- Email: [email protected]
Recent developments in earthquake prediction research in Japan
Tectonophysics - Elsevier Printed in The Netherlands
Publishing
Company,
Amsterdam
RECENT DEVELOPMENTS IN EARTHQUAKE PREDICTION RESEARCH IN JAPAN HIROO KANAMORI Earthquake (Received
Research November
Institute,
University
of Tokyo, Tokyo (Japan)
13, 1969)
SUMMARY Recent research on earthquake prediction in Japan has been carried out mainly in the framework of the “5-year Program on Earthquake Prediction Research” which was first founded by th8 government in 1965. Some of the latest experiments are precise observation of explosions for detecting a possible velocity change in a tectonically active area, laboratory study of the migration of micro-fractures in rocks under stress, and a deep drilling in the Matsushiro earthquake swarm area. Based upon the outcome of the 5-year program and the experience of several destructive earthquakes encountered during the period, a new 5-year program starting from 1969 was proposed in 1968. The new program which aims at actual predictions of destructive earthquakes emphasizes augmented instrumentation, strategy, and organization.
INTRODUCTION
The research on earthquake prediction in Japan has been carried out mainly in the framework of the “Five-Year Program on Earthquake Prediction Research” (see Rikitake, 1966). Since the development of this program and many of the earlier works on earthquake prediction have been summarized by Rikitake (1968, 1969), this paper will describe some of the latest results and a newly proposed 5-year program which follows up the old program and aims at actual predictions of major earthquakes. PRECISE
MEASUREMENT
OF SEISMIC VELOCITY
Based upon the reasonable but not fully tested idea that a detectable change of seismic velocity may occur as stress builds up in a potential earthquake focal region, precise measurements of explosion-generated seismic waves were conducted in 1968 and 1969 by a research group at Geological Survey of Japan (M. Hayakawa and S. Iizuka, personal communication, 1969) (see also Iizuka et al., 1969). An explosive of 490 kg was detonated on Oshima Island (Fig.1) on March 18, 1968. Observations were made at fivedistant stations (Okuno, A = 31.0 km; Ukihashi, A = 41.5 km; Nagasa, A = ‘70 km; Isehara A = 72 km; Tectonophysics,
9 (1970) 291-300
291
t
Norado
“-b-j km 30
w
I
ocean
.Fig.I. Location of explosion site (x) and stations. The cross-hatched circle is the epicenter of the great Kanto earthquake of 1923, and thehatched circles are the epicenters of major aftershocks within one day after themain shock. The size of the circles roughly corresponds to the magnitude of the shocks.
Miyagase, A = 81 km) in addition to two nearby stations which were used for various calibration purposes:This site was chosen for the experiment because it includes the focal region of the great Kant0 earthquake of 1923 (see Fig.1); one might naturally suspect that a gradual stress build-up, which may reflect in the seismic velocity change,is going on in this region. The seismometer system used at the stations varies from onestation to another but a typical system consists of 1 Hz vertical and transverse, and 4 Hz vertical transducers connected either to a magnetic-tape data recorder or to an electro-magnetic oscilfograI&. The time base was given by a crystal clock and the broadcast standard signals ‘(JJY). Iizuka et al. (1969) noted that the overall accu,racy of the reading is probably better than 10 msec under favourable conditions. About one year after this first.explosion, the second observation was made in I&arch 1969, by detonating two explosives (yield: 549 kg, and 499.5 kg), four days apart, at the same spot as the 1968 explosion. The nearby observations showed that the,source function was nearly the same for the 1968 explosionandforthetwo1969explosions. Fourstat-fons(Zldatq#ku, A = 105 km; Ubukawa, A = 130 km; Kamiinako, A = 92 km; Narada, A = 129 km) were added for future comparisons. No significant change was made in the 292
Tectonophysics, 9 (1970) 291~300
lsrharo
(A = 72
km)
66 Tr
69
-
L 14 *.c
I
Ukiharhi
IS S.C (A-41.5,
km)
V
Fig.2. Comparison of. 1 Hz seismograms between 1968 and 1969 explosions at two stations, Isehara and Ukihashi. Traces V show the vertical components and traces T, show the transverse components. Times are the travel times.
instrument system. It was found ihat the reproducibility of the wave-form at distant stations was remarkable as shown iti Fig.2 which shows 1 Hz vertical and transverse seismograms at two stations, The overall magnification is different for the 1968 and 1969 events but S. Iizuka (personal communication, 1969) estimated that the amplitude of the ground displacement was approximately the same for the 1968 and 1969 events. The travel time was measured for the initial onset, the first peak, and the first trough as shown in Fig.3. The difference of the travel time between the 1968 and 1969 events was then calculated at four stations and plotted in Fig.3. The traveltime difference was less than 5 msec which is comparable to the experimental error, It was therefore concluded that no detectable velocity change occurred in this region over this period. Although this experiment did not reveal a positive evidence for velocity change, it showed that the travel-time difference can be determined to an accuracy of several milliseconds for a region of about 100km extent. Whether this accuracy-is sufficient or not for a reconnaissance of the stress state in the crust depends upon the mode of stress accumulation and the property of the rocks. It is expected that this experiment, if continued with refined theory and technique, will lead to a more definite conclusion as to whether a premonitory velocity change occurs or not. Tectonophysics, 9 (1970) 291-300
293
+10
I
I
1
I
I
I
lseharo
Distance
1
I
Miyagase
(km)
Fig.3. Travel-time difference AT (travel time of 1969 event - travel time of 1968 event) plotted against distance, open circles are for the onset, crosses for the first peak and triangles for the first trough. IABORATORY
STIjDY OF ELASTIC
SHOCKS IN ROCKS
For the purpose of earthquake prediction the nature of foreshocks is particularly important. According to the fracture theory of earthquakes, the foreshock activity can be simulated in the laboratory in terms of the occurrence of shocks prior to a major rupture in a rock specimen. Mogi (1968) located a number of elastic shocks in a rock specimen under tensile stress by measuring, at several points in the Specimen, the travel times of the shock-generated waves. Fig.4 illustrates one of the geometries of the test piece he used. The ceramic transducers Ps and Pg pick up the signal generated by elastic shocks. The signals are amplified and displayed on a dual-beam oscilloscope on which the travel-time difference is measured. From the travel-time difference one-dimensional source locations in direction OL can be determined. By attaching additional transducers in a direction perpendicular to OL, Tao-dimensio~l source locations can be determined. %%ogistudied various rocks having different degrees of structural heterogeneity. The applied stress was gradually increased but it
Fig.4. Specimen, transducers, and loading system for elastic experiments. 4 and Pa are transducers for one-dimensional source location, and Pt is the transducer for triggering of the recording system. (After Magi, 1968). Fig.5. Source locations of elastic shocks in OL direction as a function of time, The specimen is granite; A, B, C, C, , C,, and C3 denote different stages, (After Magi, 1968.) 294
Tectonophysics;
9 (1970) 291-300
Force u . . . . . . . .:.
. . . . . . . . . .._.......
.._........._.. . . . . . . . . . . . . I.,. ~.~..
:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.3~m
~.‘.~.~.~.~.~.‘.~.~.~.‘.~.~.~.~.~.~.~.
~.~.~.~.~.~.~.‘.~.~.~.~.~.~.~.~.~.~.~. . . . . . . . . . . . . . ._ . . . . . . . . . . . . . . . .
..
. . :. . . :. :. . .,.. :. :.
.. ... ... . .. .... .. .. .. .... .... .... .. :. .. .. I.........................,.... 4 :.:.:_:.:.:.:.>:_:.:
f-g
I
I
I
I
~6crn~ k
30cm F
Fig. 4. Granite
(15)
Rupture 0
c .. .*.
ml
-Rupture
.
l
G
.
6l
*
l
. .
. 7
_C’ .’
l’ .Y
****
l *
. 40
+
30
l
.
. . .
. .
.
y; .
-
.
‘a .,*
.
l .*.
.
’
l
. .
. .
. .
.
.
. :qy
be
.
.* *.-
** ‘. ’ .;
.a
Cl
..0
.
. *
.
C
.. .a..
.
*< t * :
.
.
G
.
. .
x
..
.
.% .
.
l*
B 20
IO
-4
-2
0 -
2
L incm
4
6
Fig.5.
pz
was held nearly constant just before the major rupture. He found that, for relatively heterogeneous rocks, the entire fracture process can be classified into three distinct stages A, B, and C (Fig.5). The stage where the stress level is so low that no elastic shock occurs is called stage A. With a subsequent increase of stress, the elastic shock starts occurring but the locations are distributed more or less at random in the specimen. This stage is called B. In due course, the shocks start concentrating in a narrow region and finally a major rupture occurs in a neighborhood of this region. This stage is called C. For relatively homogeneous rocks no clear distinction among these stages was found and the major rupture occurred more or less suddenly. For some of the major earthquakes which were preceded by a considerable number of foreshocks, the concentration of foreshocks, corresponding to the stage C, in a region where the mainshock occurred has been found (Fig.6). Mogi (1968) also studied a migration of a source region of elastic shocks. He found that a region which is highly active at a certain stage becomes relatively inactive at the following stage, and that the activity successively migrates outwards as shown in Fig.7. Mogi compared this observation with the migration of epicenters during three successive periods in the Matsushiro earthquake swarm from 1965 to 1967 (Fig.8). He concluded that the laboratory experiment can simulate many of the important features of earthquake mechanics.
Kita-lzu (19x)) m
Aftershbck Region
Aleutian
I turw
(1963)
Foieshock
(I9651
Fig.6. Epicenter distributions of foreshocks with respect to main shock and aftershocks for four earthquakes. (After Mogi, 1968.)
Granite
t t t
04) c2 ,I
I 0I
.
I
0 -----rL
2
4
in cm
Fig.7. Successive migration of source region for stages CI, C2, and C3 in Fig.5. Main crack pattern is also shown.
v /
A
,
Fig.8. Successive migration of source region during the Matsushiro earthquake swarm: A. October 1965-May 1966; B. June 1966-December 1966; C. January 196775eptember 1967.
Tectonophysics, 9 (1970) 291-300
297
DEEP
DRILLING
IN THE MATSUSHIRO
EARTHQUAKE
SWARM AREA
A deep drilling in the ~aisushiro earthquake swarm area is now being made by the National Research Center for Disaster Prevention with the cooperation of Geological Survey of Japan, Japan Meteorological Agency, and the Earthquake Research Institute of the University of Tokyo. Thisdrilling, which is planned to reach a depth of 1.8 km, has three objectives: (I) to study physical and chemical properties of rock samples obtained at depths in the borehole, (2) to install equipment for electrical and seismic experiments in the borehole, and (3) to use the borehole for fluid injection and to study its effect on earthquake activity. The drilling was started in March 1969, and reached a depth of about 1 km in July 1969. The site is nearly in the middle of the swarm area where the depth of the basement rock having a P velocity of 6 km/set is estimated to be about 1 km. This basement rock has not been reached. Several highly fractured layers have been found. Because of these fractured layers the drilling is now slightly behind schedule. However, it is hoped that within several months the drilling will be completed and be ready for the fluid injection.
NEW S-YEAR
PROGRAM
Based upon the outcome of the 5-year program and partly accelerated by the occurrence of the Matsushiro earthquakes and the 1968 Tokachi-oki earthquake, a new 5-year’program starting from, 1969 was proposed by the Geodetic Council of Japan. This newprogram which aims at actual predictions of major earthquakes emphasizes augmented instrumentation, strategy, and organization. In order to achieve actual predictions which involve vast amounts of data from multidisciplinary observations, the establishment of an efficient organization which makes possible a rapid and smooth flow of .dataamqng different institutions andobservatories is of the utmost importance. Fig.9 schematically shows an organization for earthquake prediction proposed by the Geodetic Council of Japan. Data from three different categories are supplied to respective centers, namely, the Crustal Activity Monitoring Center, the Seismicity Monitoring Center, and the Earthquake Prediction Observation Center, These data are more or less unprocessed. The three centers which are equipped with data prqcessing facilities reduce the respective data in a comprehensive manner to that of manageable size. These processed dam_ are forwarded to the Earthquake Prediction Headquarters which is responsible for overall judgement of the data and takes necessary actions. The Geodetic Council.also proposed the following strategy for earthquake prediction (Fig.10). If an indication of an abnormal phenomenon is found in a certain area by either nation-wide routine observations or observations in special areas such as active faults and densely populated cities, thearea is designated as an “area of intensified observation”, and the observation inqhis area is considerably augmented. This intensified observation eventually enables us to determine whether this abnormal phenomenon is likely to be related to a forthcoming major earthquake or not. If it is suspected to be so, observations of all kinds are then concentrated on
298
Teetonophysics,
3 (1970) 291-300
University (micro-
Observatories
and ultramicro-
earthquakes, movement,
crustal etc.
)
Fig.9. Organization for earthquake prediction. Earthquake Prediction Headquarters and Crustal Activity Monitoring Center are established at Geographical Survey Institute (G.S.I.), Seismicity Monitoring Center, at Japan Meteorological Agency (J.M.A.), and Earthquake Prediction Observation Center, at the Earthquake Research Institute (E.R.I.) of the University of Tokyo.
Nation-wide Observation Observations in Special
Detection of symptom
Diagnosis by Intensified
Detection of Premonitory -----+
Observation
Phenomena by Concentrated
Prediction
Observation
Areas
Fig.10. Strategy for earthquake prediction.
this particular area, now designated as an “area of concentrated observation”. If some premonitory phenomenon is detected during this period of concentrated observation, a warning or prediction may be issued. The Earthquake Prediction Headquarters is responsible for various judgements necessary for taking these actions. The most important but perhaps the most difficult point in carrying out earthquake predictions according to the above strategy is to establish an objective and quantitative guideline by which various judgements can be made. Although no established guideline exists at present, Rikitake (1969) made an attempt to use, as a guideline, a probability of magnitude and occurrence-time of an earthquake in a specified area. This probability can be Tectonophysics, 9
(1970)
291-300
299
calculated on the basis of various empirical relations between magnitude and frequency, crustal movement and magnitude, magnetic anomaly and magnitude etc. Rikitake concluded that the magnitude prediction can be achieved with a fairly high accuracy whereas the prediction of occurrencetime cannot be made very accurately. Further studies along this line are obviously desirable. REFERENCES Iizuka, S., Ichikawa, K., Ito, K., Hasegawa, 1. and Hosono, T., 1969. Observations on the time variations of seismic wave velocities by explosion seismic method (2nd report). Monthly Rept. Geol. Surv. Japan, 20: 313-327 (in Japanese with English abstract). Mogi, K., 1968. Source locations of elastic shocks in the fracturing process in rocks, 1. Bull. Earthquake Res. Inst. Tokyo Univ., 46: 1103-1125: Rikitake, T., 1966. A five-year plan for earthquake prediction research in Japan. Tectonophysicp, 3: 1-15. Rikitake, T., 1968. Earthquake prediction. Earth-Sci. Rev., 4: 245-282. Rikitake, T., 1969. An approach to specific prediction of earthquakes. Tectonophysics, 8: 81-95.
300
Tectonophysics,
9 (1970) 291-300
Publishing
Company,
Amsterdam
RECENT DEVELOPMENTS IN EARTHQUAKE PREDICTION RESEARCH IN JAPAN HIROO KANAMORI Earthquake (Received
Research November
Institute,
University
of Tokyo, Tokyo (Japan)
13, 1969)
SUMMARY Recent research on earthquake prediction in Japan has been carried out mainly in the framework of the “5-year Program on Earthquake Prediction Research” which was first founded by th8 government in 1965. Some of the latest experiments are precise observation of explosions for detecting a possible velocity change in a tectonically active area, laboratory study of the migration of micro-fractures in rocks under stress, and a deep drilling in the Matsushiro earthquake swarm area. Based upon the outcome of the 5-year program and the experience of several destructive earthquakes encountered during the period, a new 5-year program starting from 1969 was proposed in 1968. The new program which aims at actual predictions of destructive earthquakes emphasizes augmented instrumentation, strategy, and organization.
INTRODUCTION
The research on earthquake prediction in Japan has been carried out mainly in the framework of the “Five-Year Program on Earthquake Prediction Research” (see Rikitake, 1966). Since the development of this program and many of the earlier works on earthquake prediction have been summarized by Rikitake (1968, 1969), this paper will describe some of the latest results and a newly proposed 5-year program which follows up the old program and aims at actual predictions of major earthquakes. PRECISE
MEASUREMENT
OF SEISMIC VELOCITY
Based upon the reasonable but not fully tested idea that a detectable change of seismic velocity may occur as stress builds up in a potential earthquake focal region, precise measurements of explosion-generated seismic waves were conducted in 1968 and 1969 by a research group at Geological Survey of Japan (M. Hayakawa and S. Iizuka, personal communication, 1969) (see also Iizuka et al., 1969). An explosive of 490 kg was detonated on Oshima Island (Fig.1) on March 18, 1968. Observations were made at fivedistant stations (Okuno, A = 31.0 km; Ukihashi, A = 41.5 km; Nagasa, A = ‘70 km; Isehara A = 72 km; Tectonophysics,
9 (1970) 291-300
291
t
Norado
“-b-j km 30
w
I
ocean
.Fig.I. Location of explosion site (x) and stations. The cross-hatched circle is the epicenter of the great Kanto earthquake of 1923, and thehatched circles are the epicenters of major aftershocks within one day after themain shock. The size of the circles roughly corresponds to the magnitude of the shocks.
Miyagase, A = 81 km) in addition to two nearby stations which were used for various calibration purposes:This site was chosen for the experiment because it includes the focal region of the great Kant0 earthquake of 1923 (see Fig.1); one might naturally suspect that a gradual stress build-up, which may reflect in the seismic velocity change,is going on in this region. The seismometer system used at the stations varies from onestation to another but a typical system consists of 1 Hz vertical and transverse, and 4 Hz vertical transducers connected either to a magnetic-tape data recorder or to an electro-magnetic oscilfograI&. The time base was given by a crystal clock and the broadcast standard signals ‘(JJY). Iizuka et al. (1969) noted that the overall accu,racy of the reading is probably better than 10 msec under favourable conditions. About one year after this first.explosion, the second observation was made in I&arch 1969, by detonating two explosives (yield: 549 kg, and 499.5 kg), four days apart, at the same spot as the 1968 explosion. The nearby observations showed that the,source function was nearly the same for the 1968 explosionandforthetwo1969explosions. Fourstat-fons(Zldatq#ku, A = 105 km; Ubukawa, A = 130 km; Kamiinako, A = 92 km; Narada, A = 129 km) were added for future comparisons. No significant change was made in the 292
Tectonophysics, 9 (1970) 291~300
lsrharo
(A = 72
km)
66 Tr
69
-
L 14 *.c
I
Ukiharhi
IS S.C (A-41.5,
km)
V
Fig.2. Comparison of. 1 Hz seismograms between 1968 and 1969 explosions at two stations, Isehara and Ukihashi. Traces V show the vertical components and traces T, show the transverse components. Times are the travel times.
instrument system. It was found ihat the reproducibility of the wave-form at distant stations was remarkable as shown iti Fig.2 which shows 1 Hz vertical and transverse seismograms at two stations, The overall magnification is different for the 1968 and 1969 events but S. Iizuka (personal communication, 1969) estimated that the amplitude of the ground displacement was approximately the same for the 1968 and 1969 events. The travel time was measured for the initial onset, the first peak, and the first trough as shown in Fig.3. The difference of the travel time between the 1968 and 1969 events was then calculated at four stations and plotted in Fig.3. The traveltime difference was less than 5 msec which is comparable to the experimental error, It was therefore concluded that no detectable velocity change occurred in this region over this period. Although this experiment did not reveal a positive evidence for velocity change, it showed that the travel-time difference can be determined to an accuracy of several milliseconds for a region of about 100km extent. Whether this accuracy-is sufficient or not for a reconnaissance of the stress state in the crust depends upon the mode of stress accumulation and the property of the rocks. It is expected that this experiment, if continued with refined theory and technique, will lead to a more definite conclusion as to whether a premonitory velocity change occurs or not. Tectonophysics, 9 (1970) 291-300
293
+10
I
I
1
I
I
I
lseharo
Distance
1
I
Miyagase
(km)
Fig.3. Travel-time difference AT (travel time of 1969 event - travel time of 1968 event) plotted against distance, open circles are for the onset, crosses for the first peak and triangles for the first trough. IABORATORY
STIjDY OF ELASTIC
SHOCKS IN ROCKS
For the purpose of earthquake prediction the nature of foreshocks is particularly important. According to the fracture theory of earthquakes, the foreshock activity can be simulated in the laboratory in terms of the occurrence of shocks prior to a major rupture in a rock specimen. Mogi (1968) located a number of elastic shocks in a rock specimen under tensile stress by measuring, at several points in the Specimen, the travel times of the shock-generated waves. Fig.4 illustrates one of the geometries of the test piece he used. The ceramic transducers Ps and Pg pick up the signal generated by elastic shocks. The signals are amplified and displayed on a dual-beam oscilloscope on which the travel-time difference is measured. From the travel-time difference one-dimensional source locations in direction OL can be determined. By attaching additional transducers in a direction perpendicular to OL, Tao-dimensio~l source locations can be determined. %%ogistudied various rocks having different degrees of structural heterogeneity. The applied stress was gradually increased but it
Fig.4. Specimen, transducers, and loading system for elastic experiments. 4 and Pa are transducers for one-dimensional source location, and Pt is the transducer for triggering of the recording system. (After Magi, 1968). Fig.5. Source locations of elastic shocks in OL direction as a function of time, The specimen is granite; A, B, C, C, , C,, and C3 denote different stages, (After Magi, 1968.) 294
Tectonophysics;
9 (1970) 291-300
Force u . . . . . . . .:.
. . . . . . . . . .._.......
.._........._.. . . . . . . . . . . . . I.,. ~.~..
:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.3~m
~.‘.~.~.~.~.~.‘.~.~.~.‘.~.~.~.~.~.~.~.
~.~.~.~.~.~.~.‘.~.~.~.~.~.~.~.~.~.~.~. . . . . . . . . . . . . . ._ . . . . . . . . . . . . . . . .
..
. . :. . . :. :. . .,.. :. :.
.. ... ... . .. .... .. .. .. .... .... .... .. :. .. .. I.........................,.... 4 :.:.:_:.:.:.:.>:_:.:
f-g
I
I
I
I
~6crn~ k
30cm F
Fig. 4. Granite
(15)
Rupture 0
c .. .*.
ml
-Rupture
.
l
G
.
6l
*
l
. .
. 7
_C’ .’
l’ .Y
****
l *
. 40
+
30
l
.
. . .
. .
.
y; .
-
.
‘a .,*
.
l .*.
.
’
l
. .
. .
. .
.
.
. :qy
be
.
.* *.-
** ‘. ’ .;
.a
Cl
..0
.
. *
.
C
.. .a..
.
*< t * :
.
.
G
.
. .
x
..
.
.% .
.
l*
B 20
IO
-4
-2
0 -
2
L incm
4
6
Fig.5.
pz
was held nearly constant just before the major rupture. He found that, for relatively heterogeneous rocks, the entire fracture process can be classified into three distinct stages A, B, and C (Fig.5). The stage where the stress level is so low that no elastic shock occurs is called stage A. With a subsequent increase of stress, the elastic shock starts occurring but the locations are distributed more or less at random in the specimen. This stage is called B. In due course, the shocks start concentrating in a narrow region and finally a major rupture occurs in a neighborhood of this region. This stage is called C. For relatively homogeneous rocks no clear distinction among these stages was found and the major rupture occurred more or less suddenly. For some of the major earthquakes which were preceded by a considerable number of foreshocks, the concentration of foreshocks, corresponding to the stage C, in a region where the mainshock occurred has been found (Fig.6). Mogi (1968) also studied a migration of a source region of elastic shocks. He found that a region which is highly active at a certain stage becomes relatively inactive at the following stage, and that the activity successively migrates outwards as shown in Fig.7. Mogi compared this observation with the migration of epicenters during three successive periods in the Matsushiro earthquake swarm from 1965 to 1967 (Fig.8). He concluded that the laboratory experiment can simulate many of the important features of earthquake mechanics.
Kita-lzu (19x)) m
Aftershbck Region
Aleutian
I turw
(1963)
Foieshock
(I9651
Fig.6. Epicenter distributions of foreshocks with respect to main shock and aftershocks for four earthquakes. (After Mogi, 1968.)
Granite
t t t
04) c2 ,I
I 0I
.
I
0 -----rL
2
4
in cm
Fig.7. Successive migration of source region for stages CI, C2, and C3 in Fig.5. Main crack pattern is also shown.
v /
A
,
Fig.8. Successive migration of source region during the Matsushiro earthquake swarm: A. October 1965-May 1966; B. June 1966-December 1966; C. January 196775eptember 1967.
Tectonophysics, 9 (1970) 291-300
297
DEEP
DRILLING
IN THE MATSUSHIRO
EARTHQUAKE
SWARM AREA
A deep drilling in the ~aisushiro earthquake swarm area is now being made by the National Research Center for Disaster Prevention with the cooperation of Geological Survey of Japan, Japan Meteorological Agency, and the Earthquake Research Institute of the University of Tokyo. Thisdrilling, which is planned to reach a depth of 1.8 km, has three objectives: (I) to study physical and chemical properties of rock samples obtained at depths in the borehole, (2) to install equipment for electrical and seismic experiments in the borehole, and (3) to use the borehole for fluid injection and to study its effect on earthquake activity. The drilling was started in March 1969, and reached a depth of about 1 km in July 1969. The site is nearly in the middle of the swarm area where the depth of the basement rock having a P velocity of 6 km/set is estimated to be about 1 km. This basement rock has not been reached. Several highly fractured layers have been found. Because of these fractured layers the drilling is now slightly behind schedule. However, it is hoped that within several months the drilling will be completed and be ready for the fluid injection.
NEW S-YEAR
PROGRAM
Based upon the outcome of the 5-year program and partly accelerated by the occurrence of the Matsushiro earthquakes and the 1968 Tokachi-oki earthquake, a new 5-year’program starting from, 1969 was proposed by the Geodetic Council of Japan. This newprogram which aims at actual predictions of major earthquakes emphasizes augmented instrumentation, strategy, and organization. In order to achieve actual predictions which involve vast amounts of data from multidisciplinary observations, the establishment of an efficient organization which makes possible a rapid and smooth flow of .dataamqng different institutions andobservatories is of the utmost importance. Fig.9 schematically shows an organization for earthquake prediction proposed by the Geodetic Council of Japan. Data from three different categories are supplied to respective centers, namely, the Crustal Activity Monitoring Center, the Seismicity Monitoring Center, and the Earthquake Prediction Observation Center, These data are more or less unprocessed. The three centers which are equipped with data prqcessing facilities reduce the respective data in a comprehensive manner to that of manageable size. These processed dam_ are forwarded to the Earthquake Prediction Headquarters which is responsible for overall judgement of the data and takes necessary actions. The Geodetic Council.also proposed the following strategy for earthquake prediction (Fig.10). If an indication of an abnormal phenomenon is found in a certain area by either nation-wide routine observations or observations in special areas such as active faults and densely populated cities, thearea is designated as an “area of intensified observation”, and the observation inqhis area is considerably augmented. This intensified observation eventually enables us to determine whether this abnormal phenomenon is likely to be related to a forthcoming major earthquake or not. If it is suspected to be so, observations of all kinds are then concentrated on
298
Teetonophysics,
3 (1970) 291-300
University (micro-
Observatories
and ultramicro-
earthquakes, movement,
crustal etc.
)
Fig.9. Organization for earthquake prediction. Earthquake Prediction Headquarters and Crustal Activity Monitoring Center are established at Geographical Survey Institute (G.S.I.), Seismicity Monitoring Center, at Japan Meteorological Agency (J.M.A.), and Earthquake Prediction Observation Center, at the Earthquake Research Institute (E.R.I.) of the University of Tokyo.
Nation-wide Observation Observations in Special
Detection of symptom
Diagnosis by Intensified
Detection of Premonitory -----+
Observation
Phenomena by Concentrated
Prediction
Observation
Areas
Fig.10. Strategy for earthquake prediction.
this particular area, now designated as an “area of concentrated observation”. If some premonitory phenomenon is detected during this period of concentrated observation, a warning or prediction may be issued. The Earthquake Prediction Headquarters is responsible for various judgements necessary for taking these actions. The most important but perhaps the most difficult point in carrying out earthquake predictions according to the above strategy is to establish an objective and quantitative guideline by which various judgements can be made. Although no established guideline exists at present, Rikitake (1969) made an attempt to use, as a guideline, a probability of magnitude and occurrence-time of an earthquake in a specified area. This probability can be Tectonophysics, 9
(1970)
291-300
299
calculated on the basis of various empirical relations between magnitude and frequency, crustal movement and magnitude, magnetic anomaly and magnitude etc. Rikitake concluded that the magnitude prediction can be achieved with a fairly high accuracy whereas the prediction of occurrencetime cannot be made very accurately. Further studies along this line are obviously desirable. REFERENCES Iizuka, S., Ichikawa, K., Ito, K., Hasegawa, 1. and Hosono, T., 1969. Observations on the time variations of seismic wave velocities by explosion seismic method (2nd report). Monthly Rept. Geol. Surv. Japan, 20: 313-327 (in Japanese with English abstract). Mogi, K., 1968. Source locations of elastic shocks in the fracturing process in rocks, 1. Bull. Earthquake Res. Inst. Tokyo Univ., 46: 1103-1125: Rikitake, T., 1966. A five-year plan for earthquake prediction research in Japan. Tectonophysicp, 3: 1-15. Rikitake, T., 1968. Earthquake prediction. Earth-Sci. Rev., 4: 245-282. Rikitake, T., 1969. An approach to specific prediction of earthquakes. Tectonophysics, 8: 81-95.
300
Tectonophysics,
9 (1970) 291-300