VOL. 17, NO. 2 REVIEWS OF GEOPHYSICS AND SPACE PHYSICS APRIL 1979 EARTHQUAKESOURCE MECHANISMS AND PLATE TECTONICS Hiroo Kanamori Seismological Laboratory, California Institute of Technology, Pasadena, California 91125 This review and the accompanying references Sykes [1978] summarized the distribution will be concerned mainly with earthquake source of intraplate earthquakes and of igneous rocks studies pertinent to plate tectonics. Studies in a plate tectonic framework. One of the on geological problems, lithospheric flexure conclusions is that intraplate earthquakes tend and mantle convection are discussed in other to be concentrated along preexisting zones of reviews in this volume. weakness within areas affected by the youngest In the earlier stage of the development orogenesis that predates the opening of the of plate tectonics, the distribution of present oceans. earthquakes and the geometrical configuration Becaus• of the difficulty of determining the of earthquake source mechanisms played an fault •ime•sion of intraplate events, the important role. As more sophisticated estimates of stress drop are still uncertain. techniques for retrieving earthquake source Existing data indicate [e.g., Kanamori and parameters (synthetic seismogram method for Anderson, 1975; Richardson et al., 1977] that both body and surface waves, inversion although the stress drop of intraplate events method, matching filter method, etc.) were may be slightly (factor of 2 or 3) higher than developed, emphasis in the last four years that of inter-plate events, no order-of-magni- was placed on the determination of not only tude difference exists between them. A the geometry of the source but also more relatively low stress drop, about 100 bars, quantitative parameters such as the stress for intraplate earthquakes is consistent with drop, the amount of slip, the complexity, inferences from current models of the plate the strength of non-isotropic components, tectonic driving mechanism. Some questions and the spectral characteristics. Also have been raised about whether intraplate significant emphasis was placed on intraplate events represent the state of stress in the earthquakes and events on continental plate plate or the effect of local topographic boundaries. features in the plate [Stein, 1978]. The results for the individual earthquakes A large number of studies were made on are of fundamental importance for developing seismotectonics of various regions. Papers and constraining various plate tectonics on this subject are listed in the reference models. For example, a number of mechanism under category "Regional Tectonics". A solutions have been included in the new recent development in this field is the study synthesis of global plate motion [Minster and of continental tectonics [e.g., Molnar and Jordan, 1978]. Since it is not practical to Tapponnier, 1975; Bird et al., 1975; Chen review the results for the individual events, and Molnar, 1977; Menke and Jacob, 1976; a fairly complete list of the papers pertinent Armbruster et al., 1978; Chandra, 1978]. to the source mechanism study is prepared and Armbruster et al. [1978] studied the listed in the reference. northwestern termination of the Himalayan The orientation and the magnitude of the mountain front and found that the composite stress associated with intraplate events are mechanisms of earthquakes in this area are among the key parameters in constraining the compatible with the north-south convergence driving mechanism of plates [e.g., Solomon et between the Indian and Eurasian plates al., 1975; Richardson et al., 1976]. Richard- inferred from plate tectonics. son et al. [1976] found that intraplate Molnar and Tapponnier [1975] concluded on stresses calculated for models in which viscous the basis of geomorphological and seismological drag at the base of the lithosphere acts in evidence that the large-scale tectonics of the direction of absolute plate velocity are Asia are a result of the India-Eurasia in much poorer agreement with observed stresses continental collision. They interpreted than are those for models in which drag resists the marked decrease in the relative motion plate motions. between India and Eurasia to be a result of the collision. Copyright 1979 by the American Geophysical Union. The amount of seismic slip along plate 337 338 boundaries has been determined from the seismic model to explain the time dependent deformation momentof large earthquakes [Kanamori, 1977]. after the 1906 San Francisco earthquake and the A substantial discrepancy between the computed 1946 Nankaido earthquake. seismic slip rate and the rate inferred from Development in the method of earthquake the instantaneous plate motion has been found location (e.g., master-event method, relative- for some plate boundaries. This discrepancy location method, joint-hypocenter determination) suggests that either a large part of plate allowed a very high resolution analysis of the motion at plate boundaries is aseismic or the geometry of various seismic zones, in particular current plate motion is substantially different Benioff zones [Billington and Isacks, 1975; from that inferred from plate tectonics. For Barazangi and Isacks, 1976; Engdahl, 1977; several earthquakes, evidence has been presented Isacks and Barazangi, 1977; Cardwell and Isacks, for the existence of large amounts of aseismic 1978]. One of the significant results is the motion suggesting that slip at plate boundaries discovery of lateral segmentation of Benioff does involve aseismic motion [Kanamori and zones in South America. Isacks and Barazangi Anderson, 1975; Thatcher, 1975; Sacks et al., found a remarkable correlation between a 1978; Kanamori and Stewart, 1979]. On the nearly flat Benioff zone beneath South America other hand, hang-up of plate motion due to and the absence of Quaternary volcanoes. subduction of buoyant oceanic lithospheres James [1978] proposed an alternative model has been suggested to account for the lack of in which the Nazca plate dips at an angle of great earthquakes along some plate boundaries about 30 ø . This model is based on the depth (e.g., the Marianas) [Kelleher and McCann, of the upper boundary of the descending plate 1976; Vogt et al., 1976]. inferred from converted seismic phases and The relation between large earthquakes and on the anelasticity structure of central Peru various global processes such as rotation of [Snoke et al., 1977; Sacks, 1977]. James the earth, the earth's polar motion including argued that whether the Benioff zone beneath Chandler wobble and transient plate motions central Peru is flat or not is not yet resolved. has been a subject of vigorous research Mechanism studies of great e•rthquakes [Anderson, 1975; Press and Briggs, 1975; suggest that truly great earthquakes occur only O'Connell and Dziewonski, 1976; Smith, 1977]. in subduction zones without actively opening It appears that the effect of earthquakes, back-arc basins while subduction zones with with their seismic component alone is too active back-arc basins are completely lacking small to affect the earth's polar motion in great earthquakes. Uyeda and Kanamori significantly. Wilson and Haubrich [1977] [1979] interpreted this correlation in terms concludes that the earthquake portion of the of either a difference in the degree of Chandler wobble excitation is smaller than intra-plate coupling or a difference in the 25% while the contribution of meteorological velocity of the landward plate. variation appears to be not less than 25%. However, in view of the possible aseismic Acknowledgments. The study was supported by component of earthquake deformation, a causal Grant No. EAR78-11973 from the Earth Sciences relation between earthquakes and Chandler Division, National Science Foundation. Contri- wobble should not be completely dismissed bution Number 3227, Division of Geological and [O'Connell and Dziewonski, 1976]. Planetary Sciences, California Institute of The response of the viscous asthenosphere Technology, Pasadena, California 91125. to the transient displacement caused by a large earthquake has been studied in order to determine the constitutive law for the REFERENCES asthenosphere and its effective viscosity [Anderson, 1975; Melosh, 1976; Savage and Source Mechanism Studies Prescott, 1978; Thatcher and Rundle, 1979]. Anderson [1975] used a simple model of an Bhattacharya, B., and N. N. Biswas, implicat- elastic lithospheric plate riding on a ions of north Pacific plate tectonics in viscous asthenosphere to show that most of central Alaska: Focal mechanisms of earth- the plate motions near plate boundaries occur quakes, Tectonophysics., 53, 99-130, 1979. during short periods of time after a large Buland, R. and F. Gilbert, Matched filtering decoupling earthquake. This accelerated for the seismic moment tensor, Geophys. Res. plate motion may have a significant effect Lett., 3, 205-206, 1976. on the excitation of Chandler wobble. Burdick, L. J., and G. R. Mellman, Inversion of Melosh [1976] used a migration pattern of the body waves from the Borrego mountain the aftershocks of the 1965 Rat Is. earthquake earthquake to the source mechanism, Seismol. to determine the constitutive law for the Soc. Am., Bull..., 66, 1485-1499, 1976. asthenosphere and concluded that a nonlinear Butler, R. G. S. Stewart, and H. Kanamori, The constitutive relation & (strain rate) • July 27, 1976 Tangshan, China earthquake- o(stress) n (n = 3 to 4) is most appropriate. A complex sequence of intraplate events, However, Savage and Prescott [1978]