Distribution Of Earthquakes

Plate tectonic theory predicts that the majority of the Earth's tectonic activity takes place at the margins of plates. It follows, then, that the location of earthquake epicenters can be used to define plate boundaries. Figure 5.2 shows the global distribution of the epicenters of large magnitude earthquakes for the period 1961-67 (Barazangi & Dorman, 1969). Although in terms of most geologic processes this represents only a very short period of observation, the relatively rapid motions experienced by plates generate very large numbers of earthquakes over a short interval of time.

1968 Merking Tectonic Plate

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 5.1 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).

Volcanic Mineral Distribution Worldwide

0 20 40 60 80 100 120 140 160 180 -160 -140 -120 -100 -80 -6 0 -40 -20 0

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

0 20 40 60 80 100 120 140 160 180 -160 -140 -120 -100 -80 -6 0 -40 -20 0

Figure 5.2 Worldwide distribution of epicenters of large magnitude earthquakes (mb > 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 window, is that prior to the setting up of the World Wide Standardized Seismograph Network in 1961 (Section 2.1.4), epicentral locations, particularly in oceanic areas, were very poorly determined. For a more detailed discussion of earthquake distribution see Engdahl et al. (1998).

Earthquakes are classified according to their focal depths: 0-70 km shallow focus, 70-300 km intermediate focus, greater than 300 km deep focus.

An important belt of shallow focus earthquakes follows the crest of the ocean ridge system (Fig. 5.2), where focal mechanism solutions indicate tensional events associated with plate accretion and strike-slip events where the ridges are offset by transform faults (Section 4.2.1). On land, shallow focus tensional events are also associated with rifts, including the Basin and Range Province of the western USA (Section 7.3), the East African Rift system (Section 7.2), and the Baikal Rift system.

All intermediate and deep events are associated with destructive plate margins. The northern, eastern and western Pacific Ocean is ringed by a belt of earthquakes which lie on planes, in places offset by transform faults, dipping at an angle of about 45° beneath the neighboring plates. These planes of earthquake foci, known as Benioff (or Benioff Wadati) zones, are typically associated with volcanic activity at the surface. The deepest events recorded lie at a depth of about 670 km. Collisional mountain belts such as the Alpine-Himalayan chain are similarly characterized by intermediate and deep focus earthquakes although, since there is no longer a Benioff zone present in such regions, the seismic activity occurs within a relatively broad belt (Fig. 10.17). Careful examination of epicenter locations has revealed, however, that some of the shallow events lie on arcuate strike-slip fault zones associated with the collisional event.

The intra-plate areas are relatively aseismic on this timescale, although occasionally large magnitude earthquakes do occur. Although insignificant in their release of seismic energy, intra-plate earthquakes are important as they can indicate the nature and direction of stress within plates (Section 12.7).

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How To Have A Perfect Boating Experience

How To Have A Perfect Boating Experience

Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.

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