"Seismo-Electromagnetic
Signals above the Ocean: from the Earth's Upper Mantle to Ionosphere
(physics, the mathematical model and the numerical simulation)
Sergey Ershov *, Irina Mikhaylovskaya **, and Oleg Novik ***
*
Keldysh Inst. for Appl. Math. of Russian Acad. Sci., Moscow, 125047,**
Moscow State University, Faculty for Math., 119899, irina@nw.math.msu.ru ;***
Moscow State Geo Prospecting University, Moscow, 117246, POB 51, Russia; onovik@online.ru, Tel. 7 095 332-2808, Fax: 7 095 7274881Abstract:
The most powerful dynamic processes transforming the Earth are located beneath the World Ocean’s floor covering about 75% of the earth surface. Seismic excitations in ocean lithosphere (i.e. disturbances of its mechanical state) accompanied by tsunami and/or earthquakes are manifestations of these large-scale processes. Is it possible to get information about geodynamic processes under the Ocean in the form of electromagnetic (EM) signals in the atmosphere?
EM signals associated by researches with seismic activity of ocean lithosphere have been detected more then once, in particular in near sea Pacific regions (the most active tectonically segment of the Earth). The lithosphere-ionosphere EM coupling is being widely discussed especially in view of EM signals recorded by satellites’ instrumentation above earthquake preparation zones (characteristic size of order of 1000 km). But the physical mechanism of these seismo-EM phenomena and connection between EM signals’ amplitudes and spectra in atmosphere, on the one hand, and these characteristics of seismic excitations of ocean lithosphere and tsunami waves, on the other hand, is not well understood, especially from a quantitative point of view needed for the monitoring and prognosis.
Basing on the main physical principles, we formulate a mathematical model of transformation of a mechanical seismic process behind the Ocean floor into EM signals above the Ocean. On the base of this model we trace numerically and graphically the generation and propagation of EM signals in a seismically disturbed moving medium including a lithosphere zone, a sea and an atmosphere zone up to the lower boundary of ionosphere. We demonstrate: the spatial modulation of a long lithosphere EM wave by a seismic wave; a “frozen” seismo-EM wave near the sea bottom (because of high electric conductivity of sea water); a rupture(!) of a diffusive seismo-EM field in the Ocean caused by a seismic wave arrival at the sea bottom; joining of counters of the broken EM field and other features of the physical mechanism of seismo-EM signals above the Ocean.
As a result, we show that measurable low frequency EM signals arise in atmosphere(e.g. hundreds of pT at the Ocean-atmosphere interface and tenths of pT near the lower boundary of ionosphere) in the course of development of a seismo-hydro-EM process initialized by a seismic excitation in the earth depth under the Ocean.
The quantitative characteristics of the geophysical field interaction process are discussed and compared with known measurements. The recommendations for the EM tsunami monitoring are given.