Electromagnetic precursors to earthquakes in the Ulf band: A review of observations and mechanisms

Abstract

Despite over 2 decades of international and national monitoring of electrical signals with the hope of detecting precursors to earthquakes, the scientific community is no closer to understanding why precursors are observed only in some cases. Laboratory measurements have demonstrated conclusively that self potentials develop owing to fluid flow and that both resistivity and magnetization change when rocks are stressed. However, field experiments have had much less success. Many purported observations of low‐frequency electrical precursors are much larger than expectations based on laboratory results. In some cases, no precursors occurred prior to earthquakes, or precursory signals were reported with no corresponding coseismic signals. Nonetheless, the field experiments are in approximate agreement with laboratory measurements. Maximum resistivity changes of a few percent have been observed prior to some earthquakes in China, but the mechanism causing those changes is still unknown. Anomalous electric and magnetic fields associated with fluid flow prior to earthquakes may have been observed. Finally, piezomagnetic signals associated with stress release in earthquakes have been documented in measurements of magnetic fields.

Electric potential variations associated with yearly lake level variations

“…Electric potential variations have been observed in some instances before earthquakes [Park et al, 1993], for example in China [Raleigh et al, 1977] or Greece [Varotsos et al, 1993]. These observations are sparse, their correlation with earthquakes in Greece is still a matter of debate [Geller et al, 1996], some may be affected by serious instrumental problems or the presence of industrial noise [Gruszow et al, 1996] and they are usually not confirmed by independent measurements using other geophysical methods.…”

Estimating the seismotelluric current required for observable electromagnetic ground signals

“…These include the electrokinetic and magnetohydrodynamic effects resulting from fluid flowing through rocks, piezomagnetism, stressinduced variations in crustal conductivity, microfracturing, and so on (Draganov et al, 1991;Park et al, 1993;Park, 1996;Fenoglio et al, 1993Fenoglio et al, , 1995Johnston, 1997;Merzer and Klemperer, 1997;Hayakawa, 1995, 1998;Molchanov et al, 2001;Egbert, 2002;Surkov et al, 2003;Simpson and Taflove, 2005). Recent efforts stemming from rock experiments performed under laboratory conditions, have attempted to unite the assortment of phenomena reported prior to large earthquakes into a single mechanism that originates from the semiconductor-like behavior of pressurized igneous and high-grade metamorphic rocks (e.g., Freund, 2007a, b;Freund et al, 2009).…”

Remote Detection of the Electric Field Change Induced at the Seismic Wave Front from the Start of Fault Rupturing

“…The seismic wave speed is about 6 km/s for the primary (P) wave and 3.5 km/s for the secondary wave [1,2] in the upper crust. There have been many reports on the electromagnetic phenomena of earthquakes and volcanic eruptions (e.g., [3][4][5][6][7]), but only a few papers have dealt with the field observation of electromagnetic variation that started just after the rupture of the fault [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Belov et al [8] detected associated electromagnetic fields about 10 seconds before the seismic wave arrival and 4 seconds after the origin time.…”