Geomagnetism and earthquake prediction

Tectonophysics - Elsevier Printed in The Netherlands

Publishing

Company, Amsterdam

GEOMAGNETISM AND EARTHQUAKE PREDICTION

T. RIKITAKE Earthquake Research (Received

Institute,

Tokyo University,

Tokyo (Japan)

October 2, 1967)

SUMMARY

A survey of existing data relevant to seismomagnetic effect leads to a conclusion that most of the classical reports are not free from contamination by errors, but according to recent study, a 5-15 y change in the geomagnetic field may well be expected to accompany an earthquake of moderate magnitude. Examples of geomagnetic changes as observed by arrays of proton precession magnetometers and rubidium magnetometers are briefly outlined. It is emphasized that the most important point for this kind of observation is the elimination of non-local changes. Repetition of precise magnetic surveys also brings out local anomalous change which seems closely related to the occurrence of earthquakes. In view of recent improvement of measuring techniques, the writer is of the opinion that it is not utterly hopeless to detect a geomagnetic change as one of the forerunning effects of earthquake occurrences.

INTRODUCTION

Whether or not an earthquake is accompanied by changes in the geomagnetic field has been one of the classical problems in geophysics since the last century. As reported by Kato (1939) and Roth6 (1948), it has often been said that marked changes in the geomagnetic field sometimes took place in association with occurrences’ of earthquakes. The writer picked up in this review a number of geomagnetic changes, which might be caused by earthquakes, from available literature as given in Table I. The amplitudes of the geomagnetic changes are listed in units of gammas even though some of them were originally given in dip or declination angles. A striking yet instructive feature of these seismomagnetic changes is certainly the steep decrease in their magnitude as years advanced as can be seen in the amplitude of geomagnetic change versus year plot in Fig.1. Most likely, such an apparent diminution in the magnitude of the geomagnetic changes does not witness the existence of a real secular decrease of seismomagnetic effect, but the secular improvements of the field measuring techniques with time. As most of data are supplied from Japanese ’ sources, the approximate periods of introducing new techniques in Japan, i.e., the advents of the Hydrographic Office-type magnetometer, Geographical Survey Institute-type magnetometer and proton precession magnetometer, Tectonophysics,

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TABLE I Geomagnetic changes that might be caused by earthquakes Earthquake

Year

Magnitude Max. change ti)

Component

Author

Mino- Owari Sakata Riku-U Susaka Hiroshima San Francisco North Izu Sanriku Shizuoka Osaka Nankai Imaichi Tokachi North Miyagi Tanabe Niigata Niigata Shizuoka Matsushiro Matsushiro

1891 1894 1896 1897 1905 1906 1930 1933 1935 1936 1946 1949 1952 1962 1962 1964 1964 1965 1965 1966

8.4 7.3 7.5 6.3 7.6 8.3 7.0 8.5 6.6 6.7 8.1 6.5 8.2 6.5 6.1 and 6.4 7.3 7.3 6.2 eq. to 5.7 eq. to 6.1

H H F F I

Kato (1939) Kato (1939) Kato (1939) Kato (1939) Kato (1939) Kato (1939) Kato (1939) Kato (1939) Kato (1939) Kato (1939) Kato (1939) Kato (1939) Kato (1939) Kato (1939) Tajima (1966) Kato (1966) Tazima (1966) Tazima (1966) Yanagihara (1966) Rikltake et al. (1966a,b,c,1967)

902 289 228 607 118 182 114 181 315 75 35 182 159 78 7 48 20 5 12 7

I

I I I I D I I I H I H H 2 F

looo$ !!Qo-

l

.

. . . loo-

’

l

l

.

. .

.

w-

. .

.

.

IO .

S-

. .

Fig.1. Geomagnetic changes reported associated with earthquake occurrences as plotted against the year. H, G and P respectively denote the approximate times when the Hydrographic Office-type magnetometer, Geographical Survey Institute-type magnetometer and proton precession magnetometer were introduced to the Japanese geomagnetic work. 60

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are indicated in Fig.1 with letters H, G and P respectively. It should also be pointed out that no proper account for correcting non-local geomagnetic changes such as magnetic storm, daily variation (S&) and the like was taken in the past. It is therefore said that Fig.1 demonstrates the year-by-year progress in the measuring and correcting techniques. All the geomagnetic changes associated with an earthquake as obtained in recent years by the most up-to-date techniques fall in a 5-15 y range, and the writer believes that changes of this order of m_agnitude may well be expected as real seismomagnetic effect. It is at present not clear whether or not any change in the geomagnetic field would be observed forerunning an earthquake although there have been a number of reports suggesting such a change. It would be natural to think that a forerunning effect, if any, is smaller than the effect that takes place during a whole seismic event. An extremely accurate observation should therefore be conducted in order to detect such a change. Judging from the present development of detecting techniques, however, the writer is of the opinion that it is not utterly hopeless to catch some geomagnetic changes prior to an earthquake of moderate magnitude. It should even be encouraged to make systematic observations of the geomagnetic field in seismically active areas as one of the disciplines of earthquake prediction research. Changes in the geomagnetic field may possibly give us continuous information about those in stress fields in the earth’s crust.

MODERN TECHNIQUES GEOMAGNETIC FIELD

Proton precession

FOR DETECTING

magnetometer

LOCAL

ANOMALOUS

CHANGES IN THE

array

It used to be no easy matter to make an accurate observation of the geomagnetic field over a long period of time because of the drift of baseline values caused by various sources. Accurate observations, i.e., with an accuracy of f 1 y, could be performed only at well-equipped standard observatories. When temporary observations were made at field stations over a seismic area in the past, it could not be helped that the accuracy of observation was substantially lowered. Since the invention of the proton precession magnetometer, which is practically free from any drift because the principle of measurement is based on an atomistic constant, the accuracy of observation over a long period has been dramatically improved. The Matsushiro Earthquake Swarm in 1965 and 1966 provided a good opportunity to observe seismomagnetic effects by an array of proton precession magnetometers (Rikitake et al., 1966a,b,c, 1967). A few months after the outbreak of the swarm activity, a magnetometer was set up roughly at the centre of the seismic activity. The seismic area being expanded later, proton precession magnetometers were added there one by one forming an array of six magnetometers covering the seismic area of about 50 km in length and 20 km in width. Fig.2 shows the five-day means of local anomalous changes in the total intensity of the geomagnetic field as observed at two stations M and H which are situated respectively at the centres of the two seismic areas separated Tectonophysics, 6 (1) (1968) 5+68

61

NW.

ckc.

Jon.

Feb.

Mar.

Apr.

MoY

Julr

July

Au9.

Sept.

Oct.

NOV.

I966

196s

Fig.2. Five-day means ‘of the local anomalous c-es in the total geomagnetic intensity values at two stations in the seismic area of the Matsushiro earthquake swarm. The bottom curve indicates the number of felt earthquakes.

FM-

oFx

FM -

oFK

Fig.3. Histograms of the daytime and n~httime weighted ~ferences in the total geomagnetic intensity value between the two observatories, about 180 km distant from one another, in central Japan. 62

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by a subsurface fault. The distance between these stations is about 6 km. A local change is defined in this case as the difference between the total intensity value at a field station and that at a permanent observatory about 180 km distant. It turns out, however, that a simple difference between the total intensity value at one of the field stations and that at the permanent observatory does not provide a value which is completely free from non-local changes. A weighted difference technique is then introduced as will be explained later. AFM and AFR in Fig.2 may be regarded as representing the essential features of the local anomalous changes in the total intensity of the geomagnetic field. It is observed in Fig.2 that, towards August 1966, when the seismic activity was violent, FM showed an increase of several gammas while FH decreased by a similar magnitude. These changes can be accounted for by supposing that the magnetization of the earth’s crust in the seismic area increased. Even if one of the field stations just outside the seismic area is taken as a reference, the total intensity values at M and H indicate almost the same changes as shown in Fig.2. The conclusion is also supported by the results of repeated magnetic surveys. The seismomagnetic change seems to be more or less recovered by the middle of October. It is not known why no marked changes in the geomagnetic field were observed when the seismic activity was even higher during April and May. It is noticed, however, that the speed of land deformation as observed by precise levellings, tiltmeters and geodimeters is very much larger during the August activity than the April one. Stacey and Westcott (1965) reported on a comparison of proton precession magnetometer observations at stations 25 km apart in England and concluded that non-local geomagnetic changes could be eliminated with a standard deviation of 0.85 y for an individual pair of measurements. Similar study in the central part of Japan made by Rikitake (1966a) revealed that the accuracy of elimination of non-local field is substantially low. When the records at a station in the Matsushiro area are compared to those at a permanent observatory (Kanozan) some 180 km south-east of the area, the standard deviation is as large as 2.6 y for a simple difference during nighttime. The standard deviation during daytime is even larger. It is found, however, that fluctuations in the dally mean values of the total intensity of the geomagnetic field at the permanent observatory are always larger than those at the field station in such a way that a proportional constant a! can be empirically introduced. The weighted difference, that is FM-CYFH, during nighttime exhibits a standard deviation of 2.2 y. In Fig.3 are shown the histograms of the weighted differences for daytime and nighttime data. It is, therefore, said that the weighted difference technique should be extensively investigated in order to see the detectability limit of the seismomagnetic effect. The changes in Fig.2 are far larger than the conceivableerror involved in the eliminating procedure of non-local changes. These findings provide reasons for the writer to believe that seismomagnetic effects of the order of 10~ or thereabout actually took place in association with the Matsushiro activity. SOme twenty sets of proton precession magnetometers of digital recording that can be conveniently used for detecting seismomagnetic effect are now under construction as a part of earthquake prediction research programme in Japan(Rikitake, 1966b). The writer alsogatheredfrom F.D. Stacey Tectonophysics, 6 (1) (1968) 59-68

63

(personal communication, 1966) that a proton precession magnetometer of similar kind is produced in England and is at work on a volcano in New Zealand. It appears to the writer, therefore, that a more clear-cut nature of seismomagnetic effect would be brought to light in the near future. Rubidium magnetometer

array

The possibility of observing a short-term geomagnetic change accompanying an earthquake cannot be at present ruled out (see Moore, 1964). In this respect, use of an optical pumping magnetometer as recently developed would be a great advantage because of the feasibility of continuous recording and high sensitivity. A very extensive observation by an array of rubidium magnetometers has been carried out by Breiner (1966). Breiner set up five magnetometers at San Bruno, Stanford, Los Gatos, Watsonville and Hollister along the San Andreas fault zone in California, the distance between each station being approximately 30 km. All the signals from these field stations are sent to Stanford by telephone cable and simultaneously recorded there. Usually, the changes of the total geomagnetic intensity at Los Gatos, a

Fig.4. Changes in the total geomagnetic intensity as observed by a rubidium magnetometer array in California. LG shows the change itself at Los Gatos, while H, St and SB indicate the changes relative to that at LG at Hollister, Stanford and San Bruno respectively. An anomalous change of 1 y or so is observed at H several minutes before 17.00. (After Breiner, 1966.) 64

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station at the middle of the array, is recorded as a reference and the differences in the total geomagnetic intensity between the other stations and Los Gatos are also recorded. Fig.4 shows, for example, the record of February 7, 1966 on which one clearly observes a change of about 1 y at Hollister for a duration of a few minutes. Breiner reported that a sudden creep of the fault took place 2 days after the change and a local earthquake of magnitude 2.8 also followed 3 days after the magnetic event. Breiner has already observed a number of changes of similar nature. When a large number of changes are brought forth, Breiner’s method would undoubtedly provide a powerful means of earthquake prediction. The writer looks forward to see a more marked change accompanied with an earthquake of greater magnitude, although the occurrence of such an earthquake is by no means something to be wished for. Two sets of rubidium magnetometers were set up at the epicentral area of the Matsushiro Earthquake Swarm towards the end of 1966. No outstanding change has been reported yet probably because of the subsiding seismic activity (T. Ykutake, personal communication, 1966).

Repetition

of magnetic

survey

Accuracy of magnetic survey has been very much improved in Japan in recent years. The Geographical Survey Institute set up 91 first-order and some 1,000 second-order magnetic stations all over Japan. Magnetic surveys have been repeated approximately every 6-10 years at the exact same stations where granite bench marks are buried. Taking into account all the conceivable errors including the error due to epoch reduction, Tazima (1966) reported that the accuracy of the first-order survey is f 4 y for the horizontal intensity, f 0.4’ for the declination and f 0.4’ for the inclination. In the course of repetition of magnetic surveys by the Geographical Survey Institute, it has been noticed that there are a number of districts in Japan where the geomagnetic secular change seems to be anomalous. What is called an anomalous change is here defined by an unusually large deviation from the geomagnetic secular variation as observed at the Kakioka Magnetic Observatory, one of the standard observatories in Japan. Anomalies

Fig.6. Anomalously large secular variation in the geomagnetic horizontal intensity at Tanabe on Kii Peninsula, Japan. The changes at the neighbouring stations, a few tens of kilometers distant from Tanabe are also shown. Occurrences of the two earthquakes in the vicinity of T&abe are indicated together with their magnitude. (After Tazima, 1966.) Tectonophysics, 6 (1)(1968)59-68

65

as large as Gy/year have been reported. It has also been pointed out that most of the anomalous areas have been attacked by earthquakes of fairly large magnitude. As Japan is generally active seismically, it is not known whether such coincidence is accidental or physically meaningful. Tazimaemphasized, however, that the anomalous secular changes usually tend to vanish after the occurrence of an earthquake. Fig.5, which is reproduced from his paper, shows an example of anomalous change as observed at a first-order magnetic station on Kii Peninsula. The horizontal intensity had been increasing there with a rate of 5 y/year over a period of 10 years. Less marked anomalies had been observed at adjacent stations. The measurements after two earthquakes, 6.1 and 6.4 in magnitude respectively, indicated a 7 y decrease in the horizontal intensity as can be observed in the figure. Although a number of similar changes have been reported, we are still in a position to accumulate more data. In the Japanese programme for earthquake prediction research, therefore, much stress is put on detecting anomalous secular changes in the geomagnetic field by accurate observations with proton precession magnetometer arrays.

THEORETICAL

BACKGROUND OF SEISMOMAGNETIC

EFFECT

Simple theory tells us that the thermal conduction process in the earth’s crust is extremely slow, so that no proper explanation of anomalous geomagnetic changes taking place within a period of 10 years or so can be sought on the basis of displacement of the Curie point isotherm caused by thermal conduction only. Occurrence of local anomalous change of thermal origin may well be expected, except in cases of volcanic activity in which nearsurface intrusion of high-temperature magmas and gases are very likely to take place (Rikitake, 1951). An alternative explanation of local anomalous change in the geomagnetic field relies on the piezomagnetic effect. It has been known that magnetic susceptibility (x) as well as remanent magnetization (J) of rocks decrease in the direction of uniaxial compression. Although experimental results are variable from author to author, decrease rates Ax/x - 10V4/bar and N/J - 3 - lo-a/bar would be typical values for basaltic rocks (Stacey, 1963; Nagata and Kinoshita, 1965). Taking for granted these values, the changes over the Matsushiro area as demonstrated in Fig.2 can be accounted for by assuming a stress change of the order of a few tens of bars at a moderate depth. All the existing experiments are made only on uniaxial compression. It is apparent that more extensive study of piezomagnetic effect under general stress conditions should be encouraged. A number of trials to calculate possible seismomagnetic effect on some reasonable assumptions have been made notably by Stacey (1963, 1964).

CONCLUDING REMARKS

It can be said that modern technology has by now come to furnish US with magnetic instruments sensitive enough to detect a possible seismomagnetic effect. The most important and troublesome point at the present 66

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stage of investigation is certainly how to lessen geomagnetic noises which are far larger than wanted signals due to seismom~etic effect. In order to have a good signal-noise ratio, therefore, it is highly desirable to make use of a large array of magnetometers and, at the same time, much stress should also be put on developing techniques for eliminating non-local geomagnetic changes. Intensive study in this line would certainly require us to handle large sets of data. It appears to the writer that basic study on piezomagnetic effect of rocks, both experimental and theoretical, is also very important for interpreting seismomagnetic effect. To the writer’s knowledge, geomagnetic study in relation to earthquake prediction is now making progress in a direction outlined above in Japan and the U.S.A. It is highly desirable that geomagnetic work on a similar line would also be carried out in as many countries as possible.

The writer thanks Dr. S. Breiner and Dr. F.D. Stacey for their help in preparing this review. REFERENCES Breiner, S,, 1966. A magnetometer array for investigation of the piezo-magnetic

effect in seismically active area. Proc. U.S.-Japan Conf. Res. Earthquake Predtction Problems, 2nd, New York, ~~~22-23. Kate, Y., 1939. Investigation of the changes in the earth’s magnetic field accolnxmying earthquakes or volcanic eruptions. Sci. Rept. Tohoku Imp. Univ., Ser.1, 27: l-100. -to, Y., 1966. Recent studies on changes accompanied by earthquakes. In: T. Nagata (Editor), Proc. Symp. Geomagnetic Changes Associated with Earthquakes and Volcanic Activities. Geophys. Inst., Tokyo Univ., Tokyo, pp.I-20 (in Japanese). Moore, G.W., 1964. Magnetic disturbances preceding the I964 Alaska earthquake. Nature, 203: 508-509. Nagata, T. and Kinoshita, I-I., 1965. Studies on piezo-magnetization, 1. Magnetization of titaniferous magnetite under uniaxial compression. J. Geomagnetism Geoelec., 1’7: 121-135. Rikitake, T., 1951. The distribution of magnetic dip in Ooshima Island and its change that accompanied the eruption of Volcano Mihara, 1950. Bull. Earthquake Res. Inst., Tokyo Univ., 29: 161-181. Rikitake, T., 196&a. Elimination of non-local changes from total intensity values of the geomagnetic field. Bull. Earthquake Res. Inst., Tokyo Univ., 44: 1041-1070. Rikitake, T., 1966b. A differential proton magnetometer - a geomagnetic project under the 5-year plan for earthquake prediction research. Bull. Earthquake Res. Inst., Tokyo Univ., 44: 1161-1178. Rikitake, T., Yamazaki, Y., Hagiwara, Y., Kawada, K., Sawada, M., Sasai, Y.,

Watanabe, T., Momose, K., Yoshino, T., Otani, K., Ozawa, K. and Sanzai, Y., 1966a. Geomagnetic and geoelectric studies of the Matsushiro earthquake swarm, 1. Bull. EarthquakeRes. Inst., Tokyo Univ., 44: 363-408. Rikitake, T., Yamazaki, Y., Hagiwara, Y., Kawada, K., Sawada, M., Sasai, Y. and Yoshino, T., 1966b. Geomagnetic and geoelectric studies of the Matsushiro earthquake swarm, 2. Bull. EarthquakeRes. Inst., Tobo Univ., 44: 409-418. Rikitake, T., Yukutake,T., Yamazaki, Y ., Sawada, M., Sasai, Y., Hagiwara, Y., Kawada, K., Yoahlno, T. and Shimomura, T., 1966c. Geomagnetic and gee-

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electric studies of the Yatsushiro earthquake swarm, 3. Bull. Earthquake Res. Inst.; Tokyo Univ., 44: 1335-1370. Rikitake, T., Yamaeaki, Y., Sawada, M., Sasai, Y., Yoshino, T., Uzawa, S. and Shimomura, T., 1967. Geomagnetic and geoelectric studies of the Matsushiro earthquake swarm, 4. Bull. Earthquake Res. Inst., Tokyo Univ., 45: 89-107. Roth& J.P., 1848. Tremblements de terre et anomalies magn&iques. Geofis: Pura Appl., 12: 134-144. Stacey, F.D., 1963. Seismomagnetic effect and the possibility of forecasting earthquakes. Nature, 200: 1083-1085. Stacey, F.D., 1964. The seismomagnetic effect. Pure Appl. Geophys. (Milan), 58: 5-22. Stacey, F.D. and Westcott, P., 1965. Seismomagnetic effect-limit of observability imposed by local yariations in geomagnetic disturbances. Nature, 2Oti: 1204-1211. Tazima, hf., 1966. Accuracy of recent magnetic survey and a locally anomalous behaviour of the geomagnetic secular variation in Japan. Thesis, Tokyo Univ., 133 pp. (unpublished). Yansgihara, K., 1966. Geomagnetic changes associated with the MatsuehIro earthquakes. In: T. Nsgata (Editor), Proc. Symp. Geomagnetic Changes Assoctated with Earthquakes and Volcanic Activities. Geophys. Inst., Tokyo Univ., Tokyo: pp.22-24 (in Japanese).

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