Hydrothermal anomalies of the Earth's surface and crustal seismicity related to Ms8.0 Wenchuan EQ

Hydrothermal anomalies related to the Ms8.0 Wenchuan earthquake (EQ) on May 12, 2009, have been widely reported. However, the reported anomalies have not been associated with multi-geosphere analysis, and space–time analysis with crustal seismicity is lacking. In this paper, the space–time variation of hydrothermal parameters, including soil moisture, soil temperature, near-surface relative humidity (RHsig995) and air temperature (TMPsfc), was first extracted and analyzed with the NCEP-FNL reanalysis dataset. The b-value (a seismic parameter from the Gutenberg–Richter law) was calculated and mapped to unravel the crustal stress and rock rupture. Our results reveal a similar time window for hydrothermal anomalies on April 20 and April 30, 2008, and these anomalies are mainly distributed along the southern and middle parts of the Longmenshan fault zone. The surface temperature anomalies lag behind the humidity anomalies, and the accelerating stress accumulation started since June 2007 and lasted for eight to nine months before the mainshock. The b-value mapping shows a segmented difference along strike of the Longmenshan fault, and that regional stress accumulated mainly in the southern parts of the F2 and F3 faults. We propose the occurrence of a complex coupling process led by crustal stress buildup before the Wenchuan EQ. The anomalies are concentrated in the southern part of the surface rupture zone. The prolonged crustal stress accumulation corresponds to the short intermittent hydrothermal response on the Earth’s surface before the Wenchuan EQ. Our findings reveal new hydrothermal anomalies in the Earth’s surface and atmosphere and explore direct link with seismogenic processes in the crust.

Evaluation of crustal inhomogeneity parameters in the southern Longmenshan fault zone and adjacent regions

Broadband teleseismic waveform data from 13 earthquakes recorded by 70 digital seismic stations were selected to evaluate the inhomogeneity parameters of the crustal medium in the southern Longmenshan fault zone and its adjacent regions using the teleseismic fluctuation wavefield method. Results show that a strong inhomogeneity exists beneath the study region, which can be divided into three blocks according to its structure and tectonic deformation features. These are known as the Sichuan-Qinghai Block, the Sichuan-Yunnan Block, and the Mid-Sichuan Block. The velocity fluctuation ratios of the three blocks are approximately 5.1%, 3.6%, and 5.1% in the upper crust and 5.1%, 3.8%, and 4.9% in the lower crust. The inhomogeneity correlation lengths of the three blocks are about 10.1 km, 14.0 km, and 10.7 km in the upper crust and 11.8 km, 17.0 km, and 11.8 km in the lower crust. The differences in the crustal medium inhomogeneity beneath the Sichuan-Yunnan Block, the Sichuan-Qinghai Block, and the Mid-Sichuan Block may be related to intensive tectonic movement and material flow in the crust and upper mantle.

High-resolution lithosphere viscosity structure and the dynamics of the 2008 Wenchuan earthquake area: new constraints from magnetotelluric imaging

SUMMARY Estimation of lithospheric viscosity remains challenging, especially for variations with spatial scales less than 100 km. Some recent studies have developed a method to determine viscosity structure from electrical conductivity models determined from magnetotelluric (MT) data. This method was initially applied to the extensional transition zone from the Great Basin to Colorado Plateau. Here, we use this approach to infer the effective lithospheric viscosity in a convergent setting by using an MT profile that crosses the eastern margin of the Tibetan Plateau. The profile extends from the Songpan-Ganzi block, crosses the 2008 Wenchuan earthquake epicentre region and ends in the Sichuan basin. The preferred viscosity structure is characterized by the middle-lower crustal viscosities in the range 2.42 × 1018 to 2.69 × 1021 Pa s below the Songpan-Ganzi block. In the Longmenshan fault zone and 2008 Wenchuan Ms8.0 earthquake area, the crustal viscosity is higher and in the range 4.32 × 1018 to 5.10 × 1021 Pa s with significant small-scale (<100 km) lateral variations. The MT-derived viscosities are consistent with previous regional-scale estimates but reveal the viscosity structure in more detail. The preferred geodynamic model can explain both the crustal deformation velocity and the small-scale lateral variations of surface topography. It implies that the crustal deformation is driven by mantle flow that results in a weak coupling of the upper and middle-lower crust beneath the eastern Tibetan Plateau. The inferred viscosity structure may help further understand the earthquake mechanisms in the Longmenshan fault zone.