Articles | Volume 5, issue 1
Nat. Hazards Earth Syst. Sci., 5, 1–10, 2005

Special issue: Precursory phenomena, seismic hazard evaluation and seismo-tectonic...

Nat. Hazards Earth Syst. Sci., 5, 1–10, 2005

  03 Jan 2005

03 Jan 2005

Near-seismic effects in ULF fields and seismo-acoustic emission: statistics and explanation

O. Molchanov1, A. Schekotov1, M. Solovieva1, E. Fedorov1, V. Gladyshev1, E. Gordeev2, V. Chebrov3, D. Saltykov3, V. I. Sinitsin3, K. Hattori4, and M. Hayakawa5 O. Molchanov et al.
  • 1Inst. of the Physics of the Earth, Russian Academy of Sciences, Bolshaya Gruzinskaya 10, 123995, Moscow D-242, Russia
  • 2Inst. of Volcanology, Petropavlovsk-Kamchatski, Kamchatka, Russia
  • 3Inst. of Geophysical Survey, Russian Academy of Sciences, Far-East Branch, Petropavlovsk-Kamchatski, Kamchatka, Russia
  • 4Marine Biosystems Research Center, Chiba University, 1-33, Yayoi, Chiba 263-8522, Japan
  • 5University of Electro-Communications, Chofu 1-5-1, Tokyo 182, Japan

Abstract. Preseismic intensification of fracturing has been investigated from occurrence analysis of seismo-acoustic pulses (SA foreshocks) and ULF magnetic pulses (ULF foreshocks) observed in Karimshino station in addition to seismic foreshocks. Such analysis is produced for about 40 rather strong and nearby isolated earthquakes during 2 years of recording. It is found that occurrence rate of SA foreshocks increases in the interval (-12, 0 h) before main shock with 3-times exceeding of background level in the interval (-6, -3 h), and occurrence probability of SA foreshocks (pA~75%) is higher than probability of seismic foreshocks (ps~30%) in the same time interval.ULF foreshocks are masked by regular ULF activity at local morning and daytime, nevertheless we have discovered an essential ULF intensity increase in the interval (-3, +1 h) at the frequency range 0.05-0.3 Hz. Estimated occurrence probability of ULF foreshocks is about 40%. After theoretical consideration we conclude: 1) Taking into account the number rate of SA foreshocks, their amplitude and frequency range, they emit due to opening of fractures with size of L=70-200 m (M=1-2); 2) The electro-kinetic effect is the most promising mechanism of ULF foreshocks, but it is efficient only if two special conditions are fulfilled: a) origin of fractures near fluid-saturated places or liquid reservoirs (aquifers); b) appearance of open porosity or initiation of percolation instability; 3) Both SA and ULF magnetic field pulses are related to near-distant fractures (r<20-30 km); 4) Taking into account number rate and activation period of seismic, SA and ULF foreshocks, it is rather probable that opening of fractures and rupture of fluid reservoirs occur in the large preparation area with horizontal size about 100-200km.