the less heterogeneous the velocity structure. The velocity distribution on the fault seems to correspond to the distribution of dislocation and rupture front estimated from an analysis of strong motion and geodetic data: the low velocity region has the large amount of dislocation and the retarded rupture front.

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The key subjects of the 1986 Joint Seismological Research in Western Nagano Prefecture (JSR'86) were to determine hypocenters of aftershocks of the 1984 Western Nagano Prefecture Earthquake and the shallow crustal structure precisely. Therefore, we deployed a temporary seismic network of closely spaced stations with a high quality in the southern foot of Mt. Ontake, the epicentral area of the 1984 earthquake. As one of the key subjects of JSR'86, a three-dimensional velocity modeling was executed using the natural earthquake data observed during the experiment. In addition to this inversion of observed data of earthquakes, we conducted an explosion seismic experiment to obtain information on the shallow crustal structure more precisely to improve hypocenter determination as well as to find characteristics of the structure.
The two-dimensional profile was derived from observed data of seismic waves by two shots at 43 temporal stations. The length of the profile was 20 km, crossing the Otaki River which may be concordant with the structural boundary as pointed out geologically and topographically. The derived time-term distribution represents the two-dimensional surface-layer thickness, and has correlations to the distribution of the Bouguer gravity anomaly and the hypocenter distribution in the area. The rim of high consistency of contour lines on the Bouguer gravity anomaly map and the time-term distribution map may coincide with the boundary of the seismically active area.

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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.

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A multi-channel digital event recording system with automatic event detection and location has been developed by using personal computers with clock frequency of 8 MHz. The system is designed to record seismic signals of more than 100 input channels with a sampling frequency of 150 Hz. The sampled data are written on a hard disk with a cassette streamer, which can copy all the data in the hard disk to a cartridge tape. The capacity of the hard disk is 20 Mbytes, which corresponds to waveform data for 40 events. Although a personal computer is used, it takes only 2 min to pick P and S wave arrivals of 27 stations and to calculate a hypocenter location. The present personal computer system was used in the seismic bservation of the 1986 Joint Seismological Research in the Western Nagano Prefecture. Waveform data of about 2, 800 events were recorded during the observation period of 52 days, and 1, 264 events that occurred in the aftershock area were precisely located in the seismic observation. Hypocenters of about 2, 000 events, including about 1, 600 local events, were determined automatically by using the data played back from the streamer tapes.

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Boulders displaced by the 1984 Western Nagano Prefecture Earthquake (M_JMA=6.8) were investigated mainly in the eastern part of the source region which had not been fully surveyed. It was found that numerous boulders were displaced over the large extent of the eastern part of the source region. It is inferred that the boulders were not displaced by high-frequency seismic waves with frequencies between 5-10 Hz, but by low-frequency seismic waves of about 1 Hz, which radiated from the smooth dislocation on the fault plane. The boulder which experienced the largest ground motions was discovered in the eastern part. The ground motions estimated from the displacement of the boulder have a velocity of 5.5 m/s and an acceleration of 3.5 g in the vertical direction, and a velocity of 3.2 m/s and an acceleration of 2.0 g in the horizontal direction.

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We have investigated the dependence of coda Q (Q_C) on frequency and lapse time using the data observed at short hypocentral distances (1-23 km) over a wide frequency range (1-64 Hz). Q_C was determined by applying a single isotropic scattering model to the bandpass-filtered seismograms from aftershocks of the 1984 Western Nagano Prefecture, Earthquake. Q_C depends on frequency, f in Hz, according to a power law, Q_C=Q₀fⁿ. We also found a strong dependence of Q_C on lapse time, in particular, for short lapse times less than 10 s. This lapse time dependence is characterized by an increase in Q₀ and a decrease in n with lapse time. We interpreted this observation as indication of depth variation of Q_S and estimated a crustal model of one-dimensional Q_S structure from Q_C measured at various lapse times. The model exposes low Q_S in a surface layer and a significant increase in Q_S with depth in the upper 2 km of the crust. Q_S in the uppermost crust is compatible with that measured at a deep borehole in the Kanto Plain, central Japan. However, we cannot attribute the lapse time dependence of Q_C to the sole effect of depth, variation of Q_S, because Q_C calculated from the Q_S model shows more gradual increase than the observation. The scattering coefficient (g₀) estimated from the amplitude ratio of S- and coda waves, as well as Q_S calculated from the energy flux model, suggest a contribution of multiple scattering to the lapse time dependence for long lapse times. For short lapse times, on the other hand, depth variation of g₀, as indicated by the change in n with lapse time, is possibly responsible for the sharp increase in Q_C with lapse time. Thus the apparent lapse time dependence of Q_C is caused by a combination of depth variation of Q_S and g₀, and multiple scattering effect.

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Measurements of total magnetic force and electric self-potential were performed at the joint observation of the explosion seismic experiment made at Ohtaki Village in Nagano Prefecture in Japan. The explosion was induced by 86 kg of gunpowder placed at a depth of 68 m. We observed an increase in total magnetic force of 3 nT at explosion time, at a point 33 m from the explosion point. Increase in the self-potential from 2 to 10 mV was also observed between the electrode couples, which were 60 m apart. These induced changes did not recover to their former levels during the hour following the explosion. The changes may have been caused by the physical stress and/or the fracture. Explanations of the mechanism for the large induced changes are attempted from the aspect of the movement of groundwater, change of remanent magnetization and the effect of steel casing. The area around the explosion experiment is the aftershocks region of the 1984 Western Nagano Prefecture Earthquake (M=6.8) and an active fault runs near the studied area (Kobayashi et al., 1985). The active fault zone has the specific structure in electromagnetic properties which may have enhanced the observed changes at the explosion.

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A gravity survey was extensively carried out around Mt. Ontake as a part of the 1986 Joint Seismological Research in Western Nagano Prefecture, central Japan (the JSR'86). Together with the overall revision of the existing data sets compiled by the Nagoya University, we completed a Bouguer anomaly map around Mt. Ontake with a high resolution of about 1 mgal. It delineates detailed gravity features in the area surrounding the focal region of the 1984 Western Nagano Prefecture Earthquake. A triangular crustal block around Mt. Ontake and a linear anomaly belt along the Atera Fault mark specific tectonic features. A characteristically strong gradient belt of Bouguer anomaly is outstanding in the middle of the triangular crustal block. The seismic fault plane of the 1984 Western Nagano Prefecture Earthquake runs along this belt and dips steeply northward. The fault plane of the largest aftershock exactly coincides with the western end of the gradient belt of Bouguer anomaly.

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