Geography Reference
In-Depth Information
the nucleation area of the selected earthquake, it is important to estimate the
quality of the selected catalog (Figure 15).
First, one can find out how the earthquake parameters recorded by the
seismological network change in time by plotting data from the selected JMA
catalog in EEDB:
1)
The magnitude time series
M
s
(t)
shows (i) quality improvement of
seismological networks in 1977 (for
M
s
< 3) and 1987 (for
M
s
< 1.5)
evident in increased number of recorded earthquakes, 3 times on
average in each year; and (ii) stabilization of the recording quality in
the end of 1997 (for all magnitudes).
2)
The
N(t)
plot shows another step of improved recording (in 2002-
2003), and an increased number of recorded events in the curve of a
higher time resolution since mid-90s, followed by stabilization in
recording quality.
3)
The
Gutenberg-Richter N(M
s
)
curve
(least squares method) shows a
linear segment in the magnitude range
M
= 3.5-6.5 corresponding to
the K = 10-16 interval of the energy scale (Utsu's [34] maximum
likelihood thresholding methods).
Thus, we conclude that the catalog completeness is the best within the
interval of 2002-2013 while the magnitudes from 3.5 to 6.5 are the best
representative.
Seismic activity A
[28] is the first characteristic of the seismic process
explored by
the
cartographic method
, which implies calculation of contour
lines for the average values of the parameter on a regular spatial grid. The
resulting map of the parameter
A
15
shows the mean long-term trend of seismic
activity normalized to K = 15 (
M
s
~ 5.5), obtained from statistically uniform
averaged seismicity data. The map (Figure 19) shows the peaks of the long-
term seismic activity for the past 20 years in the area of Fukushima to follow
the coastal line of the Fukushima Prefecture with a maximum near Hitachi
City.
To visualize the spatial distribution of the
parameter b,
another
cartographic method is used: mapping parameter changes at uniform time
intervals. In the ―fill‖ mode of contour line visualization, the program
performs spatial interpolation using the 2D Bessel function (1). The map
(Figure 12 B, a) shows a poorly pronounced zone of concentrated negative
anomalies which originates in the epicentral area of the pending Tohoku event
and strikes along 38°N in 2007 through 2011 before the main shock.
