26 Origin and Morphology of Ocean Basins
o
j . PUERTO RICO
.'"
IS.II'N 1
It'.""
altimeter residuals after
/ ' removing skylab orb~
...... .. :,-......... .
~ -.. ,--~ .,.. ~---'
lO . ~ '"
coastal area
a
o lOO ,00
K"ome/~rs
b
~
.. .
!
. " .. .
l00km
t - - ¥ -
Fig. 1.10. a Topographic profiles across trenches in various regions of the ocean. Land or island arcs
to the right. [M. Talwani , 1970, in The sea 4 [I]: 282] . b Defonnation of the sea surface measured
by satellite altimeter. Sea level is influenced by regional gravity. Trenches are well-known gravity
minimum areas. Oceanic high areas not in isostatic equilibrium show positive gravity anomalies,
therefore "anracting" sea water and thus raising sea level. [Skylab Data Catalog NASA 1974,
p. 133]. c Map of the mean ocean surface reflecting ocean bottom features such as seamounts,
trenches, ridges , and fracture zones. Land masses have been masked out. The map was developed
by the US. National Aeronautics and Space Administration (NASA), 1982. [Courtesy Jet. Prop. Lab,
Pasadena]
the Himalayas. When the depths of the quake centers are plotted below the
epicenters, they are seen to occur on planes which intersect the surface near the
trenches and dip underneath the arcs or the continents at an angle of between 15° and
75°, to a depth of about 700 km. It is reasonable to conclude that the earthquakes are
caused by friction on the upper surface of the downgoing slab of sea floor, and that
this friction ceases when the temperature becomes high enough to permit flow (Fig.
1.12).
Fig. 1.12. Relationship of plate motions to plate boundaries and earthquakes. Divergent plate hOllndary Mid-Ocean Ridge , shallow earthquakes; convergent plate bOllndary, trench, shallow, intennediate, and deep earthquakes. Lateral boundary: ji'acture zone. shallow earthquakes on active part only.
Horizontal motions are of the order of I to 10 cm/yr. [Based on a diagram by B. Isacks. J. Oliver,
and L. R. Sykes, 1968, J Geophys Res 73: 5855]
o
j . PUERTO RICO
.'"
IS.II'N 1
It'.""
altimeter residuals after
/ ' removing skylab orb~
...... .. :,-......... .
~ -.. ,--~ .,.. ~---'
lO . ~ '"
coastal area
a
o lOO ,00
K"ome/~rs
b
~
.. .
!
. " .. .
l00km
t - - ¥ -
Fig. 1.10. a Topographic profiles across trenches in various regions of the ocean. Land or island arcs
to the right. [M. Talwani , 1970, in The sea 4 [I]: 282] . b Defonnation of the sea surface measured
by satellite altimeter. Sea level is influenced by regional gravity. Trenches are well-known gravity
minimum areas. Oceanic high areas not in isostatic equilibrium show positive gravity anomalies,
therefore "anracting" sea water and thus raising sea level. [Skylab Data Catalog NASA 1974,
p. 133]. c Map of the mean ocean surface reflecting ocean bottom features such as seamounts,
trenches, ridges , and fracture zones. Land masses have been masked out. The map was developed
by the US. National Aeronautics and Space Administration (NASA), 1982. [Courtesy Jet. Prop. Lab,
Pasadena]
the Himalayas. When the depths of the quake centers are plotted below the
epicenters, they are seen to occur on planes which intersect the surface near the
trenches and dip underneath the arcs or the continents at an angle of between 15° and
75°, to a depth of about 700 km. It is reasonable to conclude that the earthquakes are
caused by friction on the upper surface of the downgoing slab of sea floor, and that
this friction ceases when the temperature becomes high enough to permit flow (Fig.
1.12).
Fig. 1.12. Relationship of plate motions to plate boundaries and earthquakes. Divergent plate hOllndary Mid-Ocean Ridge , shallow earthquakes; convergent plate bOllndary, trench, shallow, intennediate, and deep earthquakes. Lateral boundary: ji'acture zone. shallow earthquakes on active part only.
Horizontal motions are of the order of I to 10 cm/yr. [Based on a diagram by B. Isacks. J. Oliver,
and L. R. Sykes, 1968, J Geophys Res 73: 5855]