Geography Reference
In-Depth Information
projection of stereoplots in the latter case), using multi-layer coloring (in XOR
mode) of asymmetrical quadrants calculated from parameters in catalogs of
focal mechanisms; plotting focal mechanisms in the vector mode; selecting
profiles and creating a relief profile and a cross section of seismicity in a
selected visualization mode; loading whatever external raster map, as a
background, into the EEDB environment; recognizing linear structures by a set
of points distributed in space; etc. All the examples were shown using the
functions from the first column of the flow chart (Figure 15), including the
detection of aftershocks.
The functions of
detecting aftershocks
implemented in EEDB are a set of
different methods (empirical, elliptic, and interactive) and various
modifications of the elliptic method [3, 24]. The efficiency of different
algorithms for detecting aftershocks was evaluated by estimating the statistics
remaining after filtering and comparing it with random Poisson's exponential
distribution. After removal of aftershocks by the elliptic method, only 14% of
earthquakes have remained in the study area. We have estimated the number
of aftershocks (using, also, the plot of Figure 18b) to be a few hundreds of
thousands (303,640 shocks), and the aftershock process to continue, as the
seismic background has not reached the average level observed before the
event. Thus, we can draw an important conclusion that the greater part of
earthquakes in this area belongs to triggered seismicity, rather than being
independent; specifically, they are sequences of events induced by the Tohoku
earthquake. Moreover, the spatial distribution of the Tohoku aftershocks
(Figure 18a) has implications for the size of the active zone. Therefore, the
territory adjacent to the Fukushima Prefecture can be considered a part of the
nucleation area of the Tohoku event, which is suitable to retrospective search
of indicators for earthquake prediction. Applying the procedure of removing
earthquake swarms and aftershocks brings the seismicity parameters to the
stationary baseline and thus significantly improves the quality of processing.
Studying Seismicity Parameters Using the Analysis Subsystem of
Fukushima-EEDB
Investigation into seismicity anomalies can have different applications in
seismic and geodynamic research. In this Part, we show how the analysis
subsystem of Fukushima-EEDB is used for retrospective study of anomalies
preceding a great earthquake, e.g., the Tohoku
event. In addition to outlining
