ANOMALOUS CRUSTAL ACTIVITY IN THE IZU PENINSULA, CENTRAL HONSHU. Kenshiro TSUMURA

J. Phys. Earth, 25, Suppl., S 51-S 68, 1977 ANOMALOUS CRUSTAL ACTIVITY IN THE IZU PENINSULA, CENTRAL HONSHU Kenshiro TSUMURA Earthquake Research Institute, University of Tokyo, Tokyo, Japan (Received June 22, 1977; Revised October 3, 1977) Earthquake prediction research on the recent anomalous crustal activity in the Izu Peninsula is summarized. Swarm activity of shallow microearthquakes in the eastern Izu Peninsula began in August 1975 and became more active in October. The epicenters clustered near Togasa-yama at first but spread over other places in the eastern part of and east off the peninsula in early 1976. Releveling carried out in January-April 1976 has disclosed crustal uplift of 15cm centered at Hiekawatoge, several kilometers north of Togasa-yama. The uplift area, more than 30km in diameter, covered the eastern half of the peninsula. Leveling, gravimetric, and tidal data showed that this uplift had developed only during the preceding 1-1.5 year period. Rapid changes in length of base-lines were also detected by repeated geodimeter measurements. These phenomena were noted by the Coordinating Committee for Earthquake Prediction and observations were intensified. The information about the crustal activity was made public by the Committee in May 1976. On August 18, the Kawazu earthquake (M=5.4) occurred in the southern part of the uplift area. Short-term precursors except for foreshocks were not so clear in spite of the intensified observations. In the eastern part of the Izu Peninsula, Central Honshu, remarkable crustal uplift has developed since around the end of 1974, with accompanying microearthquake swarms. An indication of anomalous crustal activity was firstly noticed in increasing microseismicity in August 1975, and the crustal uplift of 15cm was found by releveling in early 1976. Rapid changes in sea level, gravity and base-line length were also found. These facts were noted by the Coordinating Committee for Earthquake Prediction (CCEP) and various observations were intensified by its cooperating institutions. The information about this anomalous activity was made public by the CCEP in May 1976. On August 18, 1976, the Kawazu earthquake (M=5.4) occurred in the southernmost part of the uplift area, and caused some property damage. Short-term precursors were not clear in spite of the intensified observations, except for the foreshock sequence during about 1.5hr prior to the main shock and slight change in groundwater temperature. The process of the research and the response of the CCEP for this anomalous activity are reviewed chronologically as an actual example of the Japanese efforts to predict earthquakes. S 51

S 52 K. TSUMURA 2. The Past Seismic Activities in the Izu Peninsula The Izu Peninsula is a special tectonic block in the Japanese Islands. It belongs to the Philippine Sea Plate, moving northwestward and colliding against the Honshu block of the Eurasian Plate (Fig. 1). There exist many active faults (e.g., MURAI and KANEKO, 1974), Holocene volcanoes (ARAMAKI, 1976), as shown in Fig. 2, and hot springs. Major seismic events in this peninsula in the past 50 years are as follows: From February to May 1930, very active earthquake swarms with the largest shock of M=6.0 occurred near Ito on the east coast, accompanied by remarkable crustal uplift amounting to about 35cm (e.g., SAMC, JMA, 1976; CRUSTAL DYNAMICS DIVISION, GSI, 1976). The Kita-Izu earthquake (M=7.1) took place on November 26 of the same year, with left lateral movement of the Tanna fault which extends Fig. 1. Location map of the Izu Peninsula and the plate boundaries. The East-off-Izu tectonic line was proposed by ISHIBASHI (1977) to explain the present uplift. about 35km in a N-S direction in the northern peninsula. Many foreshocks were observed during a period of 20 days before the main shock. This earthquake is considered to be the largest event in this peninsula since a severe earthquake (M=7) in 841. On March 21, 1934, the Minami-Izu earthquake (M=5.5) took place in the central part, between Yugashima and Kawazu. Then, the next 40 years period was seismically very quiet. On May 9, 1974, the Izu-Hanto-oki earthquake (M=6.9) occurred near the southwestern coast of the peninsula, with right lateral movement of the Irozaki fault, and activated the seismicity in the central part too (e.g., RESEARCH GROUP FOR AFTERSHOCKS, 1975). But the eastern part had been almost aseismic until the summer of 1975. There was a small seismicity gap around Kawazu for the past 50 years, surrounded by the epicenters of the 1930, 1934 and 1974 events and earthquake swarms which have frequently occurred in the area to the east, between the peninsula and Oshima Island.

Anomalous Crustal Activity in the Izu Peninsula, Central Honshu Fig. 2. Active faults and lineaments in the Izu Peninsula (MURAI and KANEKO, 1974). 1, right lateral fault; 2, scarplet; 3, lava flow; 4, volcanic cone; 5, explosion crater. The northernmost fault extending N-S is the Tanna fault which caused the 1930 Kita-Izu earthquake (M=7.1) and the southernmost one is the Irozaki-fault which caused the 1974 Izu-Hanto-oki earthquake (M=6.9). 3. Commencement and Development of Earthquake Swarms (August 1975-February 1976) Figure 3 shows the area of the present uplift and the epicentral distribution of microearthquakes until the end of 1976, together with leveling routes and temporary seismographic network in the Izu Peninsula. Figure 4 shows daily number of microearthquakes recorded at a routine station Okuno (OKN), which has been operating since November 1971, with magnification about 100K at 10Hz. The number of shocks, with S-P times about 1.5 sec at OKN, suddenly increased on August 10-12,

S 54 K. TSUMURA 1975 as shown in Fig. 4. Accordingly, two temporary stations (YGS and KWZ) were added in early September. On October 26, 1975, the number of shocks suddenly increased again and more than 100 shocks per day were observed at OKN in the following about 10 days, then suddenly decreased. The epicenters in this period clustered near Togasa-yama or Mt. Togasa and the focal depths were shallower than 10km. The largest shock of this cluster (M=3.7) occurred on November 14, just after a short quiet period (Figs. 4 and 5(A)) (ERI, 1976a). In the following months, the activity near Togasayama was gradually decreasing with some fluctuations as shown in the lowest curve Fig. 3. Map showing distributions of the crustal uplift and epicenters of microearthquakes in the Izu Peninsula. The contour lines of uplift are for the interval from 1967-69 surveys to January-April 1976 surveys (CRUSTAL DYNAMICS DIVISION, GSI, 1976). Broken lines indicate the leveling route along which surveys were carried out in 1976. Epicenters are determined by the Earthquake Research Institute temporary seismographic network (OKN, ICY, NRM, and KWZ) during the period from November 1975 to December 1976.

Anomalous Crustal Activity in the Izu Peninsula, Central Honshu Fig. 4. Number of microearthquakes of S-P time. recorded at Okuno station (OKN), for different ranges in Fig. 4, but seismically active area spread towards northeast and southwest as shown in Fig. 5(B). On February 9, 1976, a separate activity began to occur in a small area near Hokkawa on the east coast (Fig. 5(C)). Twenty-three foreshocks were recorded at OKN during the period of 9hr before the main shock (M=3.6), which caused a slight damage and accompanied by many aftershocks (Fig. 6(A)). On February 12, a swarm activity began in the area between the peninsula and Oshima Island and continued about one month (ERI, 1976b). These data on earthquake swarms were reported to the CCEP in November 1975 and February 1976. But they did not attract much attention, because such earthquake swarms are not uncommon in volcanic areas, such as the Izu Peninsula. Moreover we knew nothing about the crustal uplift in the eastern peninsula, which was going on at that time. However, further research was commenced to determine the nature of this microearthquake swarm activity. Nine base-lines centered at Togasa-yama and 4 base-lines centered at Naramoto were newly set up in January and February 1976 (CRUSTAL MOVEMENT SURVEY PARTY, ERI, 1976). The leveling survey was also started along the route from Toi on the west coast towards the east coast in January (CRUSTAL DYNAMICS DIVISION, GSI, 1976).

S 56 K. TSUMURA Fig. 5. Epicentral distributions of microearthquakes for different periods of the swarm activity. (A) November 20-December 31, 1975, (B) January 1-February 8, 1976, (C) February 9-April 30, 1976, (D) May 1-August 17, 1976, (E) August 18-31, 1976, (F) September 1-December 31, 1976. 4. Anomalous Crustal Movements Found and Response of the Coordinating Committee for Earthquake Prediction (March-May 1976) The leveling survey carried out in January to April 1976 has revealed remarkable crustal uplift of 15cm centered at Hiekawa-toge, several kilometers north of the seismically most active area, as compared to the 1967-69 surveys (CRUSTAL DY- NAMICS DIVISION, GSI, 1976), as shown in Figs. 3 and 7. The uplift area exceeds

Anomalous Crustal Activity in the Izu Peninsula, Central Honshu Fig. 6. A plot of magnitude versus time of occurrence of (A) foreshocks of the earthquake near Hokkawa (M=3.6) on February 9, 1976, (B) earthquake swarm near Kawazu on June 26, 1976, and (C) foreshocks of the Kawazu earthquake (M=5.4) on August 18, 1976. 30km in diameter, covering the eastern half of the penin sula. Although the contour lines of uplift in Fig. 3 are based on the data of 7-9 years interval, other data suggested that this uplift had developed only during 1-1.5 year period. Figure 8 is the mean sea level change of Ito tidal station relative to Aburatsubo, about 50km northeast of the former, clearly demonstrating such process of the uplift (CRUSTAL DYNAMICS DIVISION, GSI, 1976). The precise gravity survey carried out in March has also disclosed a pattern of gravity changes consistent with the leveling data as shown in Fig. 9(A). Namely, mum crustal uplift, as compared to the December 1974 result (GEODETIC SURVEY PARTY, ERT, 1976). The geodimeter measurements executed in April also disclosed very rapid and complicated horizontal crustal movements. As shown in Fig. 10, large contraction and northeast from Togasa-yama only during three months period (CRUSTAL MOVEMENTS SURVEY PARTY, ERI, 1976), while extension of similar amount has found in a slightly northern area just over the center of the uplift as compared to the 1973 or 75 data (CRUSTAL DYNAMICS DIVISION, GSI, 1976).

Anomalous Crustal Activity in the Izu Peninsula, Central Honshu S 59 Fig. 8. Relative sea level changes at Ito tital station compared to Aburatsubo (CRUSTAL DYNAMICS DIVISION, GSI, 1976). 1974-February-March 1976, (B) December 1974-June 1976, and (C) December 1974- September 1976 (HAGIWARA et al., 1976). These data were reported from the cooperating institutions to the CCEP and its Sub-Committee for Kanto Area and carefully examined at the meetings held in May. Older data about the activities in 1930's were also reexamined in detail and compared to the present data (CRUSTAL DYNAMICS DIVISION, GSI, 1976; SAMC, JMA, 1976). For instance, a striking similarity in the spatial distributions of uplift amount for the old and the new data along the leveling route on the east coast was pointed out. Since this peninsula is a volcanic area, application of the dilatancy-diffusion model (e.g., SCHOLZ et al., 1973) or empirical formulas hitherto presented (e.g., DAMBARA, 1966; RIKITAKE, 1975) were thought questionable in estimating the magnitude and time of forthcoming earthquake from the observed dimension of uplift area, although such formulas predicted a magnitude about 7 earthquake, 5 to 7 years hence for the present crustal activity. Negative evidence for the dilatancy-diffusion model was offered from the very precise observations of P wave velocity using artificial explosions on Oshima Island, which have been carried out every year since 1968. For the December 1975 experiment, any significant changes have not been detected in the travel times larger than

K. TSUMURA 10 msec at Okuno and Ukihashi, near the center of the uplift area, as compared to data at other stations in different directions (GEOLOGICAL SURVEY OF JAPAN, 1976; KAKIMI and HASEGAWA, 1977). Hagiwara (GEODETIC SURVEY PARTY, ERI, 1976) showed that the pattern of uplift mentioned above can be explained by the so-called Mogi model (MOGI, 1958), when we assume pressure increase of 100 bars in a spherical magma reservoir with diameter of about 4km, at a depth of 10km. ISHIBASHI (1977) proposed a creep dislocation model assuming aseismic movement on a fault plane westerly-dipping from a tectonic line east off the Izu Peninsula (Figs. 11(C) and 12). FUJII (1977) proposed a similar model of aseismic creep dislocation propagating towards deep crust on a westerly-dipping fault plane to explain the late westwards migration of uplift area. Assuming any model, it was thought reasonable to expect some trigger effect from the stress changes due to this uplift on the many active faults existing in the peninsula. But, geologists at the CCEP offered an opinion that there is no fault long enough to generate a magnitude 7 or larger earthquake, except for the Tanna fault, along which stress must have been released in the 1930 Kita-Izu earthquake. After discussion on these data, the CCEP concluded that the possibility of strong earthquake(s) in the uplift area could not be denied as one of presumable future sequences, but the magnitude would be less than 7. Volcanic activity was also Fig. 10. Location of the base-lines on which the geodimeter measurements were carried out in 1976 by the Geographical Survey Institute (centered at Tokunagamura, Tn) and by the Earthquake Research Institute (centered at Togasa-yama, T and Naramoto, N). Data taken from CRUSTAL DYNAMICS DIVISION, GSII (1976, 1977) and CRUSTAT, MOVE- MENTSURVEY PARTY, ERI (1976, 1977).

Anomalous Crustal Activity in the Izu Peninsula, Central Honshu suspected, but it was thought that the seismic activity was not so violent to expect imminent eruption. This information about the anomalous crustal activity was made public by the CCEP on May 25, 1976, and further intensification of observations was strongly recommended. In response to this recommendation, various kinds of observations and surveys were intensified by the cooperating institutions in the following months. Fig. 12. Vertical displacement field and horizontal displacements of the triangulation points associated with the aseismic dislocation model ((C) in Fig. 11). Parameters employed for calculation are given in the figure. The shaded rectangle and the white arrow on it are the horizontal projection of the dislocation surface and slip vector, respectively (ISHI- BASHI, 1977).

The microseismicity had been monitored mainly by the ERI temporary network consisting of 3 to 5 seismographic stations in the peninsula. But, as it has no telemetering station, the analysis had usually been delayed by one or two weeks. Two stations (JIZ and KMT) telemetered through telephone cable were newly set up by the NATIONAL RESEARCH CENTER FOR DISASTER PREVENTION (1977) and by the Japan Meteorological Agency (SAMC, SEISMOLOGICAL DIVISION, JMA, 1977) in July and October 1976 respectively. The latter has been used for real-time watching of microseismicity in the peninsula. Continuous observation of the geomagnetic total force intensity by the proton precession magnetometer was started at Sugehiki (SH) in May and the geomagnetic survey was executed in June at 18 points distributed over the eastern half of the peninsula (GEOMAGNETIC SURVEY PARTY, ERI, 1977). Geomagnetic observation by the flux-gate magnetometers and geoelectric observation were also commenced at several places (Fig. 13). Observations of groundwater were started in May-June at many places (Fig. 14). Radon concentration in groundwater was measured at 18 points including two continuous recording stations (GEOLOGICAL SURVEY OF JAPAN, 1976; WAKITA and NOTSU, 1977; WAKITA, 1977, this issue). Observations of water level, water discharge, water temperature, and geochemical investigations of groundwater were continued at more than 30 points in total. But these observation points were too concentrated in the central part of the uplift area as shown in Fig. 14 (5 points for temperature measure- Fig. 13. Distribution of the geomagnetic and the geoelectric observation points. No change in the geomagnetic total force intensity was detected at Sugehiki (SH) related to the Kawazu earthquake as referred to Kanozan on the Boso Peninsula, but probably coseismic change of several gamma was detected by repeated survey at point P, just above the epicentral zone as referred to Sugehiki and Kanozan as shown on the right side (GEOMAGNETIC SURVEY PARTY, ERI, 1977).

Anomalous Crustal Activity in the Izu Peninsula, Central Honshu S 63 Fig. 14. Distribution of the observation points of groundwater. ment near Kawazu were set up after the Kawazu earthquake in August). Active faults and lineaments, and recent volcanic activity were re-examined (e.g., HOSHINO et al., 1977). Submarine topography and structure in the area east off the Izu Peninsula were surveyed in detail by the Hydrographic Department. 5. The Kawazu Earthquake of August 18, 1976 and Related Phenomena (June-August 1976) From May to July 1976, the microseismicity was decreasing as a whole. On June 26, an earthquake swarm including at least 6 felt shocks with M=2-3 occurred near Kawazu and was noted, but as it lasted only one hour or so, then ignored (Fig. 6(B)). The microseismicity became extremely quiet in the first half month of August, especially in the area south of Amagi-san. From August, 14, the seismicity in this area recovered slightly, and the Kawazu earthquake (M=5.4 (JMA), MB=5.1 (USGS)) occurred at 0219 on August 18, with clear foreshock sequence started at 0054 and many aftershocks. At Kawazu station (KWZ), 27 foreshocks were recorded during about one and a half hour period before the main shock (Fig. 6(C)). The b- value was about 0.6. Figure 5(E) shows the epicentral distribution of aftershocks in August. The focal depths are mostly shallower than 6km (ERI, 1977). The distribution of epicenters suggests a buried fault extending from NW to SE, and this direction coincides with those of the surface faults in the vicinity. Although no evidence of surface rupture has found by field investigation (Murai, 1976, report to CCEP), right lateral strike-slip faulting is inferred from the initial motions of P waves and the aftershock distribution. Abe (1976, personal communication) estimated the

On August 23, the CCEP was held and noted the possibility of another shock(s) with similar magnitude to the Kawazu earthquake. This conclusion seems reasonable, because several strong shocks occurred successively in the 1906 earthquake swarm in the same area. Actually, three days after this notice, on August 26, the largest aftershock (M=4.5) occurred and caused slight damage. But this notice seems to have given much more social impact and lasting influence than the earthquake itself, because it caused decrease of visitors to this area of many resorts during the following several months. As the Kawazu earthquake was accompanied by a clear foreshock sequence, it might be thought that we could have issued an alarm just before the earthquake by watching the seismograms more carefully. But if so, a false alarm had also been issued at least once in June on the same basis, because the June 26 swarm in the same area was more active than the foreshock activity as compared in Fig. 6. The b-value was rather smaller (0.5) in the June swarm. We tend to imagine seismic waves of higher frequency in the foreshock sequence than in the aftershock or the ordinary sequences. But negative result was obtained by Tsujiura (1976, personal communication) for the present case. The initial half periods of P-waves of the Kawazu foreshocks are longer than or nearly the same as the aftershocks. A minute, probably coseismic, uplift of 2cm or less has found in the epicentral region (Fig. change was detected at Sugehiki as referred to Kanozan as shown in Fig. 13 (GEO- MAGNETIC SURVEY PARTY, ERI, 1977). The radon concentrations in groundwater were almost invariant throughout the present sequence (Geological Survey of Japan, 1976, report to CCEP; WAKITA and NOTSU, 1977; WAKITA, 1977, this issue). The temperature of groundwater from a well of 500m deep at Naka-Izu, about 15km north of the epicenter is reported to just after it, accompanied by the decrease of water discharge from the well (Geological Survey of Japan, 1976, report to CCEP). Other data by recording thermometer at the same place show smaller preseismic change of groundwater temperature (about Precursory changes were not clearly noticed in other elements and at other places in spite of the intensified observations. Figure 15, constructed by Yamashina (1977, personal communication), may offer one explanation why a strong earthquake selectively occurred near Kawazu in

Anomalous Crustal Activity in the Izu Peninsula, Central Honshu the uplift area. The shaded portion in Fig. 15 indicates the area where the shear strain on the existing fault plane extending NW-SE was possibly increased by such events as (B) the 1974 Izu-Hanto-oki earthquake, (C) the uplift assuming the Mogi model

with parameters estimated by Hagiwara (GEODETIC SURVEY PARTY, ERI, 1976), and (D) the uplift assuming the creep dislocation model proposed by ISHIBASHI (1977). It seems interesting that Kawazu area is always in the shaded zone in these maps. Especially, the pattern for the 1974 Izu-Hanto-oki earthquake is suggestive. If we had noticed such patterns of shear strain changes before the Kawazu earthquake, the observation network might have been modified to cover Kawazu area more densely. 6. Decaying Activity and Outlook by the Coordinating Committee for Earthquake Prediction (September 1976-) According to the leveling surveys conducted in August-September 1976, the uplift has almost stopped or turned to subsidence during a preceding half year in the eastern part, but is still continuing with smaller rate in the central part as the uppermost curves in Fig. 7 indicate. The sea level data (Fig. 8) and gravimetric data (Fig. 9) substantiate this tendency. Rapid horizontal movement has also stopped in the eastern part, but rapid extension is still observed in the central part in accordance with the vertical movement as shown in Fig. 10. The microseismicity is decreasing after the Kawazu earthquake, though swarm-like activities in short duration are sometimes observed (Figs. 4 and 5(F)). Based on these data, the CCEP, held in November, concluded that this activity was likely to decay without another strong earthquake, but as the possibility of strong shock could not be completely ruled out, the intensified observations should be continued for a while, say one year or so. 7. Concluding Remarks For the present crustal activity in the Izu Peninsula, microearthquake observation, leveling survey, geodimeter measurement, tidal observation, and precise gravimetric survey furnished useful data for the detection of anomalous activity and the medium-term prediction. The medium-term prediction by the CCEP based on these data, together with other data in which clear changes were not found, seems to have been adequate so far as the area to be remarked and the magnitude of expected earthquake concerned. However, the bases for short-term prediction of exact time and place had not been found before the Kawazu earthquake, although it is easy to point out now, after knowing the whole process, that the immediate foreshock sequence is clear and the anomalously quiet state in microseismicity preceded it is likely to be some kind of precursor. It seems more appropriate to say that the Kawazu earthquake was triggered by the crustal uplift than to express that the former was accompanied with the latter as it own precursor, when such models as the Mogi model or the creep dislocation model are adopted to explain the uplift. The present crustal activity in the Izu Peninsula was not so prominent as the

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