AFTERSHOCK ACTIVITY OF THE 1984 WESTERN NAGANO PREFECTURE EARTHQUAKE, CENTRAL JAPAN, AND ITS RELATION TO EARTHQUAKE SWARMS

J. Phys. Earth, 37, 401-416, 1989 AFTERSHOCK ACTIVITY OF THE 1984 WESTERN NAGANO PREFECTURE EARTHQUAKE, CENTRAL JAPAN, AND ITS RELATION TO EARTHQUAKE SWARMS Tooru OOIDA,* Fumihito YAMAZAKI,* Iwao FUJII," and Harumi AOKI* * Research Center for Seismology and Volcanology, School of Science, Nagoya University, Nagoya, Japan ** Takayama Seismological Observatory, School of Science, Nagoya University, Takayama, Japan (Received June 24, 1989; Revised March 7, 1990) The damaging earthquake (M=6.8) on September 14, 1984 was a strike-slip faulting trending ENE-WSW across an area of earthquake swarm near Ontake volcano, central Japan. The rupture was 12 km long, about 10 km wide and very shallow, though there was no break on the surface. The earthquake occurred near the boundary of crustal blocks between two adjacent earthquake swarms that have been markedly active since 1976. The expansion of aftershock activity to the west and to the east was obvious, though the rupture pattern was complicated. The largest aftershock of M=6.2 to the west of the main shock was conjugated with the main fault. The largest to the east was a reverse faulting. However the pressure axes of these faultings were the same as that of the main shock. The difference between these fault types might be controlled by block boundaries with different directions around the volcano. I. Introduction A magnitude 6.8 earthquake struck western Nagano Prefecture, central Japan, at 8:48 a.m. on September 14, 1984 local time (9/13/84, 23 : 48 : 49.4 UT), and caused a huge landslide on the southeast foot of Ontake volcano. The hypocentral location, from the Japan Meteorological Agency (JMA), is 35 49.3'N, 137 33.6'E. The focal depth is 2 km. The earthquake is located in a recent earthquake swarm zone to the southeast of the volcano that erupted in 1979. The area around the volcano including high mountain ranges in central Japan is a site of intense tectonic activity during the Quaternary age. The major active faults of class A are: 1) the Neodani fault system to the west; 2) the Atotsugawa fault to the north; and 3) the Atera fault system striking northwest and southeast at a distance of 20 km from the 1984 Western Nagano Prefecture earthquake (Fig. 1). There are also a number of active faults of classes B and C in the area (RESEARCH 401

402 T. 00IDA et al. Fig. 1. Location of the 1984 Western Nagano Prefecture earthquake (M= 6.8) (E), active faults, volcanoes (solid triangles) and telemetry seismic stations (open circles) used in this study. The southernmost triangle, OV, denotes Ontake volcano. The Atera fault (AT), the Atotsugawa fault (AG), and the Neodani fault (N) are active faults of class A. The activity of the Shirasu-touge fault (S) is between A and B (RESEARCH GROUP FOR ACTIVE FAULTS, 1980 a). GROUP FOR ACTIVE FAULTS, 1980a, b), but none is mapped in the vicinity of the 1984 earthquake. It is probably due to thick volcanic ejecta from the nearby volcanoes shown in Fig. 1. The nearest active fault of class A-B is the Shirasu-touge fault extending 10 km towards the main shock epicenter from the Atera fault. The activity of Ontake volcano is not intense. The last eruption on October 28, 1979, which declined soon, was the first activity ever recorded in historic times. The seismic activity prior to the eruption is traced back to 1976. It began with an earthquake clustering beneath the southern to southeastern foot of the volcano in August 1976 (SEISMIC ACTIVITY MONITORING CENTER, SEISMOLOGICAL DIVISION, J. M. A., 1977; TAKAYAMA SEISMOLOGICAL OBSERVATORY and INUYAMA SEISMOLOGICAL OBSERVATORY, NAGOYA UNIVERSITY, 1977). The activity reached its peak within ten days, but the biggest event with M=4.2 occurred about one month later. The activity lasted about three months (REGIONAL CENTER FOR EARTHQUAKE PREDICTION OBSERVATION, NAGOYA UNIVERSITY, 1980). The second swarm activity began in May 1978 and continued at least till the 1984 earthquake. This second activity spreads north-south (SCHOOL OF SCIENCE, NAGOYA UNIVERSITY, 1979) on the east

Aftershock Activity of the Western Nagano Prefecture Earthquake 403 side of the 1976 activity (AOKI, 1987). The event of M=5.3 on October 7, 1978 was the largest in this sequence (OFFICE OF INFORMATION FOR EARTHQUAKE PREDICTION, SEISMOLOGICAL DIVISION, J.M.A., 1979). The 1984 earthquake occurred at 9 km from the volcano only 5 years after the 1979 eruption. Although the seismic activity did not seem disturbed by the eruption (AOKI et al., 1980), the proximity of seismic and volcanic activities in space and time suggests some relationship between the two events. The spatial distribution of the aftershocks and its variation with time indicate a bilateral rupture trending ENE-WSW and a complicated expansion of the aftershock zone. This aftershock sequence and its relation to the earthquake swarms are analyzed in detail in this paper in order to elucidate the characteristic behavior of seismic activity in the volcanic area. 2. Seismic Activity in Central Japan The earthquakes in central Japan can be classified into the following three groups: 1) deep shocks occurring at depths of 150-350 km in the subducted Pacific plate, 2) subcrustal shocks occurring in the subducted Philippine Sea plate at depths of 20-80 km, and 3) shallow events in the upper crust at depths less than 20 km (YAMAZAKI and SUMINO, 1985). Inland earthquakes of M: 6.5 mainly occurred near or along active faults in the western to northeastern part of central Japan. Some of their aftershock activities are still observed. On the contrary, the seismic activity in the southern to eastern part is generally low, as characterized by the low seismic activity along the Atera fault (WATANABE and IIDA, 1969; IKAMI et al., 1972). The region around Ontake volcano is located at the boundary between the active and less active regions. It has been a region of earthquake swarms rather than a region of big earthquakes until the 1984 Western Nagano Prefecture earthquake. The earthquake swarms around the volcano have been precisely located on a routine basis by a telemetry seismic network since 1978. Although the observation system changed in several stages (Aom et al., 1985), the detection capability of events of M 1.5 is considered uniform and unbiased in the whole period. Therefore, we treat events of M 1.5 hereafter. The locations of telemetry seismograph stations in central Japan are shown in Fig. 1. The nearest station is located within 20 km, and the maximum azimuthal gap between stations is less than 80. The crustal velocity model used for earthquake locations is based on the Atsumi-Noto refraction study by Aom et al. (1972). It is slightly modified to give the best fit to travel times to the telemetry network for the 1982 Otaki seismic explosion which was fortunately made in the source area of the 1984 earthquake by RESEARCH GROUP FOR EXPLOSION SEISMOLOGY (1982). The final model is given in Table 1. By using the model, the main shock focus was relocated at 35 49.6'N, 137 32.7'E, and 8.4 km in depth. In order to discuss the seismic pattern within a small area in detail, we select such accurate foci that are determined by the data

404 T. OOIDA et al. Table 1. Crustal structure model. Fig. 2. The epicentral distribution of earthquake swarm (May 1, 1978 September 13, 1984). The events in the square II'K'K are divided into two areas N and S by the line JJ'. The big open circle denotes the 1984 main shock. from more than five stations with standard deviations less than 0.1 s for P-wave travel-time residuals. 3. Activity of Earthquake Swarms before the Main Shock The epicenters of the earthquake swarm from May 1, 1978 through September 13, 1984 are shown in Fig. 2. Epicenters to the southeast of Ontake volcano are distributed north-south. On the basis of difference in seismic activity between northern and southern areas, the area shown by the square II'K'K in Fig. 2 is divided into two areas N and S by the line J'J with an azimuth of N70 E, which is parallel to the fault plane of the 1984 main shock. The focal distributions of events in areas N, S, and N + S are illustrated by three vertical profiles in Fig. 3. Profiles N and S are views from the south, whereas the profile N + S, from the west. There is a significant difference in focal depths

Aftershock Activity of the Western Nagano Prefecture Earthquake 405 Fig. 3. The vertical distributions of foci in areas N, S, and N+ S in Fig. 2. Profiles N and S are views from the SSE, and profile N + S, from the WSW. Fig. 4. Space-time plots of epicenters along the ENE-WSW direction for the areas N and S. Abscissa; time since May 1, 1978 through October 31, 1984. Ordinate; distance measured from the line IK in Fig. 2. The general trend of eastward migration of the northern activity is estimated as 0.8 km/year as shown in the upper panel.

406 T. OOIDA et al. Fig. 5. Space-time plots of epicenters along the NNW-SSE direction for the area N + S. Abscissa; the same as Fig. 4. Ordinate; distance measured from the line II' in Fig. 2. between the two; most of the events in the southern area range from 6 to 14 km in depth, and those in the northern area, from 2 to 12 km. Space-time plots of epicenters for the areas N, S, and N + S during the period from May 1, 1978, through October 31, 1984, are shown in Figs. 4 and 5. A cluster of events in the southern area consists mainly of aftershocks of the M= 5.3 earthquake on October 7, 1978. The aftershock activity was dominant until June 1980. Except for the event of M= 5.3 and its aftershocks, the seismic activity in the southern area S, was very stable and significantly lower than that in the northern area N. Aftershocks of the M= 5.3 earthquake did not diffuse into the northern area, suggesting weak coupling between the northern and southern areas. One of the noticeable changes in seismicity before the damaging earthquake is the eastward migration of seismic activity at a rate of 0.8 km/year (Fig. 4), which is consistent with the view of eastward shift of the 1978 swarm from the 1976 swarm (Aom, 1987). Another change is the quiescence of seismic activity from October 1982 through March 1983 in the northern area and the subsequent clustering of events at a few places in the same area. 4. Main Shock and the Activity of Aftershocks Epicenters of the main shock and its aftershocks since September 14 through

Aftershock Activity of the Western Nagano Prefecture Earthquake 407 Fig. 6. Epicentral and depth distributions of main shock and its aftershocks. Big double circles are the main and two large aftershocks that occurred outside of the incipient aftershock zone. October 31, 1984 are plotted in Fig. 6. Aftershock activity near the main shock remained very low for the whole period. The aftershock zone is obviously circumscribed by a straight line at the southern margin. It will be pointed out later that the line agrees with the southern margin of the northern earthquake swarm before the main shock. The vertical distribution of foci viewed from the S20 E direction is shown in the lower figure, where depths are measured from the sea level. The average altitude of topographic surface is about 1 km in this area. The lower boundary of foci is shallow in the east and dips about 20 toward the west. In Fig. 7, depth distributions of aftershocks are shown for six sections. The section BB' is the focal distribution of secondary aftershocks projected onto the fault plane for the largest aftershock (M= 6.2). Sections CC' and DD' represent the aftershocks of the main shock, and the section EE', for the widespread events in the east. The section DD', the most concentrated distribution of aftershocks, suggests a fault plane with a dip of 80 or two nearly vertical fault planes; one is shallow in the south, and the other, deep in the north. The latter interpretation might be supported by the splitting of active seismic zone on the section CC' or EE', and the focal plane solution of the main shock, in which the presumable fault plane is almost

408 T. OOIDA et al. Fig. 7. The vertical distributions of aftershocks in the areas, B, C, D, E, F, and G. All the profiles are views from the WSW. vertical as shown in Fig. 10. Sections FF' and GG' represent the distribution of events associated with the late expansion of the aftershock zone. Expansion of aftershock zone was very characteristic; the largest aftershock of M= 6.2 occurred to the west at 07 :14 on September 15, 1984, 23 hr after the main shock (Fig. 6). Aftershocks of this event extended 5 km long in the N20 W direction, which was almost perpendicular to the aftershock distribution of the main shock. Most of the aftershocks were confined within a rectangular area of 15 by 5 km. Variation of aftershock activity with time is illustrated by space-time plots of aftershocks in Figs. 8 and 9. Figure 9 shows that there was a foreshock activity for about 2 hr before the largest aftershock. The aftershock activity in the area of the main shock declined when the largest aftershock occurred to the west, and gradually recovered to the previous level within about 12 hr. The recovery began

Aftershock Activity of the Western Nagano Prefecture Earthquake 409 Fig. 8. Space-time plots of the aftershocks along the ENE-WSW direction (September 1, 1984-October 31, 1984). Ordinates are the distances from the line AB which is shown in the inset in the upper left corner. at the west and expanded to the east at a rate of about 1.3 km/hr. It is noted that, after four days from the main shock, a new earthquake swarm (the westernmost seismic cluster in the area G in Fig. 7) began to occur in an area near the border of Gifu and Nagano Prefectures, where very few earthquakes had been observed before the main shock. The new seismic activity in this area may account for a stress concentration caused by the main shock. The expansion of aftershock zone to the east was not obvious until October 2, when some earthquakes began to cluster in a very quiet area adjacent to the eastern end of the aftershocks. They were regarded as foreshocks of the M= 5.3 earthquake on October 3, 1984. The event was very shallow and was accompanied by a number of aftershocks which were distributed about 3 km in length in the eastward direction. Their locations were not on the extension of the aftershock alignment but branched southward in the east as shown in Fig. 6. Focal plane solutions of the main shock and major aftershocks (M 4.0) are shown in Fig. 10. Event 1 is one of the swarm earthquakes before the main shock. The focal plane solution of the largest aftershock (event 8) is similar to that of the main shock (event 2). The distribution of aftershocks suggests that the main shock was right-lateral and the largest aftershock, left-lateral and conjugated with the former. The focal plane solution of the event 17 (M=5.3), which occurred on October

410 T. OOIDA et al. Fig. 9. Space-time plots of the aftershocks for three days (September 14-16, 1984). Data are the same as those in Fig. 8. The aftershock activity near the main shock (M=6.8) declined suddenly, when the largest aftershock (M=6.2) occurred to the west, and then recovered gradually from the west to the east as shown by a broken line in the figure. 3, 1984, was different from that of the main shock. Although most of the aftershocks had strike-slip solutions, events in the northeastern region were predominantly of dip-slip type. The average of pressure-axes, however, was N63 W for both mechanism solutions. 5. Discussion 5.1 Earthquake swarm As mentioned in Sec. 1, a number of earthquakes clustered to the southeast foot of Ontake volcano since August 1976. Although the distribution of epicenters appeared to extend from north to south on the whole, we can divide it into northern and southern areas by a study of seismicity in space and time. The earthquake swarm activity in the southern area was featured by the M=5.3 earthquake on October 7, 1978, and its aftershocks. In the northern area, however, the swarm activity was rather stable until October 1982 and was followed by a quiet period of about six months. Events of M 4.5 were very few until the main shock. It is likely that the two activities are independent of each other. Therefore, a weak stress coupling between the two areas is suggested. HORT et al. (1982) analyzed the focal

Aftershock Activity of the Western Nagano Prefecture Earthquake 411 Fig. 10. Focal plane solutions of major earthquakes (M 4.0), in the period from April 9, 1984 through January 31, 1985. Projections on upper hemispheres, where solid and open circles denote compression and dilatation respectively. The focal plane solution numbers correspond to earthquake numbers in the inset. Solid circles in the inset denote events with M 4.0 whose focal mechanism solutions were determined. plane solutions in the northern area after the eruption of Ontake volcano and found that P-axes were mainly in N70-80 W directions, whereas T-axes were dispersed with inclinations from 0 to 90. In other words, various types of faulting were equally observed in the earthquake swarm, and were distributed in a zone extending north and south. However, the average direction of P-axes is consistent with the tectonic stress in this area (Hom et al., 1982). The concentration of earthquakes and the variety of focal mechanism solutions are therefore explained by the existence of a weak zone subjected to a compressional stress in the WNW-ESE direction. The abundance of reverse faulting in the zone contrasts characteristically with the predominance of strike-slip faulting for the aftershocks immediately after the 1984 earthquake. After the occurrence of the main shock, some of the focal plane solutions of M 4.0 events indicated reverse faulting in the northeast part of the aftershock zone where the swarm earthquakes were located (Fig. 10). If it is possible to distinguish the swarm activity from the aftershock activity by the difference between focal plane solution types, we can conclude that the seismicity of the swarm activity in the northern area was not terminated but activated by the occurrence of the main shock.

412 T. OOIDA et al. 5.2 Main shock The main shock occurred in the northern area of the swarm activity. Its aftershocks expanded east-west, crossing the northern area N in Fig. 2 and extended by 5 km to the west. The length of aftershock zone within 23 hr after the main shock amounted to about 12 km. It should be noted that aftershocks located in the southern area were few. The southern margin of aftershock zone was very clear-cut; it marked a straight line trending N70 E, suggesting the boundary between the northern and southern earthquake swarm areas. The direction of the main fault is N70 E, which is different from those of major active faults in the western half of central Japan. Their average direction is NW-SE or NE-SW, which implies an E-W compressional stress in this area in the Quaternary period. However, according to the secular horizontal deformation based on the recent triangulation data, the direction of compressional stress in this region is NW-SE (GEOLOGICAL SURVEY INSTITUTE, 1982). The direction of the main fault coincides with the present tectonic stress field. 5.3 Expansion of aftershock zone The aftershock of M=6,2 that occurred about 23 hr after the main shock was exceptional in a strict sense of aftershock in several respects: 1) its focal plane solution was almost the same as that of the main shock, but their fault planes were conjugated with each other; 2) the fracture zones of the two shocks were separated by an aseismic gap which was observed in the incipient stage, but bridged soon by a weak activity between the two zones (Fig. 6); 3) according to a statistical study on the properties of aftershock sequences (UTSU, 1969), the median of difference in magnitude between the main shock and the largest aftershock is about 1.6 for M=6.8. The difference of 0.6 is too small. It is therefore suggested that the event of M=6.2 was not a simple aftershock of the earthquake of M=6.8, but an induced shock that was caused by the sudden stress concentration near the western tip of the main fault. The focal plane solution of the event of M=5.3 on October 3, 1984 near the east end of the main fault, where few events had been recognized before this shock, was a reverse fault type. This event was also regarded as induced by the similar stress concentration. 5.4 The tectonic structure A comparison between the seismic distribution and the contours of Bouguer gravity anomaly around Ontake volcano (Fig. 11) suggests that both distributions of aftershocks of the main shock and of the subsequent M=6.2 event are aligned parallel to the belt of high horizontal gradient in Bouguer gravity anomaly on the southern foot of the volcano, but do not coincide with the belt itself. According to KUBOTERA (1975), such a parallel distribution of epicenters with the belt of high gradient in Bouguer gravity anomaly was also recognized at Chijiwa caldera, Kyushu, southwestern Japan. The relation between the gravity anomaly trend and

Aftershock Activity of the Western Nagano Prefecture Earthquake 413 Fig. 11. Contour map of Bouguer gravity anomaly around Ontake volcano (adopted from SHICHI et al., 1988). The contour interval is 0.5 mgal. Epicenters of events in the period from September 14, 1984 through October 13, 1984 are plotted on the map in order to show a relation between the epicenter distribution and the Bouguer gravity anomaly. rupture direction implies a large-scale structural discontinuity beneath the volcanic area. The Bouguer gravity anomaly map (Fig. 11) suggests a buried escarpment structure with the north side down and approximately parallel with the northern margin of the seismically active region. Although we can not clearly find any fault in the source area of the 1984 earthquake, the distributions of aftershocks and the earthquake swarms suggest that the escarpment structure as revealed by the gravity survey is a seismogenic fault system. According to the geological study by KOBAYASHI et al. (1971), there is a triangular caldera-structure around Ontake volcano, but the structure suggested in our study is probably greater in scale than that proposed by them.

414 T. OOIDA et al. 6. Conclusions On the basis of routine observation, some characteristics of earthquake swarms, a damaging earthquake in 1984 and its aftershocks near Ontake volcano are summarized as follows: (1) The two seismic swarm activities beneath the southeast foot of Ontake volcano were found to be almost independent of each other. (2) An eastward migration of seismic swarm activity was recognized before the M=6.8 earthquake on September 14, 1984. (3) The prominent changes in the seismic activity before the 1984 earthquake were the quiescence that continued from October 1982 through March 1983 and the subsequent clustering of events at a few places in the northern area. (4) The 1984 earthquake occurred in the northern area of the swarm activity. The rupture was 12 km long, almost vertical and striking ENE-WSW which is different from the direction of major active faults in the western half of central Japan. (5) The southern margin of the aftershock zone agrees with the boundary between the northern and southern areas of the earthquake swarms. (6) The two peculiar aftershocks (M=6.2 and M=5.3) near the western and eastern ends of the main fault were probably induced by the stress field caused by the main shock (M=6.8). A new earthquake swarm further west was also suggested to have been induced by the main shock. The earthquake swarm in the northeastern area of the main fault is also considered to be activated by the main shock. (7) The fault of main shock, the fault of the largest aftershock and the western margin of the northeastern aftershock area were parallel to the belt of high gradient in Bouguer gravity anomaly at respective places. It is suggested that the occurrence of the earthquake swarms, the main shock and its aftershock sequence were constrained by the particular crustal structure beneath Ontake volcano. We would like to thank Messrs. M. Yamada, M. Nakamura, and R. Miyajima for assistance in seismic observation and data processing. We also thank Prof. Y. Fukao, Dr. M. Furumoto, Dr. I. Yamada and many other members of the Department of Earth Sciences, Nagoya University, and Prof. H. Mizutani, Institute of Space and Astronautical Science, for their considerable help for the data processing of aftershocks. This research was supported in part by a Grant-in-Aid for Scientific Research No. 59020202 from the Ministry of Education, Japan, to which we express our thanks. REFERENCES AOKI, H., The 1984 Western Nagano Prefecture earthquake, Proc. Earthq. Predic. Res. Symposium (1987), 109-114, 1987 (in Japanese with English abstract). AOKI, H., T. OOIDA, I. FUJII, and F. YAMAZAKI, Seismological study on the 1979 eruption of Ontake volcano, in Investigation of Volcanic Activity and Disasters Caused by the

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