Geography Reference
In-Depth Information
magnetic anomalies, etc.). To date, the Earth's Impact Structure Catalog
(EISC) has been one of the most complete published catalogs of this kind, with
2141 records. It is currently being used by many researchers and is open for
updating. It is convenient to study the space patterns of impact structures and
to analyze their parameters with an independent version of the control and
visualization system (the EISC system). By applying the EEDB software to the
EISC catalog, one can select a sample of impact structures from the original
Catalog according to different parameters (diameter, validity, etc.) and
working areas of different scales from global to local maps, and then obtain
the related cartographic information, including the locations of impact craters
(Figure 23b), geophysical and geological layers, etc. In addition to the map
visualization, the system can list the catalog in the text format, plot different
parameters, and show results of statistical data processing. The mathematical
support and the software of the EISC system allow plotting frequency
distributions of crater diameters (logarithmically proportional to impact
energy) for events from various samples, as well as different distributions of
integrated parameters with time, space, and with respect to one another. The
curves shown in Figure 24 image log-log frequency distributions of craters of
different diameters and rms deviation of the random distributions from the
regression line (variance
S
). The curve in Figure 24a shows abrupt changes at
lg
D
= 0.7 (
D
~5 km). When the curves for
D
> 5 km, 1 km <
D
< 5 km and
D
<
1 km are plotted separately, irregular distributions appear in two latter plots. It
means that the data available for
D
< 5 km craters is incomplete, because the
ancient surface structures may have been modified by erosion or sedimentation
or, they may be poorly studied, as high-resolution surveys cover only a lesser
part of the globe. The crater diameters are unevenly distributed in time (Figure
25), with age constraints missing for a half of events (50% of craters), which
all are tentatively placed in the end of the time scale. The curves of Figure 26
show progressive growth in the number of discovered craters, the number of
really discovered structures departing markedly from the exponential
distribution
N
~e
2.83+0.12·
t
predicted in the 1970s [60]. The updated time
dependence of the number of real discoveries is rather nonlinear quadratic,
obtained based on analysis of EISC catalogs:
N
~10·
t
2
+13·
t
-11.
The published catalog has been used also to plot the size of craters against
their age, which has implications for the relaxation time of impact structures.
This plot is similar to the one published in [60] and [61] and is not shown here.
