In this paper we give a review of several previously published papers on anomalous tremors observed before the 2008 Ms8.0 Wenchuan earthquake. Based on the observed time and frequency shifts between coastal and inland stations, we discussed some methods to distinguish different kinds of microseisms, and speculated that a pre-earthquake typhoon might have caused a "mainland-originated microseism" which in turn triggered the earthquake.
Based on co-seismic displacements recorded by terrestrial GPS stations and seafloor GPS/acoustic stations, the static slip model of the 2011 Mw 9.0 Tohoku earthquake was determined by inverting the data using a layered earth model. According to a priori information, the rupture surface was modeled with a geometry that is close to the actual rupture, in which the fault dip angle increases with depth and the fault strike varies with the trend of the trench. As shown by the results inferred from the joint inversion, the "geodetic" moment is 3.68 × 10 22 Nm, corresponding to Mw 9.01, and the maximum slip is positioned at a depth of 13.5 km with a slip magnitude of 45.8 m. Rupture asperities with slip exceeding 10 m are mainly distributed from 39.6 to 36.97°N, over a length of almost 240 km along the trench. The slip was mostly concentrated at depths shallower than 40 km, up-dip of the hypocenter. "Checkerboard" tests reveal that a joint inversion of multiple datasets can resolve the slip distribution better than an inversion with terrestrial GPS data only-especially when aiming to resolve slip at shallow depths. Thus, the joint inversion results obtained by this work may provide a more reliable slip model than the results of other studies that are only derived from terrestrial GPS data or seismic waveform data.
A three-dimensional crustal density model beneath North China is determined using P-wave traveltimes and gravity datum by sequential inversion method in this paper. To improve the uniqueness of the solution, we used a linear relationship between velocity and density to achieve mutual conversions and constraints between velocity difference and density difference. Algebraic reconstruction technique (ART) was used in density inversion, which highly improved the calculation speed comparing with common least squares method. The inversion results indicate that the crustal density beneath North China is extremely inhomogeneous with its distribution coherent with regional tectonics. The lower crust of Taihang mountain tectonic zone shows an obvious low density characteristic. We proposed that it may be an indicator of upwelling of mantle materials or partial melting of lowermost crust.
The mode serials of the Earth's free oscillation provide some important information on the Earth's deep structure and superconducting gravimeters (SG) can investigate the phenomena of the Earth's free oscillations with high accuracy. The great Sumatra-Andaman earthquake fully excited the Earth's free oscillations and these signals were perfectly recorded by five superconducting gravimeters in the globe. After the pre-treatment and spectral analysis on the SG observation data, we obtained the experimented mode serials of the Earth's free oscillations consisting of 147 modes with GGP station data. These observed modes were themselves some new important data for the study of the Earth's deep structure. On the basis of the discussions on some checked inner-core-sensitive modes, we distinguished three layers from the inner core, and the boundary of the upper layer was compatible with the formerly known transition zone in the inner core based on seismic body waves and supported that there were the hemispherical variation and very lower shear velocity zone in the lower inner core.
