EM Monitoring of Crustal Processes Including the Use of the Network-MT Observations |
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| Authors: | Makoto Uyeshima |
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| Institution: | (1) Earthquake Research Institute, The University of Tokyo, 1-1-1 Yayoi, Bunkyo, Tokyo 113-0032, Japan |
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| Abstract: | There are several kinds of coupling mechanisms which can convert mechanical, chemical or thermal energies due to seismic or
volcanic activities into electromagnetic energies. As a result of concentrated efforts in laboratory and theoretical research,
the basic relationship between the intensity of electromagnetic sources and changes in mechanical, chemical and thermal state
is becoming established. Also with the progress of the electromagnetic simulation techniques, it has been possible to evaluate
in situ sensitivity. Based on this progress and also due to extensive improvement in measuring techniques, many field experiments
have been performed to elucidate subsurface geophysical processes underlying the preparation stage, onset, and subsequent
healing stage of earthquakes and volcanic eruptions. In volcanic studies, many studies have reported the measurement of electromagnetic
signals which were successfully interpreted in terms of various driving mechanisms. Although there have been numerous reports
about the existence of precursory electromagnetic signals in seismic studies, only a few of them could be successfully explained
by the proposed mechanisms, whereas coseismic phenomena are often consistent with those mechanisms including the absence of
detectable signals. In many cases, one or two orders of higher sensitivity were required, especially for precursory signals.
Generally, electromagnetic methods are more sensitive to near-surface phenomena. It will be necessary to discriminate electromagnetic
signals due to these near-surface sources, which often possess no relationship with the crustal activities. Further efforts
to enhance in situ sensitivity through improvements in observation techniques and in data processing techniques are recommended.
At the same time, multi-disciplinary confirmation against the validity of electromagnetic phenomena will inevitably be necessary.
A Network-MT observation technique has been developed to determine large-scale deep electrical conductivity structure. In
the method, a telephone line network or purpose-built long baseline cables are utilized to measure voltage differences with
long electrode separations. Because of the averaging effect of the electric fields, static shift problems due to small-scale,
near-surface lateral heterogeneities can be alleviated. Several field experiments revealed regional scale deep electrical
conductivity structures related to slab subduction or its stagnation, which enable us to elucidate underlying physical processes
caused by the slab motion. The technique can also be applied to monitor the electric potential field related to crustal activities.
The annual variation of the potential field and electrical conductivity in the French Alps were interpreted to be caused by
the annual variation of lake water level. The method was also used to monitor the regional scale spatio-temporal variation
of the SP field and electrical conductivity before and at the onset of earthquakes and volcanic eruptions. |
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| Keywords: | Electromagnetic methods Earthquake Volcano Monitoring Network-MT method |
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