Geography Reference
In-Depth Information
the Altai territory), with energy classes
K = lgE
determined by T. Rautian's
technique [27]. To bring all data to the universal scale, we convert energy
classes to surface-wave magnitudes (
Ms
) using Richter's formula
K
=
4.8+1.5
·Ms
(for the global catalogs) and its variants
K
= 4.0+1.8
·Ms
for
K
<14
and
K
=8+1.1
·Ms
for
K
>=14 (applied to regional catalogs, specifically, to
events in West and East Siberia).
Ms
is recalculated from
mb
as
Ms = (mb -
2.4)/0.5556
, which is an empirical ratio, obtained from known magnitude
pairs. The length of seismic rupture (
L)
is found as
lg L = aK +c
, where
a
and
c
are empirical constants [28].
The seismological unit also includes preprocessing of the initial catalog
data to select a subset of earthquakes according to the inquiry parameters:
choice of a current catalog, time range, space range, magnitudes, etc.
Furthermore, the users can filter the selected earthquakes from aftershocks,
with three independent algorithms. The first algorithm (tentatively named a
statistical
algorithm) is based on parameters responsible for the space-time
difference between aftershocks and the main shock
(dT
and
dS)
, which have
been obtained from the available aftershock statistics and depend on the main
shock magnitude:
dT
= (
M
s
main
4) 162;
dS
= 3*
L
.
The second (
elliptical
) algorithm (Figure 6), most frequently used to
remove aftershocks, consists of several runs:
1)
run 1: estimating the density of non-aftershock events (aftershocks are
removed according to the statistically found parameters);
2)
run 2: preliminary removal of aftershocks on a rectangular grid with
the cell size proportional to the main shock magnitude;
3)
plotting an aftershock ellipse isolating the aftershocks by the
maximum-likelihood method or according to rms deviation from the
sampling center.
4)
subsequent runs: separating aftershocks level-by-level, in the elliptic
metric.
At Steps 2 and 4, the time window (
dT
) of aftershock search increases
proportionally to the ratio of the current number of aftershocks to the total
number of events within rectangular or the elliptic areas [29].
Prozorov's method has been modified by A. Mikheeva as follows:
•
all located aftershock sequences are considered simultaneously in a
single run,
•
the minimum size of the rectangular metric is set interactively,
