An Integrated Study of ULF Magnetic Field Variations in Association with the 2008 Sichuan Earthquake, on the Basis of Statistical and Critical Analyses

Abstract

The ultra-low frequency (ULF) magnetic field data at a station very close to the 2008 Sichuan earthquake (EQ) (on 12 May, 2008; M = 8.0) are extensively studied on the basis of combined statistical and natural time analyses. Two effects in ULF are treated: one is the well-known ULF radiation from the lithosphere, and the other is the non-conventional depression of ULF horizontal magnetic field. The simple statistical analysis has yielded: 1) no clear evidence of the presence of precursory ULF radiation, and 2) a significant effect of depression of ULF horizontal field a few days before the EQ (as a signature of ionospheric perturbations). The recently introduced natural time analysis has also been performed in order to study the critical features of the lithosphere and essentially new information has been brought about. The parameter "polarization", as the ratio of vertical to horizontal components, showed critical features in the time period of 17 -27 April, about one month to two weeks before the EQ as a signature of lithospheric radiation. Then, the natural time analysis has reconfirmed the presence of ionospheric perturbations a few days be-fore the EQ, together with an additional time window found on 19 -23 April, about one month before the EQ, exhibiting critical features in the ULF depression.

Indications of Ground-based Electromagnetic Observations to A Possible Lithosphere–Atmosphere–Ionosphere Electromagnetic Coupling before the 12 May 2008 Wenchuan MS 8.0 Earthquake

“…Many researchers have investigated the EM effects since the occurrence of the Wenchuan MS 8.0 EQ on 12 May 2008. These studies were mainly based on the ground-based ultra low frequency / extremely low frequency (ULF-ELF) EM data [32,[34][35][36][37][38][39][40][41] and DC-ULF geomagnetic data [42][43][44][45][46][47][48][49][50][51].…”

Criticality Analysis of the Lower Ionosphere Perturbations Prior to the 2016 Kumamoto (Japan) Earthquakes as Based on VLF Electromagnetic Wave Propagation Data Observed at Multiple Stations

“…The analysis is performed by means of the recently proposed natural time (NT) method [ 33 , 34 , 35 ]. Beyond the seismic electric signals (SES) of Varotsos’ group, the NT method has already been successfully applied on other EM variations possibly related to EQs, such as MHz-kHz EM emissions [ 8 , 36 , 37 ] and ground-observed ULF magnetic fields (e.g., [ 31 , 38 , 39 , 40 ]). Yet, this is the first attempt of application of NT analysis to VLF subionospheric propagation data.…”

Anomalous phenomena in DC–ULF geomagnetic daily variation registered three days before the 12 May 2008 Wenchuan <i>M</i>_S 8.0 earthquake

“…It seems that the pre‐EQ criticality has taken place in a wide area of the lithosphere, and the criticality is also recognized in the lower ionosphere with a small time difference of one day or so (Hayakawa et al, 2015a). The work of Hayakawa et al (2015b) and Li Q et al () shows lower ionospheric perturbations, as well as weak lithospheric radiation in the ULF frequency, by analyzing the vertical Z and horizontal H based on ground–based geomagnetic observation. There are also an increasing number of reports corresponding to these ionospheric variations, GPS TEC and f 0 F 2 , and DEMETER parameters in both the Wenchuan epicentral area and its magnetically conjugate point area (Liu JY et al , ; Xu T et al , ; Akhoondzadeh et al ; Zhu FY et al ; Zeng ZC et al ; Yu T et al, ; Zhang XM et al ; Zhang Y et al ; etc).…”