Download Lithospheric Plates, Benioff Zones, and Volcanoes

RICHARD E. STOIBER | Department of Earth Sciences, Dartmouth College, Hanover, New
MICHAEL J. CARR
j Hampshire 03755
Lithospheric Plates, Benioff Zones, and Volcanoes
ABSTRACT
Sykes, 1966). For the other localities, those
compiled by the Coast and Geodetic Survey
Earthquake focus depth contours delineate for the period 1961-1969 are used. Earthundulating surfaces which may represent the quakes with focal depths of less than 50 km
tops and bottoms of lithospheric plates 50 to were excluded from consideration in Java and
75 km thick, dipping beneath island arcs. Tonga. In the Banda Sea region, focal depths
The same data can be contoured as a single less than 90 km were excluded. Some of
surface only with difficulty and with anoma- these shallow focus quakes are not very
lous points. Active volcanoes lie above axes accurately located as to depth, and, from
of undulations which plunge down the slope published delineations of Benioff zones such
of the surface.
as those oflsacksand others (1969), itappears
that these shallow earthquakes occur in a
INTRODUCTION
broad zone extending across the entire island
The relationship of the zones of deep-focus arc. The resulting subsurface contours are
earthquakes to island arcs or continental shown for the Java area in Figure 1. They
margins was pointed out by Visser (1936) portray a surface which dips under the island.
and later by Benioff (1954). In the early In several areas, there are dip reversals which
investigations of Wadati (1935) and Berlage are indicated by depression contours. The
(1937, p. 15), contour maps of the depths surface is undulating. The undulations have
of the foci of intermediate and deep focus the form of folds which plunge down the dip.
earthquakes were constructed. This type of
A few foci of differing depths underlie a
representation showed the parallelism of the single surface location. To accommodate
zones to the deep ocean trenches and the these anomalous points, a two-surface model
increasing depth of the zones away from the was constructed (Fig. 2). Almost all the earthtrenches. More recent papers have depicted quake foci are accommodated by this model
composite cross sections perpendicular to the and dip reversals perpendicular to the trench
axes of the associated trenches (Sykes and disappear. In general, one surface of this
others, 1969) • Foci from tens or even hundreds model is roughly tangent to the tops of the
of kilometers on each side of the section large folds in the single surface model. The
have been projected onto such diagrams. In second surface is roughly tangent to the
still other studies, epicenters of different depth bottoms of these large folds. The large ampliranges have been depicted on maps by dis- tude of the folds of the one surface model
tinctive symbols (Hatherton and Dickinson, may result from concentrations of earthquake
1969). The array has then been roughly con- foci which plunge down the dip alternately
toured. All of these representations average on the upper and on the lower of the two
out much of the pattern of earthquake surfaces.
distribution.
The earthquake foci may be contoured to
represent more than two surfaces. Each new
CONTOURING OF
surface will decrease the size of the folds, but
EARTHQUAKE FOCI
the surfaces will be less continuous and
The authors have recently completed denned by fewer data. One indication of the
detailed contouring of depths of earthquake number of apparent surfaces is the number
foci in several regions: Java, the Banda Sea, of sets of focal depths at any one geographic
Tonga, and Central America. The data used location.
for Tonga are those published in discussions
In the Banda Sea drawing (Fig. 3), most
of seismological studies of the area (Sykes of the data were contoured to represent two
and others, 1969; Isacks and others, 1969; surfaces. In the southeastern part of the area,
Geological Society of America Bulletin, v. 82, p. 515-522, 4 figs., February 1971
515
EARTHQUAKE EPICENTER
SUBSURFACE CONTOUR
FOLD"AXIS
Figure 1. One-surface interpretation of the seismic data for Java. Each subsurface contour is labeled in kilometers. Datum is sea level.
A
ACTIVE VOLCANO
SUBSURFACE
UPPER
~-T-"
CONTOURS
SURFACE
LOWER SURFACE
Figure 2. Two-surface interpretation of the seismic data for Java. The depth of the earthquakes at each epicenter and each subsurface contour is
labeled in kilometers. Datum is sea level.
518
STOIBER AND CARR—PLATES, BENIOFF ZONES AND VOLCANOES
SUBSURFACE COMTOURS
—*—-•
•-!. .^r^
FIRST SURFACE '
SECOND SURFACE
A ACTIVE VOLCANO
.._.- "FOLD'AXIS
THIRD SURFACE
128°
I30C
Figure 3. Three-surface interpretation of the
seismic data for the Banda Sea. Squares, diamonds, and open diamonds indicate epicentral
locations used in contouring the upper, intermediate, and lower surface, respectively. Subsurface contours are labeled in kilometers and
depth at each epicenter is indicated in kilometers. Earthquake locations were omitted
from the central area of the map near the three
volcanoes because they would severely overcrowd the figure. Datum is sea level.
14 earthquakes were recorded which occurred
at depths greater than the depths of the two
surfaces. Contours were drawn on these epicenters and these suggest a surface subparallel
to the two. The scarcity of data and the
geological complexity of the area make it
difficult to determine the significance of this
surface. It will be of interest to examine the
data farther to the east to see if this is a
continuous surface and how it relates to the
tectonics of the area adjacent to the Banda Sea.
Examination of focal data in the Tonga
area (Fig. 4) supports the two-surface representation. In several other areas examined in
preliminary fashion, including Central
America, two-surface models are also appro-
CONTOURING OF EARTHQUAKE FOCI
519
•\ TONGA
Figure 4. Two-surface interpretation of the
earthquake foci in the Tonga area. Upper surface contours are solid, lower surface contours
are dashed. Earthquake locations used in contouring upper surface are circles; those used in
contouring lower surface are triangles. Indicated earthquake depths and contour labels are
priate. The two-surface models have several
common features: the two surfaces are crenulated; the surfaces, including the crenulations,
are remarkably conformable; and both surfaces
dip more steeply at depth. We determined
the separation between surfaces for the 100to 200-km depth range in several areas. In
Central America, Java and Tonga, the intersurface distances are approximately 50, 70,
in kilometers. Depth values are omitted from
some locations to avoid overcrowding of the
figure. Data are from Isacks and others (1969,
p. 1447); Sykes and others (1969, Table 1,
p. 1098-1099, and Table 2, p. 1102-1105);
and Sykes (1966, Table 3). Datum is sea level.
and 70 km, respectively. In the Banda Sea
area, the second surface is 50 to 75 km below
the uppermost one.
It is suggested that the surfaces represent
upper and lower surfaces of a plate which is
plunging beneath the continent or island arc.
The thickness of the lithosphere has been
estimated by several methods, and these data
have been summarized recently by Kanamori
520
STOIBER AND CARR—PLATES, BENIOFF ZONES AND VOLCANOES
and Press (1970). Values of 40, 60, 65, and
70 km are quoted from these earlier studies.
From new velocity data, they conclude that
the base of the lithosphere "is at a depth not
much different from 70 km." The 50- to 75km thickness which we found for the plunging plates compares well with these estimates.
Scholtz and Page (1970) have suggested that
plunging plates would become crenulated
because of shortening. Our model fulfills
this requirement.
CONTOURED SURFACES
AND VOLCANOES
Active volcanoes border the Pacific and
occur in island arcs. Wadati (1935) and later
Benioff (1954) pointed out that zones of
deep-focus earthquakes dip under the volcanic
chains. Kuno (1966) has related the depth
of the Benioff zone below a volcano to the
chemistry of its products. Hatherton and
Dickinson (1969) specifically relate K2O content to the depth.
Contoured seismic surfaces (Figs. 1 and 3)
illustrate a relationship between the location
of active volcanoes and these zones of earthquakes. In the one-surface Java model, most
of the active volcanoes lie above the axes of
plunging undulations in the dipping surface
(Fig. l). In the Banda Sea model, this relationship is also illustrated (Fig. 3). The
volcanoes considered are those which have
erupted in historic time and are listed in the
Volcano Catalog (Neumann van Padang,
1951). The coincidence of volcano locations
with axes is not perfect, but it is sufficiently
good that a genetic relationship between
recent volcanism, modern earthquakes, and
the morphology of lithospheric plates is
strongly suggested. We suspect that volcanoes
lie either above "fold" axes in the lithospheric
plate or are related to the upper and lower
surface concentrations of foci which are expressed by the undulations in the one-surface
model. We have found similar coincidence
of volcano locations and fold axes in Central
America (Mooser and others, 1958). In Tonga
(Fig. 4), there are only small folds in the
two-surface model beneath the volcanic chain
(Richard, 1962). The undulations in the contoured surfaces are well developed only at
greater depths.
Our investigations convince us that conformable undulating surfaces can be defined
by contouring groups of earthquake focal
depths. These surfaces are interpreted as the
top and bottom of a 50- to 75-km thick
lithospheric plate. These focal data can also
be contoured as a single surface with more
marked undulations. Volcanoes seem to be
related to fold axes on these surfaces. We are
continuing our study of these surfaces as well
as similar surfaces in other areas and their
relationship to the locations, types, frequency
of eruption and chemistry of volcanoes. Of
particular interest is the possible correlation
of explosivity and depth to the lithospheric
plate.
This study was made possible by the data
from the worldwide seismograph network
operated by Environmental Science Services
Administration. It is limited by the accuracy
of depth and epicenter location. New techniques which allow more accurate relative
locations of earthquakes in limited areas
should greatly improve the definition of the
plates.
ACKNOWLEDGMENTS
We thank Charles L. Drake for his help in
the preparation of this paper, R. W. Decker
for his advice on the manuscript, and Muawia
Barazangi for providing a computer printout
of many of the data that we used.
REFERENCES CITED
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Geol. Soc. Amer., Bull., Vol. 65, p. 385400, 1954.
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Sykes, L. R.; Isacks, B.; and Oliver, J. Spatial distribution of deep and shallow earthquakes of small magnitudes in the FijiTonga region: Seismol. Soc. Amer., Bull.,
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AUGUST 21, 1970