Page 106 - Global Tectonics

THE FRAMEWORK OF PLATE TECTONICS 93

Figure 5.1 Block diagram summarizing the principal features of plate tectonics. Arrows on lithosphere represent

relative motions. Arrows in asthenosphere may represent complementary flow in the mantle (redrawn from Isacks et
al., 1968, by permission of the American Geophysical Union. Copyright © 1968 American Geophysical Union).

Figure 5.2 Worldwide distribution of epicenters of large magnitude earthquakes (m b > 4) for the period 1961–67
(after Barazangi & Dorman, 1969, with permission from the Seismological Society of America).

The significance of 1961, as the start of this time Earthquakes are classified according to their focal

window, is that prior to the setting up of the World depths: 0–70 km shallow focus, 70–300 km intermediate
Wide Standardized Seismograph Network in 1961 focus, greater than 300 km deep focus.
(Section 2.1.4), epicentral locations, particularly in An important belt of shallow focus earthquakes
oceanic areas, were very poorly determined. For a more follows the crest of the ocean ridge system (Fig. 5.2),
detailed discussion of earthquake distribution see where focal mechanism solutions indicate tensional
Engdahl et al. (1998). events associated with plate accretion and strike-slip