Geology Reference
In-Depth Information
Fig. 11.3
Second
invariant of the strain rate
field (After Kreemer et al.
2003
)
by a mesh of triangles. For each mesh element,
they estimated the strain rate using the seismic-
ity of Quaternary faults and Kostrov's formula.
Similarly, Corredor (
2003
) determined the seis-
mic strain rate of the northern Andes region,
while Papazachos and Kiratzi (
1992
)usedthe
same technique to study the pattern of active
deformation in central Greece. At global scale,
Kreemer et al. (
2000
, 2003) used geodetic veloc-
ities, seismic moment tensors from the Harvard
CMT catalog, and Quaternary fault slip rate data
to build a model strain rate field along the major
plate boundaries, in particular along the wide
Alpine-Hymalaian collision zone. These authors
assumed a constant shear modulus
D
35 GPa,
and seismogenic depths of 20 km for the con-
tinental areas, 30 km along subduction zones,
20 km for zones of diffuse oceanic deformation,
and 7.5 km for oceanic ridges and transforms.
Their global map, which is illustrated in Fig.
11.3
,
shows the distribution of the second invariant of
the strain rate, which is defined by:
the geodetic average rate of release of seismic
moment starting from slip rates. For a system
of
n
faults having length
L
k
and dip •
k
, we can
rewrite (
11.3
) as follows:
D
M
.k
0
E
L
k
h
sin •
k
hP
u
k
i
D
S
k
hP
u
k
iD
(11.20)
where
h
is the seismogenic thickness and the
average slip rate is estimated from a combination
of geodetic data (GPS, VLBI, etc.), regional Qua-
ternary fault slip rate data, and seismic moment
tensor information from shallow earthquakes. In-
serting this expression into Kostrov's formula
(
11.4
), Kreemer et al. (
2000
, 2003) obtained
an estimate of the
geodetic
-
seismic
strain rate
tensor:
2A
X
k
1
L
k
hP
u
k
i
sin •
k
m
.k/
ij
P
©
ij
D
(11.21)
where
A
is the area of a grid cell for which the
strain rate is estimated. In their study, Kreemer
and colleagues divided the Earth's deforming
regions into 24,500 grid cells having dimension
0.6
0.5
ı
. The possibility of using geologic and
geodetic data to estimate strain rates had already
been exploited by Ward (
1998
) in a study on
the differences between geodetic, seismic, and
geologic strain rates for the US region. This
author determined the field of maximum geodetic
strain rate through the largest eigenvalues of the
geodetic strain rate tensor. The geodetic strain
rateswereusedinturntoestimatetheaver-
age geodetic moment rates. An interesting result
h
P
©
ij
P
©
ij
.
P
©
kk
/
2
i
1
2
P
©
II
D
©
12
C
©
23
C
©
31
D
.
P
©
11
P
©
22
C
©
11
P
©
33
C
©
22
P
©
33
/
(11.19)
The total annual release of seismic moment
results to be 7.7
10
21
Nm yr
1
within the shal-
low seismogenic thickness of the lithosphere, but
17 % of this total moment rate is concentrated
in areas of diffuse deformation on continents.
In their approach, Kreemer et al. (
2000
,
2003) used Brune's formula (
11.3
) to estimate
