Study Seismicity By Satellite Lithospheric Magnetic Field And Curie Isotherm Depth Inversion

The global satellite lithospheric magnetic field (LMF) model NGDC-720is used for Mainland China and surroundings. The distribution characteristics of different components and their gradients of LMF in different altitudes are studied. The topography, active tectonics, crust thickness and surface heat flux are all compared to LMF. The spatial distribution of seismicity (≥Ms5.0)over the LMF is investigated statistically, and a uniform regularity is found for different time spans, which is somewhat different with former studies. A theoretical model is built to explain this regularity, and different model parameters are tested to study the characters of the output. The Curie isotherm depth is inverted by LMF. We verify the result, and compare it with the heat flux, seismicity and active tectonics. The principle research achievements and conclusions are:1. The spatial distribution of LMF(1) Among the three components (Bx, By, Bz) of LMF, Bz dominates in the total intensity, and is also most consistent with geology and tectonics.(2) LMF at different altitudes correspond to active tectonics of different levels:the surface Bz is more consistent with level II blocks and their boundaries, while Bz at high altitudes is more consistent with level I blocks and their boundaries.(3) Main tectonics (basins, cratons, orogens, faults) show distinct distributions over Bz. Giant basins and cratons usually show obvious positive anomalies, orogens commonly show obvious negative anomalies, and most large faults are located on boundaries of obvious anomalies.(4) The surface heat flux is nearly in negative correlation with Bz at200km altitude.2. LMF and seismicity(1) The correlation between seismicity and Bz and its gradient is consistent for different time spans. For Bz at200km altitude, most earthquakes occurred in areas with Bz=-5～3nT, or areas with|▽HBz|=0.015～0.025nT.(2) Bz at200km altitude is most correlated to seismicity in the same tiem span with satellite data.53.2%earthquakes occurred in areas with Bz=-5～-3nT, and the probability for earthquakes to occur in these areas is4.7times that of the statistical mean. (3) A TESD (Two Equivalent Source Dipole) model is put forward to explain this correlation. The difference of rheological parameters caused by different Curie depth is proposed to be the key reason for the correlation.3. Curie isotherm depth inversion(1) By using of ESD forward and NCGM inversion, we get the0.5°×0.5°Curie depth distribution in the research area. The RMS error is no larger than lnT.(2) The Curie depth is consistent with the active tectonics. Giant basins usually correspond to deep Curie depth (>40km), orogens usually correspond to shallow Curie depth (<20km), and most larger faults distribute along the gradient belts of the Curie depth.(3) Statistical comparisons between the Curie depth and seismicity show that most earthquakes occurred in areas with a Curie depth of10-30km, e.g. for the seismicity between2000and2010,88%earthquakes occurred in areas with Curie depth of10-30km. The focal depth shows positive correlation with the Curie depth, and this correlation is max (the slope of the fitted line is0.25) in the same time span with satellite observations. Earthquakes with Ms>7.0usually occurred in areas with Curie depth no deeper than25km.(4) The Curie depth shows negative correlation with surface heat flux, and the slope of the fitted line is-0.5. The Curie depth inverted by LMF is consistent with that conjectured by the temperature gradient:66.3%are in an difference of±10km, and93.5%are in an difference of±20km.(5) The surface heat flux and underground temperature profile are calculated by the Curie depth. The calculated surface heat flux shows good correspondence with seismicity and volcanos. Most earthquakes or volcanos are located in high heat flux areas (>80mW/m2), or in the heat flux gradient belts.