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Seismicity and seismological method
Tectonophysics - Elsevier Publishing Company, Amsterdam Printed in The Netherlands
SEISMICITY AND SEISMOLOGICAL METHOD K. AKI Department of Geology and Geophysics, Massachusetts Institute of Technology, Cambridge, Mass., (U.S.A.) (Received October 2, 1967) SUMMARY
A straight-forward way for seismologists to contribute to earthquake prediction research is to find the state of stress inside the earth from seismograms of numerous small earthquakes taking place in the general area where a large one is expected. The success of this approach depends on the capability of seismologists in finding conditions about earthquake sources from seismograms. This in turn depends on the available theory to deal with seismic waves and on our knowledge about the wave medium. Ray theory can give us information on positions of earthquake foci, time sequence of earthquakes, geometrical aspects of stress, and some measure of energy release. Recent efforts in this direction, primarily directed toward micro-earthquakes, have produced evidence that indicates the significance of such studies in earthquake prediction research. Normal mode theory can give us additional information about the source condition, because we can incorporate an arbitrary source function in space and time in the scheme of this theory. We can, for example, measure the stress-drop in an earthquake from the spectrum of surface waves, if some additional near-field observations are available. Both ray theory and normal mode theory have their own limitations. Difficult problems arise especially when we try to obtain source conditions of micro-earthquakes, because they generate short period waves which suffer strongly from the heterogeneity of wave medium. Recent studies bearing upon these problems will be reviewed and possible approaches to overcome the above mentioned difficulties will be discussed. INTRODUCTION The word “seismicity” expresses such a picture of earthquake generating process inside the earth that can be obtained from the record of seismographs. Therefore, the quality of the picture will depend on the quality of the seismological method utilized in the analysis of seismograms. For example, Gutenberg and Richter’s (1956) monumental work Seismicity of the Earth was an outcome of a new method which enabled them to measure the magnitude of an earthquake from the seismic waves. We may say that the study of seismicity is a synthesis of results of analysis of Tectonophysics, 6 (1) (1968) 41-58
41
seismograms. As shown by Gutenberg and Richter, success of such a synthesis requires a broad underst~d~ng of regional geophysics and geology. During the last 20 years, since the Sea’smicify of the Earth was published, seismological observations considerably widened its spectral range as well as dynamic range. Methods of analysis were remarkably advanced using not only ray theory but also wave theory in the study of sources of seismic waves. Some efforts have been made to try to get a better, more detailed and precise picture of seismicity by synthesizing the outcome of such new methods. One of these efforts is directed toward finding any relation between the occurrence of large earthquakes and the activity of very small earthquakes in the same area. The idea behind this study is that large and smali earthquakes probably share the same tectonic causes, and we may learn about the stress conditions responsible for large earthquakes by studying small but numerous earthquakes. Small earthquakes with magnitudes less than 3 are called micro-earthquakes. They usually escape from the standard seismograph network, and the investigation of them requires new techniques of observations and analysis
Fig.1. Map of historic fauIt breaks and associated earthquakes in southern California region. = fault breaks associated with earthquakes. (After Allen et al., 1965.) 42
Tectonophysics,6 (1)(1968)41r58
RELATIONSHIP BETWEEN SOUTHERN CALIFORNIA
MICRO-EARTHQUAKES
AND LARGE EARTHQUAKES:
How does the micro-earthquake activity relate to the occurrence of large earthquakes? Let us first look at the area of southern California where we find some interesting discoveries made by a joint work effort of seismologists and geologists. Fig.1 shows the map of historic fault breaks and associated earthquakes in southern California reproduced from a paper by Allen et al. (1965). Most of these earthquakes are on major through-going faults. Earthquakes with magnitudes greater than 6 which were not accompanied by clear fault breaks are also located close to the major faults. Looking at only the main San Andreas fault system, we see that the fault breaks up into many branches in the southern section starting near Cajon Pass but shows a single curved trace north of it. We also see that a number of earthquakes took place along the southern branched fault zone, but a single large earthquake of 1857 is associated with the northern section.
Fig.2. Smoothed strain-release map of southern California region, Jan. 1, 193PJan. 1, 1965. Numbers of equivalent M = 3.0 earthquakes per 100 kmz. (After Allen et al., 1965.) Tectonophysics,
6 (1) (1968) 41-58
43
This correlation of geological structure with seismic activity is more clearly shown in the strain release map (Fig.%) also reproduced from Allen et al. (1965). The number shown is the sum of the square root of energy released in 29 years measured by taking the value corresponding to magnitude 3 as the unit. The data used are 10,000 earthquakes located by the Seismological Laboratory, Pasadena, California. South of the Cajon Pass, where the fault branches, the strain release is large. On the other hand, the part of the fault associated with the 1857 earthquake shows very small strain release during this period, except the small segment close to the Kern County earthquake of 1952, which was associated with the transverse fault rather than the main San Andreas fault. A similar picture showed up from a study of micro-earthquakes done by Brune and Allen (1967a). This map (Fig.31 shows the number of microearthquakes per day within 24 km of each station. Most stations are operated for 3&100 h, some of them up to 4,000 h. Again, we see a very quiet zone along the fault segment associated with the 1857 earthquake, except near the epicentre of the Kern County earthquake. To the south of this section micro-earthquake activity increases and shows a pattern nearly identical to the strain release obtained from the 2%years’ data. To the north of this section a moderate increase of activity shows up starting at Cholome where the Parkfield earthquake took place in June 1966. \ ‘\
“\
0
so
Iookm
24 km radius
J
Fig.3. Map of sites occupied in the micro-earthquake study. (After Brune and Allen, 196’7a.) Heavy numerals are numbers of micro-earthquakes per day within 24 km of station; heavy lines are major faults, reproduced from their paper. 44
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It is interesting to note that the Parkfield earthquake is associated with an unusually low stress drop, of the order of 1 bar. The estimate of the stress drop is made by applying a wave theory to the spectrum of seismic waves on the basis of the dislocation source model (Aki, 196’7). This is a powerful technique in studying the stress condition of the source, and is an outcome of efforts of many theoretical seismologists in recent years. Another low-stress-drop earthquake was found by Brune and Allen (1967b) in the Imperial Valley near the Mexican border. These examples certainly suggest that in the area of higher micro-earthquake activity, the accumulating tectonic strain may be released by many smaller earthquakes because of structural weaknesses, and that the zone of faulting during the great 1857 earthquake may be characterized by extremely low seismicity of structural strength possibly due to some locking mechanism. If a strong structure and a weak structure exist next to each other, of course, the strain release will be concentrated to the weak one. From this brief survey of southern California seismicity we may emphasize the following two points: (1) The regional pattern of micro-earthquake activity is grossly similar to the seismicity pattern of larger earthquakes.
SOOkm
Fig.4. Distribution of epicentres of earthquakes of the depths from 30 to 70 km for the period 1951-1965. (After Katsumata, 1966.) Tectonophysics,6 (1)(1968)41-58
45
(2) The response of the crust to regional tectonic stress differs from place to place even in the same general tectonic area, resulting in a complex pattern of strain concentration and of micro-earthquake .activity. RELATIONSHIP JAPAN
BETWEEN
MICRO-EARTHQUAKES
AND LARGE EARTHQUAKES:
Bearing these two points in mind, we shall turn our eyes on Japan, where everything is expected to be far more complex than in California. We shall again see, however, that the micro-earthquakes show the same activitypattern as that of larger earthquakes. Unlike California, the seismicity in Japan has a three-dimensional structure. Therefore, the maps must be given for different ranges of focal depths. Fig.4 is the map of epicentres for depths 30-70 km reproduced from Katsumata (1966), who used data of the last 15 years (1951-1965). As was pointed out by Asada, those earthquakes originating in the uppermost mantle are confined to zones along the trends of arc structure. There is a sharp boundary of activity, especially in eastern Japan, nearly along the
I;0-<30llm / M
5-sw&6o-65-70-25-60-65 . .o 00 000
0
5oohm
Fig.5. Distribution of epicentres of earthquakes of the depths from 0 to 30 km for the period 1951-1965. (After Katsumata, 1966.) 46
Tectonophysics,
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coast. The most active area in the country is the Kanto district, which is at the intersection of the Honshu arc and the Mariana arc. The seismicity map of earthquakes originating in the crust shows a markedly different pattern from the mantle seismicity. Fig.5 is again reproduced from Katsumata (1966) and shows epicentres of earthquakes with depths O-30 km for the same period (1951-1965). They occur in the land as well as in the ocean nearly uniformly, without defining clear zones. An interesting area is the Kanto district, where the mantle activity is the most vigorous in Japan. We see practically no earthquakes in this area originating in the crust. Outside this area, the activity in the crust is high. This feature is reproduced almost exactly in the micro-earthquake activity. Fig.6, reproduced from Miyamura et al. (1962), shows the epicentres of micro-earthquakes determined by the use of an array station located at Tsukuba. Most of their depths range from 35 to 60 km, and agree with the depths of larger earthquakes. The activity in the crust is extremely
Fig.6. The epicentres of micro-earthquakes determined by the observation at Tsukuba array station and the Bouguer gravity anomaly contours; Dec. 1, 1961~March 31, 1962. (After Miyamura et al., 1962.) Tectonophysics,
6 (1) (1968) 41-56
47
km
Fig.7. The epicentres of micro-earthquakes determined by the observation at Tochimoto array station and the Bouguer gravity anomaly contours; May 20-Juni 27, 1962. (After Miyamura et al., 1962.)
Fig.8. Epicentres of micro-aftershocks Lawing, Alaska, array station; May l%June
located by the observation 7, 1964. (After Hori et al., lS66.j
48
Tectonophysics, 6 (1)(1968)41-58
Km200
100
120
El0
40 NW
ALE40
00
120
160
200
Km
-100 -120 -140 -160 -160 200 220
0
o
0
240
Km
Fig.9. Vertical cross-section of the hypocentres of micro aftershocks along the profile in N 55°WS550E. The apparent concentration of foci at depths 5 km and 35 km is due to an artificial restriction in the hypocentre location. (After Hori et al., 1966.) low in the central part and increases outward to the ocean as well as to the land. This feature becomes more clear in the epicentre map (Fig.7) obtained at the array station set up about 100 km to the west of Tsukuba. We find that the micro-earthquake activity in the crust is high in the area where larger earthquakes are active in the crust. There are several other studies, made in Japan and other countries, which show also the similarity between the pattern of micro-earthquake activity and that of larger earthquakes. One such spectacular example is the micro-eartquakes after the great Alaska earthquake of 1964. Fig.8 shows the epicentres determined by an array station in the Kenai Peninsula (Hori et al., 1966). All the shocks in the oceanic side of the stations originated in the crust, but those on the continental side show the focal depths down to 200 km. This area is precisely the zone of earthquakes of the intermediate depth in the normal period. The vertical cross-section perpendicular to the coastline shows this three-dimensional structure of micro-earthquake activity more clearly (Fig.9). This three-dimensional similarity strongly supports the idea that micro-earthquakes and larger earthquakes share a common tectonic cause. If so, we should be able to infer the stress conditions responsible for large earthquakes by studying numerous micro-earthquakes.
EARTHQUAKE
GENERATING
STRESS
IN JAPAN
Now let us look at Japan again and ask about the second point we made from the observations in California, that is, what is the nature of the Tectonophysics, 6
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49
0 I
500
I
km
-
Fig.10. Distribution of the maximum pressure axes for deep earthquakes in and near Japan. (After Honda and Masatsuka, 1952.)
crustal response to the regional stress ? First of all, is there any uniform regional stress like the one in California? As is well known from Honda’s work on first motions, the deep focus earthquakes show a systematic orientation of the maximum pressure axis, nearly perpendicular to the trend of the seismic zone. This suggests the existence of a rather uniform stress field generating these earthquakes (Fig.10). On the other hand, the shallow earthquakes show a rather complex pattern as is shown in Fig. 11 (Honda and Masatsuka, 1952). A closer look, however, reveals that the earthquakes along the Japan Sea coast show systematically oriented pressure axes in roughly an E-W direction. This picture is obtained from 22 years of data prior to 1949. 50
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The pressure axes pattern in the next 7 years is shown in Fig.12, which is reproduced from lchikawa (1960~. The pattern is again complex, but the one in the Japan Sea coast shows the same general orientation found in the prior period. I If this eompfex stress pattern is due to the timely change in stress condition, we may.expect that the pattern may be more uniform, if we only use short-term data. Fig.13 shows the result obtained from 30 months data from January 1961. A method of smoothing radiation patterns is applied to many small earthquakes to obtain the average orientation of axes for various areas (Aki, 1966b). The maximum pressure axes are indicated by solid arrows. The axes are oriented reasonably systematic~ly, suggesting the existence of a uniform regional stress. About a year after the end of the period covered by this picture, the Niigata earthquake of magnitude 7.5 took place in the Japan Sea. The pressure axis of this earlobe was oriented in N 7O*W,which fits the general direction expected at the epicentre.
Mg.11. Distribution of the maximum pressure axes for shallow earthquakes for the period 1927-1949. (After Honda and Masatsuka, 1952,) Tectonophysics,6 (1)(1968)41-58
51
L
Qd0
I
500km I
Fig.12. Distribution of the maximum pressure axes for shallow earthquakes for the period 195S-1957. (After Ichikawa, 1960.)
The Niigata earthquake took place where the seismicity map prior to 1964 showed the least activity in Japan. In this sense, this earthquake is much alike to the 1857 California earthquake mentioned before. The stress drop estimated from the seismic spectrum and near-field observations amounts to 125 bar (Aki, 1966a), which is two orders of magnitude greater than those of the Parkfield earthquake (Aki, 1967) and the Imperial earthquake (Brune and Allen, 1967b) mentioned before. This suggests higher strength of the crust in the Niigata earthquake area than in the other. An area comparable to the low-stress-drop, high seismieity areas of California may be found in Matsushiro located about at the centre of Honshu. The Matsushiro earthquake swarm started in August 1965 and lasted for more than two years. At the peak of the activity, 6,000 earthquakes/day were recorded. 52
Tectonophysics, 6 (1)(l968)41-56
The first motion patterns from these small earthquakes (Fig.14, Anonymous, 1966) are remarkably consistent, and the pressure axis is roughly E-W, again agreeing with the regional stress pattern shown in Fig.13. This suggests a concentration of regional strain to a zone of weak structure. Supporting this idea is an observation that the micro-earthquake activity outside the swarm area was unusually low, indicating a stress relaxation in the neighborhood of the swarm area (Aki et al., 1966). Another interesting ‘observation comes from the comparison of seismic spectra made by S. Suyehiro (personal communication, 1967) between the micro-earthquakes prior to the swarm period and those of the swarm earthquakes. He found greater high-frequency waves in the normal period than in the swarm period. This fact may be explained by an increased dissipation due to change in wave medium, but can be attributed to the change of stress condition at the source. The observed change corresponds to greater stress drop in the normal period than in the swarm period, which fits the idea of strain concentration to the weak zone and stress relaxation outside the zone.
Fig.13. Distribution of the maximum pressure (solid arrow) and tension (dashed arrow) axes for the period 1961-1963. (After Aki, 1966b.) Tectonophysics,
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Fig.14. Push-pull distribution of the first motions of the Matsushiro earthquakes. The pattern due to different shocks are superposed together, the epicentres being put at a common point (origin). (Anonymous, 1966.)
A similar comparison was made by Aki (in preparation) between a foreshock and an aftershock of the Parkfield earthquake. Stress drop was estimated from the ratio of seismic energy to the seismic moment which was obtained from the coda waves. In this case, there was no appreciable difference in stress drop between the foreshock and the aftershock. In any case, the above two examples, the Niigata earthquake and the Matsushiro swarm, seem to suggest again that the regional tectonic stress is rather uniform, but the response of the crust is quite different depending on the structural strength or weakness, and this difference gives rise to extremely different modes of seismicity even in the same tectonic area.
FORESHOCKS
A similar point of view has been taken by Mogi, who studied extensively the fracture process of rocks in the laboratory and compared the results with the actual earthquakes. Fig.15 shows a result of his experiment (Mogi, 1962). Numerous microfractures, or elastic shocks, take place before the main rupture for Pumice, while no microfracture precedes the main rupture for Pine Resin. Trachyte and granite show intermediate nature. The experiment was made under the increasing stress at a constant rate, but similar phenomena are .observed also at a constant stress. Mogi attributes this 54
Teotonophysics,
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difference to a difference in degree of structural heterogeneity of material. With the increase of structural heterogeneity, stress concentration will take place at a greater number of points, resulting in a greater number of microfractures. These microfractures may correspond to foreshocks, which may be used as the most direct means of predicting earthquakes. An example of the foreshocks preceding the larger Matsushiro earthquake is shown in Fig.16 (Hagiwara and Rfkitake, 1967). Mogi (1963) examined 1,400 earthquakes in Japan to see whether they were preceded or not by foreshocks. Only 60 earthquakes were preceded by foreshocks, but the probability of having foreshocks shows a systematic regional pattern. Fig.17 shows the percentage of earthquakes which were preceded by foreshocks. For example, 50% of the earthquakes originating near Matsushiro were preceded by foreshocks, but the percentage was very low near the epicentre of the Niigata earthquake. In the same figure, the area where the swarm earthquakes are frequent is indicated by dashed lines. There is a clear correlation between the two patterns.
Pine Rem
(Homogenecud
Main Rupture
I
:
0 Trachyte
5 (Near@
1
1
15
lo
20
Homogeneous) I
;
OO
20
60
40
I
Granite (Heterogeneous)
Pumice Extremely
Heterugeneous)
Awlled
stressin
hg/crd
Fig.15. Microfractures preceding a main rupture for various material under increasing stress at a constant rate. (After Mogi, 1962.) Tectonophysics,
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55
Mogi points out that these patterns are similar to the distributions of Cenozoic volcanism, high terrestrial heat flow and the strongly folded tertiary zones, all of which may imply a highly fractured crust. Thus, we see again that the mode of seismicity is controlled by the structure. Mogi made another prediction based on his experiment. The slope of the magnitudefrequency relation, that is, the b-value of the Gutenberg-Richter formula varies withthe structural heterogeneity of rock samples. The b-value is greater for higher heterogeneity. Suyehiro (1964) made an observation which may be attributed to this effect. A group of foreshocks and one of aftershocks, in central Japan, showed different b-values, the value for the aftershock being significantly greater. This result may be explained, using Mogi’s result, by a structural heterogeneity produced by fracturing due to the main shock. We do not yet, however, understand completely the physics behind this effect, and also we need more observations. Nevertheless, this shows that the mode of seismicity varies not only from place to place, but also from time to time. CONCLUSION
From the above survey of the seismicity in southern California and Japan, we learn that, even within the same tectonic area with the same
13Jan.
14
15
16
b
17
Tit-t-l@-----
Fig.16. The hourley frequency of micro-earthquakes immediately before and after the Matsushiro earthquake of January 16, 1967. (After Hagiwara and Rikitake, 1967.) 56
Tectonophysics,
6 (1) (1968) 4F58
Pacific Ocean
Fig.17. Percentage of earthquakes preceded by foreshocks (solid line) and areas of active earthquake swarms (dashed line). (After Magi, 1963.)
regional stress conditions, the mode of seismic activity can be quite different from place to place. However, by improving the seismological methods dealing with seismic waves and obtaining more precise knowledge about the physics of their sources, we may be able to understand why they are so. If we can learn why, the method of prediction will naturally emerge. Finally, I must apologize for not being able to cover (because of limited time) many important works done recently in the same field, such as interesting details of seismic zones revealed by improved accuracy of hypocentre location; space-time distribution of pressure or tension axes and strain release in particular earthquake sequences; modernized data pro* cessing which may eventually lead to a complete automation of seismicity monitoring. I hope, however, that I have picked up enough of the works for convincing the reader that seismologists can contribute to the earthquake ‘prediction research.
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REFERENCES Aki, K., 1966a. Generation and propagation of Gwaves from the Niigata earthquake of June 16, 1964, 2. Estimation of earthquake moment, released energy, and stress-strain drop from the Gwave spectrum. Bull. Earthquake Res. Inst., Tokyo Univ., 44: 73-88. Aki, K., 196613.Earthquake generating stress in Japan for the years 1961-1963 obtained by smoothing the first motion radiation patterns. Bull. Earthquake Res. Inst., Tokyo Univ., 44: 447-471. Aki, K., 1967. Scaling law of seismic spectrum. J. Geophys. Res., 72: 1217-1231. Aki, K., in preparation. A statistical theory of seismic coda waves and its application to the earthquake source problem. Aki, K., Hori,hiI. and Matumoto, H., 1966. Observations of Matsushiro earthquake swarms at Nakano. Rept. Ann. Meeting Seismol. Sot. Japan, 1966, unpublished. Allen, C.R., Amand, P.St., Richter, C.R. and Nordquist, J.M., 1965. Relationship between seismicity and geologic structure in the southern California region. Bull. Seismol. Sot. Am., 55: 753-798. Anonymous, 1966. Party for seismographic observation of Matsushiro earthquakes and the Seismometric Section, Matsushiro earthquakes observed with a temporary seismographic network. Bull. Earthquake Res. Inst., Tokyo Univ., 44: 168S-1714. Brune, J.N. and Allen, C.R., 1967a. A micro-earthquake survey of the San Andreas fault system in southern California. Bull. Seismol. Sot. Am., 57: 27V296. Brune, J.N. and Allen, C.R., 1967b. A low-stress-drop, low-magnitude earthquake with surface faulting: The Imperial, California, earthquake of March 4, 1966. Bull. Seismol. Sot. Am., 57: 501-514. Gutenberg, B. and Richter, C.F., 1956. Seismicity of the Earth, 2nd ed. Princeton University Press, Princeton, N.J., 310 pp. Hagiwara, T. and Rikitake, T., 1967. Japanese program on earthquake prediction. Science, 157: 761-768. Honda, H. and Masatsuka, A., 1952. On the mechanism of the earthquakes and the stress producing them in Japan and its vicinity. Sci. Rep., Tohoku Univ., 5th Ser., 4: 4260. ,Hori, M., Matumoto, H. and Aki, K., 1966. Observation of micro-aftershocks of’the Alaska earthquake, March 28, 1964. J. Seismol. Sot. Japan, 19: 187-199. Ichikawa,.M., 1960. On the mechanism of the earthquakes in and near Japan during the period from 1950 to 1957. Geophys. Msg., 30: 355-403. Katsumata, M., 1966. Seismic activity in and near Japan. J. Seismol. Sot. Japan, 19: 23V-245. Miyamura, S., Hori, M., Aki, K., Tsujiura, M. and Matumoto, H., 1962. Simultaneous operation of two seismometer-array stations in a study of micro-earthquakes in the Kanto and Chubu region. Bull; Earthquake Res. Inst., Tokyo Univ., 40: 885-897. Mogi, K., 1962. Study of elastic shocks caused by the fracture of heterogeneous materials and its relations to earthquake phenomena. Bull. Earthquake Res. Inst.,Tokyo Univ., 40: 125-173. Mogi, K., 1963. Some discussions on aftershocks, foreshocks and earthquake swarms. Bull. Earthquake Res. Inst. Tokyo Univ., 41: 615-658. Suyehiro, S., 1964. An example of fore- and aftershock sequences and difference in the relation between magnitude and frequency of occurrence between the two sequences. Proc. U.S.-Japan Conf. Res. Earthquake Prediction Problems, 2nd, New York, pp.52-53.
58
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SEISMICITY AND SEISMOLOGICAL METHOD K. AKI Department of Geology and Geophysics, Massachusetts Institute of Technology, Cambridge, Mass., (U.S.A.) (Received October 2, 1967) SUMMARY
A straight-forward way for seismologists to contribute to earthquake prediction research is to find the state of stress inside the earth from seismograms of numerous small earthquakes taking place in the general area where a large one is expected. The success of this approach depends on the capability of seismologists in finding conditions about earthquake sources from seismograms. This in turn depends on the available theory to deal with seismic waves and on our knowledge about the wave medium. Ray theory can give us information on positions of earthquake foci, time sequence of earthquakes, geometrical aspects of stress, and some measure of energy release. Recent efforts in this direction, primarily directed toward micro-earthquakes, have produced evidence that indicates the significance of such studies in earthquake prediction research. Normal mode theory can give us additional information about the source condition, because we can incorporate an arbitrary source function in space and time in the scheme of this theory. We can, for example, measure the stress-drop in an earthquake from the spectrum of surface waves, if some additional near-field observations are available. Both ray theory and normal mode theory have their own limitations. Difficult problems arise especially when we try to obtain source conditions of micro-earthquakes, because they generate short period waves which suffer strongly from the heterogeneity of wave medium. Recent studies bearing upon these problems will be reviewed and possible approaches to overcome the above mentioned difficulties will be discussed. INTRODUCTION The word “seismicity” expresses such a picture of earthquake generating process inside the earth that can be obtained from the record of seismographs. Therefore, the quality of the picture will depend on the quality of the seismological method utilized in the analysis of seismograms. For example, Gutenberg and Richter’s (1956) monumental work Seismicity of the Earth was an outcome of a new method which enabled them to measure the magnitude of an earthquake from the seismic waves. We may say that the study of seismicity is a synthesis of results of analysis of Tectonophysics, 6 (1) (1968) 41-58
41
seismograms. As shown by Gutenberg and Richter, success of such a synthesis requires a broad underst~d~ng of regional geophysics and geology. During the last 20 years, since the Sea’smicify of the Earth was published, seismological observations considerably widened its spectral range as well as dynamic range. Methods of analysis were remarkably advanced using not only ray theory but also wave theory in the study of sources of seismic waves. Some efforts have been made to try to get a better, more detailed and precise picture of seismicity by synthesizing the outcome of such new methods. One of these efforts is directed toward finding any relation between the occurrence of large earthquakes and the activity of very small earthquakes in the same area. The idea behind this study is that large and smali earthquakes probably share the same tectonic causes, and we may learn about the stress conditions responsible for large earthquakes by studying small but numerous earthquakes. Small earthquakes with magnitudes less than 3 are called micro-earthquakes. They usually escape from the standard seismograph network, and the investigation of them requires new techniques of observations and analysis
Fig.1. Map of historic fauIt breaks and associated earthquakes in southern California region. = fault breaks associated with earthquakes. (After Allen et al., 1965.) 42
Tectonophysics,6 (1)(1968)41r58
RELATIONSHIP BETWEEN SOUTHERN CALIFORNIA
MICRO-EARTHQUAKES
AND LARGE EARTHQUAKES:
How does the micro-earthquake activity relate to the occurrence of large earthquakes? Let us first look at the area of southern California where we find some interesting discoveries made by a joint work effort of seismologists and geologists. Fig.1 shows the map of historic fault breaks and associated earthquakes in southern California reproduced from a paper by Allen et al. (1965). Most of these earthquakes are on major through-going faults. Earthquakes with magnitudes greater than 6 which were not accompanied by clear fault breaks are also located close to the major faults. Looking at only the main San Andreas fault system, we see that the fault breaks up into many branches in the southern section starting near Cajon Pass but shows a single curved trace north of it. We also see that a number of earthquakes took place along the southern branched fault zone, but a single large earthquake of 1857 is associated with the northern section.
Fig.2. Smoothed strain-release map of southern California region, Jan. 1, 193PJan. 1, 1965. Numbers of equivalent M = 3.0 earthquakes per 100 kmz. (After Allen et al., 1965.) Tectonophysics,
6 (1) (1968) 41-58
43
This correlation of geological structure with seismic activity is more clearly shown in the strain release map (Fig.%) also reproduced from Allen et al. (1965). The number shown is the sum of the square root of energy released in 29 years measured by taking the value corresponding to magnitude 3 as the unit. The data used are 10,000 earthquakes located by the Seismological Laboratory, Pasadena, California. South of the Cajon Pass, where the fault branches, the strain release is large. On the other hand, the part of the fault associated with the 1857 earthquake shows very small strain release during this period, except the small segment close to the Kern County earthquake of 1952, which was associated with the transverse fault rather than the main San Andreas fault. A similar picture showed up from a study of micro-earthquakes done by Brune and Allen (1967a). This map (Fig.31 shows the number of microearthquakes per day within 24 km of each station. Most stations are operated for 3&100 h, some of them up to 4,000 h. Again, we see a very quiet zone along the fault segment associated with the 1857 earthquake, except near the epicentre of the Kern County earthquake. To the south of this section micro-earthquake activity increases and shows a pattern nearly identical to the strain release obtained from the 2%years’ data. To the north of this section a moderate increase of activity shows up starting at Cholome where the Parkfield earthquake took place in June 1966. \ ‘\
“\
0
so
Iookm
24 km radius
J
Fig.3. Map of sites occupied in the micro-earthquake study. (After Brune and Allen, 196’7a.) Heavy numerals are numbers of micro-earthquakes per day within 24 km of station; heavy lines are major faults, reproduced from their paper. 44
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It is interesting to note that the Parkfield earthquake is associated with an unusually low stress drop, of the order of 1 bar. The estimate of the stress drop is made by applying a wave theory to the spectrum of seismic waves on the basis of the dislocation source model (Aki, 196’7). This is a powerful technique in studying the stress condition of the source, and is an outcome of efforts of many theoretical seismologists in recent years. Another low-stress-drop earthquake was found by Brune and Allen (1967b) in the Imperial Valley near the Mexican border. These examples certainly suggest that in the area of higher micro-earthquake activity, the accumulating tectonic strain may be released by many smaller earthquakes because of structural weaknesses, and that the zone of faulting during the great 1857 earthquake may be characterized by extremely low seismicity of structural strength possibly due to some locking mechanism. If a strong structure and a weak structure exist next to each other, of course, the strain release will be concentrated to the weak one. From this brief survey of southern California seismicity we may emphasize the following two points: (1) The regional pattern of micro-earthquake activity is grossly similar to the seismicity pattern of larger earthquakes.
SOOkm
Fig.4. Distribution of epicentres of earthquakes of the depths from 30 to 70 km for the period 1951-1965. (After Katsumata, 1966.) Tectonophysics,6 (1)(1968)41-58
45
(2) The response of the crust to regional tectonic stress differs from place to place even in the same general tectonic area, resulting in a complex pattern of strain concentration and of micro-earthquake .activity. RELATIONSHIP JAPAN
BETWEEN
MICRO-EARTHQUAKES
AND LARGE EARTHQUAKES:
Bearing these two points in mind, we shall turn our eyes on Japan, where everything is expected to be far more complex than in California. We shall again see, however, that the micro-earthquakes show the same activitypattern as that of larger earthquakes. Unlike California, the seismicity in Japan has a three-dimensional structure. Therefore, the maps must be given for different ranges of focal depths. Fig.4 is the map of epicentres for depths 30-70 km reproduced from Katsumata (1966), who used data of the last 15 years (1951-1965). As was pointed out by Asada, those earthquakes originating in the uppermost mantle are confined to zones along the trends of arc structure. There is a sharp boundary of activity, especially in eastern Japan, nearly along the
I;0-<30llm / M
5-sw&6o-65-70-25-60-65 . .o 00 000
0
5oohm
Fig.5. Distribution of epicentres of earthquakes of the depths from 0 to 30 km for the period 1951-1965. (After Katsumata, 1966.) 46
Tectonophysics,
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coast. The most active area in the country is the Kanto district, which is at the intersection of the Honshu arc and the Mariana arc. The seismicity map of earthquakes originating in the crust shows a markedly different pattern from the mantle seismicity. Fig.5 is again reproduced from Katsumata (1966) and shows epicentres of earthquakes with depths O-30 km for the same period (1951-1965). They occur in the land as well as in the ocean nearly uniformly, without defining clear zones. An interesting area is the Kanto district, where the mantle activity is the most vigorous in Japan. We see practically no earthquakes in this area originating in the crust. Outside this area, the activity in the crust is high. This feature is reproduced almost exactly in the micro-earthquake activity. Fig.6, reproduced from Miyamura et al. (1962), shows the epicentres of micro-earthquakes determined by the use of an array station located at Tsukuba. Most of their depths range from 35 to 60 km, and agree with the depths of larger earthquakes. The activity in the crust is extremely
Fig.6. The epicentres of micro-earthquakes determined by the observation at Tsukuba array station and the Bouguer gravity anomaly contours; Dec. 1, 1961~March 31, 1962. (After Miyamura et al., 1962.) Tectonophysics,
6 (1) (1968) 41-56
47
km
Fig.7. The epicentres of micro-earthquakes determined by the observation at Tochimoto array station and the Bouguer gravity anomaly contours; May 20-Juni 27, 1962. (After Miyamura et al., 1962.)
Fig.8. Epicentres of micro-aftershocks Lawing, Alaska, array station; May l%June
located by the observation 7, 1964. (After Hori et al., lS66.j
48
Tectonophysics, 6 (1)(1968)41-58
Km200
100
120
El0
40 NW
ALE40
00
120
160
200
Km
-100 -120 -140 -160 -160 200 220
0
o
0
240
Km
Fig.9. Vertical cross-section of the hypocentres of micro aftershocks along the profile in N 55°WS550E. The apparent concentration of foci at depths 5 km and 35 km is due to an artificial restriction in the hypocentre location. (After Hori et al., 1966.) low in the central part and increases outward to the ocean as well as to the land. This feature becomes more clear in the epicentre map (Fig.7) obtained at the array station set up about 100 km to the west of Tsukuba. We find that the micro-earthquake activity in the crust is high in the area where larger earthquakes are active in the crust. There are several other studies, made in Japan and other countries, which show also the similarity between the pattern of micro-earthquake activity and that of larger earthquakes. One such spectacular example is the micro-eartquakes after the great Alaska earthquake of 1964. Fig.8 shows the epicentres determined by an array station in the Kenai Peninsula (Hori et al., 1966). All the shocks in the oceanic side of the stations originated in the crust, but those on the continental side show the focal depths down to 200 km. This area is precisely the zone of earthquakes of the intermediate depth in the normal period. The vertical cross-section perpendicular to the coastline shows this three-dimensional structure of micro-earthquake activity more clearly (Fig.9). This three-dimensional similarity strongly supports the idea that micro-earthquakes and larger earthquakes share a common tectonic cause. If so, we should be able to infer the stress conditions responsible for large earthquakes by studying numerous micro-earthquakes.
EARTHQUAKE
GENERATING
STRESS
IN JAPAN
Now let us look at Japan again and ask about the second point we made from the observations in California, that is, what is the nature of the Tectonophysics, 6
(1) (1968)
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49
0 I
500
I
km
-
Fig.10. Distribution of the maximum pressure axes for deep earthquakes in and near Japan. (After Honda and Masatsuka, 1952.)
crustal response to the regional stress ? First of all, is there any uniform regional stress like the one in California? As is well known from Honda’s work on first motions, the deep focus earthquakes show a systematic orientation of the maximum pressure axis, nearly perpendicular to the trend of the seismic zone. This suggests the existence of a rather uniform stress field generating these earthquakes (Fig.10). On the other hand, the shallow earthquakes show a rather complex pattern as is shown in Fig. 11 (Honda and Masatsuka, 1952). A closer look, however, reveals that the earthquakes along the Japan Sea coast show systematically oriented pressure axes in roughly an E-W direction. This picture is obtained from 22 years of data prior to 1949. 50
Tectonophysics,
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The pressure axes pattern in the next 7 years is shown in Fig.12, which is reproduced from lchikawa (1960~. The pattern is again complex, but the one in the Japan Sea coast shows the same general orientation found in the prior period. I If this eompfex stress pattern is due to the timely change in stress condition, we may.expect that the pattern may be more uniform, if we only use short-term data. Fig.13 shows the result obtained from 30 months data from January 1961. A method of smoothing radiation patterns is applied to many small earthquakes to obtain the average orientation of axes for various areas (Aki, 1966b). The maximum pressure axes are indicated by solid arrows. The axes are oriented reasonably systematic~ly, suggesting the existence of a uniform regional stress. About a year after the end of the period covered by this picture, the Niigata earthquake of magnitude 7.5 took place in the Japan Sea. The pressure axis of this earlobe was oriented in N 7O*W,which fits the general direction expected at the epicentre.
Mg.11. Distribution of the maximum pressure axes for shallow earthquakes for the period 1927-1949. (After Honda and Masatsuka, 1952,) Tectonophysics,6 (1)(1968)41-58
51
L
Qd0
I
500km I
Fig.12. Distribution of the maximum pressure axes for shallow earthquakes for the period 195S-1957. (After Ichikawa, 1960.)
The Niigata earthquake took place where the seismicity map prior to 1964 showed the least activity in Japan. In this sense, this earthquake is much alike to the 1857 California earthquake mentioned before. The stress drop estimated from the seismic spectrum and near-field observations amounts to 125 bar (Aki, 1966a), which is two orders of magnitude greater than those of the Parkfield earthquake (Aki, 1967) and the Imperial earthquake (Brune and Allen, 1967b) mentioned before. This suggests higher strength of the crust in the Niigata earthquake area than in the other. An area comparable to the low-stress-drop, high seismieity areas of California may be found in Matsushiro located about at the centre of Honshu. The Matsushiro earthquake swarm started in August 1965 and lasted for more than two years. At the peak of the activity, 6,000 earthquakes/day were recorded. 52
Tectonophysics, 6 (1)(l968)41-56
The first motion patterns from these small earthquakes (Fig.14, Anonymous, 1966) are remarkably consistent, and the pressure axis is roughly E-W, again agreeing with the regional stress pattern shown in Fig.13. This suggests a concentration of regional strain to a zone of weak structure. Supporting this idea is an observation that the micro-earthquake activity outside the swarm area was unusually low, indicating a stress relaxation in the neighborhood of the swarm area (Aki et al., 1966). Another interesting ‘observation comes from the comparison of seismic spectra made by S. Suyehiro (personal communication, 1967) between the micro-earthquakes prior to the swarm period and those of the swarm earthquakes. He found greater high-frequency waves in the normal period than in the swarm period. This fact may be explained by an increased dissipation due to change in wave medium, but can be attributed to the change of stress condition at the source. The observed change corresponds to greater stress drop in the normal period than in the swarm period, which fits the idea of strain concentration to the weak zone and stress relaxation outside the zone.
Fig.13. Distribution of the maximum pressure (solid arrow) and tension (dashed arrow) axes for the period 1961-1963. (After Aki, 1966b.) Tectonophysics,
6 .(l) (1968) 41-58
53
Fig.14. Push-pull distribution of the first motions of the Matsushiro earthquakes. The pattern due to different shocks are superposed together, the epicentres being put at a common point (origin). (Anonymous, 1966.)
A similar comparison was made by Aki (in preparation) between a foreshock and an aftershock of the Parkfield earthquake. Stress drop was estimated from the ratio of seismic energy to the seismic moment which was obtained from the coda waves. In this case, there was no appreciable difference in stress drop between the foreshock and the aftershock. In any case, the above two examples, the Niigata earthquake and the Matsushiro swarm, seem to suggest again that the regional tectonic stress is rather uniform, but the response of the crust is quite different depending on the structural strength or weakness, and this difference gives rise to extremely different modes of seismicity even in the same tectonic area.
FORESHOCKS
A similar point of view has been taken by Mogi, who studied extensively the fracture process of rocks in the laboratory and compared the results with the actual earthquakes. Fig.15 shows a result of his experiment (Mogi, 1962). Numerous microfractures, or elastic shocks, take place before the main rupture for Pumice, while no microfracture precedes the main rupture for Pine Resin. Trachyte and granite show intermediate nature. The experiment was made under the increasing stress at a constant rate, but similar phenomena are .observed also at a constant stress. Mogi attributes this 54
Teotonophysics,
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difference to a difference in degree of structural heterogeneity of material. With the increase of structural heterogeneity, stress concentration will take place at a greater number of points, resulting in a greater number of microfractures. These microfractures may correspond to foreshocks, which may be used as the most direct means of predicting earthquakes. An example of the foreshocks preceding the larger Matsushiro earthquake is shown in Fig.16 (Hagiwara and Rfkitake, 1967). Mogi (1963) examined 1,400 earthquakes in Japan to see whether they were preceded or not by foreshocks. Only 60 earthquakes were preceded by foreshocks, but the probability of having foreshocks shows a systematic regional pattern. Fig.17 shows the percentage of earthquakes which were preceded by foreshocks. For example, 50% of the earthquakes originating near Matsushiro were preceded by foreshocks, but the percentage was very low near the epicentre of the Niigata earthquake. In the same figure, the area where the swarm earthquakes are frequent is indicated by dashed lines. There is a clear correlation between the two patterns.
Pine Rem
(Homogenecud
Main Rupture
I
:
0 Trachyte
5 (Near@
1
1
15
lo
20
Homogeneous) I
;
OO
20
60
40
I
Granite (Heterogeneous)
Pumice Extremely
Heterugeneous)
Awlled
stressin
hg/crd
Fig.15. Microfractures preceding a main rupture for various material under increasing stress at a constant rate. (After Mogi, 1962.) Tectonophysics,
6 (1) (1968)
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55
Mogi points out that these patterns are similar to the distributions of Cenozoic volcanism, high terrestrial heat flow and the strongly folded tertiary zones, all of which may imply a highly fractured crust. Thus, we see again that the mode of seismicity is controlled by the structure. Mogi made another prediction based on his experiment. The slope of the magnitudefrequency relation, that is, the b-value of the Gutenberg-Richter formula varies withthe structural heterogeneity of rock samples. The b-value is greater for higher heterogeneity. Suyehiro (1964) made an observation which may be attributed to this effect. A group of foreshocks and one of aftershocks, in central Japan, showed different b-values, the value for the aftershock being significantly greater. This result may be explained, using Mogi’s result, by a structural heterogeneity produced by fracturing due to the main shock. We do not yet, however, understand completely the physics behind this effect, and also we need more observations. Nevertheless, this shows that the mode of seismicity varies not only from place to place, but also from time to time. CONCLUSION
From the above survey of the seismicity in southern California and Japan, we learn that, even within the same tectonic area with the same
13Jan.
14
15
16
b
17
Tit-t-l@-----
Fig.16. The hourley frequency of micro-earthquakes immediately before and after the Matsushiro earthquake of January 16, 1967. (After Hagiwara and Rikitake, 1967.) 56
Tectonophysics,
6 (1) (1968) 4F58
Pacific Ocean
Fig.17. Percentage of earthquakes preceded by foreshocks (solid line) and areas of active earthquake swarms (dashed line). (After Magi, 1963.)
regional stress conditions, the mode of seismic activity can be quite different from place to place. However, by improving the seismological methods dealing with seismic waves and obtaining more precise knowledge about the physics of their sources, we may be able to understand why they are so. If we can learn why, the method of prediction will naturally emerge. Finally, I must apologize for not being able to cover (because of limited time) many important works done recently in the same field, such as interesting details of seismic zones revealed by improved accuracy of hypocentre location; space-time distribution of pressure or tension axes and strain release in particular earthquake sequences; modernized data pro* cessing which may eventually lead to a complete automation of seismicity monitoring. I hope, however, that I have picked up enough of the works for convincing the reader that seismologists can contribute to the earthquake ‘prediction research.
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57
REFERENCES Aki, K., 1966a. Generation and propagation of Gwaves from the Niigata earthquake of June 16, 1964, 2. Estimation of earthquake moment, released energy, and stress-strain drop from the Gwave spectrum. Bull. Earthquake Res. Inst., Tokyo Univ., 44: 73-88. Aki, K., 196613.Earthquake generating stress in Japan for the years 1961-1963 obtained by smoothing the first motion radiation patterns. Bull. Earthquake Res. Inst., Tokyo Univ., 44: 447-471. Aki, K., 1967. Scaling law of seismic spectrum. J. Geophys. Res., 72: 1217-1231. Aki, K., in preparation. A statistical theory of seismic coda waves and its application to the earthquake source problem. Aki, K., Hori,hiI. and Matumoto, H., 1966. Observations of Matsushiro earthquake swarms at Nakano. Rept. Ann. Meeting Seismol. Sot. Japan, 1966, unpublished. Allen, C.R., Amand, P.St., Richter, C.R. and Nordquist, J.M., 1965. Relationship between seismicity and geologic structure in the southern California region. Bull. Seismol. Sot. Am., 55: 753-798. Anonymous, 1966. Party for seismographic observation of Matsushiro earthquakes and the Seismometric Section, Matsushiro earthquakes observed with a temporary seismographic network. Bull. Earthquake Res. Inst., Tokyo Univ., 44: 168S-1714. Brune, J.N. and Allen, C.R., 1967a. A micro-earthquake survey of the San Andreas fault system in southern California. Bull. Seismol. Sot. Am., 57: 27V296. Brune, J.N. and Allen, C.R., 1967b. A low-stress-drop, low-magnitude earthquake with surface faulting: The Imperial, California, earthquake of March 4, 1966. Bull. Seismol. Sot. Am., 57: 501-514. Gutenberg, B. and Richter, C.F., 1956. Seismicity of the Earth, 2nd ed. Princeton University Press, Princeton, N.J., 310 pp. Hagiwara, T. and Rikitake, T., 1967. Japanese program on earthquake prediction. Science, 157: 761-768. Honda, H. and Masatsuka, A., 1952. On the mechanism of the earthquakes and the stress producing them in Japan and its vicinity. Sci. Rep., Tohoku Univ., 5th Ser., 4: 4260. ,Hori, M., Matumoto, H. and Aki, K., 1966. Observation of micro-aftershocks of’the Alaska earthquake, March 28, 1964. J. Seismol. Sot. Japan, 19: 187-199. Ichikawa,.M., 1960. On the mechanism of the earthquakes in and near Japan during the period from 1950 to 1957. Geophys. Msg., 30: 355-403. Katsumata, M., 1966. Seismic activity in and near Japan. J. Seismol. Sot. Japan, 19: 23V-245. Miyamura, S., Hori, M., Aki, K., Tsujiura, M. and Matumoto, H., 1962. Simultaneous operation of two seismometer-array stations in a study of micro-earthquakes in the Kanto and Chubu region. Bull; Earthquake Res. Inst., Tokyo Univ., 40: 885-897. Mogi, K., 1962. Study of elastic shocks caused by the fracture of heterogeneous materials and its relations to earthquake phenomena. Bull. Earthquake Res. Inst.,Tokyo Univ., 40: 125-173. Mogi, K., 1963. Some discussions on aftershocks, foreshocks and earthquake swarms. Bull. Earthquake Res. Inst. Tokyo Univ., 41: 615-658. Suyehiro, S., 1964. An example of fore- and aftershock sequences and difference in the relation between magnitude and frequency of occurrence between the two sequences. Proc. U.S.-Japan Conf. Res. Earthquake Prediction Problems, 2nd, New York, pp.52-53.
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