A comprehensive study of the observations of seismology provides widely based strong support for the new global tectonics which is founded on the hyptheses of continental drift, sea-floor spreading, transform faults, and underthrusting of the lithosphere at island arcs. Although further developments will be required to explain certain part of the seismological data, at present within the entire field of seismology there appear to be no serious obstacles to the new tectonics. Seismic phenomena are generally explained as the result of interactions and other processes at or near the edges of a few large mobile plates of lithosphere that spread apart at the ocean ridges where new surficial materials arise, slide past one another along the large strike-slip faults, and converge at the island arcs and arc-like structures where surficial materials descend. Study of world seismicity shows that most earthquakes are confined to narrow continuous belts that bound large stable areas. In the zones of divergence and strike-slip motion, the activity is moderate and shallow and consistent with the transform fault hypothesis; in the zones of convergence, activity is normally at shallow depths and includes intermediate and deep shocks that grossly define the present configuration of the down-going slabs of lithosphere. Seismic data on focal mechanisms give the relative direction of motion of adjoining plates of lithosphere throughout the active belts. The focal mechanisms of about a hundred widely distributed shocks give relative motions that agree remarkably well with Le Pichon's simplified model in which relative motions of six large, rigid blocks of lithosphere covering the entire earth were determined from magnetic and topographic data associated with the zones of divergence. In the zones of convergence the seismic data provide the only geophysical information on such movements.Two principal types of mechanisms are found for shallow earthquakes in island arcs: The extremely active zone of seismicity under the inner margin of the ocean trench is characterized by a predominance of thrust faulting, which is interpreted as the relative motion of two converging plates of lithosphere; a less active zone in the trench and on the outer wall of the trench is characterized by normal faulting and is thought to be a surficial manifestation of the abrupt bending of the down-going slab of lithosphere. Graben-like structures along the outer walls of trenches may provide a mechanism for including and transporting sediments to depth in quantities that may be very significant petrologically. Large volumes of sediments beneath the inner slopes of many trenches may correspond, at least in part, to sediments scraped from the crust and deformed in the thrusting.Simple underthrusting typical of the main zone of shallow earthquakes in island arcs does not, in general, persist at great depth. The most striking regularity in the mechanisms of intermediate and deep earthquakes in several arcs is the tendency of the compressional axis to parallel the loc...
This study presents several possible criteria for forecasting the locations of large shallow earthquakes of the near future along major plate boundaries and for assigning a crudely determined rating to those forecasts. These criteria, some of which were proposed by other investigators, are based on the past space-time pattern of large earthquakes, the lateral extent of their rupture zones, and the direction of rupture propagation. The criteria are applied in two stages. Application of the first set of these criteria to major plate boundaries along the eastern, northern, and northwestern margins of the Pacific from Chile to Japan and also to the Caribbean loop east of about 74øW results in delineation of several areas of special seismic potential along each of the boundaries. The phrase 'special seismic potential' is used in this paper only to indicate those segments of plate margins that fulfill certain specific criteria. However, if the criteria are valid, at least some and perhaps most large shallow earthquakes of the near future within the zones examined will occur near these locations. At present the validity of the criteria is not firmly established, and profound social changes based on these predictions are uncalled for, but the forecast presented here can, at the very least, serve as a guide in selecting areas for intensive study and instrumentation prior to the occurrence of a major earthquake. The criteria have greater usefulness in those regions where the rupture zones of large earthquakes have nearly covered the seismic zone in recent years and only a few gaps remain. In certain areas where additional information is available the subsequent application of a second set of supplementary criteria focuses special attention on certain of the areas delimited by the first set of criteria. Rupture zones of large shallow earthquakes as determined from aftershock locations, intensities, tsunami descriptions, and coastal uplift tend to cover plate boundaries without significant overlap. Because of this tendency to fill in the plate boundaries and because large earthquakes account for such a high percentage of the total seismic mpment released in a seismic zone, segments of the zone that have not experienced a large earthquake recently are likely locations for future shocks. Along the Kurile-Kamchatka area, the Alaska-Aleutian arc, and much of western South America, the 'large' earthquakes tend to be great earthquakes with rupture zones sometimes extending many hundreds of kilometers. Along western Middle America, however, the large earthquakes have rupture zones no larger than 100-200 km. Although great earthquakes with ruptures hundreds of kilometers in length have occurred along some segments of the Caribbean loop, certain other segments have no known history of great earthquakes. Thus there is no recognizable class of large earthquakes around the Caribbean loop. Nearly all the extensive parts of the plate boundaries examined in this study tended to be covered by rupture zones of large shallow earthquakes. Ther...
This study demonstrates the existence and determines the pattern of lateral variations of attenuation in the uppermost mantle on a worldwide scale. The evidence comes mostly from a comparison of the gross characteristics of the seismic phase Sn for over 1500 paths, which taken together cross as many regions of the earth as possible with the current configuration of the World‐Wide Standardized Seismograph Network. Sn is a seismic shear wave that propagates in the uppermost mantle and that does not penetrate the low‐velocity channel. It propagates very efficiently across the stable regions of the earth, the continental shields, and deep‐ocean basins, but propagation is very inefficient when paths cross the crests of the mid‐ocean ridge system or the concave sides of most island arcs. These observations suggest that attenuation is more pronounced in the uppermost mantle near the ridge crests and the islands arcs than in the more stable regions. If low attenuation, or high Q, correlates with high strength, the data imply that the uppermost mantle is considerably weaker under the ridge crests and the concave sides of the island arcs than it is elsewhere. Thus, the part of the strong outer shell, or lithosphere, in the mantle is discontinuous with gaps in it at the ridges and island arcs. The low attenuation for Sn for paths crossing the transform faults that connect ridge crests suggests that any gap at the transform faults is very narrow. In addition, S waves recorded at stations in and near the Mariana, New Britain, Solomon, and South American arcs from local deep shocks were found to follow the pattern previously observed in the Tongan and Japanese arcs. As was suggested for the Tongan arc, the observations imply that one piece of lithosphere has underthrust another to great depth at these arcs. Hence, data presented in this paper are in accord with recent ideas of sea‐floor spreading in which large plates of lithosphere move with respect to each other as rigid bodies, spreading apart at the ocean ridges, sliding past one another at the transform faults, and underthrusting at the island arcs.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.