| To elucidate the features of the velocity structure of the aftershock gap between the Wenchuan and Lushan earthquakes and further explain why the aftershock gap exits,explore the relation between the big earthquakes and velocity anomalies in the southeastern(SE)margin of the Tibetan Plateau(TP)and the big earthquake generation mechanism in this area and provide constraints on the different deformation models of the TP,we conducted joint inversion of body waves and surface waves and joint inversion of body waves,surface waves and gravity data respectively to obtain new 3D Vp and Vs models of the crust and upper mantle of the SE margin of the TP.For the body-wave data,we used 7190 events recorded by 102 stations for the period of 2001-2004,from May to August 2008 and April to May 2013 in the SE margin of the TP.Rayleigh wave dispersion curves at periods from 4 to 40 s are extracted from ambient seismic noise data recorded by the densely deployed array,containing 298 broadband seismometers with inter-station distance of about 15 km in the SE margin of the TP.The gravity data is Bouguer gravity anomaly,extracted from global gravity model EGM2008 provided by Bureau Gravimétrique International.At shallow depths,the patterns of velocity anomalies inverted jointly by body and surface waves coincide with the local geology.The low-velocity anomalies in the Sichuan Basin are associated with thick sediments and the high-velocity anomalies in the Longmenshan block correspond well with the Middle to Upper Triassic flysh.At the depth of 10 km,the velocity distribution is heterogeneous.There are low-,highand low-velocity anomalies from northwest to southeast in the Sichuan Basin,corresponding well with the Northwestern Depression,the Central Uplift and the Southeastern Depression in the Sichuan Basin respectively.Along the Longmenshan fault(LMSF)there exit three strong high Vp and Vs bodies from northeast to southwest,which correspond to metamorphic rocks of Xueshan plateau,Pengguan Massif and Baoxing-Kangding Massifs,respectively.There are two obviously low-velocity anomalies between the three high-velocity bodies along the LMSF zone in the upper crust(LV1 and LV2).In LV1 less coseismic slip and fewer aftershocks of the Wenchuan earthquake occurred than in the neighboring high-velocity regions.The LV2 is situated in the aftershock gap of the Wenchuan and Lushan earthquakes.In the two low-velocity regions,materials are weak in mechanical strength and it is hard to accumulate enough stress for large earthquakes to occur.So low-velocity zones may act as barriers for rupturing of earthquakes.In the SE margin of the TP,most large earthquakes occurred around the boundaries between the low-and high-velocity zones.It is interpreted that in high-velocity zones,stress can be more easily accumulated but ruptures don't easily occur.However,in low-velocity areas,materials are weak in mechanical strength and are not apt to accumulate stress.The boundaries between high-and low-velocity zones combine features of both high-and low-velocity zones.Stress can be accumulated and ruptures are apt to occur around the contrasts between high-and low-velocity regions,as a result,the contrast regions may be suitable locations to generate large earthquakes.Compared with the Vs model inverted by body waves only,the Vs model inverted jointly by body and surface waves reveals clearer low-velocity zones(LVZs)in the mid-lower crust of the SE margin of the TP.Combined with previous studies,the LVZs may represent mid-lower crustal channel flow.Our velocity models indicate that the distribution of mid-lower crustal flow is not homogeneous.The velocity models inverted jointly by body waves,surface waves and gravity data are similar to those inverted by body waves only and inverted jointly by body and surface waves in large scales but are different in some details.Some detailed features can be well delineated by adding the gravity data. |
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