Western Nagano Prefecture Earthquake Research Articles

Relationship between hypocentral distributions and Vp/Vs ratio structures inferred from dense seismic array data: a case study of the 1984 western Nagano Prefecture earthquake, central Japan

Relationship between hypocentral distributions and Vp/Vs ratio structures inferred from dense seismic array data: a case study of the 1984 western Nagano Prefecture earthquake, central Japan

Lateral inhomogeneity deduced from 3-D magnetotelluric modeling around the hypocentral area of the 1984 Western Nagano Prefecture earthquake, central Japan

Earthquake swarms have been observed since 1976 in the southeastern region of Mt. Ontake, where the 1984 Western Nagano Prefecture earthquake (M6.8) occurred. Most earthquake swarm activities show a dense distribution in a narrow area and at shallow depths. We have carried out forward 3-D magnetotelluric modeling for the data set observed around the focal region of the earthquake swarms. An initial 3-D image was constructed based on previous 2-D magnetotelluric analysis results. The resistivity structure shows remarkable lateral inhomogeneity: a low resistivity zone is located at shallow depth around the center of the study area and the low resistivity zone is surrounded by a high resistive structure and its shape is about a 4-km cube. This feature agrees well with the distribution of the degree of water saturation deduced from seismic information. Hypocenters of recent seismicity are distributed around an area between the resistivity boundary and the relatively high-resistance region. This fact strongly suggests that earthquakes in the study area are triggered by fluid (water) in the crust.

Resistivity structure around the hypocentral area of the 1984 Western Nagano Prefecture earthquake in central Japan

We carried out magnetotelluric measurements in the southeastern region of Mt. Ontake, where the 1984 Western Nagano Prefecture earthquake occurred and earthquake swarms have been observed since 1976. Most of the earthquakes have focal depths shallower than 10 km. The depth of the hypocenters increases towards the west. Our purpose is to delineate the resistivity structures down to the focal depths of the earthquakes, because the resistivity structure is very sensitive to the free water in the crust. The regional strike was estimated as N60°E. We carried out a two-dimensional analysis over two profiles across the earthquake fault of the 1984 Western Nagano Prefecture earthquake: one is in the low seismicity region of the fault (A-A′) and the other, in the high seismicity region (B-B′). The resistivity structure along the A-A′ profile is reasonably homogeneous and shows a high resistivity of more than 300 ohm·m. The resistivity structure along the B-B′ profile has a clear boundary at the center of the profile. This boundary divides the structure along the B-B′ profile into two resistivity blocks and its location coincides with that of the earthquake fault. A conductor is detected at depths greater than 8 km to the northwest of the earthquake fault. The depth to the upper boundary of the conductor coincides with those of the seismic reflectors. This structure seems to be formed by the free water dehydrated from the magma.

Stress Changes Caused by Strike-Slip Faulting and Secondary Faulting Induction in Conjugate Directions.

As regards strike-slip faultings generated inland in Japan, we occasionally observe conjugate fault activity in association with main faultings. In most cases, conjugate faults appear on the side of main faults where frictional force on conjugate faults decreases in response to the main faulting. We present two examples which typically show mutually conjugate activities. These are the pair of the 1978 Izu-Oshima-kinkai earthquake (M7.0) and the 1990 earthquake near Izu-Oshima Island (M6.5) and the pair of the 1984 Western Nagano prefecture earthquake (M6.8) and its largest aftershock (M6.2). It seems rare that conjugate fractures were propagated in regions where frictional forces on conjugate faults were expected to increase. These facts indicate that changes in frictional force plays an important role in the generation of secondary fault activity at least in inland strike-slip faults in Japan. In other words, the frictional coefficient in the Coulomb failure function may not be as small as 0.1∼0.3 but as large as 0.5∼0.7 in its value in inland strike-slip faultings in Japan.

Discrimination of fault planes from auxiliary planes based on simultaneous determination of stress tensor and a large number of fault plane solutions

It is well known that there is a large difference in focal mechanism solutions of events even if they occur within a certain small area. Considering earthquake‐generating stress to be uniform in a studied area, we developed a new method for simultaneously determining the stress tensor and the orientation of fault planes for many events by the use of polarity data of P waves. First, the number of inconsistent stations is calculated for various values of three parameters in the focal mechanism solution, with intervals of 10°. This calculation is made for all the events used. Then, four parameters defining the stress tensor are determined by a grid search using the data calculated above with the assumption that the slip direction in the fault plane should be parallel to the direction where the shear stress on the plane becomes maximum. A numerical test is made by applying the present method to an artificial data set composed of 25 events with 25 × 25 readings. It is found that a stable solution of the stress tensor is determined by the inversion and about 60% of fault planes are distinguished from auxiliary planes. There are no events whose auxiliary planes are incorrectly determined as fault planes. This method is used to estimate the earthquake‐generating stress in the aftershock area of the 1984 western Nagano prefecture earthquake, central Japan. The results obtained show that the maximum principal stress is horizontal in the direction of N80°W‐S80°E. There are strike‐slip events having fault planes almost perpendicular to the fault plane of the main shock.

長野県王滝村の天然水の水質について

To investigate the water qualities of natural waters in Ohtaki Village, where there was a large-scale landslide caused by the Western Nagano Prefecture Earthquake in 1984, water samples on river streams were taken in 1992.(1) Water qualities are classified into three types, Ca-(HCO3) 2, Ca-SO4 and Na-HCO3 as likely results by YOSIOKA et al. and OKUDA et al. .(2) Water qualities are shown two principal components PCl which shows degree of weathering and PC2 which shows Ca-(HCO3) 2 type or Ca-SO4 type.(3) Water quality values of PC1 and PC2 on Denjo-river were small before the earthquake, and larger after it and gradually tend to decrease.

Focal Mechanism Analyses of Aftershocks of the 1984 Western Nagano Prefecture Earthquake.

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.

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 NM0 (normal moveout) corrections are made for these seismograms.

Three-Dimensional P and S Wave Velocity Structure in the Focal Region of the 1984 Western Nagano Prefecture Earthquake.

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.

Gravimetric Evidences for Active Faults around Mt. Ontake, Central Japan: Specifically for the Hidden Faulting of the 1984 Western Nagano Prefecture Earthquake.

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.

Strong Ground Motion in the Source Region of the 1984 Western Nagano Prefecture Earthquake. Inferred from Displaced Boulders.

Boulders displaced by the 1984 Western Nagano Prefecture Earthquake (MJMA=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.

Hypocenter Locations by a Dense Network.

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.

A Shallow Crustal Structure as Derived from an Explosion Seismic Experiment.

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.

Minimum size of earthquakes and minimum value of dynamic rupture velocity

The very small aftershocks of the 1984 Western Nagano Prefecture, Japan Earthquake ( M jma = 6.8) were studied in order to investigate the faulting processes of small earthquakes. Earthquakes with M < −1.4 were not detected in spite of the high sensitivity of the observation system, focal distances about 1 km short, and ground noises small enough to detect earthquakes with M < −1.4. It is concluded that the minimum size of aftershocks of the 1984 Western Nagano Prefecture Earthquake is M = −1.4 ( M 0= 2 × 10 14dyn cm). The fault length ( L) of the smallest event is estimated to be 1–5 m by using relationships between seismic moment and fault length which are derived from aftershock areas. The rupture velocity of the smallest event is calculated to be 50–250 m/s from the fault length and the pulse width of the source time function. These values are much smaller than those of large earthquakes (2–3 km/s). It was deduced by Iio (1986a) that smaller earthquakes with M 0 < 10 25 dyn cm have a smaller average rupture velocity. It is then deduced that the smallest event studied here has the minimum value of average rupture velocity. The fault length of the smallest event may be a critical crack length which is predicted by a slip-weakening crack model and observed on stick-slip experiments of large-scale simulated faults. It is possible that fractures with a fault length of less than 1–5 m cannot generate seismic waves since the crack growth is quasi-static.

長野県西部地震による木曽御岳ゴルフ場での地変

長野県西部地震による木曽御岳ゴルフ場での地変

Aftershock activity of the 1984 western Nagano Prefecture earthquake, central Japan, and its relation to earthquake swarms.

Aftershock activity of the 1984 western Nagano Prefecture earthquake, central Japan, and its relation to earthquake swarms.

Anomalously high radon discharge from the atotsugawa fault prior to the western nagano prefecture earthquake ( m 6.8) of september 14, 1984

Radon alpha tracks monitored at six sites increased gradually three months before (detected by microscopic counting), and increased remarkably 2 weeks before (detected by both mechanical and microscopic counting) the western Nagano Prefecture earthquake ( M 6.8) which occurred on September 14, 1984. The monitoring sites were located about 65 km northwest of the epicenter, on the shear zone of the Atotsugawa fault. The changes in radon emanation are considered to be related to the earthquake rather than to changes in meteorological conditions.

1984年長野県西部地震の破壊域におけるS波偏向異方性

1984年長野県西部地震の破壊域におけるS波偏向異方性

1984年長野県西部地震での石の跳躍現象から推定した震源域の地震動強さ

It is important for earthquake resistant design of critical structures like nuclear power plant to evaluate characteristics of near-field ground motion. The intensity of the near-field ground motion during the 1984 Western Nagano Prefecture earthquake (M6.8) is estimated from the observed upthrow of boulders. To simulate the upthrow, the Distinct Element Method is employed, because it can take into account not only interaction but also separation of soil and boulder. According to ambient vibration test of the boulders and seismic prospecting of surface ground in the field, the parameters used in the simulation are determined. As a result of the simulation, the intensity which cause large displacement of boulder due to the upthrow is approximately estimated to be 2g in terms of peak horizontal acceleration, 150 to 200cm/s in terms of peak horizontal velocity and 1g in terms of peak vertical acceleration. These values are consistent with those of the previously observed strong-motion records in near-field.

Analysis and simulation of seismic ground motions observed by an array in a sedimentary basin.

The effects of the irregularity of a shallow subsurface interface on seismic motions are investigated using aftershock records of the western Nagano Prefecture earthquake of September 14, 1984, observed by an array in' the Kyoto basin extending from the rock outcrop near the eastern margin to the center. Comparison between low-pass-filtered seismograms at the basin sites indicates the presence of spatial waveform variation and elongation of duration of the S-wave portion (from the onset of S wave to a later part where amplitudes decrease to a certain low level). The phase and group velocities of the wavelets contained in the S-wave portion suggest that the above characteristics of seismic motion in the Kyoto basin are primarily caused by the excitation of surface waves at the eastern margin of the basin. This means that the prediction of seismic motions in a basin-like structure requires the consideration of irregular interfaces. To reproduce the recorded seismograms in the Kyoto basin, the Aki and Lamer method is extended to allow the computation of seismic responses of a two-dimensional basin-like structure with multiple irregular interfaces. The computed shear waves exhibit spatial waveform variation and elongation of duration due to surface waves generated at the eastern margin of the basin. However, the amplitude of the computed surface waves are less than the observed amplitude. To obtain better agreement, the other effects such as those of three-dimensional subsurface structure and non-plane wave front of incident motions should be included in the synthetic seismograms.