ULF Magnetic Ionosphere Precursor of Sichuan Earthquake Detected by [SQUID]2 System in LSBB-Rustrel
Résumé
One hour before the Sichuan earthquake (Mw=7.9, 2008 May 12, 06:28~TU), amplified, recurrent and well-defined ionosphere excitations at the resonance frequency of the ionosphere floor were observed by the [SQUID]2 system, an ultra low-noise magnetometer with a rejection rate better than 3fT/√Hz over 40~Hz installed within the LSBB underground laboratory of Rustrel, France, 60~km on the East of Avignon (43°56'26.04~N, 05°29'04.11~E), buried 550~m in a carbonate platform. The period of this resonance is about 80 seconds. At such extreme ULF frequency whatever is the distance from a signal source on Earth, one is in a near-field electromagnetic proximity. As a result, this non-invasive instrument detects world-wide ULF ionosphere emissions, as demonstrated by the detection of P waves arrival at the ionosphere floor for world-wide quakes M>3. The persistent excitation of May 12th started just after a magnetic jump. Two additional jumps of almost one nanoTesla were observed before the quake. These three jumps are in rather good time coincidence with appearances of luminous clouds colored like rainbows and documented by amateur videos taken in the Shaanxi Province at 500~km from epicenter, at the start of the excited regime, in the Gansu Province thirty minutes before the quake at Tian-Shui 450~km from epicenter for the second one, and again ten minutes before the quake at Mei-Xian in the Shaanxi Province. The extinction of the excited regime occurred 300 seconds after the quake, the delay it takes for a P wave to reach the ionosphere floor. The first aftershock, at 06:41, was marked by an instantaneous magnetic pulse, confirming the presence of strong electric fields associated to the quake. Comparing with similar manifestations (San Juan Bautista 1998, Kobe 1995) one can conclude that an ionosphere magnetic precursor of the Sichuan earthquake has been observed by remote sensing with much better sensitivity and time resolution than in previous cases.