We report tectonomagnetic results from the epicentral area of the 27 September 2003 Ms=7.5 earthquake in the
Altay mountains, southern West Siberia. The results were obtained in the first three survey campaigns in early
October 2003, late November 2003, and latest May-early June 2004 using an array of one continuous-measurement
station and fifteen repeated-survey sites. Changes in total geomagnetic field associated with seismic activity
were mostly of regional scale and usually did not exceed 1.5 nT. The M=4.5 aftershock of 31 May 2004 was
accompanied by an anomaly of 1-2 nT. The location of the anomaly within highly magnetic rocks can be evidence
of its piezomagnetic origin

Djadkov, P. G.

Mikheev, O. A.

Minenko, M. I.

Sobolev, O. A.

Earth-prints Repository

31
ANNALS  OF  GEOPHYSICS, VOL.  50, N.  1, February  2007
Key  words tectonomagnetic monitorng – piezo-
magnetic model – aftershock process – stress change
1. Introduction
Tectonomagnetic monitoring of ongoing geo-
dynamic and seismic activity is based mainly on
the piezomagnetic effect of rock magnetization
variations caused by changes in crustal stress. 
Many laboratory (e.g., Kapitsa, 1955; Naga-
ta, 1969; Kean et al., 1976) and modeling (e.g.,
Stacey, 1965; Hao et al., 1982; Sasai, 1983) da-
ta show that natural processes accompanied by
stress changes influence the surface geomagnet-
ic field and can generate tectonomagnetic anom-
alies which are most often as low as a few nT. 
Tectonomagnetic anomalies can be ampli-
fied by space variations in rock magnetization
associated with static magnetic anomalies (Fo-
tiady et al., 1969; Zlotnicki and Cornet, 1986;
Dyadkov, 1987; Oshiman, 1990). More evi-
dence on this point comes from field experi-
ments near Lake Baikal (Dyadkov et al., 1999).
Another mechanism proposed to explain
tectonomagnetic anomalies is based on the circu-
lation of electrical currents as a result of variable
potentials induced by fluids percolating through
solid rocks (Mizutani et al., 1976; Zlotnicki and
Le Mouel, 1990). However, the electrokinetic ef-
fects must be weaker than the piezomagnetic ef-
fects in normal geodynamic conditions (Kormilt-
sev and Retushniak, 1997). 
Detection of low tectonomagnetic anom-
alies requires high-precision and stable instru-
ments and special processing (filtering) tech-
niques to separate the signal from noise (Davis
et al., 1981; Dyadkov, 1985). 
At present, tectonomagnetic monitoring has
been successfully practised in many geodynamic
provinces (e.g., Rikitake et al., 1980; Abdulla
Some results of tectonomagnetic
monitoring in the epicentral zone 
of the M7.5 Altay earthquake,
September 27, 2003
Pyotr G. Djadkov, Oleg A. Mikheev, Maxim I. Minenko and Oleg A. Sobolev
Institute of Geophysics, Siberian Branch of the Russian Academy of Sciences (SB RAS), Novosibirsk, Russia
Abstract
We report tectonomagnetic results from the epicentral area of the 27 September 2003 Ms=7.5 earthquake in the
Altay mountains, southern West Siberia. The results were obtained in the first three survey campaigns in early
October 2003, late November 2003, and latest May-early June 2004 using an array of one continuous-measure-
ment station and fifteen repeated-survey sites. Changes in total geomagnetic field associated with seismic activ-
ity were mostly of regional scale and usually did not exceed 1.5 nT. The M=4.5 aftershock of 31 May 2004 was
accompanied by an anomaly of 1-2 nT. The location of the anomaly within highly magnetic rocks can be evi-
dence of its piezomagnetic origin. 
Mailing address: Dr. Pyotr G. Djadkov, Institute of
Geophysics, Siberian Branch of the Russian Academy of
Sciences (SB RAS), Russian Federation, Koptuyga prosp.
3, 630090 Novosibirsk, Russia; e-mail: djad@uiggm.nsc.ru
32
Pyotr G. Djadkov, Oleg A. Mikheev, Maxim A. Minenko and Oleg A. Sobolev
bekov, 1987; Johnston, 1989; Zlotnicki and Le
Mouel, 1990; Kuznetsova and Maksimchuk,
1994; Shapiro et al., 1994; Dyadkov et al., 1999;
Meloni et al., 2004).
We applied the tectonomagnetic method to
monitor the postseismic behavior of crustal
stress in the epicentral area of a large continental
earthquake. The monitoring results have essen-
tial implications for relaxation of crustal stress
after a large earthquake and the physics of tec-
tonomagnetic anomalies. 
2. Earthquake parameters and aftershocks
The Ms=7.5 Altay earthquake occurred on
27 September 2003 in the Altay mountains in
southern West Siberia (fig. 1). 
The shock of 27 September 2003 came unex-
pectedly, though prehistoric fault scarps of large
earthquakes in the region have been known to
seismologists. The last large historic event in the
vicinity of the Altay shock dates back to 1761 
or about 250 years ago (the M ∼ 8 Mongolian
earthquake). 
The Altay earthquake shows a right-lateral
strike-slip mechanism (Emanov and Leskova,
2005). The slip plane strikes in the NW direc-
tion and is traceable on the surface for a dis-
tance of up to 60 km. The depth of the main
shock is 15-18 km. Two Ms=6.5 and Ms=6.9 af-
tershocks followed in four days after the main
shock. The total length of the aftershock zone
reaches ∼80 km (fig. 1). 
3. Monitoring array and methods 
The monitoring array was deployed 5-8
days after the main shock, from 2 to 5 October
Fig. 1. Array of repeated-survey tectonomagnetic stations (triangles) and epicenters of M>3 aftershocks of the
M=7.5 Altay earthquake of 27 September 2003 (circles). Triangles show repeated-survey sites and boxed trian-
gle shows the continuous station. Double white line delineates the fault scarp produced by the Altay earthquake.
Inset shows location map of study area.
33
Some results of tectonomagnetic monitoring in the epicentral zone of the M7.5 Altay earthquake, September 27, 2003
2003, and included 15 repeated-survey sites and
a reference station Aktash for continuous total
field monitoring (fig. 1). In the period from 25
to 28 May 2004 additional repeated-survey sta-
tions (Turbaza, Medved’ and Kuk-slez) were in-
stalled in the northwestern part of the after-
shock area. The position of the Aktash continu-
ous station was chosen in a site of a permanent
seismic station of the regional network.
Total field intensity at the repeated-survey
stations was measured by Kazgeofizpribor
MM-61 proton magnetometer with a sensitivi-
ty of 0.1 nT, a long-term stability of ∼0.2 nT
and a thermal drift under ∼0.03 nT per 10°. 
The continuous station was equipped with
Geomer MV-08 total field proton magnetome-
ter of 0.01 nT sensitivity, a long-term stability
of ∼0.2 nT, and a thermal drift under ∼0.01 nT
per 10°.
In each repeated-survey site, the sensor was
positioned to an accuracy of a few millimeters
and then the magnetic field was measured at a
sampling interval of 20 s in sessions of 5-10
samples each, simultaneously with sampling at
the continuous station. The measurements were
run normally in the night time when diurnal
magnetic fluctuations are the lowest.
Note that magnetic surveys in the Altai
mountains are run in favorable conditions due
to the absence of cultural noise and low anom-
alous field in most sites. 
The time-dependent transient magnetic field
component due to ionosperic and magnetos-
pheric currents was eliminated by calculating
Fig. 2a,b.  Total field variations in the epicenter of the main shock of 27/09/2003 a) from early October to late
November 2003 and b) from late November 2003 to late May 2004. Triangles show repeated-survey sites and
boxed triangle shows the continuous station.
a
b
34
Pyotr G. Djadkov, Oleg A. Mikheev, Maxim A. Minenko and Oleg A. Sobolev
the difference between synchronous magnetic
field readings at the repeated-survey sites and at
the Aktash reference station. The standard devi-
ation of such differences for a session of 5-10
samples was as a rule below 0.1 nT. 
We estimated the space gradient of secular
geomagnetic variations for the territory of our
tectonomagnetic network using the Internation-
al Geomagnetic Reference Field (IGRF) model
(McGuire, 2004). The gradient found as the dif-
ference of values between stations of Koshagach
(extreme southeastern point) and Medved’ (ex-
treme northwestern point) was negligible (within
0.03 nT) through 2003-2004, though this esti-
mate appears not very reliable because of the
scarcity of the network of magnetic observatories
in this region of Asia. 
Repeated surveys at the fifteen stations were
carried out in three campaigns over the period
from October 2003 to June 2004: from 2 to 5 Oc-
tober 2003, or 5-8 days after the main shock; from
18 to 22 November 2003, about 50 days after the
main shock; and from 25 to 28 May 2004. Survey
was repeated again after the Ms=4.5 aftershock of
31 May 2004 at some sites in its vicinity. 
4. Observations 
Total field changes over the period from Oc-
tober 2003 to November 2003. Figure 2a shows
total field changes between the first and second
campaigns. Differential change relative to the
Aktash data was within −1 nT to +0.6 nT. The
strongest negative anomaly was measured in
the epicenter of the main shock and positive
values cluster near the northwestern termina-
tion of the aftershock area. 
Total field changes over the period from No-
vember 2003 to May 2004. An anomaly up to 2
nT emerged in the northwestern part of the af-
tershock area (fig. 2b), with its maximum near
site Rudnik. New sites of Turbaza, Medved’ and
Kuk-slez were installed during the third cam-
paign on 25-28 May 2004 and a repeated sur-
vey was planned at Rudnik and at the neighbor-
ing sites after the anomaly was detected on 25-
28 May 2004. However, an M=4.5 aftershock
had occurred on 31 May 2004 before the re-
peated survey was set up. 
Total field changes associated with the 31
May 2004 aftershock. A repeated survey was
set up on 2-3 June 2004 at nine sites, including
the new sites of Turbaza, Medved’ and Kuk-
slez, in the region of the 31 May 2004 after-
shock to detect the related anomalies. Total
field intensity was monitored over a short peri-
od from latest May to earliest June 2004 and
showed a prominent anomaly of 2 nT (fig. 3a) a
few kilometers northeast of the aftershock. 
5. Discussion and conclusions
Interpretation of regional-scale magnetic
variations from October to November 2003
Fig. 3a,b. a) Tectonomagnetic anomaly and b) anom-
alous magnetic field in the epicenter area of the M=4.5
aftershock of 31 May 2004 (asterisk). Triangles show
repeated-survey sites and boxed triangle shows the
continuous station.
a
b
35
Some results of tectonomagnetic monitoring in the epicentral zone of the M7.5 Altay earthquake, September 27, 2003
(fig. 2a) is challenging because rock magnetiza-
tion, piezomagnetic coefficients, and the crustal
structure in the central part of the region remain
poorly explored. Moreover, low values of static
magnetic anomalies Fa indicate low magnetiza-
tion in the upper crust. The 1 nT negative anom-
aly near the main shock requires further inves-
tigation and modeling. 
Anomalies of piezomagnetic origin are
marked by their spatial association with static
anomalies generated by highly magnetic rocks.
We observed this association in the case of the
tectonomagnetic anomaly related to the M=4.5
aftershock of 31 May 2004 (Fig. 3a,b). Stations
Rudnik, Badan, and Turbaza fall within the edge
of the static anomaly Fa produced by high-mag-
netization rocks (see the anomalous field Fa in
fig. 3b). Thus, the aftershock-related anomaly
may be a piezomagnetic one. 
Stress change presumably responsible for
the observed tectonomagnetic anomalies was
estimated by forward modeling using a soft-
ware for 3D imaging of heterogeneous magne-
tization and piezomagnetic patterns (Sobolev
and Djadkov, 2003). Finite-element solutions
for piezomagnetic anomalies were obtained
based on the formalism from (Hao et al., 1982).
Following Hao et al. (1982) we assumed that
magnetization change in stress-normal direc-
tion is half the magnitude of change in stress-
parallel direction and is of the opposite sign. 
The piezomagnetic anomalies associated
with the magnetic field change after the 31 May
2004 aftershock were computed assuming near-
ly horizontal principal extension stress in the N-
S direction. We attribute this extension to the
aftershock which released the regional com-
pression stress due to the India/Eurasia colli-
sion and related northward indentation of India.
An extension environment was likewise in-
ferred from the focal mechanisms of the main
shock, the largest aftershocks (Emanov and
Leskova, 2005) and the aftershock of 31 May
2004 (Leskova, pers. comm.). Modeling was
performed for an input magnetic susceptibility
in the region of highly magnetic rocks of 0.024
SI and input piezomagnetic coefficients of
1×10−3 MPa−1. The change in horizontal stress
that can generate the recorded anomalies in the
input conditions is within 2-4 MPa. 
Figure 4a,b presents a version of a 3D piezo-
magnetic model for the crustal block within the
territory of fig. 3b. The anomaly-producing body
(dark part of the finite-element grid in fig. 4a) has
a magnetic susceptibility of 0.024 SI. It is located
at depths between 1 km and 15 km and is as-
sumed to keep its geometry invariable in the ver-
tical dimension. The remaining part of the block
is nonmagnetic (white part of the finite-element
grid in fig. 4a). The components of the total field
F are 55000 nT (Z), 20000 (X) and 0 nT (Y). The
change in N-S horizontal stress (uniform exten-
sion) is assumed to be 4 MPa. The calculated
piezomagnetic anomaly is shown in fig. 4b.
The computed (fig. 4b) and measured (fig.
3a) features generally agree and the sites of Tur-
Fig. 4a,b. A version of a 3D piezomagnetic model
for the crustal block within the territory of fig. 3b. Fi-
nite-element grid (a) with the anomaly-producing
body of a magnetic susceptibility of 0.024 SI shown
by dark and computed piezomagnetic anomaly asso-
ciated with the 31 May 2004 aftershock, M=4.5 (b). 
a
b
36
Pyotr G. Djadkov, Oleg A. Mikheev, Maxim A. Minenko and Oleg A. Sobolev
baza and Badan fall into the computed anomaly,
but the model predicts a positive anomaly in the
Rudnik site contrary to the pattern in fig. 3a. This
discrepancy may be due to a wrong assumption
of the subsurface geometry of high-magnetic
rocks and the too rough approximation of uni-
form stress change. Further improvement of the
model requires including stress data based on a
3D dislocation model of the aftershock. 
The piezomagnetic origin of the tectono-
magnetic anomaly near the aftershock of 31
May 2004 appears less ambiguous due to the
spatial association of the anomaly with highly
magnetic rocks. The anomaly between October
and November 2003 (fig. 2a) occurred in the
zone where rocks (at least in the shallow sub-
surface) are low magnetic, and interpretation of
its origin requires further studies.
Acknowledgements
The study was supported by grant 03 05
65418 from the Russian Foundation for Basic
Research, a grant from the Integration Projects
16, 87, 6.18 of the Siberian Branch of the Russ-
ian Academy of Sciences and a grant for Project
3 of Program 16 of the Russian Academy of
Sciences.
We greatly appreciate the constructive criti-
cism of the reviewers Dr. J. Zlotnicki and Dr. Y.
Tanaka.
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Some results of tectonomagnetic monitoring in the epicentral zone of the M7.5 Altay earthquake, September 27, 2003

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Some results of tectonomagnetic monitoring in the epicentral zone of the M7.5 Altay earthquake, September 27, 2003

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