The attenuation characteristics and site response are calculated respectively for each individual tectonic unit in Sichuan (Sichuan Basin,west Sichuan plateau and Panzhihua-Xichang area),using digital waveform data recorded by regional seismic networks and relevant seismic phase data collected from China Seismograph Network.The frequency dependent Q(f) is obtained by the iterative grid-search technique described by Atkinson and Mereu based on trilinear geometrical spreading model.The source spectra are determined by the model of Brune and the site responses of seismic stations are derived by Moya's method using genetic algorithms.Comparison to conventional M_L estimates shows that the network local magnitude bias is quite significant at low and intermediate magnitudes.The bias at the j^th station for the i^th event is defined as ΔM_ij=M_ij-M_i, where ΔM_ij is the station magnitude and M_i the network-average value.For comparison,we mapped the spatial distribution of biases by digital seismograms recorded from 10535 earthquakes of magnitude 2.5≤M_L≤4.9 that occurred in Sichuan from January 1,2009 to June 30,2015.Based on the above data,the attenuation characteristics,site response and their effects on magnitude determination in Sichuan are analyzed.Our results demonstrate that the associated model for regional quality factor for frequencies can be expressed as Q₁(f)=450.6f^{0.513 4} for Sichuan Basin,Q₂(f)=136.6f^{0.581 3} for west Sichuan Plateau and Q₃(f)=101.9f^{0.666 3} for Panzhihua-Xichang area.Site response results indicate that different stations show different amplifications.Maps of biases appear to be different,but with similar dominant spatial distribution.For stations in Sichuan Basin,their greater magnitudes are functions of low attenuation in structure and amplification effects of both seismic stations and basin effects.For stations in west Sichuan Plateau,the possible causes of these lower magnitudes are severe dependence upon source region due to extreme lateral variations in either structure or path effect attenuation.For stations in Panzhihua-Xichang area,broken medium caused by strong tectonic activity or large earthquakes and heat flow up-welling along active faults may be the main reasons of low magnitude values when earthquakes occur in western Sichuan and eastern Tibetan region.And the greater magnitudes for earthquakes along the Longmen Mountains appear to be well correlated with edge effect of sedimentary basin on strong ground motion.In our study,stations magnitude biases appear to be extremely correlated with tectonic structures and different regions when seismic rays passing through,magnitudes are affected significantly by lateral variations in attenuation characters rather than site responses.

Stress changes due to the co-seismic slip on the source fault of the 2008 M_W7.9 Whenchuan earthquake and delayed response of inelastic deformation in the lower crust and upper mantle have an important role in the seismicity in Longmenshan area. After the Wenchuan earthquake,seismicity shows progressively increasing in a wide region. However,the south segment of the Longmenshan Fault did not show any significant change in seismicity,where positive Coulomb failure stress change(ΔCFS)was estimated under the elastic half-space model. Under such a background,the 2013 M_W6.6 Lushan earthquake occurred. This paper presents some preliminary results based on seismicity analysis and stress analysis using lithology models in which the lower crust and the upper mantle are suggested to be viscoelastic. The Wenchuan earthquake resulted in a miner negative coseismic ΔCFS in the hypocenter region of the Lushan earthquake. As a result of inelastic response the estimated ΔCFS reached the order of 0.2～0.4bar,a value sufficient to trigger earthquakes in critically loaded faults. We thus conclude that the Lushan earthquake provides a case of inelastic triggering of the Wenchuan earthquake. The 1970 M6.2 Dayi earthquake caused an obvious Coulomb stress shadow in its source area,which partly overlaps to the seismic gap between the ruptures of the Lushan and Wenchuan earthquakes. The stress shadow still exists although the area has been loaded by both the Wenchuan and Lushan earthquakes. We thus suggest that it is less likely that a great earthquake,which ruptures the entire gap,may occur in the near future if there are no other unknown factors.

To investigate the mechanism of seismicity due to impoundment of a reservoir,we propose a method for integrated study on regional/local seismicity and stress by both surface loading and pore pressure diffusion.As an example,possible role of Zipingpu Reservoir on nearby seismicity was studied in detail.The epicenter of the M_S 8.0 Wenchuan earthquake on May 12 of 2008 is very close to the Zipingpu Reservoir.On one hand,several statistical properties including event rate(n),cumulative energy release(E),seismic b-value in the magnitude-frequency relation,and spatial correlation length(SCL)of earthquakes occurring in Zipingpu area from 2004.8 to 2008.5.11 were estimated in detail.On the other hand,we quantitatively examined change of Coulomb Stress(ΔCFS)due to the impoundment of the reservoir.Both weight loading and pore pressure diffusion resulted in significant ΔCFS on the underlying Yingshu-Beichuan and Guanxian-Mianzhu Faults,which are considered as the source faults of the Wenchuan earthquake.Some clear correlations were verified between the local seismicity and stress change,thus we concluded that the impoundment of Zipingpu Reservoir clearly affected the local seismicity and it is worthwhile to further study if the effect played a role in triggering the Wenchuan earthquake.

The M_S 8.0 Wenchuan earthquake and its 2216 aftershocks were relocated using the double difference algorithm.The horizontal and vertical errors of the 2061 relocated hypocenters are approximately 1～2km and 2～3km,respectively.The epicenter of main shock is approximately 31.00°N,103.38°E,the focal depth is about 13km and the seismogenic structure is the central fault of Longmenshan Fault zone.The total length of spatial distribution of aftershocks along the strike of the fault is about 330km and the predominance distribution of focal depth is 3～20km,which shows obviously the characteristic of segmented activity.The seismicity of the southern part mainly concentrates on the central fault of Longmenshan Fault zone,and some earthquakes occurred on the range-front and range-back faults;the dip of the three faults seems to become gentler gradually from west to east,forming imbricate ruptures.The central fault and Pingwu-Qingchuan Fault of the Longmenshan Fault zone are involved in the seismogenic process,and the seismic rupture is both of thrust napping and right-lateral strike-slipping.

2741 aftershocks of the May 12,2008 M_S8.0 Wenchuan earthquake from May 12 to June 26 were relocated using the double difference algorithm to investigate tentatively the seismotectonic deformation and rupture process of the earthquake.Most relocated aftershocks occurred in the upper crust at depths of 0～20km,and fewer in the lower crust from depth of 25 to 40km,which may correspond to faulting in the lower crust.An aseismic layer appears beneath the Longmenshan thrust nappe zone between 20 and 25km depth after relocation,which may be correlated with the detachment for thrusting of the nappe zone.Spatial distribution of the relocated aftershocks and focal mechanism solutions show that the earthquake ruptured from south to north unilaterally with segmentation characteristics.The seismic belt formed by the aftershocks may be divided into two segments by the Gaochuan stepover in Anxian.The width of the seismic belt becomes narrower from south to north in map view,implying that the dip of the seismic fault may become steeper northward.The focal mechanism solutions suggest that rupturing in the south segment where the initial rupture point locates may be dominated by thrust slip with less component of strike slip,while it may have both thrust slip and dextral strike slip in the north segment.From Beichuan to the northeast end of the rupture zone,the seismic belt cannot be correlated with any fault in the north segment of the Longmenshan thrust nappe zone,even it cuts athwart the Qingchuan Fault.We can conclude that no controlling fault in the north segment of the Longmenshan thrust nappe zone was involved in earthquake generating and rupturing.The hypocenters in this part delineate a near vertical rupturing plane perpendicular to the strike of the seismic belt,and the focal mechanism solutions show great dextral strike slip component.