SEISMIC AND ASEISMIC SLIP ALONG SUBDUCTION ZONES AND THEIR TECTONIC IMPLICATIONS Hirao Kanamori Seismological Laboratory, California Institute of Technology, Pasadena, California 91125 Abstract. Results of detailed mechanism Introduction studies of great earthquakes are used togeth­ er with their repeat times to determine the In the theory of plate tectonics, litho­ amount of seismic slip along various subduc­ spheric subduction is one of the major tec­ tion zones. Comparison of the seismic slip tonic processes related to the formation and with the rate of plate motion suggests that, evolution of island arcs. The deep and in­ in Chile, and possibly Alaska, the seismic termediate earthquake zones along island arcs slip rate is comparable to the rate of plate delineate the geometry of the subducting motion while, in the Kuriles and Northern lithosphere, and major shallow earthquake Japan, the seismic slip constitutes only a activity along island arcs is a manifestation very small portion, approximately 1/4, of of the mechanical interaction between the the total slip. In the Sanriku region, and subducting and the overriding lithospheres. to the south of it, the relative amount of The geometrical agreement of focal mechanisms seismic slip is even smaller. These results of earthquakes along the Circum-Pacific belt suggest that in Chile and Alaska the coupling led McKenzie and Parker [1967] to the con­ and interaction between the oceanic and con­ cept of rigid plates dividing the earth's tinental lithosphere are very strong, re­ surface. Isacks and Molnar [1969] used the sulting in great earthquakes with a very geometry of the compression and tension axes large rupture zone, and in break-off of the of deep and intermediate earthquakes to in­ undergoing lithosphere at shallow depths. fer the stress distribution in the descending In the Kuriles and Northern Japan, the oce­ lithosphere. Stauder [1968] found tensional anic and continental lithosphere are largely mechanisms for earthquakes along the trench decoupled, so that the slip becomes largely axis in the Aleutians and interpreted them aseismic, and the rupture length of earth­ as fractures due to bending of the oceanic quakes reduced. The reduced interaction at lithosphere. Besides these geometrical ar­ the inter-plate boundary may allow the oce­ guments, studies of physical processes as­ anic lithosphere to subduct more easily and sociated with large earthquakes provided to form a continuous Benioff zone extending important information concerning the dynamics to depths. It may also facilitate ridge of plate subduction [Plafker, 1972; Kanamori, subduction beneath island arcs, which may 197lb; Abe, 1972]. Kanamori [197lb] proposed play an important role in the formation of a model of gradual thinning and weakening of marginal seas such as the Japan Sea. The the ocean-continent lithospheric boundary to decoupling is also evidenced by silent or account for the differences in the maximum tsunami earthquakes [e.g., the 1896 Sanriku dimension of rupture zones among different earthquake], great intra-plate normal-fault island arcs. It was suggested that such earthquakes [e.g., the 1933 Sanriku earth­ differences in the coupling between the oce­ quake], and crustal deformation. A natural anic and continental lithospheres may control extension of this concept of inter-plate various tectonic processes at island arcs. decoupling is the spontaneous sinking of the The argument was based on detailed studies oceanic lithosphere with a consequent re­ of great earthquakes through analysis of treating subduction. Retreating subduction long-period surface waves (periods of 200 may be an important mechanism in the forma- to 300 sec). Such long-period waves repre­ t ion of marginal seas such as the Philippine sent the overall crustal deformation at Sea, and explains the complete lack of major plate boundaries more directly than conven­ shallow earthquake activity along some sub­ tional short-period seismic waves. Kelleher duction zones such as the Izu-Bonin-Mariana et al. [1974] found a good correlation be­ arc. tween the maximum dimension of rupture zones KANAMORI 163 of great earthquakes and the width of the area of lithospheric contact at various sub­ duction zones. This paper extends the pre­ vious paper [Kanamori, 197lb] by including more recent data from great earthquakes. Inclusion of these recent data strengthens the conclusions of the previous paper. It is almost certain that the degree of mechan­ ical coupling (or decoupling) between the oceanic and continental lithospheres varies among different island arcs; along some sub­ duction zones the coupling is very strong, \ --~ so that the plate motion is almost entirely oo taken up by seismic slip. It is relatively !2!;'-~ weak elsewhere and the seismic slip repre­ • Ms -20° sents only a very minor part of the plate I 1923 8.2 9 2 1933 8.3 10 motion. Along some subduction zones, the 3 1944 8.2 II 4 1946 8.2 12 -40° boundary is entirely decoupled and the plate CJ\J 5 1952 8.3 13 motion seems to take place aseismically. It 6 1960 8.3 14 7 1963 8.2 15 is proposed that such coupling and decoupling 8 1964 8.5 of plates may play an essential role in the -60° evolution of island arcs and marginal seas. Summary of Characteristics of Great Earth­ Figure 1. Mechanisms and rupture zones of large quakes along the Circum Pacific Belt earthquakes along the Circum Pacific belt for which detailed studies of long-period waves have Figure 1 shows the rupture zones and the been made. Mechanism diagrams show the stereo­ mechanisms of major earthquakes for which graphic projection of the lower hemisphere. Dark detailed studies were made by using long­ and white quadrants indicate compression and dila­ period surface waves. Rupture zones of other tation respectively. References are: 1923 Kanto major earthquakes have been mapped by Fedotov [Kanarnori, 197lc]; 1933 Sanriku [Kanamori, 197la]; [1965], Mogi [1968a, b], Sykes [1971], and 1944 Tonankai [Kanamori, 1972a]; 1946 Nankaido Kelleher et al. [1973]. Except for the 1933 [Kanarnori, 1972a]; 1952 Kamchatka [Kanamori, 1976); Sanriku earthquake and the 1970 Peruvian 1960 Chile [Kanamori and Cipar, 1974]; 1963 Kurile earthquake, the mechanisms of these earth­ Is. [Kanamori, 1970a]; 1964 Alaska [Kanamori, quakes suggest low-angle thrust faulting, 1970b]; 1965 Rat Is. [Wu and Kanamori, 1973); 1966 which is consistent with the plate motion Peru [Abe, 1972]; 1968 Tokachi-Oki [Kanamori, along the Circum Pacific belt. The 1933 197ld];--:i:"969 Kurile Is. [Abe, 1973); 1970 Peru Sanriku earthquake is an exceptionally large [Abe, 1972); 1973 Nemuro-Oki [Shimazaki, 1975a]; normal-fault earthquake which occurred be­ 1974 Peru [unpublished]. neath the axis of the Japan Trench [Kanamori, 197la]. It is interpreted as a lithospheric overall amount of displacement at the source normal-fault which cuts through the entire is a more adequate parameter for the present thickness of the lithosphere. The 1970 Pe­ discussion. Great earthquakes in Chile, ruvian earthquake is another normal-fault Alaska, the Aleutians, and Kamchatka have event [Abe, 1972] within the oceanic litho­ very large rupture lengths and very large M0 sphere. These two events are therefore ranging from 1029 to 1030 dyne-cm, while intra-plate earthquakes and do not represent earthquakes in Peru and Japan have small rup­ slip between the oceanic and continental ture lengths and Mo, about 1028 dyne-cm. lithospheres. Kanamori [197lb] and Kelleher et al. [1974] Two other important features in Figure 1 argued that this remarkable difference in the are (1) a remarkable regional variation of characteristics of great earthquakes reflects rupture lengths of great earthquakes despite regional differences in the contact zone be­ their nearly identical earthquake magnitude, tween the oceanic and continental lithospheres. and (2) a nearly complete lack of large shal­ As regards (2), two mechanisms are possible. low activity along the Izu-Bonin-Mariana arc, First, as a result of weakening and decoupling despite its typical island arc features such at the interface, the subduction has become as a deep trench, volcanic activity and a nearly completely aseismic. In this case, Benioff zone. Regarding (1), it is now wide­ the subduction is still taking place but with­ ly known [e.g., Kanamori and Anderson, 1975] out any major seismic activity. The second that earthquake magnitude Ms is not a mean­ mechanism involves a buoyant oceanic litho­ ingful parameter for very large earthquakes. sphere [Vogt, 1973; Kelleher and McCann, 1976); The seismic moment Mo which represents the part of the oceanic lithosphere is less dense 164 KANAMORI than elsewhere and is not capable of subduct­ 50°r-~~~---,-~~~~--,-,,-.---~~.--~, ing under the opposing lithosphere, and little N or no subduction is now taking place. This mechanism is attractive in that it explains the arcuate feature of island arcs, some of Eurasia the characteristic distr:Loutions of large earthquakes and volcanoes along island arcs and the regional variations in the shape of the Benioff zone [Kelleher and Mccann, 1976]. However, these geometrical arguments alone are not enough to fully evaluate these pos­ sibilities. It is hoped that recent progress in long-period seismology will provide more direct clues to the understanding of these problems. Seismic and Aseismic Slip Among various subduction zones, histori­ cal earthquake data is most complete for southwest Japan. It is well known that along the Nankai trough in southwest Japan (Figure 2), large earthquakes have occurred very regularly in time (about once every 125 years) and space [Imamura, 1928; Ando, 1975a]. This regularity in time and space may justify the use of relatively recent data in esti­ mating the seismic slip rate along various other subduction zones. N For Chile, along the rupture zone of the 1960 great Chilean earthquake, historic records suggest a repeat time of the order Repeat time of a century [Lomnitz, 1970, Kelleher, et al., ~ 120 yrs 147 y 1973]; large earthquakes occurred in 1970, Ansei I 1837, 1737, and 1575.