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confident correction of static shifts, (2) The interpretation is conducted in a narrow
model class bounded by the limits of a hypothesis being tested. So, the inversion
stability is enhanced, but its resolution is decreased.
These features were clearly demonstrated in the course of interpreting the MT
soundings conducted in the Baikal rift zone. We avoided the uncertainties that come
about from corrections of static shifts. We obtained rather reliable evidence in favor
of the mantle diapir model. However, we missed many details in the structure of
the Baikal rift. Their examination requires other approaches and a better quality of
field data (wide frequency range of MT-variations, integrated MV and MT studies,
robust processing techniques, considerably denser observations). But what we have
to stress is that magnetotellurics due to its unique possibilities settled the long-term
discussion about the deep structure of the Baikal rift.
12.7 Geoelectric Model of the Cascadia Subduction Zone
In 1978, on the initiative of Vanyan, a global geoelectric project was organized
under the auspices of the International Association of Geomagnetism and Aeronomy
(IAGA) with the aim of studying deep electric conductivity characterizing melt-
ing processes in the asthenosphere. Work on this project, named ELAS (ELectrical
conductivity of the ASthenosphere), was conducted throughout the world, and the
objectives of the projects were extended. It had included investigations of the con-
ductivity in the lithosphere and asthenosphere (Electrical conductivity of the Litho-
sphere and ASthenosphere). The ELAS project brought geoelectrics to the forefront
of modern geodynamics.
One of the most important events in the history of the ELAS project was the
EMSLAB experiment (ElectroMagnetic Study of the Lithosphere and Astheno-
sphere Beneath the Juan de Fuca plate) conducted from 1985 to 1988 by US,
Canadian, and Mexican geophysicists on the Pacific coast of North America in the
Cascadia subduction zone (where the Juan de Fuca microplate subducts under the
northwestern continental margin).
Figure 12.40 shows the network of the EMSLAB observations (Wannamaker
et al., 1989a). Almost the entire Juan de Fuca plate and adjacent part of the Pacific
orogenic belt were covered by MV soundings with a spacing of 50-100 km. MT
soundings were performed mainly on an E-W profile near the town of Lincoln.
This profile was named the Lincoln line. Spacings of MT and MV soundings on the
Lincoln line were about 5 km (39 MTS sites) in a period range of 0.01-500 s and
10 km (15 deep MTS sites) in a period range of 50-10000 s.
Researchers involved in the EMSLAB experiment had hoped to obtain new infor-
mation on the state and structure of the crust and upper mantle in the Cascadia
subduction zone.
The first geoelectric models of the Cascadia subduction zone were constructed
either by means of the Backus-Gilbert smoothing method (Jiracek et al., 1989)
or manually, by the trial-and-error approach (Wannamaker et al., 1989b; Vanyan
et al., 1988). These models were vulnerable to criticism, but they showed that
