Geography Reference
In-Depth Information
|u|
/ (2
) ≤ 0.03
(2)
Thus, if the confidence probability is 0.95 (an allowable statistical
reliability of a conclusion), the quintile
|u|
0.95
=
u
0.975
has the value 1.96. Based
on (2), one can estimate the accuracy of the obtained values of
b
. For this
purpose, one can contour earthquake numbers inside the cells in EEDB, i.e.,
draw the sample size isolines (Figure 12 A, c). So, for the 0.4×0.6º averaging
grid the error is 6-18% (32 ≤
n
≤ 256) in the axial BRZ area, but reaches 35%
(
n
= 8) outside the lake. To improve the accuracy and keep the same cell
resolution, the parameter can be calculated using an averaging window with
double overlap. The error in thus obtained estimates reduces to 4-6% (243 ≤
n
≤ 729) over the whole BRZ area (dark gray) and even to 2 % (
n
≥ 729) at some
sites (black)
An advantage of interpreting
b
cartograms is in implications for elastic
energy buildup in the crust (stress sources):
b
= 1.8 (
γ
= 1) if all potential
elastic energy is released in an earthquake and
b
= 0.9 (
γ
= 0.5) if the release
is incomplete. Thus, the
b
value has bearing on relative amount of released
elastic energy [41], and changes of the parameter in time correspond to
changes in lithospheric stress and strain [42]. Proceeding from these
assumptions, one may conclude that an increase in
b
observed, for example, in
the central BRZ during the 1996-2003 interval records stress release in the
area. The growth of average
b
values appears also in the time series (Figure
13).
Studying Spatial Seismicity Patterns with GIS EEDB
The use of GIS EEDB has revealed important features in the spatial
patterns of seismicity at the junction of orogens and stable blocks with rigid
lithospheric elements. In the case of Central Asia, for instance (Figure 14),
earthquakes tend to the East Tien-Shan and its borders with the adjacent rigid
tectonic units of Tarim and Dzungaria, both with low seismicity. The
hypocenter pattern in the vertical cross section along the A-B profile likewise
indicates a high seismic activity of the Tien-Shan against the nearly aseismic
Tarim and Dzungaria areas. The earthquake origin depths in this part of
Central Asia are within 30-40 km.
