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region under study consists of rigid (seismically inactive) or soft structures
(Figure 37). Such comparison is necessary both to model a detailed crust
structure by detecting seismic block boundaries or individual faults, and to
study regional seismicity.
Another example shows how a gravity map can be used to detect the
pattern of aseismic or weak seismic zones (Figures 38a), which have
implications for interaction of rigid tectonic structures with orogens. This
interaction was visualized using a special EEDB method suggested by P.
Dyadkov for analyzing seismicity anomalies (activity and quiescence)
proceeding from indicators of rigid zones (Figures 14, 38b-c).
Figure 38. Spatial distribution of aseismic and weak seismic zones: a)
gravity
anomalies
according to [72] and ENDDB tools (dark color shows
gravity
lows); b)
distribution of average annual total energy for 1972-1998 taken as the norm; c) maps
of aseismic areas (black) and zones of seismic quiescence (gray) in Central Asia, at
every 4 years [24]. The maps were compiled using data of the combined earthquake
catalog COMPLEX (M ≥ 4, 1972-2000, 24-46º N; 70-102º E).
In conclusion, we illustrate the application of the method to the rigid
structure previously identified (Figures 16-18, 22) by the seismicity analysis of
Japan area along 38°N. The gravity map and vertical cross section AA show a
prominent high in the epicenter of the
pending
Tohoku earthquake (Figure
39).
The examples of Figures 37, 39, as well as other evidence of relationship
between the gravity and seismicity patterns [79], show that earthquakes are
generally confined to prominent features of the gravity field. They are, for
instance, regional and local gravity gradients at boundaries between blocks
with different physical and structural properties, including active seismic
boundaries. Note that regional features of the gravity field of a deep (most
likely mantle) origin correspond to main structural elements of a zone bounded
by seismic belts [79]. This determines the prospects of our future research.
