Geology Reference
In-Depth Information
obtained by Ward (
1998
) was that the average
release of seismic moment was 73-86 % of the
average geodetic moment rate in California and
70 % outside this state. Given the unlikelihood
of a high rate of aseismic creep, he concluded
that the earthquake catalogues failed to be repre-
sentative of the long-term deformation. However,
in the next section we shall see that the correct
application of Kostrov's formula to the determi-
nation of the seismic deformation requires some
caution, because the Gutenberg-Richter law (
10.
seismic moment) demand a rigorous statistical
analysis in the estimation of annual seismic rates.
Similarly, using
observed
geologic slip rates for
estimating the annual rate of seismic moment re-
lease through (
11.20
) should require an in-depth
analysis of the statistical properties of the seismic
slip data set. Therefore, the correct application of
Kostrov's and Brune's formulae to the analysis of
lithospheric deformation is still an open problem
that demands further studies.
Figure
11.4
shows an example of subducting slab
cross-section as outlined by its seismicity.
Seismicity of Wadati-Benioff zones can be
divided in three categories. At shallow depths,
up to
80 km, slip of the bending lithosphere
beneath the accretionary wedge produces very
strong events that may have devastating effects
on the people living in the forearc region. An ex-
ample is the large Sumatra-Andaman earthquake
of 26 December 2004, which had magnitude
M
w
D
9.1 and was accompanied by a tsunami that
caused more than 283,000 deaths (for a general
description of this event, see Lay et al.
2005
).
Figure
11.5
illustrates the typical geological set-
ting and focal mechanism associated with this
class of events.
The other two sources of seismicity are associ-
ated with intra-plate earthquakes that reflect two
modes of slab deformation. The first of them is
referred to as downdip deformation and can be
either
downdip shortening
or
downdip extension
.
In both cases, the
P
or
T
axes are parallel to the
local slab dip and the fault strike is approximately
normal to the local downdip direction. Isacks
and Molnar (
1971
) were the first to perform
a systematic study of this kind of seismicity,
with the objective of studying the distribution
of stresses within the sinking lithosphere. They
observed that there were remarkable systematic
differences between intermediate-depth (between
70 and 300 km) and deep solutions. The deep
earthquakes were dominated by down-dip com-
pression, whereas the intermediate-depth solu-
tions showed a variety of focal mechanisms, in-
cluding downdip shortening, downdip extension,
and many solutions that could not be classified as
downdip deformation. Isacks and Molnar (
1971
)
suggested that this distribution of seismicity was
a consequence of the gravitational instability of
the lithosphere, which is more dense than the
surrounding mantle and sinks into the astheno-
sphere under its own weight. Therefore, slabs are
subject to downdip extension when they pene-
trate the rheologically weak asthenosphere, and
downdip compression when they reach regions of
the upper mantle characterized by higher strength
(hence resistance to penetration), in particular the
transition zone.
11.4 Bending of Slabs
Seismicity supplies an important source of data
for the study of the geometry and deformation of
subducting slabs. As pointed out by Isacks et al.
(
1968
) in a seminal paper, almost all the global
seismicity in the deep and intermediate range,
and most of the world's shallow earthquakes are
associated with island arcs or arc-like structures.
Oliver and Isacks (
1967
), in a study of the Fiji-
Tonga region, were among the first to recognize
the existence in the upper mantle of an anomalous
zone whose thickness was of the order of 100 km
and whose upper surface was characterized by
elevated seismicity up to depths of about 700 km.
The zone was anomalous in that attenuation of
seismic waves was low and seismic velocities
were high relative to those of the surrounding
asthenosphere. Subsequent studies confirmed the
existence of such anomalous regions beneath all
the world's island arcs, and they are today known
as
Wadati
-
Benioff
zones. These zones delineate
the geometry of slabs sinking into the mantle
as a consequence of their negative buoyancy.
